ESTRO 2021 Abstract Book

S1403

ESTRO 2021

imaging system. The signal-noise relationship was measured in Catphan projection images at varying exposures to inform the amount of non-uniform Gaussian noise to be added to original projection data to simulate lower exposures. Signal-to-noise ratio (SNR) was measured in simulated Catphan images and compared to the real acquisitions for validation. 3 different dose levels were simulated for the 5 images of patients treated for brain tumours with the ProBeam system – 100%, 30%, and 10% of the clinical standard (1.8 mGy CTDI). The developed reconstruction method corrected the projection images for exposure and beam hardening – using angle-dependent corrections to correct for shifts of the bowtie filter during gantry rotation. Filtered backprojection reconstruction was performed using The Reconstruction Toolkit (RTK) (Rit et al. 2014). VGA was performed on the reconstructed images by 3 experienced observers, grading visibility of bone, ventricle, air cavities, and external contour. Results Simulated and acquired Catphan projection images had good agreement in SNR – demonstrating adequate simulation of quantum noise due to lower mAs exposure. Figure 1 shows two patient images reconstructed at 100%, 30% and 10% of clinical exposure. Figure 2 shows results of the VGA on all patient images. Visibility of bone and external contour consistently highly rated even at the lowest exposure; air cavities scored similarly but their visibility was slighted degraded at lower doses; ventricle visibility was consistently moderate to poor, and depended on the specific patient – likely due to variable ventricle size - some were not visible even at 100% exposure.