Image Quality Assessment, Artifacts and Troubleshooting
Quality is a gate, not a footnote
Assess four things in order: signal strength, centration, motion/blink integrity, and segmentation correctness. A scan that fails any of these cannot support a diagnosis of thinning, oedema or progression. Quality indices are displayed by the instrument; below threshold, repeat rather than interpret.
Recognising the common artifacts
- Mirror (fold-over) artifact: a duplicate, inverted image of the retina reflected across the zero-delay line, usually from poor signal or a structure exceeding the imaging range. Frequently mistaken for a detachment or a second retina.
- Motion artifact: horizontal or vertical stair-stepping, broken vessels, duplicated fovea; vertical shifts produce abrupt layer jumps.
- Blink/truncation: a wedge of missing data, often with a falsely bright or blank band.
- Decentration/obliquity: elliptical foveal pit, apparent thickening of inner layers, off-axis cut through the disc.
- Shadowing: posterior signal loss beneath vessels, hard exudates, dense haemorrhage or a floater - the shadow means the underlying tissue is not assessable, not absent.
- Projection artifact: superficial hyperreflective material (epiretinal membrane, vessel) cast onto deeper planes in en-face or slab views.
- Segmentation error: lines drifting off the RPE over drusen or PEDs, giving false thickness values; off-centre scans and low signal are the usual culprits.
Troubleshooting ladder
- Is the patient positioned and fixating? 2. Is the scan centred? 3. Is focus optimal and the lens/lid artefact cleared? 4. Is the signal adequate - if not, consider dilation, lubricant, or repeating on another day if media opacity is dense? 5. Re-scan if an artifact crosses the lesion of interest.
Rule of thumb: if a feature appears on only one B-scan and is accompanied by a second, inverted copy of the retina, call it mirror artifact until proven otherwise.