Author: Ross I. Berbeco, Vera Birrer, Raphael Bruegger, Pablo Corral Arroyo, Roshanak Etemadpour, Dianne M. Ferguson, Rony Fueglistaller, Thomas C. Harris, Yue-Houng Hu, Matthew W. Jacobson, Mathias Lehmann, Nicholas Lowther, Daniel Morf, Marios Myronakis 👨🔬
Affiliation: Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology, Dana Farber/Brigham and Women's Cancer Center, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Brigham and Womens Hospital, Dana Farber Cancer Institute, Harvard Medical School, Brigham and Women's Hospital, Varian Imaging Laboratory, Dana-Farber Cancer Institute 🌍
Purpose: Multi-layer flat-panel imagers can improve for clinical image-guided radiotherapy applications, including the enhanced visualization of soft tissue and a reduction in image artifacts. Each imager layer captures photons from a common beam source and simultaneously hardens the x-ray spectrum for subsequent layers. The readouts of each layer can be combined for multi-energy analysis to identify, enhance, or suppress specific materials. This study focuses on the optimization of a dual-layer kV imager (DLI) design to achieve effective energy separation.
Methods: A 140 kVp x-ray spectrum was simulated in the Monte Carlo (MC) GEANT4 Application for Tomographic Emission (GATE) toolkit to study energy separation in a DLI. The simulation simultaneously captured high- and low-energy images, on bottom and top imager layers, respectively. Thallium doped cesium iodide (CsI:Tl) scintillators’ thicknesses and inter-layer filters were varied to optimize separation. A cylindrical water phantom was used for performance evaluation. Energy separation metrics that were used in this study include absolute mean energy difference and Intersection over Union (IoU).
Results: To determine the optimal thickness of each imager layer for improved energy separation, we varied the scintillator thicknesses from 0.3 mm to 1 mm in increments of 0.1 mm. Our MC simulations show that top/bottom scintillator thicknesses of 0.3mm/0.3mm, 0.7mm/0.8mm, and 1mm/1mm provides energy separations of 11.06 keV, 18.62 keV, and 22.85 keV, respectively. For comparison, a conventional dual-source approach using 140 kVp and 80 kVp beams achieves an energy separation of approximately 14 keV with a standard single-layer imager.
Conclusion: We successfully developed a MC simulation tool to calculate energy separation in kV dual-layer imagers. The results will be used in the design of a novel DLI. Future work will focus on experimental validation and evaluating simulated image quality to ensure that the optimized DLI configuration is effective and clinically viable.