In this work we relate the CT dose index volume (CTDIvol) readily displayed in all CT units with an actual Peak Skin Dose (PSD) value. Measurements of the CTDI and skin exposure were performed simultaneously using an ionization chamber and radiochromic film. The ionization chamber was placed in the center of the CTDI head phantom while the films were placed on the outer surface. A total of five different x-ray tube currents including the maximum allowed value (expressed in units of mAs) were used to irradiate the phantom. For each mAs value, simultaneous readings of the CTDI100 from the ion chamber, the CTDIvol from the scanner console and the peak skin exposure value were obtained. Both the chamber and films were calibrated at NIST in an x-ray beam that matched the characteristics of the CT unit x-ray beams. Results are presented for CT units from four different manufacturers. Peak skin dose values of up to 160 mGy in a single scan were found in some scanners, and a variation of a factor of 2 in the maximum allowed exposure was found among default protocols by different manufacturers. A scanner-specific relation between CTDIvol and peak skin dose is found as a result of the method. Clinics may apply the described method to characterize their scanners as well as to explain and define appropriate limits of mAs to avoid excessive skin exposure values for a given patient examination.
Pub Type: Journals
CT, CTDI, chamber, dose limit, dosimetry, skin exposure