JACR: Bismuth shielding not worth the downsides

While bismuth shielding has been used to reduce dose from CT, the disadvantages, including wasted radiation dose and degradation of image quality, outweigh the benefits, according to an article published in the December issue of the Journal of the American College of Radiology.

Cynthia H. McCollough, PhD, of the Mayo Clinic in Rochester, Minn.; Jia Wang, PhD, also of the Mayo Clinic; and Lincoln L. Berland, MD, of the University of Alabama in Birmingham wrote that both patients and providers feel good taking steps to reduce radiation dose. “Proponents of bismuth shields therefore consider their use a win-win situation: dose is reduced, and diagnostically acceptable image quality is maintained.

“However, using bismuth (or any other attenuating element) shielding within the scan range violates a fundamental rule of radiation protection by wasting radiation,” the authors wrote. They explained that when the shield attenuates photons coming from the anterior direction, radiation dose to the superficial organs is reduced, but when the tube irradiates the patient from the posterior direction, the shield attenuates photons that are exiting the patient.

“Because these photons have already passed through and delivered dose to the patient, stopping them as they leave the patient produces no dose reduction,” wrote McCollough et al. “However, removing photons exiting the patient does remove a substantial amount of useful signal for creating the CT images.”

This radiation loss also applies to photons from lateral directions that may be partially attenuated, so during more than half of each 360° rotation of the CT gantry, useful photons are lost and image noise is increased, according to the authors.

The other problem with bismuth shielding identified in the article arises when shields are combined with automatic exposure control systems. These systems adjust scanner output based on the expected level of attenuation, and use a localizer CT radiograph to estimate patient attenuation. Based on this localizer CT radiograph, the tube current may automatically be raised to compensate for the expected attenuation. If shielding is applied before the CT radiograph, current will automatically be raised thus countering any benefit of the shield, yet waiting until after the CT radiograph results in degraded image quality.

Rather than bismuth shielding, the authors favor a simpler approach: manual tube current reduction. By reducing tube current, there is a proportional reduction in dose, and since no photons are wasted, this method is more dose efficient.

While studies have confirmed that using bismuth shields still leaves images that are diagnostically acceptable, the acceptability of tube current reduction has not properly been assessed, according to the authors.

“Although their use is well intentioned, and we applaud the imaging community's efforts toward safe and effective use of CT imaging, bismuth shields waste radiation dose to patients and degrade image quality,” concluded the authors. “Equally or more effective alternatives exist to minimize radiation dose to the breast, thyroid and lens of the eye without degrading image quality or wasting dose.” Z-axis tube current modulation and organ-based tube current modulation could replace the use of bismuth shields, added McCollough et al.