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Radiographic Quality :Radiographic Quality :Radiographic Quality :

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ผศ. สุพจน เอื้ออภิสิทธิ์วงค หนวยเวชศาสตรนิวเคลยีรภาควชิารังสีวิทยา คณะแพทยศาสตร

มหาวิทยาลัยเชียงใหม[email protected] โทร. 053-94-6194

mailto:[email protected]

เนือ้หา : Radiographic Quality :Radiographic Quality :

• Film Factors

• Geographic Factors

• Subject Factors

• Clinical considerations for improved radiographic quality

Radiographic Quality :Radiographic Quality :• Refer to the fidelity with which the anatomic structure

under examination is high quality imaged on the radiograph

• High quality radiograph to make accuracy diagnosis

• The quality of radiograph is directly related to the technologist’s understanding of basic principles of x-ray and of factors affecting radiographic quality

• The x-ray technologist is provided with all physical tools required to produce high quality of radiograph

Film Factors

Characteristic curve• Density• Contrast• Speed• Latitude

Film processing• Time• Temperature

Geometric Factors

Distortion

Magnification

penumbra

Subject Factors

Contrast

Thickness

Density

Atomic number

shape

Kilovoltage

motion

Radiographic QualityRadiographic Quality

Film Factors



• Processed unexposed x-ray film⇒ lucent

• Processed exposed x-ray film⇒ opaque various shades of gray → heavily exposed film appears black

• Intensity of exposure of film and blackness after processing is called sensitometry

• Normally RTs not involve in sensitometrymeasurement but knowledge of sensitometry aspects of radiographic film is essential



Film Factors



• The principal measurement in sensitometry :The exposure to x-ray film and the percentage of light transmitted through the processed film

D = log I0/It

I0 : light incident

It : light transmitted through film

• Density (D) :The degree of blackening of an x-ray film

- clear area (low exposure) → low density- black area (high exposure) → high density

Dbone = log 1500/480

Dsoft = log 1500/2

= 0.5

= 2.9


Film Factors



Radiographic film contains densities ranging from near 0-4

Toe

ShoulderSt

raig

ht lin

e po

rtion

Base and fog densities

Diagnostically useful densities

Base density ∼ 0.05

Fog density ∼ 0.05 – 0.15


Film Factors



• The principal measurement in sensitometry :The exposure to x-ray film and the percentage of light transmitted through the processed film

Density VS Exposure → Characteristic curve

• Toe : Dmin

• Straight-line : most useful range of density area for proper exposure

• Shoulder : Dmax

Toe

Shoulder

Stra

ight

line

porti

on

Base and fog densities

Diagnostically useful densities


Film Factors




Film Factors



• The different of black and white densities level in image radiographic film called radiographic contrast

• Sharp differences in densities → high contrast

• Small differences in densities and not distinct → low contrast

High contrast Low contrast


Film Factors



• Radiographic contrast

• Film contrast- inherent in film → density- processing of film

• Judgment of technologist- film selection : type of examination

required ; non or screen film- expose film so that the range of densities lie

in diagnostically useful range 0.5 – 2.5

• Subject contrast- size shape and x-ray attenuating

characteristics of subject being examined

In clinical setting, best standardize film contrast and alter subject contrast according to the need of examination


Film Factors



Two different types of x-ray filmSlope A > B ; Film A higher contrast than film B

average gradient = (D2 – D1) / (LRE2 – LRE1)

• average gradient dose at 2 point that density level 0.25 and 2.0 above base + fog density divided by LRED0.25 and LRED2.0


Film Factors



Two different types of x-ray filmSlope A > B ; Film A higher contrast than film B

average gradient

= (D2 – D1) / (LRE2 – LRE1)

• Dbase = 0.06 • Dfog = 0.11

• Dbase+fog = 0.06 + 0.11 = 0.17

• D2 = 2.0 + 0.17 = 2.17

• D1 = 0.25 + 0.17 = 0.42

• LRE2 = 2.15, LRE1 = 1.35

average gradient = (2.17 – 0.42) / (2.15 - 1.35) = 2.19


Film Factors



• Speed (sensitivity) :The ability of an x-ray film to respond to a minimum quantity of x-ray exposure

Screen film 1 mR VS Non screen film X mR

Speed = 1 / # of Roentgens produce D of 1

Two different types of x-ray film

• exposure A 10 mR and B 20 mRFilm A twice faster speed than film B

more

less

• exposure A < B ; Film A faster speed than film B

If X = 25 mR → D =1 Speed = 1/ 25 mR = 1 / 25 x 10-3 R = 40


Film Factors



• Latitude :The range of exposures over which the x-ray film will respond with densities in diagnostically useful range 0.5 – 2.5

• B wider range of exposure than A

⇒ B wider latitude than A

Film wide latitude ⇒ long gray scale

Film narrow latitude ⇒ short gray scale

• Latitude and contrast are inversely proportional


Film Factors



• Film processing :Proper film processing is required for optimum film contrast .

Film agitation during development

Composition of processing chemical

Factor affecting the degree of development

Development temperature

Development time

• Degree of development pronounced effect on the level of film fog and on the density from a giving exposure at a given film speed


Film Factors



• Development time is varied → characteristic curve will vary ; both in shape & position

Contrast, speed and fog level

• Speed & fog ↑ when time ↑

• Contrast ↑ then ↓ when time ↑

Manufacturer’s development time

• Maximum contrast

• Relative high speed

• Low level of fog


Film Factors



• Development temperature is varied →characteristic curve will vary ; both in shape & position

Contrast, speed and fog level

• Speed & fog ↑ when Temp ↑

• Contrast ↑ then ↓ when Temp ↑

Time (min)

3

4

5

6

7

Temp (°F)

74

71

68

64

60

Automatic processor time 90 s, a variation of 5 s can significant changes in film quality

• Radiography is similar in many ways to taking a photograph

• The sharpness of the image on the wall is a function of a number of geometry factors

Geometric Factors

Distortion

Magnification

penumbra

• Distortion• Magnification

• penumbra


• All the radiograph larger than object

• Clinical examination :* As low as a magnification as possible*

Magnification Factor (MF)

Geometric Factors

Distortion

Magnification

penumbraMF =

Image size

Object size

MF = TFD

TOD

TFD = 14.7 cm, TOD = 12.5 cm MF = 14.7 / 12.5 = 1.176


Geometric Factors

Distortion

Magnification

penumbra

• Magnification of an object positioned off the center, in the same plane


• Magnification factor will be the same for object positioned off the center, in the same plane, as lying on the center axis if OFD is the same and the object is essentially flat

Geometric Factors

Distortion

Magnification

penumbra

• Target to film distance change

• Chest 72″• Routine 40″• Special 36″

MF = TFD

TOD

TFD ; A = 16 cm, B = 8 cm OTF = 4MFA = 16/12 = 1.33

MFB = 8/4 = 2


• Unequal magnification of different portion of the same object

• Distortion can hinder proper interpretation of radiograph

• Factors contribute to image distortion

Geometric Factors

Distortion

Magnification

penumbra

1. The size and shape of the object

2. The position of the object


Geometric Factors

Distortion

Magnification

penumbra

1. Object size and shape

Thick : Thin object ⇒ Distort : Not

OFD Thick object > Thin object


Geometric Factors

Distortion

Magnification

penumbra

1. Object size and shape

Center : disc : sphere ⇒ circle : circle

lateral : disc : sphere ⇒ circle : elipse


Geometric Factors

Distortion

Magnification

penumbra

2. Object position

• Object and film plane are parallel ⇒ image undistorted

• Object and film plane are not parallel ⇒ image distorted

• Proper patient positioning is not maintain

• Foreshortened ⇒ increases as the angle of inclination increases


Geometric Factors

Distortion

Magnification

penumbra

2. Object position


• An inclined object not locate on center

• Lateral position from the central x-ray beam

• Object’s angle of inclination ⇒ Foreshortened

The degree of distortion


Geometric Factors

Distortion

Magnification

penumbra

2. Object position


• Multiple object positioned at various Object to Film Distance

⇒ Spatial distortion

• Only one image should be seen ⇒ superimposition of image

• OFD ↑ → Magnification ↑

→ More lateral shift


• The x-rays emitted from a 0.3 – 2.0 mm square source in the target ⇒ penumbra (Geometric unsharpness)

Geometric Factors

Distortion

Magnification

penumbra

effective focal spot, TOD and OFD

Small penumbra• Small effective focal spot• Short Object to film distant• Long Target to object distant

P = FSeff (OFD / TOD)

P : PenumbraFSeff : effective focal spotOFD : Object to Film DistanceTOD : Target to Object Distance

TOD

OFD


• A : Geometric Factors

Distortion

Magnification

penumbra

Small penumbra• Small effective focal spot• Short Object to film distant• Long Target to object distant

P = FSeff (OFD / TOD)

PA = 1.6 * (80/5)= 25.6 mm

TOD

OFD

FSeff : 1.6 mmOFD : 80 cmTOD : 5 cm

• B : FSeff : 1.6 mmOFD : 10 cmTOD : 75 cm

PB = 1.6 * (10/75)= 0.21 mm


• Heel effect : varying intensity across the x-ray field due to attenuation of x-rays in the heel of anodeGeometric Factors

Distortion

Magnification

penumbra• Penumbra in anode

side < cathode side

• effective focal spot not constant across radiograph ⇒ anode side < cathode side

• Large effective focal spot• Short Target to film distant• Long Object to film distant

• Image to cathode side higher geometric unsharpness


• Patient : select the properly radiographic technique for patient’s size, shape, composition and positioning

Radiographic contrast = Film contrast x Subject contrast

Factors affect subject contrast

1. Patient thickness2. Tissue density3. Effective atomic number4. Object shape5. Kilovoltage

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage


• Patient thickness :⇒ Thick body section attenuate > thin body section

• Subject contrast directly proportional to the relative number of x-rays exiting adjoining sections of the body

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

• 1000 x-ray incident on each section contrast = 1

Contrast = 800/300= 2.67

• 1000 x-ray incident on each section contrast = 1 Contrast = 800/300 = 2.67


• Tissue density (g/cm3) :important factor affecting subject contrast

Bone : tissue : Lung = 1.85 : 1 : 0.32

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

ชนิดของสารตัวกลาง ความหนาแนน

g/cm3

Fat 0.91

Soft Tissue 1.00

Lung 0.32

Bone

Barium

1.85

56

• 1000 x-ray incident on each section contrast = 1 Contrast = 900/400 = 2.25


• Effective atomic number (Zeff) :• Photoelectric effect vary greatly with Zeff• Compton scattering independent with Zeff

• Photoelectric effect important In diagnostic x-ray energies ⇒ subject contrast greatly influenced by Zeff of the tissue being radiograph

ชนิดของสารตัวกลาง เลขอะตอมยังผล ความหนาแนน

g/cm3

Fat 6.3 0.91

Soft Tissue 7.4 1.00

Lung 7.4 0.32

Bone 13.8 1.85

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage


• The anatomical structure under investigation influences the radiographic quality⇒ geometry and subject contrast

• A : a structure having a form of a truncated wedge that would coincide with the x-ray beam

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

⇒ Maximum subject contrast

• Other anatomical shapes have reduced subject contrast because of change in thickness that they present to incident x-ray photon beam

Absorption unsharpness


• Kilovoltage :Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

Series of radiographs of Al step wedge at 40-100 kVp

• Technologist not control patient’s factors• Magnitude of subject contrast is greatly

control by kVp of operation and also influences film contrast


• Kilovoltage :Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

Low kVp→ high subject contrast ⇒ Short scale contrast

high kVp→ low subject contrast⇒ long scale contrast

Low kVp technique more desirable than high kVp technique but

Two disadvantage of low kVp technique• High patient dose• Loss of exposure latitude

• Use of contrast medium result in extremely high subject contrast, ↑ photoelectric absorption


• Movement of either the patient or the x-ray tube during the x-ray exposure result in blurring radiographic image⇒ loss of radiographic quality

Motion unsharpness

• Normally, motion of x-ray tube not a problem : in tomography tube move in precise geometric pattern during exposure to blur the images of structures on either side of the plane of interest

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

• Reciprocating bucky

• Cause reexamination during procedures


• Motion unsharpness can reduce by carefully instruction the patient by x-ray technologist

• Use shortest possible exposure time• Restrict patient motion by instruction or

restraining device• Use a large TFD• Use a small OFD

Subject Factors

Contrast

Thickness

Density

Atomic number

Motion

Shape

Kilovoltage

Geometric unsharpness

• Guideline for reducing Motion unsharpness


• Anatomical structure under investigation be placed as close as to the film

Patient positioning

Imaging devices

Selection of technique factors

• Anatomical structure lie in a parallel plane of the film

• Center of x-ray beam incident on the center of the structure

• Patient effectively restrained in order to minimize motion unsharpness

Technologist :• Good knowledge of human anatomy• Radiographic positioning → minimum image

distortion and maximum image resolution


Clinical consideration for improved Clinical consideration for improved Radiographic QualityRadiographic Quality

• Use intensifying screen at lease IF 20Patient positioning

Imaging devices


• Speed of film – screen increases →image resolution decreases ⇒ film quality decreases

• Direct exposure film always has lower contrast than screen film

• low contrast imaging procedures allow for a wider margin of error in producing an acceptable radiograph

• Direct exposure film has higher contrast than screen film when exposed without screen

Clinical consideration for improved Clinical consideration for improved Radiographic QualityRadiographic Quality

• Prior to each examination, technologist requires to exercise good judgment in selecting optimum radiographic technique factors ; kVp, mA, time

Patient positioning

Imaging devices


• Use exposure time as short as possible- high frequency < three phase < single phase- reciprocity law

• kVp primarily influences the quality of the x-ray beam but it also affects on quantity

* kVp ↑ ⇒ penetration and amount of x-ray ↑

• mAs affects only the radiation quantityChanging mAChanging kVp


Table shows Principal factors affecting the making of radiography

หนังสอือานประกอบหนังสอือานประกอบ

Techniques of Veterinary Radiography 5th.Ed.1993.Joe P. Morgan

Radiography in Veterinary Technology.1and 2th Ed 1994,1999 Lisa M. Lavin.

Radiographic Photography and Imaging process.1980. David Jenkin

Radiation Image and Exposure. 2th Ed 2004. Terri L. Fauber.

Physics of radiology. 1993. Anthony B. Wolbarst

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Radiographic Quality - med.cmu.ac.th004_radiographic quality... · ray and of factors affecting radiographic quality • The x-ray technologist is provided with all physical ... •