CT is a powerful diagnostic technology, but it is also associated with risks. Ionizing radiation can damage cells and act as a weak carcinogen. The challenge is to find the optimal x-ray dose for each CT exam. “About 80 million CTs are performed each year in the United States,” says John M. Boone, PhD. “The higher the dose, the better the images, but we also want the lowest possible dose so that patients are protected from unnecessary radiation exposure. The key is to find the balance between acceptable image quality and acceptable doses.”
The Journal of the American College of Radiology (JACR) recently devoted an entire issue to CT safety. Dr. Boone and other specialists from the University of California, Davis contributed three studies to the special issue. In one of the JACR papers, the need to improve dose calculations was discussed. Manufacturers use phantoms along with instruments to measure radiation, but different companies use phantoms of different sizes, making comparisons problematic in some cases. To prevent these problems, a new metric, the size-specific dose estimate (SSDE), is recommended. “SSDEs can provide a better way to estimate patient doses and can help compare scanners from different companies,” says Dr. Boone. SSDEs may also address the need to more accurately estimate CT doses from a range of patient sizes, especially in pediatric patients.
The second JACR paper addressed the challenges of optimizing different CT machines. Automatic exposure protocols must be set up, but this can vary doses based on tissue thickness. “Transferring these settings between machines can be difficult and time consuming,” Dr. Boone says. To overcome this issue, Dr. Boone and colleagues developed equations to translate settings among three different CT machines. This approach can provide a more efficient way to manage CT protocols between different scanners.
The third JACR paper outlined how practitioners can lower radiation doses during interventional procedures using CT guidance. CT can help ensure that needles are precisely located. The study recommends using ultrasound instead of CT to guide some biopsies. Other recommendations include reducing scan lengths, lowering tube currents using dose-efficient scanning modes, and limiting the number of guidance scans.
The special JACR issue is part of an ongoing effort to reduce CT risks. Dr. Boone notes that five University of California medical centers are collaborating to improve CT protocols and education by partaking in the university’s Dose Optimization and Standardization Endeavor. “Our goal with this initiative is to educate radiologists and medical physicists so that the practice of CT becomes safer and more consistent,” he says. “Safer alternatives are needed if higher doses are being used to get results that could be obtained with less radiation.”
Seibert JA, Boone JM, Wootton-Gorges SL, Lamba R. Dose is not always what it seems: where very misleading values can result from volume CT dose index and dose length product. J Am Coll Radiol. 2014;11:233-237. Available at: www.jacr.org/article/S1546-1440(13)00663-7/pdf.
McKenney SE, Seibert JA, Lamba R, Boone JM. Methods for CT automatic exposure control protocol translation between scanner platforms. J Am Coll Radiol. 2014;11:285-291. Available at: www.jacr.org/article/S1546-1440(13)00667-4/abstract.
Lamba R. Radiation dose optimization for CT-guided interventional procedures in the abdomen and pelvis. J Am Coll Radiol. 2014;11:279-284. Available at: www.jacr.org/article/S1546-1440(13)00657-1/abstract.
AAPM Task Group 96. The Measurement, Reporting and Management of Radiation Dose in CT (Report#96). Available at: http://aapm.org/publications.
McCollough CH, Leng S, Yu L, Cody DD, Boone JM, McNitt-Gray MF. CT Dose index and patient dose: they are not the same thing. Radiology. 2011;259:311-316.
AAPM Task Group 204. Size Specific Dose Estimates (SSDE) in Pediatric and Adult CT Examinations. 2012. Available at: http://aapm.org/publications.