Purpose: To demonstrate how Half-Value Layer (HVL) and Effective Energy (Eₑ(ff)) are spatially-dependent in the x-y plane and the z axes of wide CT beams.
Methods: HVL in the x-y plane and z-axis, bowtie profiles in the x-y plane, and dose profiles along the z-axis were measured for all tube voltages (80-135 kVp) and bowtie filters in two 320-slice Canon CT scanners. All measurements were conducted in service engineer mode with the X-ray tube parked at the 12 o’clock position in the gantry. For HVL and bowtie profile measurements, 6-cm³ and 0.6-cm³ ion chambers were used, respectively. The dose profile along the z-axis was measured using OSL dosimeters and corrected for energy dependence by using calculated correction factors. Using HVL measurements, the effective energies for both scanners were estimated using SpekCalc, an x-ray spectrum generator.
Results: Along the x-y plane, Scanner 2 had higher HVLs (4.27-11.3 mm-Al) and Eₑ(ff) (39-71.2 keV) than Scanner 1 (4.10-8.40 mm-Al, 38.4-63.7 keV). Along the z-axis, the anode side had a higher HVL (5.17-11.2 mm-Al) and lower dose (5.63 mGy, which was the minimum dose at 80 kVp) than the cathode side of the x-ray tube (3.92-10.2 mm-Al, 57.8 mGy, which represented the peak dose at 135 kVp). The bow-tie profile showed a symmetrical curve for all energies for both scanners.
Conclusion: Differences were observed for HVL and effective energy as a function of position within wide X-ray beams. Furthermore, Scanner 2 had a harder beam spectrum than Scanner 1. These findings imply differences in image quality and dosimetry in wide CT beams scanned with a single volumetric rotation. Individuals assessing image contrast, CT number accuracy, OSL dosimetry, or Monte Carlo simulations must consider the spatial dependence of HVL and effective energy within X-Ray beams.