Digital Image Processing

Overview

Nothing on the monitor came off the detector looking like that. Between the exposure and the picture sits a chain of operations, some automatic and some yours, and the ARRT outline lists preprocessing and postprocessing as separate items. The chapter runs on one sequence: readout, digitizing, pre-processing, histogram analysis, the look-up table, post-processing, presentation. Pre-processing is the machine; post-processing is the person.

Digitizing is two processes, sampling and quantization. A 14-bit digital radiograph records 16,384 shades of gray while the human eye separates about 30, and windowing hands those 30 visible shades to whichever slice of the recorded range you choose. Window width sets contrast, narrow meaning short scale and high contrast; window level sets brightness.

Histogram analysis is automatic but runs on what you gave it. It wants four clearly collimated margins and rarely tolerates fewer than three, and the wrong anatomical menu applies the wrong reference histogram. Digital systems display images made across a wide range of exposure factors, and over- and underexposure are not readily recognizable from the image, which is why exposure is judged from the deviation index: 0 is on target, and the scale is logarithmic, so +3 is roughly twice the target exposure and -3 roughly half.

The chapter ends at the archive. Lossless compression up to 3:1 is generally considered acceptable; lossy can pass 100:1 but discards data, which is why it is unacceptable for archiving mammography. DICOM is the standard that lets different manufacturers' systems exchange images, not a storage device, and the FDA renamed the PACS device category to MIMPS in 2021; the 2027 specifications list both, as PACS/MIMPS.