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First linac-mounted photon counting detector for image guided radiotherapy: Planar image quality characterization
Sean Hood1, Matthew Newall2, Phil Butler3
1Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, Australia.
Medical Physics
|November 29, 2024
Summary
Photon-counting detectors (PCDs) show improved image quality for image-guided radiotherapy (IGRT). This study demonstrates enhanced spatial resolution and contrast with linac-mounted PCDs compared to traditional flat-panel detectors (FPDs).
Area of Science:
- Medical Physics
- Radiotherapy Imaging
- Detector Technology
Background:
- Image-guided radiotherapy (IGRT) using cone-beam computed tomography (CBCT) is limited by suboptimal soft-tissue contrast and spatial resolution of energy-integrating flat-panel detectors (FPDs).
- Spectral computed tomography (CT) with high-resolution photon-counting detectors (PCDs) offers potential improvements in tumor delineation through enhanced soft-tissue contrast, spatial resolution, dose efficiency, and CT number accuracy.
Purpose of the Study:
- To present the first linac-mounted photon-counting detector (PCD) for spectral cone-beam CT (CBCT) in image-guided radiotherapy (IGRT).
- To fundamentally characterize and compare the planar image quality of a linac-mounted PCD against a conventional flat-panel detector (FPD) regarding 2D spatial resolution, noise, and contrast.
Main Methods:
- A Medipix3RX-based PCD was integrated into an Elekta linac's kilovoltage (kV) X-ray volume imaging (XVI) system, synchronized with the pulsed kV source.
- 2D spatial resolution and noise were assessed using pre-sampling modulation transfer function (MTF) and normalized noise power spectrum (NPS) with an RQA5 spectrum and a fluoroscopy phantom.
- Spectral planar images of phantom inserts (calcium and iodine) were acquired at various kVp settings (60-120 kV) and optimally energy-weighted to maximize contrast, with dynamic flat-field corrections applied.
Main Results:
- The Medipix3RX PCD demonstrated linear energy calibration up to 60 keV.
- Limiting resolution improved from 2 lp/mm (FPD) to 5 lp/mm (PCD), with higher pre-sampling MTF across all frequencies for the PCD.
- Energy-weighted contrast for a 60 mg/cc calcium insert increased significantly with the PCD compared to the FPD at both 60 kV and 120 kV.
Conclusions:
- Successful integration of a Medipix3RX-based PCD with a linac-mounted kilovoltage imaging system was achieved.
- Initial characterization revealed superior planar image quality, including improved MTF and energy-weighted contrast, compared to FPDs.
- Addressing sensor response variations during acquisition is crucial for realizing the full potential of linac-mounted spectral CBCT.
