Reducing Cone-Beam Artifacts In Gamma Knife CBCT Images Using a Line-Arc-Line Scan Trajectory
Abstract
Purpose
An important component of the patient positioning and monitoring in Gamma Knife treatments, is the integrated CBCT system. However, the current circular arc scan trajectory and the geometric constraints on the CBCT system imposed by the machine, causes noticeable cone-beam artifacts towards the top of the head for some patients. Our simulations of various scan trajectories have previously shown that a line-arc-line trajectory is promising for reducing cone-beam artifacts in Gamma Knife CBCT images. The objective here is to confirm experimentally the simulation results.
Methods
Experiments are conducted on a Gamma Knife system, modified to allow axial movement of the patient table while scanning. Projection stacks corresponding to various combinations of 200-degree arcs at fixed axial positions and of linear scans in the axial direction taken at the first and last gantry angles, including the line-arc-line trajectory, are acquired for a Catphan 503 and an anthropomorphic head phantom. The CBCT volumes are then reconstructed with an iterative algorithm based on weighted least squares.
Results
Reconstructions with the line-arc-line trajectory show the best image quality among the investigated trajectories, while only adding 100 projections to the current 334-projection arc scan. This new trajectory only requires movement of the table before and after the standard arc. Compared to the images from the single-arc trajectory, cone-beam artifacts are greatly reduced: the superior edge of the Catphan is 93% sharper in the axial direction and the uniformity of material inserts is increased by up to 25%. For the head phantom, 2% of all voxels show a difference of more than 100 HU when comparing the reconstructions from the line-arc-line and single-arc trajectories.
Conclusion
Experiments show that a line-arc-line scan trajectory is compatible with the Gamma Knife CBCT system and improves the image quality by removing virtually all cone-beam artifacts.