Bismuth Shields for CT Dose Reduction: Do They Help or Hurt?

Bismuth shielding has been used to reduce the dose from CT to superficial radiosensitive organs, such as the breast, lens of the eye, and thyroid [1]. Bismuth-impregnated latex shields are placed over the organ of interest to atenuate the x-ray beam entering the atient. Although studies have conrmed a reduction in superficial dose 2-5], the disadvantages of using bisuth shielding have not been widely ecognized. We summarize here the rimary arguments for and against its se in CT. The key reason for using bismuth hields is to reduce dose to organs beieved to be more sensitive to radiaion damage. Also, they do so in a elatively “low-tech, low-cost” maner. Patients and providers feel good bout acting to keep radiation doses s low as reasonably achievable. Proonents of bismuth shields therefore onsider their use a win-win situaion: dose is reduced, and diagnosically acceptable image quality is aintained. However, using bismuth (or any ther attenuating element) shielding ithin the scan range violates a funamental rule of radiation protection y wasting radiation. The shield atenuates photons coming from the nterior direction, reducing radiation ose to the superficial (anterior) orans. However, when the tube irradites the patient from the posterior diection, the shield also attenuates the hotons exiting the patient. Because hese photons have already passed hrough and delivered dose to the paient, stopping them as they leave the atient produces no dose reduction. owever, removing photons exiting he patient does remove a substantial mount of useful signal for creating he CT images. Similarly, photons from lateral directions are partially attenuated after exiting the patient. Therefore, during more than half of each 360° rotation of the CT gantry, the shield attenuates useful photons, thus wasting radiation dose. This leads to a noise increase across the entire image, not just in the region near the shield [4,6-8]. In addition, these highly attenuating shields cause beam hardening and streak artifacts, which increase CT numbers. Although the altered CT numbers and streaks may not be visible across the whole image at certain display settings, CT numbers throughout the image are typically increased [7-9]. Studies that have favored the use of bismuth shielding have generally suggested that the image quality is qualitatively “diagnostically acceptable” with the shields and that quantitative assessment of observer performance was unaffected or minimally affected. However, these studies have failed to assess the qualitative and quantitative acceptability of a much simpler approach; namely, the same dose reduction to the radiosensitive organs can be achieved simply by reducing the tube current proportionately. That is, for a 30% decrease in breast dose, the tube current should be reduced by 30%. This extends the dose reduction from only the anterior surface to the entire volume examined. And because no photons are wasted, it is more dose efficient, achieving the same level of image noise as with bismuth shielding even though the tube current is markedly decreased [10]. It is simple, with no additional materials (ie, the shields) or special scanner features required, and technologists do not need to spend time positioning the shields or sterilizing the shields between pa-