Resident Physics Lectures The Radiographic Image & Geometry George David, MS, FAAPM, FACR Associate Professor Department of Radiology Medical College of Georgia

Contrast difference in density between areas on the radiograph Contrast depends on subject contrast receptor contrast scatter

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

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

Scatter Reduces contrast Produces unwanted density Mostly a result of Compton interactions Increases with kVp part thickness field size collimation reduces scatter

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

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

You Already Know Some Imaging Statistics

Noise & a Die You throw the die 6 times. Is this die rigged?

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

Raindrops When it first starts to rain, one can see where each drop landed After a few minutes, sidewalk looks uniformly wet

X-Ray Images Are Created One Photon at a Time Many Photons Few Photons Credit: Sprawls.org

Quantum Mottle Appearance Cause 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

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

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

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

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

Magnification Defined Focal Spot size of image -------------------- size of object Object Image

Using Similar Triangles Focal Spot size of image Magnification = -------------------- size of object h H Object Image focus to image distance H Magnification = ---------------------------- = --- focus to object distance h

Optimizing Image Quality by Minimizing Magnification * Optimizing Image Quality by Minimizing Magnification focus to receptor distance H magnification = --------------------------------------- = --- focus to object distance h Focal Spot h H Object Image Minimize object-receptor distance Maximize focal-receptor distance

Ever-present Imaging Artifact Occurs whenever we image 3D object in 2D Work-around Multiple views ? ?

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

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 Area source focal spot Image

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

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

Sources of Unsharpness Geometry Motion minimized by short exposure times Absorption absorber may not have sharp edges round or oval objects

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

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

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

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

Motion Unsharpness Caused by motion during exposure of Effect Patient Tube Receptor Effect similar to penumbra Minimize by immobilizing patient short exposure times

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

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

100% (1) 80% (0.8) 40% (0.4) 0% (0.0) Loss of Contrast 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