1. Resident Physics Lectures
The Radiographic Image & Geometry
George David, MS, FAAPM, FACR
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. 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

8. Image Quality: 3 Definitions
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

9. You Already Know Some Imaging Statistics

10. Noise & a Die
You throw the die 6 times. Is this die rigged?

11. Noise & a Die You throw the die 6 million times. Is this die rigged?
1 million 2’s
1 million 3’s
1 million 1’s
2 million 6’s
1 million 4’s

12. Raindrops
When it first starts to rain, one can see where each drop landed
After a few minutes, sidewalk looks uniformly wet

13. X-Ray Images Are Created One Photon at a Time
Many Photons
Few Photons
Credit: Sprawls.org

14. Quantum Mottle Appearance Cause 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

16. Hint: One photo has one more stack than the other
Noise & Money
Which photo has more stacks of money? ?
Hint: One photo has one more stack than the other

17. Hint: One photo has one more stack than the other
Noise & Money
Which photo has more stacks of money? ?
Hint: One photo has one more stack than the other

18. Noise & Image Quality 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

19. 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

20. Magnification Defined
Focal Spot
size of image
size of object
Object
Image

21. Using Similar Triangles
Focal Spot
size of image
Magnification = size of object h H
Object
Image
focus to image distance H Magnification = = focus to object distance h

22. Optimizing Image Quality by Minimizing Magnification*
Optimizing Image Quality by Minimizing Magnification
focus to receptor distance H magnification = = focus to object distance h
Focal Spot h H
Object
Image
Minimize object-receptor distance
Maximize focal-receptor distance

23. Ever-present Imaging Artifact
Occurs whenever we image 3D object in 2D
Work-around
Multiple views ? ?

24. 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

25. 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
Area source focal spot
Image

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

27. 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

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

29. 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

31. 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

32. 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

33. 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

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

35. Inverse Square Law
Intensity a 1/d2
intensity inversely proportional to square of distance
if distance 2X, intensity drops by 4X
Assumptions
point source
no attenuation d

36. Loss of Contrast as a Result of Unsharpness
as sharpness decreases so does contrast
less sharp system blurs dark & light areas together
maximum density decreases
minimum density increases
at very high frequency image will be uniform gray

37. 100% (1) 80% (0.8) 40% (0.4) 0% (0.0) Loss of Contrast
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