Resident Physics Lectures

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 I • difference in x-ray intensity transmitted through various parts of subject • Depends on • thickness difference • density difference • atomic number difference • radiation quality (kVp, HVL) 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. Low kVp High kVp * kVp & Exposure Latitude • kVp affects latitude • Increasing kVp • decreases contrast • increases latitude • kVp must match latitude requirements of exam

8. Optical Density 2.0 .25 log rel. exp. Latitude 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

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

10. Optical Density log relative exposure Optical Density log relative exposure Exposure Latitude Low Contrast High Latitude Higher kVp 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 • 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. Quantum Mottle • Math (cont.) • fractional fluctuation greatest when # of photons is smallest 10 100 ---- > --------- (.1 > .01) 100 10,000 • 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 • 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. FocalSpot Object Receptor Similar Triangle Review A B b a h H c C Object Receptor a b c h---- = --- = --- = --- A B C H

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

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

25. Using Similar Triangles FocalSpot size of image Magnification = -------------------- size of object focus to receptor distance Hmagnification = -------------------------------- = --- focus to object distance h 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 Hmagnification = --------------------------------------- = --- focus to object distance h FocalSpot • Minimize magnification • Minimize object-receptor distance • Maximize focal-receptor distance h H Object Receptor (image)

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 • minimize focal spot size • maximize source to image distance • minimize object to image distance 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 BUTnot the sum! • larger than largest component • largest component controls unsharpness • improvement in smaller components don’t help much

33. Sharpness & Resolution • Sharpness • 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 Shape Distortion X-RayTube X-RayTube Image Image Distortion Types minimal distortion when object near central beam & close to receptor

35. 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 image • zone of unsharpness called • geometric unsharpness • penumbra • edge gradient 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 sourcefocal spot SOD Object SID OIDP = F x ------- SOD OID P

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

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-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

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 • heat vs. resolving power • exposure time vs. resolving power • Focal Spot Size most critical for • magnification • mammography

43. Resolution 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 0 Frequency