Room: Exhibit Hall | Forum 9
Purpose: To evaluate the shape distortion and image artifact caused by a moving object with varying frequency and direction using a prototype rapid kV CBCT system designed for a ring-shaped medical linear accelerator (Halcyon, Varian Medical System).
Methods: A moving cylindrical section, with a spherical lesion of 5.9 cmÂ³, mounted onto a motion phantom (CIRS, Norfolk, VA) was used in this study. The object motion was programmed with various sinusoidal motion frequencies to simulate different patient breathing rates. Four bpm (breaths per minute) settings (4, 5, 7.5, and 15), two motion amplitude settings (Â±20 and Â±5 mm), and two rapid CBCT scanning protocols (Thorax and Thorax Fast) were used for this evaluation. Motion in superior-inferior (SI) and anterior-posterior (AP) directions were investigated separately to simplify the quantification and reduce the complexity of this study. Imaging artifacts and relative object volume distortions were evaluated as a function of the projection acquisition speed and motion frequencies.
Results: Higher frequency motion introduced severe artifacts seen as an increase in streaking on the CBCT images. With expectation, larger motion amplitude led to more streaking artifact for the same bpm setting. Both the Thorax and Thorax Fast imaging protocols yielded comparable contour volumes for high frequency motion. The fast imaging protocols helped reduce the motion artifact for all tested frequencies. The Thorax Fast imaging protocol yielded more shape distortions represented by a larger contoured volume for low frequency motion (4 and 5 bpm). However, the Thorax fast protocol was able to better capture the full extent of motion compared to the slower thorax protocol.
Conclusion: Appropriate CBCT strategies should be applied for patients with different respiratory patterns. Image distortion and potential misalignment of the patient may occur with fast 3D CBCT acquisitions. Further investigations are needed to quantify the effect of full 3D motion.
Funding Support, Disclosures, and Conflict of Interest: This research was funded by Varian Medical Systems