Purpose: To evaluate the imaging performance of an on-board helical kVCT system on a tomotherapy unit for a variety of imaging parameters and setup conditions.
Methods: A commonly used CT image quality phantom was scanned while varying the selection of available parameters (Anatomy, Mode, and Body Size) as well as phantom positioning and size. Image quality metrics including noise, uniformity, contrast, and CT number consistency were evaluated and compared as a function of parameter and setup variations.
Results: Use of Thorax (120 kVp) and Pelvis (140 kVp) anatomy protocols improved noise, uniformity, and contrast metrics by 10 – 20% compared to images acquired with Head (100 kVp) protocols. No significant differences in CT number consistency were observed regardless of Anatomy choice. Use of Fine mode (50 mm beam width at isocenter, 240-600 mAs/rotation) improved noise and contrast metrics by 20 – 30% compared to Normal mode (100 mm, 192-480 mAs/rotation) and by nearly a factor of 2 compared to Coarse mode (140 mm, 144-360 mAs/rotation) for otherwise identical protocols. Uniformity and CT number consistency were also improved for Fine mode. Increasing Body Size (mAs/rotation) generally improved each metric. Vertical and lateral phantom shifts of up to ± 6 cm degraded noise and contrast metrics by up to 30% relative to isocenter while also worsening uniformity and CT number consistency. Noise, uniformity, and contrast were also degraded by up to a factor of 3.5 with use of annuli to increase phantom diameter (32 cm vs. 20 cm). Despite variation in image characteristics among the investigated changes, all metrics were within applicable vendor specifications for use in image-guided (IGRT) and adaptive radiotherapy (ART).
Conclusion: This work suggests that this system can provide sufficient image quality required for IGRT and ART implementation in tomotherapy treatments for a variety of protocols and setup conditions.
Funding Support, Disclosures, and Conflict of Interest: John Bayouth has ownership interest in MR Guidance, LLC, which has business activity with a company that also utilizes IGRT technology (ViewRay, Inc). Additionally, while this project received no external funding, the data was collected on a Radixact system (Accuracy, Inc.) provided to UW-Madison under a research agreement (Bayouth, PI).
Tomotherapy, Helical CT, Image Guidance