MO-D-134-10: Using HVL and KVp to Portray An X-Ray Source for Dose Calculations in CT.
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.