Comparative analysis of radiation dose and image quality between thyroid shielding and unshielding during CT examination of the neck

OBJECTIVE. The purpose of this study is to evaluate the feasibility of thyroid shielding by measuring radiation dose, CT attenuation, and noise of superficial neck structures during CT examination. SUBJECTS AND METHODS. We divided 84 patients without abnormalities seen on CT into two groups depending on whether shielding with a cotton spacer was applied over the thyroid. On CT images, we measured the CT attenuation and noises in the strap and sternocleidomastoid (SCM) muscles. The superficial radiation dose was measured using a head CT dose phantom containing ionization chambers located at the 3, 6, 9, and 12 o'clock positions. RESULTS. With thyroid shielding, the CT attenuation was significantly increased (shielded strap and SCM muscles, 117.6 ± 19.2 HU and 113.7 ± 31.8 HU, respectively; unshielded strap and SCM muscles, 84.1 ± 12.2 HU and 78.4 ± 10.1 HU, respectively; p < 0.05), whereas noise was unaffected (shielded strap and SCM muscles, 7.2 ± 4.2 HU and 10.8 ± 4.9 HU, respectively; unshielded strap and SCM muscles, 8.6 ± 4.9 HU and 10.7 ± 6.6 HU, respectively; p > 0.05). On the phantom study, the shield significantly reduced the superficial unshielded dose at the 12 o'clock position only (27.5% reduction; p < 0.01). CONCLUSION. Below the shielded surface, thyroid shielding significantly reduced the superficial radiation dose of the neck without a remarkable noise increase while increasing CT attenuation.