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Bismuth shield affecting CT image quality and radiation dose in adjacent and distant zones relative to shielding
Ying-Lan Liao1, Nan-Ku Lai2, Yeu-Sheng Tyan2
1Health Physics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan.
Background:
To quantify image quality and radiation doses in regions adjacent to and distant from bismuth shields in computed tomography (CT).
Methods:
An American College of Radiology accreditation phantom with four solid rods embedded in a water-like background was scanned to verify CT number (CTN) accuracy when using bismuth shields. CTNs, image noise, and contrast-to-noise ratios (CNRs) were determined in the phantom at 80-140 kVp. Image quality was investigated on image portions in the zones adjacent (A zone) to and distant (D zone) from a bismuth shield. Surface radiation doses were measured using thermoluminescent dosimeters. Streak artefacts were graded on a 3-point-scale.
Results:
Changes in CTN caused by a bismuth shield resulted in changes in X-ray spectra. CTN changes were more apparent in the A zone than in the D zone, particularly for a low tube voltage. The degrees of CTN changes and image noise were proportional to the thickness of the bismuth shields. A 1-ply bismuth shield reduced surface radiation doses by 7.2%-15.5%. The overall CNRs were slightly degraded, and streak artefacts were acceptable.
Conclusions:
Using a bismuth shield could result in significant CTN changes and perceivable artefacts, particularly for a superficial organ close to the shield, and is not recommended for quantification CT examinations or follow-up CT examinations.
