Abstract
Standard dose evaluation methods have not yet been developed for helical scanning in x-ray computed tomography. This report describes a new concept in dose evaluation, the hilical computed tomography dose index(HTDI). The center of a cylindrical phantom, separated from the patient couch, is set as the rotational center. A pencil-type ionization chamber is inserted into the phantom, and the integral dose is evaluated for a change in the scanning range. Measurement is performed at the center of the phantom and 1 cm inward from the surface of the phantom. The integral dose is substituted into an arithmetic expression to calculate the HTDI. The obtained HTDI is directly comparable to the computed tomography dose index(CTDI)obtained using conventional dose evaluation system, and can therefore serve as a dose index specific to helical scanning.
