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  • CT Dose Index and Patient Dose : They Are Not the Same Thing

    Radiology: Volume 259: Number 2-May 2011

    Cynthia H. McCollough, PhD Shuai Leng, PhD LifengYu, PhD Dianna D. Cody, PhD John M. Boone, PhD Michael F. McNitt-Gray, PhD

    In 1981, Shope et al (1) published "A Method for Describing the Doses Delivered by Transmission X-ray Computed Tomography." In that article, they introduced the computed tomogra-phy (CT)/dose index (CTDI) as a metric to quantify the radiation output from a CT examination consisting of multiple contiguous CT scans (ie, multiple ad-jacent transverse rotations of the x-ray tube along the patient longitudinal axis). A new dosimetric method was required for CT because the irradiation geometry was quite different from that of other x-ray modalities in use at that time; namely, the x-ray tube irradiated only a narrow section of the anatomy while it made a full rotation around the patient and did so for multiple rotations along the length of the patient. The CTDI method sought to create an "index" to reflect the average dose to a cylindri-cal phantom in the central region of a series of scans. The word "index" was specifically included in CTDI's name to distinguish the quantity from the radia¬tion dose absorbed by a patient.