Optimal combination of anti-scatter grids and software correction for CBCT imaging

Area of Science:

• Medical Physics
• Radiotherapy Imaging
• Image Reconstruction

Background:

• Cone beam computed tomography (CBCT) is crucial for image-guided radiotherapy but suffers from reduced soft tissue contrast and inaccurate Hounsfield units due to scattered radiation.
• Previous work introduced anti-scatter grids (ASGs) as a solution, with an alternative being iterative scatter correction using a scatter point spread function (PSF).

Purpose of the Study:

• To compare the effectiveness of ASGs with varying selectivity, with and without iterative scatter correction, in improving CBCT image quality.
• To develop a novel method for measuring scatter point spread functions (PSFs).

Main Methods:

• Scatter PSFs were modeled using bivariate Gaussian functions and estimated via transmission measurements with different ASG designs.
• An iterative scatter correction algorithm was implemented, convolving primary estimates with the scatter PSF until convergence.
• Image quality was assessed using phantoms and a virtual human, quantifying contrast-to-noise ratio (CNR) and Hounsfield unit (HU) accuracy.

Main Results:

• Higher ASG selectivity and iterative correction led to improved CNR, with the best results for head and neck phantom (CNR 3.9) and pelvis phantom (CNR 1.5).
• HU accuracy improved significantly, with HU RMSD as low as 6 HU for head and neck and 27 HU for pelvis using high-selectivity ASGs and iterative correction.
• Reconstructed CBCT scans with iterative correction and high-selectivity grids showed minimal average HU difference (59 ± 48 HU) compared to planning CT.

Conclusions:

• The optimal strategy for scatter mitigation in CBCT involves combining ASGs with selectivity >9 and iterative scatter estimation.
• This combined approach enhances image quality across various scenarios without increasing patient radiation dose.
• The proposed PSF measurement method contributes to improved CBCT image quality.