Radiographic Image Production and Evaluation
Image qualities
A diagnostic image balances brightness, contrast, spatial resolution, distortion, and noise. In film-screen, mAs mainly controls density and kVp controls contrast. In digital imaging, mAs controls receptor exposure and quantum noise; displayed brightness is adjusted by windowing. kVp still affects contrast and scatter.
Digital processing
A digital image is a matrix of pixels with a bit depth. Spatial resolution depends on pixel pitch, focal spot, OID, and motion. Contrast resolution depends on bit depth, detector, and dose. MTF describes resolution; DQE describes dose efficiency. The histogram and LUT map raw values to display; window level sets brightness and window width sets displayed contrast.
Exposure indicators
EI and DI help judge receptor exposure, not patient dose. At a baseline of 1000, doubling exposure raises EI by about 300. Underexposure produces noise; overexposure raises dose without necessarily changing brightness.
Artifacts and QA
Grid cutoff, motion, detector lag, dead pixels, and collimation errors are common. Repeat analysis identifies correctable problems.
Traps
A bright image is not proof of correct exposure. Increasing window width lowers displayed contrast. EI is not a dose meter. Spatial resolution is not improved by windowing.
Rule of thumb: If brightness is acceptable but noise is excessive, suspect underexposure; check EI/DI and repeat only if diagnostic criteria are not met.