CT Scanning: Principles, Patient - University at Albany€¦ · CT Scanning: Principles, Patient ... The Essential Physics of Medical Imaging. 2012, 3rd Edition, Lippincott Williams

CT Scanning: Principles, Patient Doses, Benefits &Potential Risks

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Keith Strauss, MSc, FAAPM, FACR

Cincinnati Children’s Hospital

University of Cincinnati College of Medicine

CT SCANNING; PRINCIPLES, PATIENT DOSES, BENEFITS & POTENTIAL RISKS

Acknowledgement

Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging. 2012, 3rd Edition, Lippincott Williams & Wilkins, Philadelphia

• provided many of the tables and figures used in this presentation.

Introduction

• Basic principles of CT scanning

• Strengths of CT

• Comparison to other imaging modalities

• Patient dose

• Dose indices

• Comparison to radiation doses of other imaging modalities

• Benefits and potential risks

• Potential risks: deterministic vs stochastic

• Message of popular press must be interpreted

COMPUTED TOMOGRAPHY

• Advantages• Cross Sectional image format

• Superior low contrast resolution• Radiography:

10% contrast

• CT: 0.3%

contrast

Essential Physics of Medical Imaging ©

MODES OF CT ACQUISITION• Cone Beam Acquisition

• 16 cm along longitudinal axis can be acquired with a single axial acquisition

• Entire exam can be single axial acquisition



DOSE REDUCTION TECHNIQUES

• Retrospective Gating• X-rays continuously on• Only data during end diastole

used to form image

x-ray on

data for reconstruction



• Prospective Gating• X-rays on at end diastole• All end diastole data used• No wasted dose• Intravenous contrast only

x-ray on

EKG

data for reconstruction


DOSE REDUCTION TECHNIQUES• Dose Modulation

• X-Y Modulation• Tube current increased for thick anatomy (b)• Tube current decreased for thin anatomy (a)


MODES OF CT ACQUISITION• Dose Modulation• X – Y - Z axis Modulation Essential Physics of Medical Imaging ©



• Iterative Reconstruction (IRC)• By reconstructing the image multiple times, IRC can

extract more signal from scan data compared to Filtered Back Projection

• Image quality is improved• Patient dose can be lowered until image quality is

unchanged.


DOSE REDUCTION TECHNIQUES• Iterative Reconstruction (IRC)

• Superiority of IRC images seen on the right• The effect of the display Window Width on the

displayed contrast demonstrated between vertical images.

RECENT CT SCANNER FEATURES

• Collimator (MDCT)• Transverse ~ cubic voxel

‘Square’ pixels on axial

• Cubicle pixels on

Coronal and Sagittal images


IMAGE DISPLAY

• Multi-planar Reconstruction (MPR)Create Sagittal (bottom left),

Coronal (top right), or

Volume Rendered 3-D

Reconstruction

• Need:

• Cubicle Voxel

• 0.5 mm recon-

struction

Diagnostic Radiology Physics (IAEA)


IMAGE DISPLAY

• CT 3-D images of abdomen/pelvis: Pseudo-colored • Intravenous contrast• Colonography with air contrast

Diagnostic Radiology Physics (lAEA)

600 mAs

150 mAs 300 mAs

900 mAs

DISPLAYED IMAGE


Advantages/Disadvantage of Other Imaging Modalities vs CTMRI: Great Tissue Discrimination

• Image of chemistrya. Sagittal T1-weightedb. Axial FLAIR: infarctc. T1-weighted contrastd. T1-weighted with fat

saturatione. MIP: time-of-flight MR

angiogramf. Gadolinium enhanced

• No Ionizing Radiation• Energy deposition occurs• Radio waves


Advantages/Disadvantage of Other Imaging Modalities vs CT• Ultrasound: poor sharpness

• Image from deposition of mechanical waves (sound)a. Sagittal obstetrical US image of 5.5 month fetusb. Diameter of fetal head measurementc. 3-D image rendering of

fetus with cleft palated. Doppler color-flow

imaging: aneurysmseen in carotid artery

• No Ionizing Radiation• Low levels of mechanical

energy believed to haveno biological effect.


Advantages/Disadvantage of Other Imaging Modalities vs CT• Radiography: Best sharpness• Projection radiographs require relatively small doses of

ionizing radiationa & b. AP and LAT chest x-ray: one of smallest doses

for one of the most common exams.c. LAT cervical spine: neck injury after trauma


Advantages/Disadvantage of Other Imaging Modalities vs CT

• Radiography: Poor soft tissue discrimination• Projection radiographs of extremities require smal-ler

doses of ionizing radiation than images of trunkd, e, f, g. Wrist, Foot, Knee, Hipg. Metal objects are well seen on radiographs


Advantages/Disadvantage of Other Imaging Modalities vs CTMammography: best radiographic images

• Projection radiographs of breasts require larger doses of ionizing radiation due to low energy xrays

a. Digital image of glandularfatty tissues

b. Tomogram of breast ellim-inates tissue superpositionImproves cancer detection

c. Preferred screening test for early detection ofcancer.


Advantages/Disadvantage of Other Imaging Modalities vs CTNuclear Medicine: image of physiology

• Planar 2-D maps of 3-D radioisotope distribution.

• Multiple metastatic lesionsnot detectable by radiographyare seen.

• Ionizing radiation is used.• Patient dose determined by

biological and physical half life of radioisotope and howmuch activity is injected.


Advantages/Disadvantage of Other Imaging Modalities vs CT• Nuclear Medicine: high contrast but poor

indication of diseased organ• Hybrid Imaging: combine nuclear camera with higher

resolution modality: CT or MRIa. Small cell lung cancerb. Images before and after

chemotherapyC & d. PET/CT fusion images


Advantages/Disadvantage of Other Imaging Modalities vs CT• Sharpness:

1. Mammography S/F2. Mammography Dig3. Radiography: S/F4. Fluoroscopy:5. Radiography: Dig6. Ultrasound7. CT8. MRI9. Nuclear Medicine

Air Kerma

• Dose to air at the entrance plane of the patient.

• Ability of x-ray machine to produce energy (radiation).

• Energy carried by x-rays.

• Must specify a location from the source

• Units: Gray, mGy, µGy, nGy . . .

• Replaces exposure (Roentgens)

• 1 mR = 8.7 µGy

Dose

• Dose occurs when x-rays pass through the patient’s body and interact by:

• Photoelectric effect

• Compton scattering

• Energy of x-ray is transferred (unloaded) to KE of charged particles in tissue.

• Units: Gray, mGy, µGy, nGy . . .

• Replaces rad

• 1 mrad = 10 µGy

Equivalent Dose

• Dose adjusted for the type of carrier of the energy

H = Quality Factor x Dose

where Q = 1 for x & beta (electrons) rays

10 – 20 for protons, Alpha particles

•Units: Sievert, mSv, µSv, nSv . . .

Effective Dose

Effective Absorbed Dose is the absorbed dosegiven to the whole body of the patient that

wouldresult in the same biological damage as the actual clinical dose given to a fraction of thepatient’s whole body.

•Units: Sievert (Sv) or milliSievert (mSv)• For x-rays:

1 rem = 10 mSv; 1 Sv = 100 rem

Effective Dose Limitations

• Effective dose can be used to facilitate a comparison of biological effects between diagnostic exams of different types.

• Effective doses are more than 10x less than equivalent or organ doses. Quoting CT doses in E dose (mSv) gives the user a false sense of security!

CT SCANNER DOSE INDICES

Displayed CTDIvol

• Standardized method to estimate and compare the radiation output of two different CT scanners to same phantom.

does not represent . . .

Patient dose!!

PATIENT ABSORBED DOSE

• Effect of Multiple Scan

• When a Patient asks . . .“What is my Dose”

They typically mean

“What is my risk!

• CTDI100 CTDIw & CTDIvol address only dose, not dose and volume of tissue irradiated.

So what is SSDE?:•Estimates the peak soft tissue dose of the patient at the center of the scan length.

• Adjusts for patient size and varying attenuation from overlying tissue thickness.

• Uses average scan radiationoutput: CTDIvol

• Useful first approximationof organdose?

Clinical Applications of SSDE

50

100

100

100

100

Adapted from McCollough

AGE vs PATENT SIZE

Same age patients vary dramatically in size.

•Abdomens of:

• Largest 3 year olds and

• Smallest adults are

the same size.

•Patient cross section

size, not age, should

be used.

Comparison of Radiation Doses• Radiation dose is a dose of ionizing radiation

• Tissue molecules ionized• Heavy concentration of ionization believed to

cause biological damage

Average U.S. background radiation 3 mSv (300 mrem)

Comparison of Radiation Doses

• Magnetic Resonance Imaging (MRI)• Image of chemistry of organs• Not an image of anatomy• NO ionizing radiation; NO radiation dose• Energy is used• Radio waves (low

energy photons)Essential Physics of Medical Imaging ©

Comparison of Radiation Doses• Ultrasound

• Image of anatomy• NO ionizing radiation; NO radiation dose• Energy is used• Low energy mechanical waves



• Radiography• Cost effective image of anatomy• Low levels of x-ray energy: relative low doses• Effective dose of 0.2 – 0.3 mSv cervical spine• Extremity images: < 0.1 mSv• PA & LAT Chest: 0.1 mSv• AP KUB < 1 mSv • LAT lumbar spine: 1 – 2 mSv



• General Fluoroscopy: UGI, LGI, VCUG, etc.• Imaging of moving anatomy• Low range of x-ray energies: Effective Doses• 0.1 – 0.2 mSv for toddlers• 0.2 – 0.3 mSv for primary

age• 0.3 – 0.4 mSv small teens• 0.4 – 0.5 mSv large teens• 0.7 – 0.8 mSv adults



• CT Studies• Excellent imaging of soft tissue anatomy

• Moderate Effective Doses• 0.5 – 1 mSv Head CT • 2 – 3 mSv Chest CT• 2 – 4 mSv Abdomen CT• 2 – 3 mSv Pelvis CT• 5 – 10 mSv Coronary

Angiography



• Nuclear Medicine Studies• Excellent physiological studies

• Moderate Effective Doses• 2 – 9 mSv Tc-99m: Common

studies• 3 – 4 mSv Tc-99m DI & MDP:

bone scans• 5 – 14 mSv F-18: PET studies• 15 – 20 mSv Ga-67 citrate: Tumor studies



• Interventional Fluoroscopy/Angiography• Imaging of moving anatomy

• Moderate to high range: Effective Doses• 1 – 3 mSv simple diagnostic study• 3 – 5 mSv complex diag-

nostic study• 5 – 60 mSv Radiology

intervention• 10 – 230 mSv Cardiology

intervention

All clinical studies of patients involving ionizing radiation must be clinically justified

•What is the clinical question to be answered?

•Is the imaging modality the best choice with respect to the clinical question?

•What other alternatives exist for managing the patient?

Benefits and Potential Risks

100

• Identified benefits are:• Real• Immediate• Small child who has fallen and struck their head

could die in 24 hours from a brain bleed.

• Potential risks are:• Hypothetical• Delayed in some cases up to 30 years


• Deterministic Injuries• Injury required to multiple cells• Threshold Dose: 2,000 mGy skin erythema

3,000 mGy hair loss6,000 mGy permanent erythema7,000 mGy permanent hair loss

• Below: no injury expressed• Above: Probability of injury dose

Severity of injury dose• First symptoms delayed days – weeks• Cataracts, epilation, necrosis, & erythema

Potential Risks

• Stochastic Injuries• Can occur from injury to single cell• Probability of injury dose• Severity of injury independent of dose• Neoplasms• Solid tumors delayed up to 30 years

• Stochastic injuries more likely in children

• Deterministic injuries more likely in adults

Potential Risks

• CT: Radiation induced cancer deaths based on age and gender

• Cancer death rate per 1,000 individuals exposed to 10 mGy

Infants more than 3 times more susceptible than adults.

Females ~ 60% more suspect-able than males.

Stochastic Risk to a GENERIC Patient

100


• Deterministic injuries demonstrate a cumulative effect

• Due to sub lethal repair, fractionated doses separated by time are not as damaging as a single dose = sum of fractionated doses.

• Stochastic injuries do not demonstrate a cumulative effect

• The stochastic risk from a 20th CT scan is no greater than the risk from the 1st CT scan!


• Stochastic risks (cancer induction from radiation) are hypothetical predictions

• No data exists that proves no risk• No data exists that proves a risk• Data in table to the

right allow predic-tion of risk. This isnot gospel truth assome scientistsclaim.

Interpret the Message


When the media like the NY Times publishes this type of sensationalism, patients read the article and become fearful of radiation!

Sensational scare tactics sell

Radiation must be respectedand properly used, not feared

Interpret the Message

What is More Realistic?


• Imaging plays an important role in the diagnosis and treatment of diseases today.

• Today’s production of clinical images is very sophisticated.

• Remain abreast of changes in imaging; consult with radiologists and ask questions.

• Order the best exam for your patient, it may be different today than in the past.

Conclusions

• Seek the diagnostic examination that can best answer the clinical question about your patient.

• Do not avoid, out of fear, the use of ionizing radiation when it is clinically justified.

• Interpret carefully the media’s message about imaging risks.

• Potential risks of ionizing radiation are much lower than portrayed in the media.

Conclusions

Please join us for our next webinar!

Creating a Culture of Safety in Radiology

July 22nd, 2015 12:00pm – 1:00pm

Dr. Kimberly Applegate

Register here: http://www.nyimagesafe.org/

Did you miss the last one?

Image Gently: Radiation Exposure to Children From Medical Imaging- Is there a Problem?

Presenter: Dr. Marilyn Goske

Register to watch the recording here: http://www.nyimagesafe.org/

Public Health Live!

Pediatric Computed Tomography (CT) Scans, Radiation, and Risks: Having an Informed Dialogue

June 18, 2015 9:00 am- 10:00 am

Presenter: Donald P. Frush, MD, FACR, FAAP

Register to watch here:

http://www.nyimagesafe.org

http://www.ualbanycphp.org/eval/SPHeval.cfm?ID=243

Evaluation:

Website (handouts, recording, evaluation link):

http://www.nyimagesafe.org/


Documents

CT Scanning: Principles, Patient - University at Albany€¦ · CT Scanning: Principles, Patient ... The Essential Physics of Medical Imaging. 2012, 3rd Edition, Lippincott Williams