Resident Physics Lectures

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 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. 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 • Credit: Sprawls.org Few Photons Many Photons

14. 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

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

17. 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. FocalSpot Object Receptor Similar Triangle Review A B b a h H c C Object Receptor a b c h---- = --- = --- = --- A B C H

20. Magnification Defined FocalSpot size of image --------------------size of object Object Image

21. Using Similar Triangles FocalSpot size of image Magnification = -------------------- size of object focus to image distance HMagnification = ---------------------------- = --- focus to object distance h h H Object Image

22. * Optimizing Image Quality by Minimizing Magnification focus to receptor distance Hmagnification = --------------------------------------- = --- focus to object distance h FocalSpot • Minimize object-receptor distance • Maximize focal-receptor distance h H Object Image

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 Area 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

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

27. Focal Spot Size • Trade-off • 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-RayTube X-RayTube X-RayTube

31. 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

32. 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

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

34. Motion Unsharpness • Caused by motion during exposure of • 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. Loss of Contrast 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