A CBCT study of the gravity-induced movement in rotating rabbits

Abstract

Fixed-beam radiotherapy systems with subjects rotating about a longitudinal (horizontal) axis are subject to gravity-induced motion. Limited reports on the degree of this motion, and any deformation, has been reported previously. The purpose of this study is to quantify the degree of anatomical motion caused by rotating a subject around a longitudinal axis, using cone-beam CT (CBCT). In the current study, a purpose-made longitudinal rotating was aligned to a Varian TrueBeam kV imaging system. CBCT images of three live rabbits were acquired at fixed rotational offsets of the cradle. Rigid and deformable image registrations back to the original position were used to quantify the motion experienced by the subjects under rotation. In the rotation offset CBCTs, the mean magnitude of rigid translations was 5.7 ± 2.7 mm across all rabbits and all rotations. The translation motion was reproducible between multiple rotations within 2.1 mm, 1.1 mm, and 2.8 mm difference for rabbit 1, 2, and 3, respectively. The magnitude of the mean and absolute maximum deformation vectors were 0.2 ± 0.1 mm and 5.4 ± 2.0 mm respectively, indicating small residual deformations after rigid registration. In the non-rotated rabbit 4DCBCT, respiratory diaphragm motion up to 5 mm was observed, and the variation in respiratory motion as measured from a series of 4DCBCT scans acquired at each rotation position was small. The principle motion of the rotated subjects was rigid translational motion. The deformation of the anatomy under rotation was found to be similar in scale to normal respiratory motion. This indicates imaging and treatment of rotated subjects with fixed-beam systems can use rigid registration as the primary mode of motion estimation. While the scaling of deformation from rabbits to humans is uncertain, these proof-of-principle results indicate promise for fixed-beam treatment systems.

Evaluating the Reproducibility of Mouse Anatomy under Rotation in a Custom Immobilization Device for Conformal FLASH Radiotherapy

“…Our study is similar in some respects to previously published work evaluating anatomic deformation under rotation of rabbits and human volunteers in the context of a proposed compact and economical fixed beamline clinical radiotherapy system with a geometry analogous to our FLASH-EXACT preclinical system (12,13). Those studies FIG.…”

Development and commissioning of a full‐size prototype fixed‐beam radiotherapy system with horizontal patient rotation

“…This situation does not represent the real use case, where patients undergoing horizontal rotation will be subject to varying amounts of vertical displacement (shown in Fig. ) and internal organ deformation due to gravity . Three essential steps would need to be taken to deliver the 3D conformal treatments described in this manuscript to human patients.…”

The adaptation and investigation of cone-beam CT reconstruction algorithms for horizontal rotation fixed-gantry scans of rabbits

“…However, previous studies have shown that horizontal rotation of patients can lead to anatomical deformation (Whelan et al, 2017;Buckley et al, 2019). Similar anatomic deformation has also been shown in single organ and animal studies, where the anatomical deformation caused by horizontal rotation led to artefacts in the fixed-gantry CBCT reconstruction (Feain et al, 2016;Barber et al, 2018). Shieh et al developed an algorithm for compensating for gravity induced motion for fixed-beam CBCT scans which was tested on anaesthetised rabbits (Shieh et al, 2018).…”

“…This study used fixed-gantry and rotating-gantry rotational projection images of anaesthetised rabbits, which have been previously acquired (Barber et al, 2018;Shieh et al, 2018) . The different fixed-gantry CBCT reconstruction algorithms are compared with the conventional rotating-gantry CBCT scans to determine which algorithms successfully compensate for horizontal rotational motion during fixed-gantry CBCT scans.…”

“…However, this algorithm A c c e p t e d M a n u s c r i p t compensated for only the rigid motion, and not for any complex deformable motion. It has been shown previously that for both humans (Whelan et al, 2017) and rabbits (Barber et al, 2018;Shieh et al, 2018), the non-rigid deformation makes up a significant component of the gravity-induced motion caused by horizontal rotation. Additionally, the authors reported that for one of the rabbits, the reconstruction of the volume using their motion compensation algorithm had significant motion blur, caused by the large motion that occurred during rotation.…”