Geometric Factors

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

2. Magnification Defined FocalSpot size of image --------------------size of object Object Film (image)

3. Using Similar Triangles FocalSpot size of image Magnification = --------------------size of object h focus to film distanceHMagnification = ---------------------------------- = ---focus to object distanceh H Object Film (image)

4. Using Similar Triangles FocalSpot size of image Magnification = -------------------- size of object h focus to film distanceHmagnification = ---------------------------------- = --- focus to object distanceh 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 distanceHmagnification = ---------------------------------- = --- focus to object distanceh FocalSpot • Minimize magnification • Minimize object-film distance • Maximize focal-film distance h H Object Film (image)

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

7. Penumbra Line sourcefocal spot • Latin for “almost shadow” • 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 Film Image

8. True Magnification m = geometric magM = true mag f • Function of ratio of focal spot to object size (f / d) • true & geometric magnification equal only when object very large compared to focal spot 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 sourcefocal spot SOD Object SID OIDP = F x ------- SOD OID P

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

11. a  1mm d1 1 m d2 10 cm 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:

12. Motion Unsharpness • Caused by motion during exposure of • 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-RayTube X-RayTube X-RayTube

14. 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

15. Trade-offGeometry vs. Intensity F • 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 SOD SID OID P

16. FocalSpot h H Object Film Magnification Types • 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

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 • 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 0 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