1. Geometric Factors Focal Spot Object Film Object b a h c Film B A H C
a b c h ---- = --- = --- = --- A B C H

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

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

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

5. Optimizing Image Quality*
focus to film distance H magnification = = focus to object distance h
Focal Spot h H
Object
Film
(image)
Minimize magnification
Minimize object-film distance
Maximize focal-film distance

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

7. 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 film
zone of unsharpness called
geometric unsharpness
penumbra
edge gradient
Line source focal spot
Film Image

8. True Magnification
m = geometric mag M = true mag
Function of ratio of focal spot to object size (f / d)
true & geometric magnification equal only when object very large compared to focal spot f a d b
M=m + (m-1) X (f / d)

9. Penumbra Calculation Minimizing Penumbra
Minimize object-film 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

10. Magnification m = geometric mag M = true mag M = m = (a+b) / ad
m = (a+b) / a
M=m + (m-1) X (f / d)
Finite sized focal spot
M = m = (a+b) / a
Infinitely small focal spot
Geom = True
m = geometric mag M = true mag a b

11. For general radiography purposes the geometric unsharpeness dominates the other components
Therefore the unsharpeness will increase with increasing magnification. To keep magnification small (close to m=1) requires the image receptor to be as close as possible to the patient and the focus patient distance to be large.
Typical conditions are:
a  1mm
d1  1 m
d2  10 cm

12. Motion Unsharpness Caused by motion during exposure of Effect
patient
tube
film
Effect
similar to penumbra
Minimize by
immobilizing patient
short exposure times

13. 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

14. Inverse Square Law
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
Intensity a 1/d2 d

15. 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

16. Magnification Types Geometric Magnification True Magnification
Focal Spot
Object
Film h H
Geometric Magnification
assumes point source
calculated from similar triangles
True Magnification
takes into account finite size of focal spot
focal spot is area (not point) source

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

18. Film Construction Radiographic Film has two basic parts. Base Emulsion
Most film has two layers of emulsion so it is referred to as Double Emulsion Film

19. Emulsion
The emulsion is the heart of the film. The x-rays or light from the intensifying screens interact with the emulsion and transfer information to the film
The emulsion consists or a very homogeneous mixture of gelatin and silver halide crystals about 3 to 5 µm thick.

20. Silver Halide Crystals
98% Silver Bromide
2% Silver Iodide
Tabular shape used most commonly for general radiography.
About 1µm thick for screen film exposure.

21. Silver Halide Crystals
The differences in speed, contrast and resolution depend upon the process by which the silver halide crystals are manufactured and by the mixture of these crystals into the gelatin.
Size and concentration of crystals have a primary influence on speed.

22. Producing the Latent Image
The resulting silver grain is formed.
Silver halide that is not irradiated remain inactive. The irradiated and non-irradiated silver halide produces the latent image.

23. Types of X-ray Film Two main types:
Screen film used with intensifying screens.
Single emulsion- emulsion on one side of base.
Double emulsion used with two screens.
Direct exposure film or non-screen film.
Special purpose: Duplication, Cine, Dental

24. Film Dosimeters OD
Dose (log)

26. Optical density
X-ray film is a negative recorder – increased light (or x-ray) exposure causes the developed film to become darker
Degree of darkness is quantified by the OD, measured with a densitometer
Transmittance and OD defined as:

27. OD examples T OD Comment 1.0000 Perfectly clear (does not exist)
Perfectly clear (does not exist)
0.7760
0.11
Unexposed film (base + fog)
0.1000 1
Medium gray
0.0100 2
Dark
0.0010 3
Very dark; requires hot lamp
3.6
Maximum OD used in medical radiography

29. Contrast
Contrast of a radiographic film is related to the slope of the H&D curve:
Regions of higher slope have higher contrast
Regions of reduced slope (e.g., the toe and shoulder) have lower contrast
A single number, which defines the overall contrast of a given type of radiographic film, is the average gradient

31. Average gradient OD1 = 0.25 + base + fog OD2 = 2.0 + base + fog
Average gradients for radiographic film range from 2.5 to 3.5

33. Scattered radiation
For virtually all radiographic procedures except mammography, most photon interactions in soft tissue produce scattered x-ray photons
Detection of scattered photons causes film darkening but does not add information content to the image

35. Effect of collimation
As the field of view is reduced, the scatter is reduced
An easy way to reduce the amount of x-ray scatter is by collimating the x-ray field to include only the anatomy of interest and no more

36. Antiscatter grid
An antiscatter grid is placed between the patient and the screen-film cassette
The grid uses geometry to reduce the amount of scattered reaching the detector

38. Antiscatter grid (cont.)
Antiscatter grid is composed of a series of small slits, aligned with the focal spot, that are separated by highly attenuating septa
Primary x-rays have a higher chance of passing through the slits unattenuated by the adjacent septa
Septa (grid bars) are usually made of lead; openings (interspaces) between the bars can be made of carbon fiber, aluminum, or even paper

41. Bar Phantom Setup
                                                    
Who is this?

42. The Rise & Fall of Joe Camel

43. Focal Spot Size Varies with Technique
Electron beam focuses more poorly at high mA or low kV
Blooming
increase of focal spot size with increasing mA
more of a problem at low kV’s
more blooming perpendicular to cathode-anode axis
kilovoltage effects
size decreases slightly with increasing kVp
size always measured & specified at particular technique

44. 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

45. 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

46. Modulation Transfer Function (MTF)
How well information reproduced (fraction of contrast retained) at various input spatial frequencies

47. Modulation Transfer Function (MTF)
Fraction of contrast reproduced as a function of frequency
Freq. =
line pairs / cm 1
MTF
50%
Recorded
Contrast (reduced by blur)
Contrast provided
to film
frequency

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

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

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

51. 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

52. Component MTF Each component of imaging system has its own MTF
each component retains a fraction of contrast as function of frequency
System MTF is product of MTF’s for each component.

53. Modulation Transfer Function (MTF)
Since MTF is between 0 and 1, composite MTF <= MTF of poorest component
1/2 * 1/3 * 1/4 * 1/5 = ?
? < 1/5

54. Film MTF Resolves 10-20 line pairs per mm
MTF ~ 1 for clinical applications

55. Focal Spot MTF Function of magnification
Deteriorates with increased magnification
all clinical imaging involves some magnification
The larger the focal spot, the more deterioration of MTF with increased magnification

56. Imaging Screen MTF improves with magnification Why?
magnification of an object of given frequency reduces frequency seen by screen

57. Focal Spot & Screen MTF focal spot MTF degrades with magnification
screen MTF improves with magnification
film MTF unaffected with magnification
best system resolution
detail screens
minimize magnification
BUT detail screens decrease speed
increase exposure time
more potential for motion

58. Imaging Physics Wrap Up

59. Object Motion independent of magnification depends on
exposure time
velocity
If motion < 0.1 mm usually doesn’t contribute to unsharpness
If motion > 1 mm, it generally dominates

60. Quantum Mottle
statistical fluctuation in # of x-ray photons used by imaging system to form image
quantum mottle independent of geometric (radiographic) magnification
for a given density the same # of photons must strike a given area of screen/film

61. Quantum Mottle
quantum mottle directly affected by photographic (optical) magnification
less photons per unit area of image

62. Quantum Mottle
influences perception of low-contrast objects with poorly defined borders
not well addressed by MTF
measures sharp high contrast borders
geometric (radiographic) magnification may improve visibility of low contrast objects
quantum mottle noise does not increase

63. Skin Exposure & Magnification
Patient closer to x-ray tube
Increases exposure
grids not used with magnification work because of air gap
Decreases exposure
No bucky factor loss (3-6)
Grid
Film

64. Skin Exposure & Magnification
Increase is less than calculated by inverse square law
for 2X mag skin not twice as close
routine radiography also involves some magnification
smaller volume of patient exposed
high speed screen/film sometimes used for magnification
Grid
Film

65. Ilse, we’ll always have physics.
You Must Remember This
Ilse, we’ll always have physics.
The End