Abstract
PurposeTo assess clinically relevant image quality metrics (IQMs) of helical fan beam kilovoltage (kV) fan beam computed tomography (CT). Methods and MaterialskVCT IQMs were evaluated on an Accuray Radixact unit equipped with helical fan beam kVCT to assess the capabilities of this newly available modality. kVCT IQMs were evaluated and compared to a kVCT simulator and linear accelerator-based cone beam CTs (CBCT) using a commercial CBCT image quality phantom. kVCTs were acquired on the Accuray Radixact for all combinations of kVp and mAs in fine mode using a 440-mm field of view (FOV). Evaluated IQMs were spatial resolution, overall uniformity, subject contrast, contrast-to-noise ratio (CNR), and effective slice thickness. Imaging dose was assessed for planar kV imaging. ResultsOn this kVCT system spatial resolution and contrast were consistent across all settings with 0.28 ± 0.03 lp/mm and 9.8% ± 0.7% (both 95% confidence interval). CNR strongly depended on selected mode (views per rotation) and body size (mA per view) and ranged between 7.9 and 34.9. Overall uniformity was greater than 97% for all settings. Large FOV was not found to substantially affect the IQMs whereas small FOV affected IQMs due to its effect on pitch. Technique-matched CT simulator scans were comparable for uniformity and contrast, while spatial resolution was higher (0.43 ± 0.06 lp/mm), and CNR was between 4% (140 kVp) and 51% (100 kVp) lower. For kV-CBCT, spatial resolutions ranging from 0.37 to 0.44 lp/mm were achieved with comparable contrast, CNR, and uniformity to kVCT. All kVCT scans exhibit imaging artifacts due to helical acquisition. Clinical acquisitions of megavoltage (MV) CT, kV-CBCT, and kVCT on the same patient showed improved and comparable image quality of kVCT compared to MVCT and kV-CBCT, respectively. ConclusionsHelical fan beam kVCT allows for daily image guidance for localization and setup verification with comparable performance to existing kV-CBCT systems. Scan parameters must be selected carefully to maximize image quality for the desired tasks. Due to the large effective slice thicknesses for all parameter combinations, kVCT scans should not be used for simulation or planning of stereotactic procedures. Finally, improved image quality over MVCT has the potential to greatly improve manual and automated adaptive monitoring and planning.
Highlights
Image-guided radiation therapy (IGRT) has enabled significant setup margin reduction and has increased the effectiveness of motion management [1,2,3] contributing to improved outcomes and reduced toxicities [4,5]
This study aims to compare the performance of this second generation (Gen2) x-ray imaging system for planar and volumetric imaging with the Gen1 system and to assess its clinical performance by comparing helical fan beam Kilovoltage fan beam computed tomography (kVCT) with a technique-matched computed tomography (CT)-simulator, CTsimulator kVCT using standard simulation protocols, and two other linear accelerator-based cone beam CT systems
Spatial resolution decreased from an average of 1.14 lp/mm to 0.97 lp/mm, contrast increased from an average of 40% to 47%, and contrast-to-noise ratio (CNR) increased on average by 9.4%
Summary
Image-guided radiation therapy (IGRT) has enabled significant setup margin reduction and has increased the effectiveness of motion management [1,2,3] contributing to improved outcomes and reduced toxicities [4,5]. Intra-modality variations in image quality, artifacts, and limitations between kilovoltage and megavoltage CT or fan beam and cone beam CT impact DIR quality [12,13,14]. While these variations can be sufficiently small with no significant loss of quality for kilovoltage CT-CBCT registrations [15,16,17], significant differences can exist for kVCT-MVCT registrations that require manual intervention or post-processing to enhance registration and contour propagation accuracy [18,19]. Volumetric kilovoltage imaging is the most desirable technique to increase accuracy and precision for online or offline adaptive radiation therapy workflows that aim to automate image registration, contouring, dose accumulation, and re-planning [20,21,22]. kV-CBCTs can be limited in their field of view (FOV) and suffer from specific cone beam artifacts that can limit their use in areas having multiple air pockets
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