Author: Gregory J Bootsma, Tunok Mondol 👨🔬
Affiliation: University Health Network, Princess Margaret Cancer Center 🌍
Purpose:
To evaluate the impact of prototype fluence field modulation (FFM) cone-beam CT (CBCT) on dose reduction and image quality using Monte Carlo simulations. This study particularly looks at the impact of scatter and potential mitigation strategies.
Methods:
Monte Carlo CBCT simulations were performed using the GPUMCI framework to simulate a prototype system that uses an advanced computer-controlled x-ray shutter to create multiple exposures for each projection angle. We investigated two reconstruction acquisitions with and without region of interest (ROI) FFM:
Full-Field Exposure (BG3x): The entire field-of-view (FOV) was subjected to uniform exposure.
ROI-Based Exposure (ROI3x): The collimator was adjusted to expose the ROI with a similar fluence as in BG3x while limiting background exposure to one-third.
Dose deposition and contrast-to-noise ratio (CNR) were compared for both conditions. Scatter and off-focal radiation (OFR) were analyzed using GPUMCI-simulated scatter projections. A novel scatter correction method was investigated, applying polynomial surface fitting to estimate scatter from the collimator shadow. This enabled correction of the total signal within the ROI, improving image quality.
Results:
Dose Reduction: ROI3x reduced total dose by 33% compared to BG3x.
Image Quality Improvement: ROI3x increased CNR by 11%, with scatter-OFR correction providing an additional 2 - 3% improvement, resulting in a total 14% CNR gain.
Scatter Correction: The scatter surface fit estimated from signal behind the collimator improved image quality and reduced scatter artifacts introduced in reconstructed FFM CBCT data.
Conclusion:
FFM-CBCT significantly reduces dose while improving image quality for in the selected ROI. Scatter estimation via surface fitting provides a simple correction strategy for FFM-CBCT. These findings deepen the understanding of FFM-CBCT's underlying physics and support the ongoing development and optimization of prototype FFM-CBCT systems for improved clinical imaging applications.