Program Information
A CBCT Study of the Gravity-Induced Deformation in Rotating Lagomorph Subjects
J Barber1,2*, C Shieh2 , W Counter2 , J Sykes1,2 , P Bennett2 , S Heng3 , P White3 , S Corde3 , M Jackson3 , V Ahern1 , P Keall2 , I Feain2 , (1) Western Sydney Local Health District, Sydney, NSW, (2) University of Sydney, Sydney, NSW, (3) Prince of Wales Hospital, Sydney, NSW,
Presentations
TU-C2-GePD-J(A)-6 (Tuesday, August 1, 2017) 10:00 AM - 10:30 AM Room: Joint Imaging-Therapy ePoster Lounge - A
Purpose: On fixed-gantry radiotherapy systems, subject rotations are required to acquire CBCT, but with increased complexity due to gravity induced motion-blur. The purpose of this work is to quantify the degree of anatomical deformation caused by rotating a subject around a longitudinal axis. The degree of deformation is compared to the nominal respiratory deformation of the subject from 4DCBCT.
Methods: CBCT scans of live rabbits positioned at fixed angular positions were collected using a purpose-made longitudinal rotating immobilization cradle and a Varian TrueBeam kV imaging system. Three rabbits were imaged in 45° increments over two rotations and a 4DCBCT was also acquired at zero degree angular increment to evaluate respiratory motion. The CBCT images were each registered to the zero increment CBCT. Manual rigid registration of first the cradle and then the rabbit thorax was performed, with priority on boney anatomy. A deformable registration (multi-pass B-spline) was performed on a bounding box covering the thorax. Resulting deformation vector fields (DVF) were evaluated.
Results: In the non-rotated rabbit 4DCBCT, respiratory diaphragm motion up to 5 mm was observed. In the rotated position CBCTs, the mean magnitude of rigid registrations was 5.7 ± 2.7 mm across all rabbits and both rotations. The translational motion was reproducible for multiple rotations of each rabbit, with rabbit 3 exhibiting maximum difference of 2.8 mm. Deformation mean and absolute maximum vectors were 0.2 ± 0.1 mm and 5.4 ± 2.0 mm respectively, indicating small residual deformations after rigid registration.
Conclusion: The principle motion of the rotated subjects was translational due to movement of internal tissues inside the animal. The deformation of the anatomy under rotation was found to be equal in scale to normal physiological motion. While the scaling to larger animals is uncertain, these proof-of-principle results indicate promise for fixed-gantry treatment systems.
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