Thyroid dose reduction shield with the generation of less artifacts used for fast chest CT examination

Abstract

Fast chest CT scans with high-pitch factors are effective to reduce motion artifacts, but the over-ranging effect may increase harmful radiation exposure to organs which are located near the outside of the imaging area. This becomes a particular problem during chest CT imaging when managing the radiation dose to the radio-sensitive thyroid, which is located around the upper end of the imaging area. The use of an X-ray shielding material can reduce the exposure dose to the thyroid. However, when the shielding is performed using metal foil, metal artifacts definitely appear, and they interfere with the CT image. To solve this problem, we developed a novel less-artifact thyroid shield in which fine bismuth oxide particles are embedded in low-density polyurethane (PU) foam. This study aims to present the usefulness of our X-ray shield through experimentation using a human body phantom. Three kinds of thyroid shields were used: a commercially-available lead foil as a reference, investigational clay products to evaluate the proper amount of the fine bismuth oxide particles, and bismuth PU shields which are proposed in this study. Furthermore, a bismuth PU shield with an added PU spacer was fabricated for the further suppression of the artifacts. Under clinical conditions, shielding ability and image quality were evaluated. Thyroid doses were measured using small-type OSL dosimeters, and the dose reduction rates were determined. In addition, the image qualities of the thyroid and lung regions were evaluated using a contrast-to-noise ratio (CNR) and an artifact index (AI). When the thyroid is outside the scanning region, thyroid exposure without the shield was caused by over-ranging X-rays and showed 11 mGy. When applying bismuth with an effective mass thickness of 0.12 g/cm2 to the bismuth PU shield, the thyroid exposure dose was reduced by half. In addition, we confirmed that if this shield enters the scanning region through accident, the AI would be less than 10. Also, when using our thyroid shield, the occurrence of metal artifacts was drastically suppressed. In conclusion, our thyroid shield can reduce the radiation exposure dose in half while maintaining acceptable image quality. This is a valuable tool in solving the issue of radiation exposure when performing fast chest CT scans. The shield has breathability and flexibility, and patients can apply the shield to their necks by themselves.