Using HVL and KVp to Portray An X-Ray Source for Dose Calculations in CT
M Sommerville1*, Y Poirier2, A Kouznetsov3, M Tambasco4, (1) SAN DIEGO STATE UNIVERSITY, San Diego, CA, (2) UNIVERSITY OF CALGARY, Calgary, AB, (3) UNIVERSITY OF CALGARY, Calgary, AB, (4) SAN DIEGO STATE UNIVERSITY, San Diego, CAMO-D-134-10 Monday 2:00PM - 3:50PM Room: 134
Purpose: To show that the nominal peak tube voltage potential (kVp) and measured half-value layer (HVL) are sufficient to generate energy spectra and fluence profiles for fast and accurate machine-specific dose computations in Computed Tomography (CT).
Methods:Spatial variation of the x-ray source spectrum was found by measuring HVL across the internal bowtie filter axis and using the nominal kVp settings and third-party software Spektr to generate the spectra. The beam fluence was calculated by multiplying in-air dose measurements along the filter axis with the integral product of the spectra and the in-air NIST mass-energy attenuation coefficients. Dose calculations were performed using a previously validated in-house hybrid deterministic and stochastic kV x-ray dose computation algorithm (kVDoseCalc). To ensure dose convergence while minimizing calculation time, we examined the sensitivity of kVDoseCalc to the number of photons seeded. We modeled the source of a Philips Brilliance Big Bore CT scanner for 90, 120, and 140 kVp settings. Doses measured using a Farmer-type Capintec ion chamber (0.6 cc) placed in a cylindrical poly methyl methacrylate (PMMA) phantom were compared to those computed with kVDoseCalc.
Results:The number of photons seeded required to keep the average statistical uncertainty in dose less than .1% was found to be 1.25 million. The average percent difference between calculation and measurement pooled over all 12 positions in the phantom was found to be 1.68%, 1.60%, and 1.25% for 90, 120, and 140 kVp, respectively. The maximum percent difference between calculation and measurement was less than 3.64% pooled over all energies and measurement positions. Thirty-one out of a total of 36 simulation conditions were within the experimental uncertainties associated with measurement reproducibility and chamber volume effects.
Conclusion:Our source characterization technique, which derives incident fluence and spectra from measurements of HVL across the bowtie profile, is sufficient for accurate machine-specific CT dose computations.