A novel off-axis scanning method for an enlarged ellipse cone-beam computed tomography field of view

Current on-board imaging systems commonly used by modern linear accelerators (LINACs) have a limited field of view (FOV) for a cone-beam CT (CBCT) scan, which is typically less than 50 cm. Consequently, truncation artifacts often occur for large patients. The goal of this work is to investigate a novel method to increase the FOV for current on-board CBCT systems.

When a large patient is scanned with CBCT, any region outside the FOV is only partially sampled within a short range of projection angles, and at other angles no x-ray beams may pass through that region. To increase the sampling rate for the region outside the FOV, we have designed a new source trajectory by shifting the center of rotation during a CBCT scan. This resulted in a reduced sampling rate at the central area and increased sampling rate at the edges. The tradeoff led to a more balanced sampling for an enlarged FOV. An iterative algorithm was also developed to reconstruct the CT image under the new sampling scheme using a compressed sensing technique.

The method was validated by numerical simulations mimicking a Varian Trilogy CBCT system, and it was found that artifact-free images could be obtained with the FOV as large as 80 cm.

The new CT scanning trajectory can be easily realized under current clinical setup with little modification of the control system, and this can be useful for treating obese patients.