Megavoltage cone-beam CT (MVCBCT) is the most recent addition to the in-room CT systems developed for image-guided radiation therapy. The first generation MVCBCT system consists of a treatment x-ray beam produced by a conventional linear accelerator equipped with a flat panel amorphous silicon detector. The objective of this study was to evaluate the physical performance of MVCBCT in order to optimize the system acquisition and reconstruction parameters for image quality. MVCBCT acquisitions were performed with the clinical system but images were reconstructed and analyzed with a separate research workstation. The geometrical stability and the positioning accuracy of the system were evaluated by comparing geometrical calibrations routinely performed over a period of . The beam output and detector intensity stability during MVCBCT acquisition were also evaluated by analyzing in-air acquisitions acquired at different exposure levels. Several system parameters were varied to quantify their impact on image quality including the exposure (2.7, 4.5, 9.0, 18.0, and ), the craniocaudal imaging length (2, 5, 15, and ), the voxel size (0.5, 1, and ), the slice thickness (1, 3, and ), and the phantom size. For the reconstruction algorithm, the study investigated the effect of binning, averaging and diffusion filtering of raw projections as well as three different projection filters. A head-sized water cylinder was used to measure and improve the uniformity of MVCBCT images. Inserts of different electron densities were placed in a water cylinder to measure the contrast-to-noise ratio(CNR). The spatial resolution was obtained by measuring the point-spread function of the system using an iterative edge blurring technique. Our results showed that the geometric stability and accuracy of MVCBCT were better than over a period of . Beam intensity variations per projection of up to 35.4% were observed for a MVCBCT acquisition. These variations did not cause noticeable reduction in the image quality. The results on uniformity suggest that the cupping artifact occurring with MVCBCT is mostly due to off-axis response of the detector and not scattered radiation. Simple uniformity correction methods were developed to nearly eliminate this cupping artifact. The spatial resolution of the baseline MVCBCT reconstruction protocol was approximately . An optimized reconstruction protocol was developed and showed an improvement of 75% in CNR with a penalty of only 8% in spatial resolution. Using this new reconstruction protocol, large adipose and muscular structures were differentiated at an exposure of . A reduction of 36% in CNR was observed on a larger (pelvic-sized) phantom. This study demonstrates that soft-tissue visualization with MVCBCT can be substantially improved with proper system settings. Further improvement is expected from the next generation MVCBCT system with an optimized megavoltage imaging beamline.
This research was partly funded by Siemens Oncology Care Systems. Two of the authors (O.M. and J.F.A.) wish to acknowledge a scholarship from the National Sciences and Engineering Research Council of Canada (NSERC).
II. MATERIAL AND METHODS
II.A. MVCBCT systems and research workstation
II.B. Geometric stability and absolute positioning accuracy
II.C. Beam output, detector response, and CT number calibration factor
II.D. Image quality
II.D.1. Uniformity and cupping artifact
II.D.2. Spatial resolution
II.D.3. Contrast-to-noise ratio
III.A. Geometric stability and absolute positioning
III.B. Stability of x-ray beam output and flat panel detector readout
III.C. Image quality
III.C.2. Spatial resolution
III.C.3. Contrast-to-noise ratio
IV.A. Geometric stability and absolute positioning
IV.B. Beam output and detector sensitivity
IV.C. Image quality
IV.C.2. Spatial resolution
IV.C.3. Contrast-to-noise ratio
IV.D. MVCBCT physical performance compared to other 3DCT-IGRT systems
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