Characterization of Free-Breathing Gated CBCT Imaging on C-Arm Linac with Hypersight
Abstract
Purpose
An improved imager and software (HyperSight on a Varian TrueBeam linac) feature a larger detector panel and increase the maximum gantry rotation speed for CBCT imaging to 1.5 revolutions per minute (rpm). While enabling faster acquisition, its impact on free-breathing gated CBCT (gCBCT), which is widely used in respiratory-gated radiotherapy, remains uncharacterized. The gCBCT restricts gantry rotation and kV acquisition to predefined gating windows, introducing complex mechanical dynamics (accelerations, decelerations, and “setback” maneuvers). This study evaluates the impact of increased gantry speed on gCBCT scan time, imaging dose, and image quality.
Methods
A total of 42 gCBCT scans were acquired on a HyperSight-equipped linear accelerator using gantry rotation speeds of 1.0 and 1.5 rpm. Respiratory motion was simulated using a CIRS motion platform with cos⁴(x) waveforms spanning breathing periods of 3-6 s and duty cycles of 30-60% (32 scans), and with five patient-specific breathing traces (10 scans). Scan time and imaging dose were quantitatively compared between scans acquired with the two gantry speeds. Image quality was evaluated using a Catphan phantom on a QUASAR motion platform.
Results
For typical clinical breathing periods (≥4 s) and duty cycles (30–60%), 1.5 rpm gantry rotation reduced gCBCT scan time relative to 1.0 rpm by 15–30% with a 5–15% increase in imaging dose. For faster breathing ( 50%), both scan time and imaging dose increased at 1.5 rpm. Image quality was comparable between the two gantry speeds.
Conclusion
The mechanical and temporal constraints of gCBCT limit the benefits of faster gantry rotation. Under typical clinical conditions, HyperSight’s increased gantry speed shortens gCBCT acquisition with a modest dose increase, but may prolong scans in patients with rapid breathing and high duty cycles. These results provide initial guidance for using HyperSight clinically and optimizing gCBCT protocols.