CT scatter spectra transmission data of 80, 100, 120 and 140 kVp primary beams for various shielding materials.

Abstract

The shielding of computed tomography (CT) suites has commonly relied on the assumption that the primary beam has the same beam quality and thus penetrability as the scattered radiation. This report expands on a preliminary work that showed scattered radiation from patients having an overall reduced beam quality, with published transmission data for 120 kVp and 140 kVp through lead. Beam quality data of patient scatter spectra for 80 kVp and 100 kVp are uniquely provided herein using the same methodology, expanding the diagnostic energy range. The mean energy of scatter radiation spectra across this 80-140 kVp diagnostic range was found to have a reduction of 13.4-17.9% compared to a primary beam with a defined 9.8 mm Al added filtration. A DOSXYZnrc Monte Carlo program using the EGSnrc photon and electron transport code was subsequently used to simulate the transmission of scattered spectra of all 80, 100, 120 and 140 kVp beams through various commonly used shielding materials, including lead, concrete, steel, plate glass and gypsum wallboard. Transmission data and Archer fitting coefficients for this scattered radiation were calculated and show a reduction in transmission over the range of practical shielding thicknesses for these materials. Transmission through lead was significantly reduced in comparison to the National Council of Radiological Protection (NCRP) and British Institute of Radiology (BIR) methodologies using primary beam spectra, with transmissions reduced between 40.4 and 63.9% for 120 kVp and between 38.1 and 42% for 140 kVp beams over a 0.44-2.64 mm thickness range. The use of CT scatter spectra and their resultant transmission data is recommended for optimal shielding design.