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CBCT Image Reconstruction to Compensate for Gravity-Induced Motion During Patient Rotation

M Gardner1*, E Debrot1, O Dillon1, C Shieh2, S Downes3, M Jackson3, P Liu1, P Keall1, (1) University of Sydney, Eveleigh, NSW, AU, (2) Sydney Neuroimaging Analysis Centre, Eveleigh, NSW, (3) Nelune Comprehensive Cancer Centre, Prince of Wales Hospital, Randwick, NSW, AU,

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Fixed gantry linacs provide opportunity for more affordable radiotherapy, which would be suited to low- and middle-income countries where the demand for radiotherapy is not being met. However, for fixed gantry linacs the patient needs to be rotated instead. CBCT scans during patient rotation have significant motion artefacts and motion compensation methods are needed. The aim of this work is to develop image reconstruction methods that compensate for gravity-induced motion during patient rotation.


Methods: A phantom that allowed gravitation-induced deformation was created by surrounding a solid block with pieces of foam, such that as the phantom rotates motion will occur. CBCT scans were acquired with this phantom placed in a custom patient rotation system, known as the Nano-X device, which uses inflated airbags to support the patient during rotation, limiting motion. Motion compensated reconstructions were created by dividing the projections into 8 equal angular bins of width 45° which were used as the different reconstruction phases. PICCS was then used to generate volume estimates for each phase, and DVFs were calculated between these volumes to estimate 4D-DVFs. Motion compensated FDK (MCFDK) using the 4D-DVFs were then used to generate a volume for each phase. The SSIM and MSE between the 0° phase volume and the reference CBCT were compared with the standard FDK.


Results: The MCFDK had a -35-4% change in MSE and 0.0075-0.1778% increase in SSIM when compared with the standard FDK, indicating the MCFDK was more accurate than the normal FDK. This was true even when the volume was moved from isocentre in the anterior and transverse directions.


Conclusion: This work shows that accurate volume reconstruction is possible for fixed gantry CBCT scans taken using the Nano-X device. Further development of this prototype will allow for cheaper radiotherapy devices to meet the global demand for radiotherapy.

Download ePoster [PDF]

Funding Support, Disclosures, and Conflict of Interest: This project is supported by a NHMRC Development Grant. PJK is an inventor of intellectual property assigned to Leo Cancer Care.

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