Artefacts in digital imaging – Learn Radiology Physics

Digital systems (CR and DR) have largely eliminated many film-based artefacts but have introduced new ones related to detector technology, data processing, and system calibration.

I’m not going to be able to cover all artefacts here. I have tried to classify artefacts into groups and mention the main artefacts seen in each.

The classification I’ve used is as follows:

Category	Source	Examples
Detector artefacts	Faults or non-uniformities in the detector	Dead pixels, line defects, ghosting
Processing artefacts	Errors in image correction, enhancement, or display	Edge enhancement halos, banding, histogram errors
Exposure-related artefacts	Improper exposure or grid alignment	Saturation, noise, moiré pattern
Mechanical / environmental	External factors affecting acquisition	Dust on CR plate, cassette damage, RF interference

Let’s look at each of these in some more detail.

Detector Artefacts

A. Dead or Defective Pixels

Individual detector elements (DELs) may fail, producing constant bright or dark spots.
Line artefacts occur when an entire column or row of pixels fails.
Systems use interpolation from neighbouring pixels to correct single-pixel faults, but large clusters require service calibration.

B. Non-Uniform Response

Variations in detector gain or scintillator thickness cause brightness non-uniformity (shading).
Corrected by flat-field calibration (gain and offset maps).
If calibration data are outdated → residual “mottle” or “pattern” artefacts appear.

C. Ghosting / Lag

Residual charge or trapped electrons from a previous exposure cause faint “after-images”.
More common in CR plates or a-Se detectors under high dose or rapid re-exposure conditions.
Mitigated by plate erasure (CR) or by delaying re-exposure.

Processing Artefacts

A. Edge Enhancement Artefacts

Occur when high-frequency filters over-emphasise sharp boundaries.
Appear as bright or dark “halos” along edges of bones or devices.
Recognised by symmetrical light–dark outlines.
Can mimic fractures or lines in chest radiographs.
Reduce by using standard or “soft tissue” processing modes.

B. Banding or Step Artefacts

Appear as horizontal or vertical bands of varying brightness.
Caused by readout instability, detector timing errors, or software glitches.
May also result from partial flat-field calibration failure.

Exposure-Related Artefacts

A. Saturation (Clipping)

Occurs when detector elements receive exposure above their measurable limit.
Pixels reach maximum value (white) → loss of anatomical information.
Common in DR with metallic implants or very high exposure regions.
Prevent by appropriate technique and using exposure index (EI) feedback.

B. Underexposure

Too few photons → high quantum noise → image graininess.
The image may still look acceptable after processing (due to automatic brightness correction), masking the underexposure.

C. Grid-Related Artefacts

Moiré pattern: interference between the grid line frequency and the CR scanning frequency, producing wavy bands.
Avoid by using stationary grids with non-parallel orientation or high-frequency grids (>100 lines/inch).
Grid cutoff: improper alignment or SID causes uneven exposure, most visible at image edges.

D. Double Exposure (CR)

Failure to erase a plate properly before reuse results in overlapping images.
Prevent by complete erasure and visual inspection before reloading.

Mechanical and Environmental Artefacts

Source	Description / Appearance	Prevention
Dust or debris on CR plate	White or dark specks, reproducible location	Regular plate cleaning
Damaged cassette or light leaks	Streaks or fogging	Cassette replacement
RF interference / electrical noise	Regular line patterns or speckled noise	Equipment shielding
Temperature / humidity extremes	Affects phosphor sensitivity	Controlled storage conditions

Artefacts Unique to CR vs DR

System	Common Artefacts	Notes
CR	Laser jitter, moiré pattern, ghosting, plate dust	Artefacts arise during readout and erasure stages
Indirect DR	Light scatter, pixel dropouts, calibration errors	Caused by scintillator or electronics
Direct DR (a-Se)	Lag, dead pixels, saturation	Fewer artefacts overall but more sensitive to static and heat

Key Takeaways and Exam Tips:

An artefact is any spurious image feature unrelated to the patient.
Major categories: detector, processing, exposure, mechanical/environmental.
Recognising artefacts prevents misdiagnosis and unnecessary repeat exposures.
Many artefacts mimic pathology e.g. edge halos (fracture mimic).
Common exam question: “List common artefacts in computed and digital radiography and describe their causes.”