Image Production: Image Evaluation
Core concepts
Image evaluation asks two questions: is the image diagnostic, and does it meet acceptance criteria? Assess correct patient and part, positioning and alignment, collimation, marker placement, exposure (brightness/density), contrast, spatial resolution, distortion, and artifacts.
In digital imaging, receptor exposure is decoupled from display brightness. Read the exposure indicator: CR reports an EI; DR reports a deviation index, DI = 10 log(EI/EI target). DI 0 is on target, +3 is about twice the target exposure, and −3 is about half. An overexposed digital image can look perfectly acceptable on the monitor, so never judge dose by appearance alone.
Noise comes from quantum mottle when too few photons reach the receptor; electronic noise dominates at very low exposure. Contrast depends on kVp, scatter, part thickness, and window/level settings. Motion and misalignment cause blur; tube angulation causes shape distortion (foreshortening or elongation); excess OID causes magnification.
Common artifacts include grid lines, panel or plate defects, clothing and jewelry, processing errors, and ghosting. Repeat analysis classifies rejects as technique, positioning, motion, equipment, or processing.
Pathology alters attenuation: additive processes (sclerosis, effusion, mass) need more exposure; destructive processes (osteoporosis, emphysema, lytic lesions) need less.
Traps
- Calling a digital image "too dark" when it is windowed incorrectly.
- Treating EI or DI as a measure of patient dose - it is an exposure index.
- Missing collimation that excludes required anatomy.
- Blaming motion on the patient without checking immobilization.
- Overlooking a missing or reversed side marker.
Rule of thumb
If DI falls outside roughly ±3, investigate the cause before repeating, and document it.
Example: DI = −3 on a lateral knee means the receptor received about half the target exposure - check collimation and mAs.