KEYWORDS: Monte Carlo methods, Fluctuations and noise, Computed tomography, Photons, Gold, Aluminum, Image filtering, Optical filters, X-rays, Signal attenuation
The bowtie filter is an essential element of computed tomography scanners. Implementation of this filter in a Monte Carlo dosimetry platform can be based on Turner’s method, which describes how to measure the filter thickness and relate the x-ray beam as a function of bowtie angle to the central beam. In that application, the beam hardening is accounted for by means of weighting factors that are associated to the photons according to their position (fan angle) and energy. We assessed an alternative approximation in which the photon spectrum is given a fan angle-dependent scaling factor. The aim of our investigation was to evaluate the effects on dose accuracy estimation when using the gold standard bowtie filter method versus a beam scaling approximation method. In particular, we wanted to assess the percentage dose differences between the two methods for several water thicknesses representative for different patients of different body mass index. The largest percentage differences were found for the thickest part of the bowtie filter and increased with patient size.
KEYWORDS: Monte Carlo methods, Fluctuations and noise, Aluminum, Signal attenuation, Gold, X-rays, Optical filters, Optical simulations, X-ray computed tomography, Scanners
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.
Purpose: To estimate conversion factors for calculating effective dose (E) and organ dose taking tube current modulation (TCM) and patient size into account in adult thorax and abdomen CT examinations.
Method: 99 consecutive adult patients were included in this study. All examinations were performed with TCM (CareDose 4D. Siemens Definition Flash) at 120 kVp and 110 (thorax) and 200 (abdomen) reference mAs. E and organ dose were estimated with PCXMC 2.0 (STUK. Helsinki. Finland). using an extension of the software from a planar geometry to spiral acquisitions of aCT scanner. This software accounts for patient size by rescaling the anthropomorphic phantom to actual patient weights and heights.
E and organ doses were normalized to the CTDivol as reported in the patient's report. These conversion factors (dE and dorgan were studied as a function of different patient metrics: lateral and anterior-posterior (AP) diameter. sum of the lateral and AP diameter, area of a cross section image and effective diameter.
Results:. No trend was found for any of the metrics neither forE nor for the organs investigated (lungs. breasts. stomach and liver). Average value ± 2 standard deviation were calculated. For a thorax examination, the average dE was 0.57 ± 0.14 mSv/mGy. dlungs was 1.26 ± 0.28 mGy/mGy and dbreasts was 1.29 ± 0.40 mGy/mGy. For an abdomen scan dE was
0.82 ± 0.18. mSv/mGy. d,tomooh was 1.42 ± 0.26 mGy/mGy. dliver was 1.42 ± 0.30 mGy/mGy.
Conclusion:. For the scanner studied, average conversion factors, which account for TCM and patient size, were
proposed. This is a first step towards patient-specific dosimetry.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.