1. Resident Physics Lectures
The Radiographic Image & Geometry
George David
Associate Professor
Department of Radiology
Medical College of Georgia

2. Contrast difference in density between areas on the radiograph
Contrast depends on
subject contrast
receptor contrast
scatter

3. Subject Contrast *
difference in x-ray intensity transmitted through various parts of subject
Depends on
thickness difference
density difference
atomic number difference
radiation quality (kVp, HVL) I IS IL
Subject Contrast = IS / IL

4. Subject Contrast & Radiation Quality*
high kVp = lower subject contrast
long scale contrast (less difference between areas receiving varying amounts of radiation)
low kVp = high subject contrast
short scale contrast (more black & white; more difference between areas receiving varying amounts of radiation)
low kVp increases patient dose

6. Scatter Reduces contrast Produces unwanted density
Mostly a result of Compton interactions
Increases with
kVp
part thickness
field size
collimation reduces scatter

7. kVp & Exposure Latitude*
kVp affects latitude
Increasing kVp
decreases contrast
increases latitude
kVp must match latitude requirements of exam
High kVp
Low kVp

8. Exposure Latitude With Film
range of incident radiation intensities which produce desired film density
Latitude & contrast vary inversely
high contrast = low latitude
low contrast = high latitude
log rel. exp.
Optical
Density
.25
2.0
Latitude

9. Speed & Contrast Contrast controls slope of characteristic curve *
Lower Contrast
High Latitude
log relative exposure
Optical
Density
Higher Contrast
Low Latitude
log relative exposure
Optical
Density
Whites whiter, blacks blacker

10. Exposure Latitude For low contrast film increasing kVp causes
log relative exposure
Optical
Density
Low Contrast
High Latitude
Higher kVp
log relative exposure
Optical
Density
High Contrast
Low Latitude
Lower kVp
For low contrast film
shallow slope
greater exposure latitude
wider mAs range produces proper film density
increasing kVp causes
decreased contrast (slope)
increased latitude

11. Film/Screen Limited Latitude
Film required proper radiation exposure

13. High Digital Latitude

14. Image Quality
ability of image receptor to record each point of image as point on the display
Influenced by
radiographic mottle
also called noise
sharpness
resolution

15. Image Quality: What is it?
Depends only on intrinsic, objective physical characteristics of imaging system
Can be measured independent of observer
Quantitative
Whatever observer says it is
Subjective perception of image
Defined by observer’s ability to achieve an acceptable level of performance for a specified task.
Courtesy Ralph Schaetzing, Carestream Health

16. Quantum Mottle Appearance Cause Math random x-ray emission
irregular density variations in mid-density areas exposed to uniform x-ray fields
Cause
random x-ray emission
statistical fluctuations in # of quanta / unit area absorbed by receptor
Math
related to square root of total number of photons interacting with receptor

17. Numerator is square root of denominator
Quantum Mottle
Math (cont.)
fractional fluctuation greatest when # of photons is smallest
> (.1 > .01) ,000
throw a dice 12 times or 12,000 times; variation from expected 1/6 for each face will probably be more for 12 throws!
Numerator is square root of denominator

18. Quantum Mottle
Best visualized on good-quality high contrast radiograph
Poor detail (blurring) may mask quantum mottle
Raising kilovoltage while maintaining the same receptor exposure results in:
lower patient exposure
lower mAs for
fewer x-ray photons
higher quantum mottle

19. Speed Film Digital Measure of sensitivity to light Faster speed means
less light (or radiation) required to achieve same image density (darkness)
Image produced with less radiation
Increased quantum mottle (noise) at same density
Digital
No “fixed” speed (Sprawls)
Can produce images with good contrast over wide range of receptor exposure
Receptor exposure dictates image noise

21. Noise & Speed Cause of noise (quantum mottle)
statistical fluctuation in # of x-ray photons forming image
Ability to see high contrast objects limited by image sharpness
High noise reduces visibility of low contrast objects
most important diagnostic information here

22. Similar Triangle Review
Focal Spot
Object
Receptor A b a B h H c C
Object
Receptor
a b c h ---- = --- = --- = --- A B C H

23. Magnification Defined
Focal Spot
size of image
size of object
Object
Film
(image)

24. Using Similar Triangles
Focal Spot
size of image
Magnification = size of object h H
Object
Film
(image)
focus to film distance H Magnification = = focus to object distance h

25. Using Similar Triangles
size of image
Magnification = size of object
Focal Spot h
focus to receptor distance H magnification = = focus to object distance h H
Object
Film
(image) SO
size of image = size of object X Magnification
focus to receptor dist. size of image = size of object X focus to object dist

26. Optimizing Image Quality*
focus to receptor distance H magnification = = focus to object distance h
Focal Spot h H
Object
Receptor
(image)
Minimize magnification
Minimize object-receptor distance
Maximize focal-receptor distance

27. Automatic Artifact ? ? Occurs whenever we image a 3D object in 2D
Work-around
Multiple views ? ?

28. Sharpness Ability of receptor to define an edge Sharpness and Contrast
unsharp edge easier to detect under conditions of high contrast
sharp edge are less visible under conditions of low contrast
One cause of unsharpness
Penumbra
Shadow caused by finite size of focal spot

29. Minimizing Geometric Unsharpness
minimize focal spot size
maximize source to image distance
minimize object to image distance
Minimize
maximize
minimize

30. Sources of Unsharpness
Geometry
Motion
minimized by short exposure times
Absorption
absorber may not have sharp edges
round or oval objects

32. Total Unsharpness combination of all the above BUT not the sum!
larger than largest component
largest component controls unsharpness
improvement in smaller components don’t help much

33. Sharpness & Resolution
ability of imaging system to record sharply defined margins or abrupt edges
Resolving Power (Resolution)
ability to record separate images of small objects very close together

34. Relative Position Distortion
Distortion Types
X-Ray Tube
Image
Shape Distortion
X-Ray Tube
Image
Relative Position Distortion
minimal distortion when object near central beam & close to receptor

35. Penumbra Latin for “almost shadow” region of partial illumination
also called edge gradient
region of partial illumination
caused by finite size of focal spot
smears edges on image
zone of unsharpness called
geometric unsharpness
penumbra
edge gradient
Line source focal spot
Image

36. Penumbra Calculation Minimizing Penumbra
Minimize object-receptor distance (OID)
Maximize source-object distance (SOD)
Makes focal spot appear smaller
Minimize focal spot size F
Line source focal spot
SOD
Object
SID
OID P = F x SOD
OID P

37. Motion Unsharpness Caused by motion during exposure of Effect
patient
Tube
Receptor
Effect
similar to penumbra
Minimize by
immobilizing patient
short exposure times

38. Absorption Unsharpness
Cause
gradual change in x-ray absorption across an object’s edge or boundary
thickness of absorber presented to beam changes
Effect
produces poorly defined margin of solid objects
X-Ray Tube
X-Ray Tube
X-Ray Tube

39. Inverse Square Law
Intensity a 1/d2
intensity of light falling on flat surface from point source is inversely proportional to square of distance from point source
if distance 2X, intensity drops by 4X
Assumptions
point source
no attenuation
Cause
increase in exposure area with distance d

40. Trade-off Geometry vs. Intensity
maximize SID to minimize geometric unsharpness but
doubling SID increases mAs by X4
increased tube loading
longer exposure time
possible motion
going from 36 to 40 inch SID requires 23% mAs increase F
SOD
SID
OID P

41. Off-Axis Variation
focal spot measurements normally made on central ray
apparent focal spot size changes in anode-cathode direction
smaller toward anode side
larger toward cathode side
less effect in cross-axis direction

42. Focal Spot Size Trade-off Focal Spot Size most critical for
heat vs. resolving power
exposure time vs. resolving power
Focal Spot Size most critical for
magnification
mammography

43. Resolution Units lines or line pairs per distance
1 mm
Units
lines or line pairs per distance
such as lead bars separated by equally wide spaces
Expresses limiting resolution
Limiting resolution implies high contrast situation
does not indicate how well system preserves contrast
4 lines (line pairs) per mm

44. Modulation Transfer Function (MTF)
value between 0 and 1
MTF = 1 indicates all information reproduced at this frequency
MTF = 0 indicates no information reproduced at this frequency

45. MTF If MTF = 1 all contrast reproduced at this frequency
Contrast provided
to film
Recorded
Contrast

46. MTF If MTF = 0.5 half of contrast reproduced at this frequency
Contrast provided
to film
Recorded
Contrast

47. MTF If MTF = 0 no contrast reproduced at this frequency
Contrast provided
to film
Recorded
Contrast

48. MTF as sharpness decreases so does contrast
less sharp system blurs dark & light areas together
maximum density decreases
minimum density increases
at very high line pairs per mm film will be uniform gray

49. Modulation Transfer Function (MTF)
100% (1)
80% (0.8)
Lowest Frequency
40% (0.4)
0% (0.0)
Highest Frequency
Fraction of contrast reproduced decreases at increasing frequency because lines and spaces blur into one another

50. MTF
Combines concepts
sharpness
resolution
contrast 1
MTF
Frequency