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Approximately 250 computed tomography (CT) systems were surveyed in a nationwide study to determine the average radiation dose resulting from a typical adult head procedure. The multiple scan average dose (MSAD) was selected as the dose descriptor. For the typical adult CT head procedure, the MSAD was generally within 2.2-6.8 rads (22-68 mGy). Variations in dose by a factor of two or more were often seen for a given manufacturer and model. These dose ranges indicate a potential to reduce dose by carefully selecting imaging techniques. Overall, variations in dose can result from differences in the user's choice of technique (desired image quality) or from actual differences in scanner performance (caused by differences in collimation, filtration, or geometry). To use CT appropriately, a facility should consider dose as well as image quality in selecting optimal techniques for typical modes of operation.

Bone cortical thickness and density were measured in rat femurs in vitro by a photon absorptiometry technique. A scan perpendicular to the long bone axis using photons from highly collimated 109Cd or 125I sources yielded the bone wall thickness and attenuation at mid-shaft. A second scan after the bone was rotated axially 90 degrees was taken to measure the wall or shaft thickness identically responsible for the mid-shaft attenuation of the first scan. The mid-shaft attenuation of the second scan was due to the bone thickness derived from the first scan; thus two complementary measurements are derived by this technique. The bone density is then directly calculated with an estimated accuracy of 10% (coefficient of variation) using empirically determined mass attenuation coefficients. Measurements of mass attenuation coefficients of ashed and dried samples were compared with calculated coefficients for estimated bone composition. Results indicate that the mass attenuation coefficient for bone in vivo can be well estimated such that bone density can be accurately derived from this technique.