ESTRO 2025 - Abstract Book

S2982

Physics - Image acquisition and processing

ESTRO 2025

References: [1] K. Son et al. Improving the accuracy of the effective atomic number (EAN) and relative electron density (RED) with stoichiometric calibration on PCD-CT images. Sensors . 2022 , 22 , 9220 [2] Rutherford RA et al. Measurement of effective atomic number and electron density using an EMI scanner. Neuroradiology . 1976 , 11 , 15-21 [3] Schneider U et al. The calibration of CT Hounsfield units for radiotherapy treatment planning. Phys. Med. Biol. 1996 , 41 , 111–124 Funding: This work was supported by internal fund of Electronics and Telecommunications Research Institute(ETRI). [24BR1610, Next-Generation Mammography: Development of Personalized Precision Breast Cancer Diagnostic Core Technology Utilizing Radiomics, AI, and Targeted Contrast Agents]

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Proffered Paper Linac-mounted photon counting detector for image guided radiotherapy: Image quality characterisation Sean Hood 1 , Nicholas Hardcastle 2 , Owen Dillon 3 , Marco Petasecca 1 , Peter Metcalfe 1 , Matthew Newall 4 , Saree Alnaghy 4,1 1 School of Physics, University of Wollongong, Wollongong, Australia. 2 Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Australia. 3 ACRF Image X Institute, University of Sydney, Sydney, Australia. 4 Radiation Oncology, Nelune Comprehensive Cancer Centre, Randwick, Australia Purpose/Objective: On-board spectral cone-beam computed tomography (CBCT) using high resolution photon-counting detectors (PCDs) could improve soft tissue contrast and tumour delineation for CBCT-based image-guided radiotherapy (IGRT). This feasibility study presents the first linac-mounted PCD and compares the planar image quality of the PCD with an Elekta flat panel detector (FPD). Material/Methods: A Medipix3RX based PCD was energy calibrated and integrated with the kV imaging system on an Elekta linac. Eight chips were tiled into a linear array of 1024 × 128 pixels covering a physical area of 112.64 × 14.08 mm 2 , with a resolution of 110 µm 2 per pixel 1 . The PCD was synchronised with the kV source and the FOV at the linac isocentre was extended to 80 × 150 mm 2 by translating the detector on a linear stage along the longitudinal direction of the linac couch, acquiring images in a step-and-shoot mode. Energy calibration was performed using monoenergetic emissions from I-125 (27.5 keV), Am-241 (59.5 keV), and Gd-153 (41.5 keV) sources with the PCD. A phantom with line pairs was imaged with both the PCD and FPD, and the modulation transfer function (MTF) and noise power spectrum (NPS) were measured according to the IEC standard and compared 2 . Spectral planar images of contrast inserts from the CIRS Multi-Energy CT QA phantom were acquired and optimally energy-weighted to maximise contrast. Results: Figure 1 shows the prototype PCD detector system integrated with the linac. Figure 2 (A) shows the limiting resolution observed on the fluoroscopy phantom was 2 lp/mm for the FPD and 5 lp/mm for the PCD. The MTF was improved across all spatial frequencies comparing the PCD to the FPD, however the normalised NPS of the latter was lower at higher frequencies. Figure 2 (B) shows the energy weighted PCD images increased contrast in the projection of a 60 mg/cc calcium insert by a factor of 1.52 at 120 kV compared to the FPD, however contrast improvements for other concentrations and tube voltages were minimal. Figure 2 (C) shows the differential spectra obtained from threshold scans of Am-241 radioisotope.