Biomeditsiinitehnoloogia

8/13/2019 Biomeditsiinitehnoloogia

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Biomedical Engineering and

Medical Physics

YBB0050


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Table of contents

Table of contents ........................................................................................................................ 2Table of figures .......................................................................................................................... 5

Introduction ................................................................................................................................ 71. Professional societies ........................................................................................................... 12

1.1 Biomedical Engineering Societies in the World ............................................................ 121.1.1 American Institute for Medical and Biological Engineering (AIMBE).................. 121.1.2 IEEE Engineering in Medicine and Biology Society (EMBS) ............................... 131.1.3 Canadian Medical and Biological Engineering Society.......................................... 141.1.4 European Society for Engineering in Medicine (ESEM)........................................ 141.1.5 French Groups for Medical and Biological Engineering ........................................ 141.1.6 International Federation for Medical and Biological Engineering (IFMBE).......... 151.1.7 International Union for Physics and Engineering Sciences in Medicine (IUPESM).......................................................................................................................................... 151.1.8 International Council of Scientific Unions (ICSU)................................................. 15

2. Biomedical Sensors .............................................................................................................. 172.1 Variable Resistance Sensor ............................................................................................ 172.2 Strain Gauge................................................................................................................... 182.3 Inductance Sensors ......................................................................................................... 20

2.3.1 Mutual Inductance ................................................................................................... 202.3.2 Variable Reluctance ................................................................................................ 20

2.4 Linear Variable Differential Transformer ...................................................................... 212.5 Capacitive Sensors ......................................................................................................... 212.6 Sonic and Ultrasonic Sensors......................................................................................... 22

2.6.1 Velocity Measurement ............................................................................................222.6.2 Magnetic Induction ................................................................................................. 222.6.3 Doppler Ultrasound ................................................................................................. 23

2.7 Accelerometers ............................................................................................................... 242.8 Force............................................................................................................................... 252.9 Measurement of Fluid Dynamic Variables .................................................................... 252.10 Pressure Measurement.................................................................................................. 252.11 Measurement of Flow................................................................................................... 272.12 Temperature ................................................................................................................. 292.13 Metallic Resistance Thermometers .............................................................................. 302.14 Thermistors................................................................................................................... 31

2.15 Thermocouples ............................................................................................................. 323. Physiological signals ............................................................................................................ 35

3.1 Electrocardiogram ECG................................................................................................. 353.1.2 The ambulatory ECG .............................................................................................. 383.1.3 Patient Monitoring................................................................................................... 383.1.4 High-Resolution ECG ............................................................................................. 38

3.2 Electromyography EMG ............................................................................................... 393.2.1 The Origin of Electromyograms ............................................................................. 393.2.2 Electromyographic Recordings ............................................................................... 40

3.2.2.4 Variation................................................................................................................... 423.2.3 Single-Fiber EMG................................................................................................... 42

3.2.4 Macro EMG............................................................................................................. 433.3 EEG Electroencephalography ........................................................................................ 43


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3.3.1 History..................................................................................................................... 433.3.2 EEG Recording Techniques .................................................................................... 44

3.4 Magnetoencephalography MEG ............................................................................... 453.4.1 MEG Recording Device .......................................................................................... 45

3.5 Mapping-Based on EEG or MEG .................................................................................. 46

3.6 Digital Biomedical Signal Acquisition and Processing ................................................. 473.6.1 Acquisition .............................................................................................................. 473.6.2 Signal Processing .................................................................................................... 483.6.3 Digital Filters........................................................................................................... 483.6.4 Signal Averaging..................................................................................................... 493.6.5 Spectral Analysis ..................................................................................................... 52

4. X-Ray Equipment................................................................................................................. 544.1 Production of X-Rays ..................................................................................................... 54

4.1.1 X-Ray Tube ............................................................................................................. 544.1.2 Generator ................................................................................................................. 564.1.3 Image Detection: Screen Film Combinations ......................................................... 56

4.1.4 Image Detection: X-Ray Image Intensifiers with Televisions ................................ 574.1.5 Biomedical Imaging ................................................................................................594.1.6 Image Detection: Digital Systems........................................................................... 59

4.2 Computed Tomography.................................................................................................. 604.2.1 Instrumentation........................................................................................................ 604.2.2 Data-Acquisition Geometries .................................................................................. 624.2.3 First Generation: Parallel-Beam Geometry............................................................. 624.2.4 Second Generation: Fan Beam, Multiple Detectors................................................ 634.2.5 Third Generation: Fan Beam, Rotating Detectors................................................... 634.2.6 Fourth Generation: Fan Beam, Fixed Detectors...................................................... 634.2.7 Fifth Generation: Scanning Electron Beam ............................................................ 644.2.8 Spiral/Helical Scanning...........................................................................................644.2.9 X-Ray System ......................................................................................................... 654.2.10 Computer System.................................................................................................. 68

4.3 Magnetic Resonance Imaging (MRI) ............................................................................. 694.3.1 Fundamentals of MRI.............................................................................................. 694.3.2 Fundamentals of MRI Instrumentation ................................................................... 704.3.3 Static Field Magnets ................................................................................................ 704.3.4 Gradient Coils ......................................................................................................... 714.3.5 Radiofrequency Coils .............................................................................................. 724.3.6 Functional MRI ....................................................................................................... 73

4.4 Positron-Emission Tomography (PET).......................................................................... 744.4.1 Background ............................................................................................................. 744.4.2 PET Theory ............................................................................................................. 754.4.3 Physical Factors Affecting Resolution.................................................................... 79

5. Ultrasound ............................................................................................................................ 825.1 Transducers .................................................................................................................... 82

5.1.1 Transducer Materials ............................................................................................... 825.2 Scanning with Array Transducers .................................................................................. 835.3 Ultrasonic Imaging ......................................................................................................... 845.4 Blood Flow Measurement Using Ultrasound................................................................. 865.5 Single Sample Volume Doppler Instruments................................................................. 86

5.6 Color Flow Mapping ...................................................................................................... 876. LASERS IN MEDICAL DIAGNOSTICS........................................................................... 88


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6.1 History ............................................................................................................................ 886.2 Wavelengths of different lasers...................................................................................... 886.3 Characteristics of a typical helium-neon laser ............................................................... 896.4 Absorption characteristics of tissue constituents. .......................................................... 906.5 Ophthalmology............................................................................................................... 90

6.6 Holography..................................................................................................................... 916.7 Pulse Oximetry ............................................................................................................... 91

6.7.1 Limitations .............................................................................................................. 916.8 Blood flow velocity measurements ................................................................................ 92

6.8.3 Measuring principle................................................................................................. 926.9 Lasers in Cardiovascular Diagnostics ............................................................................ 94

6.9.3 Method for optical self-mixing ............................................................................... 946.9.4 Pulse profile and pulse wave velocity..................................................................... 976.9.5 Pulse wave velocity measurement......................................................................... 1006.9.6 Blood flow measurements ..................................................................................... 101

7. Clinical engineer: safety, standards and regulations .......................................................... 104

7.1 What Is a Clinical Engineer?........................................................................................ 1047.2 Evolution of Clinical Engineering................................................................................ 1047.3 Hospital Organization and the Role of Clinical Engineering....................................... 106

7.3.1 Governing Board (Trustees).................................................................................. 1067.3.2 Hospital Administration ........................................................................................ 107

7.4 Major Functions of a Clinical Engineering Department .............................................. 1077.4.1 Technology Management...................................................................................... 1077.4.2 Risk Management.................................................................................................. 1077.4.3 Technology Assessment........................................................................................ 1087.4.4 Facilities Design and Project Management ........................................................... 1087.4.5 Training ................................................................................................................. 108

7.5 The Health Care Delivery System................................................................................ 1087.5.1 Major Health Care Trends and Directions ............................................................ 109

7.6 Technology Assessment............................................................................................... 1097.6.1 Technology Assessment Process........................................................................... 109

7.7 Risk Management........................................................................................................ 1108. Home care and rehabilitation ............................................................................................. 112

8.1 Introduction .................................................................................................................. 1128.2 Rehabilitation Concepts ............................................................................................... 1128.3 Engineering Concepts in Sensory Rehabilitation ......................................................... 1138.4 Engineering Concepts in Motor Rehabilitation............................................................ 116

8.5 Engineering Concepts in Communications Disorders ................................................. 1178.6 Appropriate Technology .............................................................................................. 1178.7 The Future of Engineering in Rehabilitation................................................................ 1188.8 Future Developments ................................................................................................... 118


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Table of figures

Figure 1. Examples of displacement sensors. ..........................................................................18Figure 2. Strain gauges on a cantilever (konsool) structure to provide temperature

compensation.................................................................................................................... 19Figure 3. Fundamental structure of an accelerometer. ............................................................. 24Figure 4. See the structure of an unbonded strain gauge pressure sensor................................ 26Figure 5. Fundamental structure of an electromagnetic flowmeter. ........................................ 27Figure 6. Structure of an ultrasonic Doppler flowmeter with the major blocks of the electronic

signal processing system. ................................................................................................. 29Figure 7. Common forms of thermistors. ................................................................................. 31Figure 8. Circuit arrangement for a thermocouple showing the voltage-measuring device. ... 33Figure 9. The 12-lead ECG. ..................................................................................................... 36Figure 10. Simulated currents and extracellular potentials of frog sartorius muscle fiber

(radius a = 50m). ............................................................................................................ 40Figure 11. EMG needle electrodes. .......................................................................................... 41Figure 12. MUP amplitude and duration.................................................................................. 41Figure 13. Measurement of interpotential interval (IPI). ......................................................... 43Figure 14. EEG measurement .................................................................................................. 44Figure 15. Schematic diagram of a multisensor MEG system (left) along with a detection coil

and SQUID in a single channel (right)............................................................................. 46Figure 16. A whole-head MEG system with 148 recording channels operated in a

magnetically shielded room. ............................................................................................46Figure 17. General block diagram of the acquisition procedure of a digital signal. ................ 48Figure 18. General block diagram of a digital filter. The output digital signaly(n) is obtained

from the inputx(n) by means of a transformation T[] which identifies the filter. .......... 48Figure 19. Equivalent frequency response for the signal-averaging procedure for differentvalues of N. ...................................................................................................................... 51

Figure 20.Enhancement of evoked potential (EP) by means of averaging technique. The EEGnoise is progressively reduced, and the EP morphology becomes more recognizable asthe number of averaged sweeps (N) is increased. ............................................................ 52

Figure 21. X-ray tube ............................................................................................................... 55Figure 22.X-ray image intensifier. .......................................................................................... 58Figure 23. Schematic drawing of a typical CT scanner installation, consisting of (1) control

console, (2) gantrystand, (3) patient table, (4) head holder, and (5) laser imager.(Courtesy of Picker International, Inc.)............................................................................ 61

Figure 24. Typical CT images of (a) brain, (b) head showing orbits, (c) chest showing lungs,and (d) abdomen............................................................................................................... 61

Figure 25. Four generations of CT scanners illustrating the parallel- and fan-beam geometries[Robb, 1982]..................................................................................................................... 62

Figure 26. The major internal components of a fourth-generation CT gantry are shown in aphotograph with the gantry cover removed (upper) and identified in the line drawing(lower). (Courtesy of Picker International, Inc.) .............................................................. 64

Figure 27.Photograph of the slip rings used to pass power and control signals to the rotatinggantry. (Courtesy of Picker International, Inc.) ............................................................... 65

Figure 28. Spiral scanning causes the focal spot to follow a spiral path around the patient asindicated. (Courtesy of Picker International, Inc.) ........................................................... 65


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Figure 29. (a) A solid-state detector consists of a scintillating crystal and photodiodecombination. (b) Many such detectors are placed side by side to form a detector arraythat may contain up to 4800 detectors.............................................................................. 67

Figure 30. Gas ionization detector ........................................................................................... 67Figure 31.The computer system controls the gantry motions, acquires the x-ray transmission

measurements, and reconstructs the final image. The system shown here uses 12 68000-family CPUs. (Courtesy of Picker International, Inc.)..................................................... 68

Figure 32. Schematic drawing of a superconducting magnet .................................................. 71Figure 33. Birdcage resonator. ................................................................................................. 72Figure 34. Functional MR image demonstrating activation of the primary visual cortex. ...... 73Figure 35. Functional MRI mapping of motor cortex for preoperative planning. ................... 74Figure 36.The MRI image shows the arteriovenous malformation (AVM) as an area of signal

loss due to blood flow. ..................................................................................................... 75Figure 37. The physical basis of positron-emission tomography............................................. 76Figure 38. Most modern PET cameras are multilayered with 15 to 47 levels or transaxial

layers to be reconstructed. ................................................................................................ 77

Figure 39. The arrangement of scintillators and phototubes is shown..................................... 78Figure 40.Factors contributing to the resolution of the PET tomograph. The contribution most

accessible to further reduction is the size of the detector crystals.................................... 79Figure 41. The evolution of resolution. ................................................................................... 80Figure 42. Resolution astigmatism in detecting off-center events. .......................................... 80Figure 43. Array-element configurations and the region scanned by the acoustic beam......... 83Figure 44. Schematic representation of the signal received from along a single line of sight in

a tissue. ............................................................................................................................. 84Figure 45. Completed M-mode display obtained by showing the M-lines side by side. ......... 85Figure 46. Schematic representation of a heart and how a 2D image is constructed by

scanning the transducer. ................................................................................................... 85Figure 47. Operating environment for the estimation of blood velocity.................................. 86Figure 48. Primary components of a laser................................................................................ 89Figure 49. Measuring principle of blood flow velocity. ......................................................... 92Figure 50. Method for optical self-mixing. .............................................................................. 94Figure 51. Method for optical self-mixing . ............................................................................ 95Figure 52. Pigtail Diode Laser: ................................................................................................95Figure 53. Mixed signal amplitude dependent from laser current. .......................................... 96Figure 54. Measured dependence of a self-mixing interference on the distance between laser

and target (first five maximums) and the spectrum of laser diode................................... 96Figure 55. Minimum registered optical power, reflected back to laser cavity, when self-mixing

output signal S/N ratio was 1, was 19 pW. ...................................................................... 97Figure 56. Measured pulse profile at the arm artery. ............................................................... 98Figure 57. Frame of recorded pulse profile signal. .................................................................. 99Figure 58. Processed pulse profile amplitude at the arm artery and processing algorithm...... 99Figure 59. Pulse wave velocity measurement ........................................................................100Figure 60. Recorded ECG, pulse profile and processed pulse profile signals. ...................... 100Figure 61. Pulse delay measured from different locations of human body and processing

algorithm. ....................................................................................................................... 101Figure 62. Block diagram of the equipment for blood flow measurements........................... 102Figure 63. Blood flow measurements signals. .................................................................... 103Figure 64. Differences between measured and calculated Doppler frequencies.................... 103

Figure 65. Diagram illustrating the range of interactions of a clinical engineer.................... 105Figure 66. Double-edged sword concept of risk management............................................... 111


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Introduction

Biomedical Engineering is no longer an emerging discipline; it has become an important vital

interdisciplinary field. Biomedical engineers are involved in many medical ventures. They areinvolved in the design, development and utilization of materials, devices (such as pacemakers,lithotripsy, etc.) and techniques (such as signal processing, artificial intelligence, etc.) forclinical research and use; and serve as members of the health care delivery team (clinicalengineering, medical informatics, rehabilitation engineering, etc.) seeking new solutions fordifficult heath care problems confronting our society. To meet the needs of this diverse bodyof biomedical engineers, this handbook provides a central core of knowledge in those fieldsencompassed by the discipline of biomedical engineering as we enter the 21st century. Before

presenting this detailed information, however, it is important to provide a sense of theevolution of the modern health care system and identify the diverse activities biomedicalengineers perform to assist in the diagnosis and treatment of patients.

Evolution of the Modern Health Care System

Before 1900, medicine had little to offer the average citizen, since its resources consistedmainly of the physician, his education, and his little black bag. In general, physiciansseemed to be in short supply, but the shortage had rather different causes than the currentcrisis in the availability of health care professionals. Although the costs of obtaining medicaltraining were relatively low, the demand for doctors services also was very small, since manyof the services provided by the physician also could be obtained from experienced amateurs inthe community. The home was typically the site for treatment and recuperation, and relatives

and neighbors constituted an able and willing nursing staff. Babies were delivered bymidwives, and those illnesses not cured by home remedies were left to run their natural, albeitfrequently fatal, course. The contrast with contemporary health care practices, in whichspecialized physicians and nurses located within the hospital provide critical diagnostic andtreatment services, is dramatic.The changes that have occurred within medical science originated in the rapid developmentsthat took place in the applied sciences (chemistry, physics, engineering, microbiology,

physiology, pharmacology, etc.) at the turn of the century. This process of development wascharacterized by intense interdisciplinary cross-fertilization, which provided an environmentin which medical research was able to take giant strides in developing techniques for thediagnosis and treatment of disease. For example, in 1903, Willem Einthoven, the Dutch

physiologist, devised the first electrocardiograph to measure the electrical activity of theheart. In applying discoveries in the physical sciences to the analysis of biologic process, heinitiated a new age in both cardiovascular medicine and electrical measurement techniques.

New discoveries in medical sciences followed one another like intermediates in a chainreaction. However, the most significant innovation for clinical medicine was the developmentof x-rays. These new kinds of rays, as their discoverer W. K. Roentgen described them in1895, opened the inner man to medical inspection. Initially, x-rays were used to diagnose

bone fractures and dislocations, and in the process, x-ray machines became commonplace inmost urban hospitals. Separate departments of radiology were established, and their influencespread to other departments throughout the hospital. By the 1930s, x-ray visualization of

practically all organ systems of the body had been made possible through the use of barium

salts and a wide variety of radiopaque materials. X-ray technology gave physicians a powerfultool that, for the first time, permitted accurate diagnosis of a wide variety of diseases and


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injuries. Moreover, since x-ray machines were too cumbersome and expensive for localdoctors and clinics, they had to be placed in health care centers or hospitals. Once there, x-raytechnology essentially triggered the transformation of the hospital from a passive receptaclefor the sick to an active curative institution for all members of society. For economic reasons,the centralization of health care services became essential because of many other important

technological innovations appearing on the medical scene. However, hospitals remainedinstitutions to dread, and it was not until the introduction of sulfanilamide in the mid-1930sand penicillin in the early 1940s that the main danger of hospitalization, i.e., cross-infectionamong patients, was significantly reduced. With these new drugs in their arsenals, surgeonswere able to perform their operations without prohibitive morbidity and mortality due toinfection. Furthermore, even though the different blood groups and their incompatibility werediscovered in 1900 and sodium citrate was used in 1913 to prevent clotting, full developmentof blood banks was not practical until the 1930s, when technology provided adequaterefrigeration. Until that time, fresh donors were bled and the blood transfused while it wasstill warm.Once these surgical suites were established, the employment of specifically designed pieces of

medical technology assisted in further advancing the development of complex surgicalprocedures. For example, the Drinker respirator was introduced in 1927 and the first heart-lung bypass in 1939. By the 1940s, medical procedures heavily dependent on medicaltechnology, such as cardiac catheterization and angiography (the use of a cannula threadedthrough an arm vein and into the heart with the injection of radiopaque dye for the x-rayvisualization of lung and heart vessels and valves), were developed. As a result, accuratediagnosis of congenital and acquired heart disease (mainly valve disorders due to rheumaticfever) became possible, and a new era of cardiac and vascular surgery was established.Following World War II, technological advances were spurred on by efforts to developsuperior weapon systems and establish habitats in space and on the ocean floor. As a by-

product of these efforts, the development of medical devices accelerated and the medicalprofession benefited greatly from this rapid surge of technological finds. Consider thefollowing examples:

1. Advances in solid-state electronics made it possible to map the subtle behavior of thefundamental unit of the central nervous system the neuron as well as to monitor various

physiologic parameters, such as the electrocardiogram, of patients in intensive care units.

2. New prosthetic devices became a goal of engineers involved in providing the disabled withtools to improve their quality of life.

3. Nuclear medicine an outgrowth of the atomic age emerged as a powerful andeffective approach in detecting and treating specific physiologic abnormalities.

4. Diagnostic ultrasound based on sonar technology became so widely accepted that ultrasonicstudies are now part of the routine diagnostic workup in many medical specialties.

5. Spare parts surgery also became commonplace. Technologists were encouraged toprovide cardiac assist devices, such as artificial heart valves and artificial blood vessels, andthe artificial heart program was launched to develop a replacement for a defective or diseasedhuman heart.

6. Advances in materials have made the development of disposable medical devices, such asneedles and thermometers, as well as implantable drug delivery systems, a reality.


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7. Computers similar to those developed to control the flight plans of the Apollocapsule wereused to store, process, and cross-check medical records, to monitor patient status in intensivecare units, and to provide sophisticated statistical diagnoses of potential diseases correlatedwith specific sets of patient symptoms.

8. Development of the first computer-based medical instrument, the computerized axialtomography scanner, revolutionized clinical approaches to noninvasive diagnostic imaging

procedures, which now include magnetic resonance imaging and positron emissiontomography as well.

The impact of these discoveries and many others has been profound. The health care systemconsisting primarily of the horse and buggy physician is gone forever, replaced by atechnologically sophisticated clinical staff operating primarily in modern hospitals designedto accommodate the new medical technology. This evolutionary process continues, withadvances in biotechnology and tissue engineering altering the very nature of the health caredelivery system itself.

The Field of Biomedical Engineering

Today, many of the problems confronting health professionals are of extreme interest toengineers because they involve the design and practical application of medical devices andsystems processes that are fundamental to engineering practice. These medically relateddesign problems can range from very complex large-scale constructs, such as the design andimplementation of automated clinical laboratories, multiphasic screening facilities (i.e.,centers that permit many clinical tests to be conducted), and hospital information systems, tothe creation of relatively small and simple devices, such as recording electrodes and

biosensors, that may be used to monitor the activity of specific physiologic processes in eithera research or clinical setting. They encompass the many complexities of remote monitoringand telemetry, including the requirements of emergency vehicles, operating rooms, andintensive care units. The American health care system, therefore, encompasses many

problems that represent challenges to certain members of the engineering profession calledbiomedical engineers.

Biomedical Engineering: A Definition

Although what is included in the field of biomedical engineering is considered by many to bequite clear, there are some disagreements about its definition. For example, consider the terms

biomedical engineering, bioengineering, and clinical (ormedical ) engineering which havebeen defined in Pacelas Bioengineering Education Directory [Quest Publishing Co., 1990].While Pacela defines bioengineering as the broadumbrella term used to describe this entirefield,bioengineering is usually defined as a basic researchorientedactivity closely related to

biotechnology and genetic engineering, i.e., the modification of animalor plant cells, or partsof cells, to improve plants or animals or to develop new microorganisms forbeneficial ends.In the food industry, for example, this has meant the improvement of strains of yeast forfermentation. In agriculture, bioengineers may be concerned with the improvement of cropyields by treatment of plants with organisms to reduce frost damage. It is clear that

bioengineers of the futurewill have a tremendous impact on the quality of human life. Thepotential of this specialty is difficult to imagine. Consider the following activities of

bioengineers: Development of improved species of plants and animals for food production


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Invention of new medical diagnostic tests for diseases Production of synthetic vaccines from clone cells Bioenvironmental engineering to protect human, animal, and plant life from toxicants and

pollutants Study of protein-surface interactions

Modeling of the growth kinetics of yeast and hybridoma cells Research in immobilized enzyme technology Development of therapeutic proteins and monoclonal antibodies

In reviewing the above-mentioned terms, however, biomedical engineering appears to havethe most comprehensive meaning. Biomedical engineers apply electrical, mechanical,chemical, optical, and other engineering principles to understand, modify, or control biologic(i.e., human and animal) systems, as well as design and manufacture products that canmonitor physiologic functions and assist in the diagnosis and treatment of patients. When

biomedical engineers work within a hospital or clinic, they are more properly called clinicalengineers.

Activities of Biomedical Engineers

The breadth of activity of biomedical engineers is significant. The field has movedsignificantly from being concerned primarily with the development of medical devices in the1950s and 1960s to include a more wide-ranging set of activities. As illustrated below, thefield of biomedical engineering now includes many new career areas.

These areas include: Application of engineering system analysis (physiologic modeling, simulation, and control)to biologic problems Detection, measurement, and monitoring of physiologic signals (i.e., biosensors andbiomedicalinstrumentation) Diagnostic interpretation via signal-processing techniques of bioelectric data Therapeutic and rehabilitation procedures and devices (rehabilitation engineering) Devices for replacement or augmentation of bodily functions (artificial organs) Computer analysis of patient-related data and clinical decision making (i.e., medicalinformatics and artificial intelligence) Medical imaging, i.e., the graphic display of anatomic detail or physiologic function The creation of new biologic products (i.e., biotechnologyand tissue engineering)

Typical pursuits of biomedical engineers, therefore, include: Research in new materials for implanted artificial organs Development of new diagnostic instruments for blood analysis Computer modeling of the function of the human heart Writing software for analysis of medical research data Analysis of medical device hazards for safety and efficacy Development of new diagnostic imaging systems Design of telemetry systems for patient monitoring Design of biomedical sensors for measurement of human physiologic systems variables Development of expert systems for diagnosis of disease Design of closed-loop control systems for drug administration

Modeling of the physiologic systems of the human body Design of instrumentation for sports medicine


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Development of new dental materials Design of communication aids for the handicapped Study of pulmonary fluid dynamics Study of the biomechanics of the human body Development of material to be used as replacement for human skin

Biomedical engineering, then, is an interdisciplinary branch of engineering that ranges fromtheoretical, nonexperimental undertakings to state-of-the-art applications. It can encompassresearch, development, implementation, and operation. Accordingly, like medical practiceitself, it is unlikely that any single person can acquire expertise that encompasses the entirefield. Yet, because of the interdisciplinary nature of this activity, there is considerableinterplay and overlapping of interest and effort between them.For example, biomedical engineers engaged in the development of biosensors may interactwith those interested in prosthetic devices to develop a means to detect and use the same

bioelectric signal to power a prosthetic device. Those engaged in automating the clinicalchemistry laboratory may collaborate with those developing expert systems to assist clinicians

in making decisions based on specific laboratory data. The possibilities are endless. Perhaps agreater potential benefit occurring from the use of biomedical engineering is identification ofthe problems and needs of our present health care system that can be solved using existingengineering technology and systems methodology. Consequently, the field of biomedicalengineering offers hope in the continuing battle to provide high-quality health care at areasonable cost; if properly directed toward solving problems related to preventive medicalapproaches, ambulatory care services, and the like, biomedical engineers can provide the toolsand techniques to make our health care system more effective and efficient.


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1. Professional societies

The field of biomedical engineering, which originated as a professional group on medicalelectronics in the late fifties, has grown from a few scattered individuals to very well-

established organization. There are approximately 50 national societies throughout the worldserving an increasingly growing community of biomedical engineers. The scope of

biomedical engineering today is enormously diverse. Over the years, many new disciplinessuch as molecular biology, genetic engineering, computer-aided drug design, nanotechnology,and so on, which were once considered alien to the field, are now new challenges a

biomedical engineer faces. Professional societies play a major role in bringing togethermembers of this diverse community in pursuit of technology applications for improving thehealth and quality of life of human beings. Intersocietal cooperations and collaborations, bothat national and international levels, are more actively fostered today through professionalorganizations such as the IFMBE, AIMBE, CORAL, and the IEEE. These developments arestrategic to the advancement of the professional status of biomedical engineers. Some of theself-imposed mandates the professional societies should continue to pursue include promoting

public awareness, addressing public policy issues that impact research and development ofbiologic and medical products, establishing close liaisons with developing countries,encouraging educational programs for developing scientific and technical expertise in medicaland biologic engineering, providing a management paradigm that ensures efficiency andeconomy of health care technology [Wald, 1993], and participating in the development of new

job opportunities for biomedical engineers.

1.1 Biomedical Engineering Societies in the World

Globalization of biomedical engineering (BME) activities is underscored by the fact that thereare several major professional BME societies currently operational throughout the world. Thevarious countries and continents to have provided concerted action groups in biomedicalengineering are Europe, the Americas, Canada, and the Far East, including Japan andAustralia. while all these organizations share in the common pursuit of promoting biomedicalengineering, all national societies are geared to serving the needs of their localmemberships. The activities of some of the major professional organizations are described

below.

1.1.1 American Institute for Medical and Biological Engineering (AIMBE)

The United States has the largest biomedical engineering community in the world. Majorprofessional organizations that address various cross sections of the field and serve over20,000 biomedical engineers include (1) the American College of Clinical Engineering, (2)the American Institute of Chemical Engineers, (3) The American Medical InformaticsAssociation, (4) the American Society of Agricultural Engineers, (5) the American Society forArtificial Internal Organs, (6) the American Society of Mechanical Engineers, (7) theAssociation for the Advancement of Medical Instrumentation, (8) the Biomedical EngineeringSociety, (9) the IEEE Engineering in Medicine and Biology Society, (10) an interdisciplinaryAssociation for the Advancement of Rehabilitation and Assistive Technologies, (11) theSociety for Biomaterials, (12) Orthopedic Research Society, (13) American Society ofBiomechanics, and (14) American Association of Physicist in Medicine. In an effort to unify


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all the disparate components of the biomedical engineering community in the United States asrepresented by these various societies, the American Institute for Medical and BiologicalEngineers (AIMBE) was created in 1992. The AIMBE is the result of a 3-year effort funded

by the National Science Foundation and led by a joint steering committee established by theAlliance of Engineering in Medicine and Biology and the U.S. National Committee on

Biomechanics. The primary goal of AIMBE is to serve as an umbrella organization for thepurpose of unifying the bioengineering community, addressing public policy issues,identifying common themes of reflection and proposals for action, and promoting theengineering approach in societys effort to enhance health and quality of life through the

judicious use of technology [Galletti, 1994].

AIMBE serves its role through four working divisions: (1) the Council of Societies, consistingof the 11 constituent organizations mentioned above, (2) the Academic Programs Council,currently consisting of 46 institutional charter members, (3) the Industry Council, and (4) theCollege Fellows. In addition to these councils, there are four commissions, Education, PublicAwareness, Public Policy, and Liaisons. With its inception in 1992, AIMBE is a relatively

young institution trying to establish its identity as an umbrella organization for medical andbiologic engineering in the United States. As summarized by two of the founding officials ofthe AIMBE, Profs Nerem and Galletti:What we are all doing, collectively, is defining a focus for biological and medicalengineering. In a society often confused by technophobic tendencies, we will try to assertwhat engineering can do for biology, for medicine, for health care and for industrialdevelopment, We should be neither shy, nor arrogant, nor self-centered. The public has greatexpectations from engineering and technology in terms of their own health and welfare. Theyare also concerned about side effects, unpredictable consequences and the economic costs.Many object to science for the sake of science, resent exaggerated or empty promises of

benefit to society, and are shocked by sluggish or misdirected flow from basic research touseful applications. These issues must be addressed by the engineering and medicalcommunities.

1.1.2 IEEE Engineering in Medicine and Biology Society (EMBS)

The Institute of Electrical and Electronic Engineers (IEEE) is the largest internationalprofessional organization in the world and accommodates 37 different societies under itsumbrella structure. Of these 37, the Engineering in Medicine and Biology Society representsthe foremost international organization serving the needs of nearly 8000 biomedicalengineering members around the world. The field of interest of the EMB Society is

application of the concepts and methods of the physical and engineering sciences in biologyand medicine. Each year, the society sponsors a major international conference whilecosponsoring a number of theme-oriented regional conferences throughout the world. Agrowing number of EMBS chapters and student clubs across the major cities of the worldhave provided the forum for enhancing local activities through special seminars, symposia,and summer schools on biomedical engineering topics. These are supplemented by EMBSsspecial initiatives that provide faculty and financial subsidies to such programs through thesocietys distinguished lecturer program as well as the societys Regional ConferenceCommittee.Other feature achievements of the society include its premier publications in the form of threemonthly journals (Transactions on Biomedical Engineering, Transactions on Rehabilitation

Engineering, and Transactions on Information Technology in Biomedicine) and a bi-monthlyEMB Magazine (theIEEE Engineering in Medicine and Biology Magazine).


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EMBS is a transnational voting member society of the InternationalFederation for Medicaland Biological Engineering.

1.1.3 Canadian Medical and Biological Engineering Society

The Canadian Medical and Biological Engineering Society (CMBES) is an associationcovering the fields of biomedical engineering, clinical engineering, rehabilitation engineering,and biomechanics and biomaterials applications. CMBES is affiliated with the InternationalFederation for Medical and Biological Engineering and currently has 272 full members. Thesociety organizes national medical and biological engineering conferences annually in variouscities across Canada. In addition, CMBES has sponsored seminars and symposia onspecialized topics such as communication aids, computers, and the handicapped, as well asinstructional courses on topics of interest to the membership. To promote the professionaldevelopment of its members, the society as drafted guidelines on education and certificationfor clinical engineers and biomedical engineering technologists and technicians. CMBES iscommitted to bringing together all individuals in Canada who are engaged in interdisciplinary

work involving engineering, the life sciences, and medicine. The society communicates to itsmembership through the publication of a newsletter as well as recently launched academicseries to help nonengineering hospital personnel to gain better understanding of biomedicaltechnology.

1.1.4 European Society for Engineering in Medicine (ESEM)

Most European countries are affiliated organizations of the International Federation forMedical and Biological Engineering (IFMBE). The IFMBE activities are described in anothersection of this chapter. In 1992, a separate organization called the European Society forEngineering in Medicine (ESEM) was created with the objective of providing opportunitiesfor academic centers, research institutes, industry, hospitals and other health careorganizations, and various national and international societies to interact and jointly exploreBME issues of European significance. These include (1) research and development, (2)education and training, (3) communication between and among industry, health care

providers, and policymakers, (4) European policy on technology and health care, and (5)collaboration between eastern European countries in transition and the western Europeancountries on health care technology, delivery, and management. To reflect this goal the ESEMmembership constitutes representation of all relevant disciplines from all European countrieswhile maintaining active relations with the Commission of the European Community andother supranational bodies and organizations.

The major promotional strategies of the ESEMs scientific contributions include its quarterlyjournal Technology and Health Care, ESEM News, the Societys Newsletter, a biennialEuropean Conference on Engineering and Medicine, and various topic-oriented workshopsand courses. ESEM offers two classes of membership: the regular individual (active orstudent) membership and an associate grade. The latter is granted to those scientific andindustrial organizations which satisfy the society guidelines and subject to approval by theMembership and Industrial Committees. The society is administered by an AdministrativeCouncil consisting of 13 members elected by the general membership.

1.1.5 French Groups for Medical and Biological Engineering

The French National Federation of Bioengineering (Genie Biologique et Medical, GMB) is amultidisciplinary body aimed at developing methods and processes and new biomedical


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materials in various fields covering prognosis, diagnosis, therapeutics, and rehabilitation.These goals are achieved through the creation of 10 regional centers of bioengineering, calledthe poles. The poles are directly involved at all levels, from applied research through theindustrialization to the marketing of the product. Some of the actions pursued by these polesinclude providing financial seed support for innovative biomedical engineering projects,

providing technological help, advice, and assistance, developing partnerships amonguniversities and industries, and organizing special seminars and conferences. The informationdissemination of all scientific progress is done through the Journal of Innovation andTechnology in Biology andMedicine.

1.1.6 International Federation for Medical and Biological Engineering (IFMBE)

Established in 1959, the International Federation for Medical and Biological Engineering(IFMBE) is an organization made up from an affiliation of national societies includingmembership of transnational organizations. The current national affiliates are Argentina,Australia, Austria, Belgium, Brazil, Bulgaria, Canada, China, Cuba, Cyprus, Slovakia,

Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, Japan, Mexico,Netherlands, Norway, Poland, South Africa, South Korea, Spain, Sweden, Thailand, UnitedKingdom, and the United States. The first transnational organization to become a member ofthe federation is the IEEE Engineering in Medicine and Biology Society. At the present time,the federation has an estimated 25,000 members from all of its constituent societies.The primary goal of the IFMBE is to recognize the interests and initiatives of its affiliatedmember organizations and to provide an international forum for the exchange of ideas anddissemination of information. The major IFMBE activities include the publication of thefederations bimonthly journal, the Journal of Medical and Biological Engineering andComputing, the MBEC News, establishment of close liaisons with developing countries toencourage and promote BME activities, and the organization of a major world conferenceevery 3 years in collaboration with the International Organization for Medical Physics and theInternational Union for Physical and Engineering Sciences in Medicine. The IFMBE alsoserves as a consultant to the United Nations Industrial Development Organization and hasnongovernmental organization status with the World Health Organization, the United Nations,and the Economic Commission for Europe.

1.1.7 International Union for Physics and Engineering Sciences in Medicine (IUPESM)

The IUPESM resulted from the IFMBEs collaboration with the International Organization ofMedical Physics (IOMP), culminating into the joint organization of the triennial World

Congress on Medical Physics and Biomedical Engineering. Traditionally, these twoorganizations held their conferences back to back from each other for a number of years.Since both organizations were involved in the research, development, and utilization ofmedical devices, they were combined to form IUPESM. Consequently, all members of theIFMBEs national and transnational societies are also automatically members of the IUPESM.The statutes of the IUPESM have been recently changed to allow other organizations to

become members in addition to the founding members, the IOMP and the IFMBE.

1.1.8 International Council of Scientific Unions (ICSU)

The International Council of Scientific Unions is nongovernmental organization created to

promote international scientific activity in the various scientific branches and theirapplications for the benefit of humanity.


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ICSU has two categories of membership: scientific academies or research councils, which arenational, multidisciplinary bodies, and scientific unions, which are international disciplinaryorganizations. Currently, there are 92 members in the first category and 23 in the second.ICSU maintains close working relations with a number of intergovernmental andnongovernmental organizations, in particular with UNESCO. In the past, a number of

international programs have been launched and are being run in cooperation with UNESCO.ICSU is particularly involved in serving the interests of developing countries. Membership inthe ICSU implies recognition of the particular field of activity as a field of science. AlthoughICSU is heralded as a body of pure scientific unions to the exclusion of cross andmultidisciplinary organizations and those of an engineering nature, IUPESM, attained itsassociate membership in the ICSU in the mid-1980s. The various other international scientificunions that are members of the ICSU include the International Union of Biochemistry andMolecular Biology (IUBMB), the International Union of Biological Sciences (IUBS), theInternational Brain Research Organization (IBRO), and the International Union of Pure andApplied Biophysics (IUPAB). The IEEE is an affiliated commission of the IUPAB and isrepresented through the Engineering in Medicine and Biology Society [ICSU Year Book,

1994].


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2. Biomedical Sensors

Physical variables associated with biomedical systems are measured by a group of sensorsknown as physical sensors. Sensors for these variables, whether they are measuring

biomedical systems or other systems, are essentially the same.

There is, however, one notable exception regarding the similarity of these sensors: thepackaging of the sensor and attachment to the system being measured.Mechanisms inherent in this tissue for trying to eliminate the sensor as a foreign body; sensorsused for fluidic measurements such as pressure and flow are quite different from systems formeasuring pressure and flow in nonbiologic environments.

Comparison of Displacement Sensors

SensorElectrical

VariableMeasurement

CircuitSensitivity Precision Range

Variable resistor ResistanceVoltage divider,

ohmmeter,High Moderate Large

bridge, current source

Foil strain gauge Resistance Bridge Low Moderate Small

Liquid metal strain gauge Resistance Ohmmeter, bridge Moderate Moderate Large

Silicon strain gauge Resistance Bridge High Moderate Small

Mutual inductance coils Inductance Impedance bridge, Moderate Moderate Moderate

inductance meter to high to low to large

Variable reluctance Inductance Impedance bridge, High Moderate Large

inductance meter

LVDT Inductance Voltmeter High High High

Parallel plate capacitor Capacitance Impedance bridge, Moderate Moderate Moderate

capacitance meter to high to large

Sonic/ultrasonic Time Timer circuit High High Large

Table 1

2.1 Variable Resistance Sensor

One of the simplest sensors for measuring displacement is a variable resistor.The resistance between two terminals on this device is related to the linear or angulardisplacement of a sliding tap along a resistance element. Precision devices are available thathave a reproducible, linear relationship between resistance and displacement.

These devices can be connected in circuits that measure such resistance as an ohmmeter orbridge, or they can be used as a part of a circuit that provides a voltage that is proportional tothe displacement. Such circuits include the voltage divider or driving a known constantcurrent through the resistance and measuring the resulting voltage across it. This sensor issimple and inexpensive and can be used for measuring relatively large displacements.


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Figure 1. Examples of displacement sensors.(a) Variable resistance sensor, (b) foil strain gauge, (c) linear variable differential transformer

(LVDT), (d) parallel plate capacitive sensor, and (e) ultrasonic transit time displacementsensor.

2.2 Strain Gauge

Another displacement sensor based on an electrical resistance change is the strain gauge. If a

long narrow electrical conductor such as a piece of metal foil or a fine gauge wire is stretchedwithin its elastic limit, it will increase in length and decrease in cross-sectional area. Becausethe electric resistance between both ends of this foil or wire can be given by

A

lR

=

, where

- is the electrical resistivity of the foil or wire material,- l is its length- A is its cross-sectional area, this stretching will result in an increase in resistance.The change in length can only be very small for the foil to remain within its elastic limit, sothe change in electric resistance will also be small.The relative sensitivity of this device is given by its gauge factor , which is defined as


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llRR /// = , where- R is the change in resistance when the structure is stretched by an amount l.Foil strain gauges are the most frequently applied and consist of a structure such as shown inb.A piece of metal foil that is attached to an insulating polymeric film such as polyimide that

has a much greater compliance than the foil itself is chemically etched into the pattern shownin b. When a strain is applied in the sensitive direction, the direction of the individualelements of the strain gauge, the length of the gauge will be slightly increased, and this willresult in an increase in the electrical resistance seen between the terminals. Since thedisplacement or strain that this structure can measure is quite small for it to remain within itselastic limit, it can only be used to measure small displacements.

If the strain gauge is attached to one surface of the beam as shown in Fig, a fairly largedisplacement at the unsupported end of the beam can be translated to a relatively smalldisplacement on the beams surface. It would be possible for this structure to be used tomeasure larger displacements.

Figure 2. Strain gauges on a cantilever (konsool) structure to provide temperaturecompensation.

(a) cross-sectional view of the cantilever and (b) placement of the strain gauges in a halfbridge or full bridge for temperature compensation and enhanced sensitivity.


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Its sensitivity is roughly the same as a foil or wire strain gauge, but it is not as reliable. Themercury can easily become oxidized or small air gaps can occur in the mercury column.These effects make the sensors characteristics noisy and sometimes results in completefailure.

Another variation on the strain gauge is the semiconductor strain gauge. Thesedevices are frequently made out of pieces of silicon with strain gauge patterns formedusing semiconductor microelectronic technology. The principal advantage of thesedevices is that their gauge factors can be more than 50 times greater than that of thesolid and liquid metal devices. They are available commercially, but they are a bitmore difficult to handle and attach to structures being measured due to their small sizeand brittleness.

A more compliant structure that has found applications in biomedical instrumentationis the liquid metal strain gauge. Instead of using a solid electric conductor such asthe wire or metal foil, mercury confined to a compliant, thin wall, narrow boreelastomeric tube is used. The compliance of this strain gauge is determined by the

elastic properties of the tube. Since only the elastic limit of the tube is of concern, thissensor can be used to detect much larger displacements than conventional straingauges.

2.3 Inductance Sensors

2.3.1 Mutual Inductance

The mutual inductance between two coils is related to many geometric factors, one of which

is the separation of the coils. Thus, one can create a very simple displacement sensor byhaving two coils that are coaxial but with different separation. By driving one coil with an acsignal and measuring the voltage signal induced in the second coil, this voltage will be relatedto how far apart the coils are from one another. When the coils are close together, the mutualinductance will be high, and so a higher voltage will be induced in the second coil; when thecoils are more widely separated, the mutual inductance will be lower as will the inducedvoltage.The relationship between voltage and separation will be determined by the specific geometryof the coils and in general will not be a linear relationship with separation unless the changeof displacement is relatively small. Nevertheless, this is a simple method of measuring

separation that works reasonably well provided the coils remain coaxial. If there is movementof the coils transverse to their axes, it is difficult to separate the effects of transversedisplacement from those of displacement along the axis.

2.3.2 Variable Reluctance

A variation on this sensor is the variable reluctance sensor wherein a single coil or two coilsremain fixed on a form which allows a high reluctance slug to move into or out of the coil orcoils along their axis. Since the position of this core material determines the number of fluxlinkages through the coil or coils, this can affect the self-inductance or mutual inductance ofthe coils. In the case of the mutual inductance, this can be measured using the technique

described in the previous paragraph, whereas self-inductance changes can be measured usingvarious instrumentation circuits used for measuring inductance. This method is also a simple


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method for measuring displacements, but the characteristics are generally nonlinear, and thesensor generally has only moderate precision.

2.4 Linear Variable Differential Transformer

By far the most frequently applied displacement transducer based upon inductance is thelinear variable differential transformer (LVDT). This device is illustrated in c and isessentially a three-coil variable reluctance transducer. The two secondary coils are situatedsymmetrically about the primary coil and connected such that the induced voltages in eachsecondary oppose each other. When the core is located in the center of the structureequidistant from each secondary coil, the voltage induced in each secondary will be the same.Since these voltages oppose one another, the output voltage from the device will be zero. Asthe core is moved closer to one or the other secondary coils, the voltages in each coil will nolonger be equal, and there will be an output voltage proportional to the displacement of the

core from the central, zero-voltage position. Because of the symmetry of the structure, thisvoltage is linearly related to the core displacement.When the core passes through the central, zero point, the phase of the output voltage from thesensor changes by 180 degrees. Thus, by measuring the phase angle as well as the voltage,one can determine the position of the core. The circuit associated with the LVDT not onlymeasures the voltage but often measures the phase angle as well. LVDTs are availablecommercially in many sizes and shapes. Depending on the configuration of the coils, they canmeasure displacements ranging from tens of micrometers through centimeters.

2.5 Capacitive Sensors

Displacement sensors can be based upon measurements of capacitance as well as inductance.The fundamental principle of operation is the capacitance of a parallel plate capacitor as givenby

d

AC

=

, where

- is the dielectric constant of the medium between the plates,- d is the separation between the plates,- A is the cross-sectional area of the plates.

Each of the quantities in Eq. can be varied to form a displacement transducer. By moving oneof the plates with respect to the other, shows us that the capacitance will vary inversely withrespect to the plate separation. This will give a hyperbolic capacitance-displacementcharacteristic. However, if the plate separation is maintained at a constant value and the platesare displaced laterally with respect to one another so that the area of overlap changes, this canproduce a capacitance-displacement characteristic that can be linear, depending on the shapeof the actual plates.The third way that a variable capacitance transducer can measure displacement is by having afixed parallel plate capacitor with a slab of dielectric material having a dielectric constantdifferent from that of air that can slide between the plates. The effective dielectric constant for

the capacitor will depend on how much of the slab is between the plates and how much of the


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region between the plates is occupied only by air. This, also, can yield a transducer with linearcharacteristics.

The electronic circuitry used with variable capacitance transducers, is essentially the same asany other circuitry used to measure capacitance. As with the inductance transducers, this

circuit can take the form of a bridge circuit or specific circuits that measure capacitivereactance.

2.6 Sonic and Ultrasonic Sensors

If the velocity of sound in a medium is constant, the time it takes a short burst of that soundenergy to propagate from a source to a receiver will be proportional to the displacementbetween the two transducers.This is given by

D=cT, where

- c is the velocity of sound in the medium,- T is the transit time,-d is the displacement. A simple system for making such a measurement is shown in e. A briefsonic or ultrasonic pulse is generated at the transmitting transducer and propagates throughthe medium. It is detected by the receiving transducer at time T after the burst was initiated.The displacement D can then be determined.In practice, this method is best used with ultrasound, since the wavelength is shorter, and thedevice will neither produce annoying sounds nor respond to extraneous sounds in theenvironment.Small piezoelectric transducers to generate and receive ultrasonic pulses are readily available.

The electronic circuit used with this instrument carries out three functions:(1)generation of the sonic or ultrasonic burst,(2) detection of the received burst, and(3) measurement of the time of propagation of the ultrasound.

An advantage of this system is that the two transducers are coupled to one another onlysonically. There is no physical connection as was the case for the other sensors described inthis section.

2.6.1 Velocity Measurement

Velocity is the time derivative of displacement, and so all the displacement transducersmentioned above can be used to measure velocity if their signals are processed by passingthem through a differentiator circuit. There are, however, two additional methods that can beapplied to measure velocity directly.

2.6.2 Magnetic Induction


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If a magnetic field that passes through a conducting coil varies with time, a voltage is inducedin that coil that is proportional to the time-varying magnetic field. This relationship is givenby

dt

dN

= , where

- v is the voltage induced in the coil- N is the number of turns in the coil- is the total magnetic flux passing through the coil (the product of the flux density and areawithin the coil).

Thus a simple way to apply this principle is to attach a small permanent magnet to an objectwhose velocity is to be determined, and attach a coil to a nearby structure that will serve asthe reference against which the velocity is to be measured. A voltage will be induced in thecoil whenever the structure containing the permanent magnet moves, and this voltage will berelated to the velocity of that movement. The exact relationship will be determined by thefield distribution for the particular magnet and the orientation of the magnet with respect tothe coil

2.6.3 Doppler Ultrasound

When the receiver of a signal in the form of a wave such as electromagnetic radiation orsound is moving at a nonzero velocity with respect to the emitter of that wave, the frequencyof the wave perceived by the receiver will be different than the frequency of the transmitter.This frequency difference, known as the Doppler shift, is determined by the relative velocityof the receiver with respect to the emitter and is given by

cffd /0 = , where

df - is the Doppler frequency shift,

0f - is the frequency of the transmitted wave,

v is the relative velocity between the transmitter and receiver,c is the velocity of sound in the medium.

This principle can be applied in biomedical applications as a Doppler velocimeter. Apiezoelectric transducer can be used as the ultrasound source with a similar transducer as the

receiver. When there is no relative movement between the two transducers, the frequency ofthe signal at the receiver will be the same as that at the emitter, but when there is relativemotion, the frequency at the receiver will be shifted according to Eq.

The ultrasonic velocimeter can be applied in the same way that the ultrasonic displacementsensor is used. In this case the electronic circuit produces a continuous ultrasonic wave and,instead of detecting the transit time of the signal, now detects the frequency differencebetween the transmitted and received signals. This frequency difference can then be convertedinto a signal proportional to the relative velocity between the two transducers.


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2.7 Accelerometers

Acceleration is the time derivative of velocity and the second derivative with respect to timeof displacement. Thus, sensors of displacement and velocity can be used to determineacceleration when their signals are appropriately processed through differentiator circuits. In

addition, there are direct sensors of acceleration based upon Newtons second law andHookes law.

A known seismic mass is attached to the housing by an elastic element. As the structure isaccelerated in the sensitive direction of the elastic element, a force is applied to that elementaccording to Newtons second law. This force causes the elastic element to be distortedaccording to Hookes law, which results in a displacement of the mass with respect to theaccelerometer housing. This displacement is measured by a displacement sensor.The relationship between the displacement and the acceleration is found by combiningNewtons second law and Hookes lawa = k/m x, where- x is the measured displacement,- m is the known mass,- k is the spring constant of the elastic element,- a is the acceleration.Any of the displacement sensors described above can be used in an accelerometer. The mostfrequently used displacement sensors are strain gauges or the LVDT.

Figure 3. Fundamental structure of an accelerometer.


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One type of accelerometer uses a piezoelectric sensor as both the displacement sensor and theelastic element. A piezoelectric sensor generates an electric signal that is related to thedynamic change in shape of the piezoelectric material as the force is applied. Thus,piezoelectric materials can only directly measure time varying forces. A piezoelectricaccelerometer is, therefore, better for measuring changes in acceleration than for measuring

constant accelerations. A principal advantage of piezoelectric accelerometers is that they canbe made very small, which is useful in many biomedical applications.

2.8 Force

Force is measured by converting the force to a displacement and measuring the displacementwith a displacement sensor. The conversion takes place as a result of the elastic properties of amaterial. Applying a force to the material distorts the materials shape, and this distortion canbe determined by a displacement sensor. For example, the cantilever structure shown in Fig.

could be a force sensor. Applying a vertical force at the tip of the beam will cause the beam todeflect according to its elastic properties. This deflection can be detected using a displacementsensor such as a strain gauge as described previously.

A common form of force sensor is the load cell. This consists of a block of material withknown elastic properties that has strain gauges attached to it. Applying a force to the load cellstresses the material, resulting in a strain that can be measured by the strain gauge. ApplyingHookes law, one finds that the strain is proportional to the applied force. The strain gaugeson a load cell are usually in a half-bridge or full-bridge configuration to minimize thetemperature sensitivity of the device. Load cells come in various sizes and configurations, andthey can measure a wide range of forces.

2.9 Measurement of Fluid Dynamic Variables

The measurement of the fluid pressure and flow in both liquids and gases is important inmany biomedical applications. These two variables, however, often are the most difficultvariables to measure in biologic applications because of interactions with the biologic systemand stability problems.

2.10 Pressure Measurement

Sensors of pressure for biomedical measurements such as blood pressure consist of a structuresuch as shown in Fig. In this case a fluid coupled to the fluid to be measured is housed in achamber with a flexible diaphragm making up a portion of the wall, with the other side of thediaphragm at atmospheric pressure.When a pressure exists across the diaphragm, it will cause the diaphragm to deflect. Thisdeflection is then measured by a displacement sensor. The displacement transducer consists offour fine-gauge wires drawn between a structure attached to the diaphragm and the housing ofthe sensor so that these wires serve as strain gauges. When pressure causes the diaphragm todeflect, two of the fine-wire strain gauges will be extended by a small amount, and the other

two will contract by the same amount. By connecting these wires into a bridge circuit, a


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voltage proportional to the deflection of the diaphragm and hence the pressure can beobtained.Semiconductor technology has been applied to the design of pressure transducers such that theentire structure can be fabricated from silicon. A portion of a silicon chip can be formed into adiaphragm and semiconductor strain gauges incorporated directly into that diaphragm to

produce a small, inexpensive, and sensitive pressure sensor. Such sensors can be used asdisposable, single-use devices for measuring blood pressure without the need for additionalsterilization before being used on the next patient. This minimizes the risk of transmittingblood-borne infections in the cases where the transducer is coupled directly to the patientsblood for direct blood pressure measurement.

Figure 4. See the structure of an unbonded strain gauge pressure sensor.

In using this type of sensor to measure blood pressure, it is necessary to couple the chambercontaining the diaphragm to the blood or other fluids being measured. This is usually doneusing a small, flexible plastic tube known as a catheter, that can have one end placed in anartery of the subject while the other is connected to the pressure sensor. This catheter is filledwith a physiologic saline solution so that the arterial blood pressure is coupled to thediaphragm.This external blood-pressure-measurement method is used quite frequently in the clinic andresearch laboratory, but it has the limitation that the properties of the fluid in the catheter and

the catheter itself can affect the measurement. For example, both ends of the catheter must beat the same vertical level to avoid a pressure offset due to hydrostatic effects.Also, the compliance of the tube will affect the frequency response of the pressuremeasurement. Air bubbles in the catheter or obstructions due to clotted blood or othermaterials can introduce distortion of the waveform due to resonance and damping.These problems can be minimized by utilizing a miniature semiconductor pressure transducerthat is located at the tip of a catheter and can be placed in the blood vessel rather than beingpositioned external to the body.Such internal pressure sensors are available commercially and have the advantages of a muchbroader frequency response, no hydrostatic pressure error, and generally clearer signals thanthe external system.

Although it is possible to measure blood pressure using the techniques described above, thisremains one of the major problems in biomedical sensor technology.


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Long-term stability of pressure transducers is not very good. This is especially true forpressure measurements of venous blood, cerebrospinal fluid, or fluids in the gastrointestinaltract, where pressures are relatively low.Long-term changes in baseline pressure for most pressure sensors require that they befrequently adjusted to be certain of zero pressure. Although this can be done relatively easily

when the pressure transducer is located external to the body, this can be a major problem forindwelling pressure transducers. Thus, these transducers must be extremely stable and havelow baseline drift to be useful in long-term applications.The packaging of the pressure transducer is also a problem that needs to be addressed,especially when the transducer is in contact with blood for long periods.Not only must the package be biocompatible, but it also must allow the appropriate pressureto be transmitted from the biologic fluid to the diaphragm. Thus, a material that ismechanically stable under corrosive and aqueous environments in the body is needed.

2.11 Measurement of Flow

The measurement of true volummetric flow in the body represents one of the most difficultproblems in biomedical sensing. The sensors that have been developed measure velocityrather than volume flow, and they can only be used to measure flow if the velocity ismeasured for a tube of known cross-section. Thus, most flow sensors constrain the vessel tohave a specific cross-sectional area. The most frequently used flow sensor in biomedicalsystems is the electromagnetic flow meter.

Figure 5. Fundamental structure of an electromagnetic flowmeter.

This device consists of a means of generating a magnetic field transverse to the flow vector ina vessel. A pair of very small biopotential electrodes are attached to the wall of the vessel


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such that the vessel diameter between them is at right angles to the direction of the magneticfield. As the blood flows in the structure, ions in the blood deflect in the direction of one orthe other electrodes due to the magnetic field, and the voltage across the electrodes is given by

u = Blv, where

- B is the magnetic field,- l is the distance between the electrodes,- v is the average instantaneous velocity of the fluid across the vessel.

If the sensor constrains the blood vessel to have a specific diameter, then its cross-sectionalarea will be known, and multiplying this area by the velocity will give the volume flow.Although dc flow sensors have been developed and are available commercially, the mostdesirable method is to use ac excitation of the magnetic field so that offset potential effectsfrom the biopotential electrodes do not generate errors in this measurement.Small ultrasonic transducers can also be attached to a blood vessel to measure flow. In this

case the transducers are oriented such that one transmits a continuous ultrasound signal thatilluminates the blood.Cells within the blood diffusely reflect this signal in the direction of the second sensor so thatthe received signal undergoes a Doppler shift in frequency that is proportional to the velocityof the blood. By measuring the frequency shift and knowing the cross-sectional area of thevessel, it is possible to determine the flow.

The oscillator generates a signal that, after amplification, drives the transmitting transducer.The oscillator frequency is usually in the range of 110 MHz. The reflected ultrasound fromthe blood is sensed by the receiving transducer and amplified before being processed by adetector circuit. This block generates the frequency difference between the transmitted andreceived ultrasonic signals. This difference frequency can be converted into a voltageproportional to frequency, and hence flow velocity, by the frequency to voltage convertercircuit.


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Figure 6. Structure of an ultrasonic Doppler flowmeter with the major blocks of the electronicsignal processing system.

Another method of measuring flow that has had biomedical application is the measurement ofcooling of a heated object by convection. The object is usually a thermistor placed either in ablood vessel or in tissue, and the thermistor serves as both the heating element and thetemperature sensor.In one mode of operation, the amount of power required to maintain the thermistor at atemperature slightly above that of the blood upstream is measured. As the flow around thethermistor increases, more heat is removed from the thermistor by convection, and so morepower is required to keep it at a constant temperature.Relative flow is then measured by determining the amount of power supplied to thethermistor.

2.12 Temperature

There are many different sensors of temperature, but three find particularly wide applicationto biomedical problems. Table summarizes the properties of var

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