Radiologic Imaging and Image Evaluation
Core concepts
Image evaluation asks you to judge diagnostic quality and to recognize artifacts and errors. The pillars are density/brightness, contrast, spatial resolution, distortion, and noise. Understand how technical factors-kVp, mAs, source-to-image distance, and patient thickness-affect these. Higher kVp generally lowers contrast and widens the exposure latitude; mAs controls quantity of photons and thus receptor exposure. Recognize the inverse square law and the relationship between distance and receptor exposure.
Traps in questions
Separate quantum mottle (too few photons, appears noisy) from scatter (adds fog, lowers contrast). Distinguish motion unsharpness from geometric unsharpness caused by focal spot size, object-to-image distance, and source-to-object distance. Remember that magnification increases with greater object-to-image distance and with shorter source-to-image distance. Do not confuse overexposure with high contrast-these are different concepts. Know that collimation improves contrast by reducing scatter and that grids reduce scatter at the cost of increased dose or mAs.
Recognizing errors
Look for positioning errors (rotation, clipping anatomy, wrong centering) versus exposure errors (underexposure, saturation) versus processing or equipment artifacts (grid lines, detector defects). Each has a distinct corrective action.
Worked rule of thumb
If an image looks noisy, suspect insufficient photons (mAs/distance). If it looks flat and gray, suspect scatter or high kVp. Match the appearance to the physics before choosing a fix.