The impact of off‐centered positioning on cone beam CT image quality for image‐guided brain SRS treatment

Purpose/objectives Cone beam computed tomography (CBCT) is widely used in image‐guided positioning for patients with brain metastases during stereotactic radiosurgery (SRS). A major issue with CBCT is the decrease in image quality due to scatter, especially when the isocenter is off‐centered for treating peripheral‐metastases. This affects image clarity and CT numbers in skull‐reconstruction. The study aims to measure the scatter‐to‐primary ratio in projection‐images based on gantry‐angle for off‐centered isocenter. It also examines the effects of scatter on CBCT image quality and alignment‐accuracy for off‐centered isocenter. Materials/methods A Catphan‐600 phantom was imaged using the head protocol across five different FOV(2–18 cm in SI dimension) to analyze the effect of scatter on CBCT image quality in both centered and off‐centered isocenter Positions. Scatter‐fractions were measured for the Catphan‐600 relative to Field of View (FOV) size and for a head‐phantom in relation to gantry rotation at an off‐centered isocenter. Computed tomography (CT) and CBCT images alignment of the head phantom was compared between centered and off‐centered isocenter placements. Results Increasing FOV from 2 to 18 cm significantly reduced image quality; the contrast‐to‐noise ratio (CNR) decreased by 1.7 times with this FOV increase. CNR decreased by 3 times at a 6 cm off‐center position and the skull CT‐number decreased by 350 HU. For the Catphan‐600 at an 18 cm FOV, the scatter‐to‐primary ratio reached 0.67. The scatter‐fraction for the head phantom was 1.8 times higher when fully in‐field compared to partially in‐field. The mean alignment accuracy for the head phantom was 0.3 ± 0.2 mm for isocenter and 0.8 ± 0.3 mm for off‐center isocenter. Conclusion Quantitative analysis of CBCT image quality in phantoms reveals substantial image degradation with larger FOV and off‐centered isocenter positioning. The methods in this study provide insights and a framework for refining scatter correction algorithms to achieve precise scatter adjustments. These developments can be used to enhance CBCT image quality and improve target‐alignment‐accuracy.