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Resident Physics Lectures

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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 * 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 kVp & Exposure Latitude
* kVp affects latitude Increasing kVp decreases contrast increases latitude kVp must match latitude requirements of exam High kVp Low kVp

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

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

10 Exposure Latitude For low contrast film increasing kVp causes
log relative exposure Optical Density Low Contrast High Latitude Higher kVp log relative exposure Optical Density 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 Cause Math 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 Math related to square root of total number of photons interacting with receptor

17 Numerator is square root of denominator
Quantum Mottle Math (cont.) fractional fluctuation greatest when # of photons is smallest > (.1 > .01) ,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 Digital 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 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

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

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

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

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

33 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

34 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

35 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 Line source focal spot 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 source focal spot SOD Object SID OID P = F x SOD OID P

37 Motion Unsharpness Caused by motion during exposure of Effect
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-Ray Tube X-Ray Tube X-Ray Tube

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-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

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

43 Resolution Units lines or line pairs per distance
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 Frequency

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