Which CT artefact is caused by the finite width of the X-ray beam and occurs when a high-density object (eg metallic prosthesis) partially occupies a voxel along with adjacent soft tissue?

• A Beam hardening artefact
• B Ring artefact
• C Helical pitch artefact (windmill artefact)
• D Partial volume averaging artefact ✓

Explanation

Partial volume averaging (PVA) artefact occurs when a voxel contains two or more tissues of differing density — the CT scanner assigns a single averaged HU value to the entire voxel. Thin structures (eg a small haemorrhage adjacent to bone) may appear either falsely denser (soft tissue + bone averaged) or falsely less dense than their true value. PVA is worse with thicker image slices and smaller lesions. It is minimised by using thinner CT slices (eg 1–3 mm reconstructions). Beam hardening artefact creates dark streaks between dense objects (eg skull base). Ring artefact is from a malfunctioning detector element. Helical windmill artefact results from under-sampling of high-contrast edges.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.