CT dose: how to measure, how to reduce.

The fundamental radiation dose parameter in computed tomography (CT) is the CT dose index (CTDI), which is an integral under the radiation dose profile of a single axial scan normalized to the nominal x-ray beam width. It estimates the average dose from a multiple-scan examination and is a directly measurable and standardized quantity. From this information, the dose length product (DLP) is calculated, which estimates the total dose delivered over a specific scan length. Finally, effective dose can be estimated and used to reflect the risk of a non-uniform exposure in terms of a whole-body exposure. To manage dose from CT while maintaining diagnostic image quality, scanner manufacturers have implemented tube current modulation, which may occur angularly around the patient, along the long axis of the patient, or both. Dose reductions of 20 to 50% have been reported using tube current modulation schemes. In the past two decades, the capabilities of CT imaging have expanded tremendously, including narrower image widths, improved temporal and spatial resolution, shorter scan times, and cardiac imaging techniques. Yet, the dose per typical exam (e.g., routine abdominal CT) has decreased by a factor of two or more over the same time period. Therefore, patients should be reassured that the benefits of medically-justified and appropriately-performed CT examinations are associated with radiation doses that continue to decrease as technology continues to evolve.