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Purpose: To estimate the consequences on dosimetric applications when a CT bowtie filter is modeled by means of
full beam hardening versus partial beam hardening.
Method: A model of source and filtration for a CT scanner as developed by Turner et. al. [1] was implemented.
Specific exposures were measured with the stationary CT X-ray tube in order to assess the equivalent thickness of Al
of the bowtie filter as a function of the fan angle. Using these thicknesses, the primary beam attenuation factors were
calculated from the energy dependent photon mass attenuation coefficients and used to include beam hardening in
the spectrum. This was compared to a potentially less computationally intensive approach, which accounts only
partially for beam hardening, by giving the photon spectrum a global (energy independent) fan angle specific
weighting factor.
Percentage differences between the two methods were quantified by calculating the dose in air after passing several
water equivalent thicknesses representative for patients having different BMI. Specifically, the maximum water
equivalent thickness of the lateral and anterior-posterior dimension and of the corresponding (half) effective diameter
were assessed.
Results: The largest percentage differences were found for the thickest part of the bowtie filter and they increased
with patient size. For a normal size patient they ranged from 5.5% at half effective diameter to 16.1% for the lateral
dimension; for the most obese patient they ranged from 7.7% to 19.3%, respectively. For a complete simulation of
one rotation of the x-ray tube, the proposed method was 12% faster than the complete simulation of the bowtie filter.
Conclusion: The need for simulating the beam hardening of the bow tie filter in Monte Carlo platforms for CT
dosimetry will depend on the required accuracy.
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