What is CTDI?
The computed tomography dose index (CTDI) represents a set of radiation dose descriptors developed to explain the radiation dose to a phantom from a series of rotations of the x-ray tube  . CTDI 100 represents the radiation exposure to a 100-mm pencil ionization chamber. When this chamber is placed at the center or near the periphery of a cylindrical CT phantom, the CTDI w can be calculated as a weighted average, giving twice as much weight to the peripheral measurement as to the central one. This weighted average can then be adapted for axial or helical scanning to produce the volume CTDI, denoted as CTDI vol . The CTDI vol is typically used to compare CT protocols on different scanner equipment and as a cutoff for accreditation. However, because it is a phantom-based calculation, it does not incorporate features of the patient, such as size, age, gender, and the anatomic region of interest, which can cause significant variation in actual absorbed patient dose. It is critical to remember that none of the CTDI descriptors represents actual dose absorbed by the patient as a result of a CT examination.