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Imaging & Therapeutic Technology |
1 From the Department of Radiology, David Geffen School of Medicine, University of California, Los Angeles, Box 951721, Los Angeles, CA 90095-1721. From the AAPM/RSNA Physics Tutorial at the 2001 RSNA scientific assembly. Received July 18, 2002; revision requested August 13 and received August 29; accepted August 30. Address correspondence to the author (e-mail: mmcnittgray@mednet.ucla.edu).
This article describes basic radiation dose concepts as well as those specifically developed to describe the radiation dose from computed tomography (CT). Basic concepts of radiation dose are reviewed, including exposure, absorbed dose, and effective dose. Radiation dose from CT demonstrates variations within the scan plane and along the z axis because of its unique geometry and usage. Several CT-specific dose descriptors have been developed: the Multiple Scan Average Dose descriptor, the Computed Tomography Dose Index (CTDI) and its variations (CTDI100, CTDIw, CTDIvol), and the dose-length product. Factors that affect radiation dose from CT include the beam energy, tube current–time product, pitch, collimation, patient size, and dose reduction options. Methods of reducing the radiation dose to a patient from CT include reducing the milliampere-seconds value, increasing the pitch, varying the milliampere-seconds value according to patient size, and reducing the beam energy. The effective dose from CT can be estimated by using Monte Carlo methods to simulate CT of a mathematical patient model, by estimating the energy imparted to the body region being scanned, or by using conversion factors for general anatomic regions. Issues related to radiation dose from CT are being addressed by the Society for Pediatric Radiology, the American Association of Physicists in Medicine, the American College of Radiology, and the Center for Devices and Radiological Health of the Food and Drug Administration.
© RSNA, 2002
Index Terms: Computed tomography (CT), radiation exposure • Dosimetry • Radiations, exposure to patients and personnel • Radiations, measurement
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