Radiation dose to the thyroid gland and breast from multidetector computed tomography of the cervical spine: does bismuth shielding with and without a cervical collar reduce dose?

PURPOSE

This study aimed to assess the radiation dose reduction that could be achieved using an in-line bismuth shielding over the thyroid gland and breast and to determine the effect of a cervical spine collar on thyroid dose reduction and image noise when performing computed tomography of the cervical spine using automatic tube current modulation.

MATERIALS AND METHODS

An anthropomorphic phantom was scanned using a commercially available 64-channel computed tomographic scanner. A standardized trauma cervical spine protocol was used. Scans were obtained with and without a standard cervical spine immobilization collar and with and without bismuth-impregnated thyroid and breast shields. Thermoluminescent dosimeters were placed over the thyroid gland and breasts for each scan. A paired t test was used to determine whether the skin entry dose differed significantly between the shielded and unshielded thyroid and breast and to determine whether placing the thyroid shield over a cervical immobilization collar resulted in a significant dose reduction. Region of interest of pixel values was used to determine image noise.

RESULTS

The average measured skin entry dose for the unshielded thyroid gland was 21.9 mGy (95% confidence interval, 18.9-4.7). With a bismuth shield applied directly over the skin, the dose to the thyroid gland was reduced by 22.5% (P < 0.05). With the bismuth shield applied over the cervical spine collar, the dose reduction to the thyroid was 10.4%, which was not statistically significant (P = 0.16) compared with the dose reduction without the cervical collar. Skin entry dose over the breasts was significant, although they were outside the primary scan range. Without bismuth shielding, the skin entry dose was 1.5 mGy, and with bismuth shielding, the dose was significantly reduced by 36.6% (P < 0.01). Image noise increased most when shielding was used with an immobilization collar.

CONCLUSIONS

There is a significant dose reduction to the thyroid gland and breasts when a bismuth shield is placed on the skin. The dose saving achieved by placing the shield on the cervical collar is approximately halved compared with placement on the skin, and this did not reach statistical significance, and this was accompanied by an increase on image noise. Bismuth shields should not be used in combination with cervical immobilization collars.