1) Mode of irradiation. In contrast to planar radiography, which is planar by nature, CT involves multi-angular irradiation of the patient. As a result, dose is distributed with essentially uniform intensity throughout the scan plane rather than with the decreasing intensity with depth characteristic of radiography. In general, a CT examination of a given section of anatomy delivers a dose that is substantially higher than its radiographic equivalent. For example, the effective doses delivered by radiography and CT of the (PA) chest are approximately 2 and 800 mrem, respectively. Order-of-magnitude dose disparities between CT and radiography exist for other examination types.
2) Dose to extraneous tissues. Relatively high doses are delivered to tissues included in the scan plane but not of clinical interest, such as the breast in thoracic CT. Breast dose during such procedures lies in the range of 2 to 10 rad—in comparison with an average mean glandular dose of approximately 200 millirad per view in mammography. Dose to all tissues in the field of view at CT as well as those irradiated by secondary radiation (internal and external scatter and tube leakage) contribute to the patient's effective dose.
3) Irradiated tissue volume. With the advent of helical MDCT and subsecond gantry rotation times, and the option of contiguous or overlapping scans, greater scan lengths are achievable in increasingly less time—resulting in a concomitant increase in the average total volume of irradiated tissue. Further, the requirement for interpolation of transmission profiles from neighboring scans in helical scanning in turn necessitates additional rotations of the gantry at the extremes of the scan range, such that the exposed tissue volume is greater than the reconstructed volume. This type of dose augmentation is exacerbated as the aperture width increases.
4) Nature of CT image formation. Modalities that use image-recording media with limited dynamic range—such as screen-film radiography—have associated with them limits to the dose that can be recorded without loss of information. CT, however, is an inherently digital-imaging modality for which there is no such dose penalty. Image quality in CT will increase with increasing dose as the level of Poisson-distributed noise decreases.
5) Nonoptimized scanning protocols. The NRBP UK CT dose survey1 demonstrated that patient-efficient doses for the same examination could vary by up to a factor of 10 among institutions. However, this magnitude of variability represents a significant improvement in the findings of the 1991 survey by the same group in which dosewise variation on the order of a factor of 40 was found. This change is, in part, the result of emergent awareness of the radiation burden imposed by CT and the application of dose-mitigating strategies based on patient age, body habitus, and the tissue type to be imaged.
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