Radiation dose in CT.

The energy deposited in the patient by the rotating x-ray beam in computed tomography produces more uniform absorbed dose values within the section of imaged tissue than those produced in conventional radiologic procedures. The dose values within a specific section are determined by factors such as voltage, current, scan time, scan field, rotation angle, filtration, collimation, and section thickness and spacing. For routine dose determinations, a pencil ionization chamber is usually employed with a plastic phantom. Dose for a specific patient can be determined with thermoluminescent dosimeters placed on the patient. Multiple-scan procedures normally increase the dose in a specific section by less than a factor of two. Typical multiple-scan average doses are in the range of 40-60 mGy for head scans and 10-40 mGy for body scans. Integral dose, however, is directly proportional to the number of sections in an examination. When examination factors are changed to reduce dose, the image noise increases. An optimum protocol is one that results in a balance between dose and image quality.