The trouble with

Abstract

The computed tomography dose index is typically measured using a long pencil ion chamber with cylindrical polymethyl methacrylate (PMMA) dosimetry phantoms. While this metric was useful in the era of single slice CT scanners with collimated slice thicknesses of or less, the efficiency of this metric in multi‐slice CT scanners with wide collimated x‐ray beams is unknown. Monte Carlo simulations were used to assess the efficiency of the parameter for wider beam collimations. The simulations utilized the geometry of a commercially available CT scanner, with modeled polyenergetic x‐ray spectra. Dose spread functions (DSFs) were computed along the length of diam rods placed at several radii in infinitely long diam (head) and diam (body) PMMA phantoms. The DSFs were used to compute radiation dose profiles for slice thicknesses from . efficiency was defined as the fraction of the dose along a PMMA rod collected in a length centered on the CT slice position, divided by the total dose deposited along an infinitely long PMMA rod. For a slice thickness, a x‐ray spectrum, and the PMMA head phantom, the efficiency of the was 82% and 90% for the center and peripheral holes, respectively. The corresponding efficiency values for the body phantom were 63% and 88%. These values are reduced by only 1% when a slice thickness was studied, so the use of for wide x‐ray beams is no less valid than its use for beam widths. However, these data illustrate that the efficiency of the measurement even with beam widths is low and, consequently, dose computations which are derived from this metric may not be as accurate as desirable.