Abstract
Purpose: CT bowtie filters modulate X‐ray intensity across the fan beam, compensating for decreasing absorption due to shorter X‐ray path lengths through patient body periphery. The effect of replacing static CT beam‐shaping filter geometry by a filter geometry dynamically adapted to the attenuation of objects of elliptic cross‐section has been evaluated with regard to its potential for signal homogenization and radiation exposure reduction. Methods: A theoretical model of a beam‐shaping filter that adapts its attenuation profile to subjects of elliptic cross‐section as a function of fan‐and projection‐angle has been developed. For comparison, the static geometrical shape of the bowtie filter of a real CT system has been measured at different fan angles (0°–21°) using Compton spectroscopy. Both bowtie models were implemented in a Geant4 Monte‐Carlo simulation of MDCT, and attenuation profiles of a water phantom of elliptic cross‐section (16.0 cm semimajor, 12.5 cm semiminor axes) and maps of dose absorbed within the phantom were simulated for projections a.p. to lateral in steps of 10° (2×108 photons/projection). Exploiting phantom symmetry, total spatial dose distributions within the phantom were reconstructed and used to evaluate relative radiation exposure reduction achievable by replacing static with dynamic bowtie filtration in CT. Results: With dynamic filtration, attenuation profiles of the elliptical phantom become constant, i.e. independent of fan‐and projection‐angle, whereas the measured static filter produces asymmetric profiles. Compared to a CT acquisition without any beam‐shaping filter, measured static and proposed dynamic beam‐shaping filter allow reducing total deposited dose by 33% and 48%, respectively. Thus, an additional radiation exposure reduction by about 15% can be achieved with a filter dynamically adapted to subject shape. Conclusion: Homogenization of the signal and a significant additional reduction of radiation exposure could be achieved by replacing static with dynamic beam‐shaping filtration for CT examinations of subjects of elliptic cross‐section. This research project was funded by the German Federal Ministry of Education and Research (BMBF) within the collaboration project: Innovative Methods for the Optimization of Radiological Applications in Biomedical Imaging, grant numbers 02NUK008G/B
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