Characterization of imaging performance of a novel helical kVCT for use in image‐guided and adaptive radiotherapy

Abstract

ClearRT helical kVCT imaging for the Radixact helical tomotherapy system recently received FDA approval and is available for clinical use. The system is intended to enhance image fidelity in radiation therapy treatment planning and delivery compared to the prior MV‐based onboard imaging approach. The purpose of this work was to characterize the imaging performance of this system and compare this performance with that of clinical systems used in image‐guided and/or adaptive radiotherapy (ART) or computed tomography (CT) simulation, including Radixact MVCT, TomoTherapy MVCT, Varian TrueBeam kV OBI CBCT, and the Siemens SOMATOM Definition Edge kVCT. A CT image quality phantom was scanned across clinically relevant acquisition modes for each system to evaluate image quality metrics, including noise, uniformity, contrast, spatial resolution, and CT number linearity. Similar noise levels were observed for ClearRT and Siemens Edge, whereas noise for the other systems was ∼1.5–5 times higher. Uniformity was best for Siemens Edge, whereas most scans for ClearRT exhibited a slight “cupping” or “capping” artifact. The ClearRT and Siemens Edge performed best for contrast metrics, which included low‐contrast visibility and contrast‐to‐noise ratio evaluations. Spatial resolution was best for TrueBeam and Siemens Edge, whereas the three kVCT systems exhibited similar CT number linearity. Overall, these results provide an initial indication that ClearRT image quality is adequate for image guidance in radiotherapy and sufficient for delineating anatomic structures, thus enabling its use for ART. ClearRT also showed significant improvement over MVCT, which was previously the only onboard imaging modality available on Radixact. Although the acquisition of these scans does come at the cost of additional patient dose, reported CTDI values indicate a similar or generally reduced machine output for ClearRT compared to the other systems while maintaining comparable or improved image quality overall.

The Impact of Parameter Selection and Setup Conditions on Image Quality of an On-Board Helical Kilovoltage Computed Tomography System

“…Of the 10 unique protocols previously evaluated, five were reevaluated in this study. The comparison of applicable IQM between this study and Tegtmeier et al [5] is shown in Table 1. Based on these values, the application of the updated reconstruction software resulted in a further reduction in noise by over 20% relative to the previous software for the Thorax and Pelvis anatomy protocols and by over 30% for the head anatomy protocol.…”

“…Two recent publications included an analysis of image quality for this system in comparison to CT simulation and other on-board CT systems (cone-beam CT and MVCT) for several image quality metrics [ 4 , 5 ]. These previous studies concluded that this system performed well in comparison to the other systems for IQM important to the implementation of IGRT and ART, as the contrast was consistent with a CT simulator and overall image quality was greatly improved when compared to MVCT.…”

“…However, this previous work was performed with the reconstruction software (Version 3.0.0.11), which was included with the initial system released in spring 2021. As the methods for this study were reproduced by Tegtmeier et al, a direct comparison of IQM between the updated and previous reconstruction algorithm was possible [ 5 ]. Of the 10 unique protocols previously evaluated, five were reevaluated in this study.…”

“…Lastly, the mode defines the longitudinal beam width, helical pitch, and views-per-rotation (Fine, Normal, or Coarse). Changes to this parameter have been observed to most impact image quality as variation in the beam width influences the scatter signal while variation in views-per-rotation impacts output (mAs/rotation) and patient dose [ 4 , 5 ]. For additional information on system and protocol specifications, the reader is referred to the manufacturer's guide [ 6 ].…”

“…A field of view of 440 mm with an aluminum bowtie filter was used regardless of the additional parameter selections to ensure identical filtration and detector-collimator offset across all comparisons. For each acquisition, image quality metrics (IQM) including noise, uniformity, contrast, CT number constancy, and spatial resolution were evaluated as outlined by Tegtmeier et al [ 5 ]. The noise was defined as the standard deviation of pixel values within some homogenous region of interest (ROI) and was reported as a percentage of the mean pixel value within this ROI [ 8 ].…”