Evaluation of CBCT Protocol Optimization Following Upgrade from aSi1200 to Hypersight Imaging Panel
Abstract
Purpose
To quantitatively and qualitatively assess CBCT image quality and achievable dose optimization following an upgrade from an aSi1200 to HyperSight imaging panel.
Methods
Head, thorax, pelvis, large pelvis, and spotlight CBCT protocols were modified to reduce exposure by 20-80% of default by varying acquisition frame rates and mAs per frame. Protocol exposure was characterized using ion chamber air kerma measurements. Catphan 604 and anthropomorphic phantoms were imaged before and after the panel upgrade from aSi1200 to HyperSight. Quantitative imaging metrics including HU constancy and uniformity, low contrast visibility, spatial resolution and spatial integrity were determined using a script analyzing Catphan images. Anthropomorphic phantom images were qualitatively reviewed by therapists and physicists to determine appropriate image quality for patient alignment. All images were reconstructed using the FDK algorithm.
Results
Low contrast visibility demonstrated a strong and significant dependence on total mAs across all protocols (r>0.86, p20% relative exposure except for some low-dose aSi large pelvis protocols, with HyperSight exhibiting modestly reduced variability compared to aSi1200 that did not impact clinical acceptability. Spatial resolution and spatial integrity were insensitive to dose reduction. Qualitative assessment showed that dose reductions of 22-49%, depending on anatomic site, preserved adequate image quality for patient alignment, with comparable limits between HyperSight and aSi1200 panels. Projection number reduction introduced more pronounced aliasing artifacts than mAs/frame for both systems.
Conclusion
CBCT imaging dose reductions of 20%-50% were achieved across common IGRT protocols without compromising image quality. Low contrast visibility was the primary dose-limiting quantitative metric, while panel type did not significantly influence achievable dose reduction. These findings highlight the need to combine quantitative analysis with qualitative clinical review during CBCT optimization.