ESR News July 2015
The use of shielding for patient dose reduction in CT
John Damilakis, Professor of Medical Physics
CT is a valuable imaging method that can be used to examine organs and tissues, detect abnormalities, guide procedures and perform biopsies. However, the clinical usefulness of CT has to be weighed against the risks associated with radiation dose. Because of the increased use of CT and the potential radiogenic risks, dose management actions are required to achieve diagnostic image quality with the lowest patient radiation dose possible. Several techniques and tools have been developed for CT dose optimisation. Do radio-protective shields have the potential to decrease patient radiation doses from CT?
Female breast tissue is considered to be among the most radiosensitive of the human body. The radiosensitivity of human organs and tissues is considered in the International Commission on Radiological Protection (ICRP) reports by the tissue weighting factor in calculations of the effective dose. The ICRP increased the weighting factor for breast tissue from 0.05 in 1991 to 0.12 in 2007. In-plane bismuth breast shields have been used to reduce breast dose from chest CT and coronary CT. Studies show that bismuth shielding is capable of reducing radiation dose from 30% to 60% for the breast, but it increases noise from 20% to 40%. Other methods of reducing patient dose, such as automatic exposure control (AEC) and organ-based tube current modulation, can reduce breast dose without affecting image quality. A review article on the effects of bismuth shielding on breast dose and image quality has been published recently (1).
Exposure to ionising radiation can kill or damage cells in the eye lens. Consequences include lens opacities and cataracts. The dose threshold for ophthalmologically detectable opacities following x-ray exposure may range from none (no threshold) to about 1Gy. In head CT scans, proper gantry angulation may exclude the eyes from the primarily exposed region and reduce eye lens dose by more than 80%. When eye lenses cannot be excluded from the scan range, the use of bismuth shields has been proposed to protect the eye lens. Studies show that the mean eye lens dose reduction achieved by these garments ranges from about 20% to about 40% (2, 3). No influence on image quality has been observed if shields are placed 1cm above the eyes (4). Ngaile et al have found that the use of lead shields could reduce the eye lens dose by 44% without considerable compromise in image quality (5). Other areas of application of shields are thyroid shields for the neck, cervical spine and chest CT, and testes shields for abdominopelvic CT. Both bismuth and lead shields reduce thyroid dose considerably, by 12% to 57% (5, 6). Lead testis shielding is capable of reducing testes dose by about 90% (6). Thus, Hohl et al recommend testis shields for routine use (7).
AEC systems are now implemented in all multi-detector CT scanners to reduce patient dose. Use of bismuth shielding together with AEC activation during a CT examination needs careful consideration. The American Association of Medical Physicists states in its position statement on the use of bismuth shielding for the purpose of dose reduction in CT scanning that ‘Applying bismuth shielding together with automatic exposure controls systems, such as tube current modulation, leads to unpredictable and potentially undesirable levels of dose and image quality’(8). It is known that different CT vendors have developed AEC systems which are based on different principles. Research work is needed to investigate the proper use of in-plane shielding when AEC is activated for the different AEC technologies developed by CT vendors.
References
1. Colletti P, Micheli O, Lee K. To shield or not to shield: application of bismuth breast shields. AJR 2013;200:503-507
2. Keil B, Wulff J, Schmitt R et al Protection of eye lens in computed tomography – dose evaluation on an anthropomorphic phantom using thermo-luminescence dosimeters and Monte-Carlo investigations. Rofo 2008;180(12):1047-1053
3. Perisinakis K, Raissaki M, Tzedakis A, Theocharopoulos N, Damilakis J. Reduction of eye lens radiation dose by orbital bismuth shielding in pediatric patients undergoing CT of the head: A Monte Carlo study. Medical Physics 2005;32(4):1024
4. Raissaki M, Perisinakis K, Damilakis J, Gourtsoyiannis N. Eye-lens bismuth shielding in paediatric head CT: artefact evaluation and reduction. Pediatr Radiol 2010;40(11):1748-54
5. Ngaile JE, Uiso CBS, Msaki P, Kazema R. Use of lead shields for radiation protection of superficial organs in patients undergoing head CT examinations. Radiat Prot Dosimetry 2008;130:490-8
6. Hidajat N, Scroder RJ, Vogl T, Schedel H, Felix R. The efficacy of lead shielding in patient dosage reduction in computed tomography. Rofo 1996:165(5):462-5
7. Hohl C, Mahnken AH, Das M et al Radiation dose reduction to the male gonads during MDCT: the effectiveness of a lead shield. AJR 2005;184(1):128-30
8. American Association of Physicists in Medicine (2012). AAPM position statement on the use of bismuth shielding for the purpose of dose reduction in CT scanning. http://www.aapm.org/publicgeneral/BismuthShielding.pdf. Accessed July 6, 2015.