12.1 Digital & Processing Artifacts

What Counts as an Artifact

An artifact is any density, shadow, band, or pattern on a radiograph that does not represent the patient's anatomy. Artifacts mimic pathology, obscure real findings, and are a leading cause of repeat exposures and unnecessary dose. On the ARRT Radiography exam, Image Production items typically describe an appearance and ask you to identify the cause and the correct corrective action. It helps to sort digital artifacts into three families: acquisition/detector artifacts (the receptor hardware), processing (software) artifacts (histogram/LUT handling), and exposure/handling artifacts (technique and physical care of the receptor). This section teaches the artifact appearance-to-cause mapping without re-deriving the digital-characteristics theory from Chapter 10.

Computed Radiography (CR) Artifacts

Computed radiography (CR) uses a reusable photostimulable phosphor (PSP) imaging plate that stores a latent image until a laser scans it in the reader. Because the plate is reused thousands of times, several artifacts are unique to it.

• Ghost / incomplete-erasure artifact: residual signal from a prior exposure is superimposed on the new image. The fix is to run the plate through the eraser; plates unused for 24-48 hours should be re-erased because background and scatter radiation slowly fog them.
• Light leak / fog: a cracked cassette or exposure to bright light before reading produces a fogged band of increased density along one edge.
• Backscatter (lead-lining) artifact: a faint reversed shadow of a dense object such as the cassette hinge or latch appears because scatter from behind the plate exposes it. A faint reversed lead-marker image on the opposite side is the classic clue. Lead backing inside the cassette prevents it.
• Moiré (aliasing) artifact: evenly spaced light/dark lines appear when a stationary grid's line frequency interferes with the laser scanning frequency of the CR reader. Correct it by aligning the grid lines parallel to the plate's scan direction, using a higher grid frequency, or removing the grid.

Direct Radiography (DR) Artifacts

Digital (direct) radiography (DR) reads a flat-panel detector electronically, so its artifacts are hardware and calibration based.

• Dead / dropped pixels: non-functioning detector elements show as fixed white or black dots or lines that appear in the same location on every image. They are masked by flat-field (gain/offset) calibration, which maps defective elements and interpolates neighboring values.
• Detector drop-out / delamination: physical damage, dropped panels, or moisture create dark blotches; the panel must be serviced.
• Ghost (lag) artifact: residual charge carries over from a previous exposure; adequate cycle time and routine calibration reduce it.

Exposure, Handling & Processing Artifacts

These are technique-related and appear on both CR and DR:

• Motion blur: voluntary or involuntary movement blurs recorded detail. Shorten exposure time (high mA/short time), give clear breathing instructions, and immobilize.
• Double exposure: two superimposed images when a plate is exposed twice before reading (CR).
• Saturation: grossly overexposed regions read as featureless white with lost data that cannot be recovered by windowing.
• Quantum mottle (noise): a grainy, mottled image from underexposure; the system amplifies sparse signal, producing a persistently negative deviation index (DI).
• Histogram-analysis (segmentation / data-recognition) errors: the processor builds a histogram of the values of interest and applies a look-up table (LUT). Improper collimation, off-centering, unexpected prostheses or scatter, or choosing the wrong anatomic algorithm cause the software to misread the histogram, so the image prints too dark, too light, or with wrong contrast. Correct collimation and correct exam/body-part selection are the fixes.

Artifact → Cause → Correction

Grid, Scatter, and Foreign-Object Artifacts

Beyond moiré, a grid-line artifact — sharp fine lines mapping the grid strips — appears when a reciprocating Bucky fails to move during exposure or a low-frequency stationary grid is imaged; the grid mechanism, not the processing software, is at fault (grid cutoff itself is covered in Chapter 11). Foreign-object artifacts from jewelry, snaps, hairpins, dentures, or ECG leads left in the field mimic pathology and force repeats, so patient preparation is a frontline artifact-prevention step. A brief clinical scenario: a wrist image returns grainy with a DI of -4; the technologist recognizes quantum mottle from underexposure, raises mAs to drive the DI toward 0, and does not simply brighten the display — brightening amplifies noise without adding signal. Recognizing that the cause is too few photons, not a display setting, is exactly the reasoning the Image Production domain rewards.

Common exam trap: a very good-looking dark image with a high exposure index / positive DI is overexposure (dose creep), not a better image — digital systems hide overexposure by rescaling brightness. Always read the EI/DI together with the visual receptor exposure rather than judging brightness alone.