Experimental characterization and mitigation of scatter induced spectral contamination in photon counting CBCT for radiotherapy.

backgroundPhoton counting detectors (PCDs) have the potential to enable spectral cone beam computed tomography (CBCT) in radiotherapy, which may improve tissue visualization, tissue characterization, and dose calculation accuracy. However, spectral imaging with PCDs such as material decomposition require highly quantitative raw data that accurately reflects X-ray attenuation through the imaged object. PURPOSE: In CBCT, X-ray scatter is the primary cause of degraded raw-data fidelity. This study experimentally characterizes the effect of scatter on PCD counts and energy spectra and investigates a hardware-driven approach to mitigate scatter contamination in photon counting CBCT (pcCBCT).

methodsA photon-counting detector with a cadmium telluride (CdTe) sensor and two adjustable energy thresholds was integrated into a benchtop X-ray imaging system mimicking the CBCT geometry of C-arm linacs. Scatter contamination as a function of energy was characterized using phantoms of varying thicknesses and the beam-stop method. Bias in the detected energy spectrum due to scatter, as well as the scatter-to-primary ratio (SPR) as a function of detected energy, were quantified. The effect of increased X-ray fluence due to scatter on pulse pile up was measured under X-ray fluence conditions relevant to image-guided radiotherapy. To mitigate scatter, all experiments were repeated using a 2D antiscatter grid (ASG) prototype, which also served as a reference for quantifying scatter-induced bias in contaminated raw data. The effect of scatter contamination on the energy-specific CT number accuracy was evaluated in pcCBCT scans acquired with two energy bins.

resultsScatter bias increased counts by up to 96% in the 25-50 keV range in the normalized energy spectra. The mean energy of the spectrum shifted to lower energies by up to 8 keV as a result of scatter bias. Average scatter to primary ratio (SPR) was up to a factor of 5 higher at 35 keV than the SPR at 100 keV. SPR reached 4 in the 25-60 keV range. The incremental pulse pile up related count underestimation attributable to scatter-induced fluence was < 3%. The 2D ASG reduced SPR by 90%-94% across all energies and phantom configurations. The efficacy of 2D ASG in SPR reduction varied only 3% across 25-120 keV range. Without scatter mitigation, mean CT-number loss was 453 and 390 HU in the 25-60 keV and 60-120 keV bins, respectively. The 2D ASG reduced CT number loss to 34 and 22 HU in the respective energy bins.

conclusionsScatter fraction is substantially higher at lower energies and biases the energy spectrum toward lower energies under CBCT imaging scenarios relevant to radiotherapy. Consequently, CT number degradation is significantly greater at lower energies, which may challenge the quantitative utilization of low-energy data in future pcCBCT systems and spectral imaging applications. The effect of scatter-induced fluence increase on pulse pile up is deemed small. The 2D ASG effectively reduced scatter contamination across the entire energy spectrum and improved the fidelity of raw data for pcCBCT imaging.