Magnetic-Field-Induced Forces and Motion of GAGG:Ce Scintillation Crystals in a 1.5 T Linac–MRI Environment
Janusz Winiecki, Agnieszka Orzechowska, Sebastian Maleszka, Paulina Michalska, Yuriy ZorenkoAbstract
Introduction
Hybrid MR-Linac systems combine MV-photon beam delivery with strong magnetic fields, creating fringe-field regions (magnetic field gradients). While dosimetric effects of magnetic fields are well understood, the mechanical behaviour of paramagnetic scintillation materials used for radiation detection has not been systematically investigated. This study provides theoretical estimates and qualitative experimental observations magnetic-field-induced forces acting on GAGG:Ce scintillation crystals in a clinical 1.5T system.
Material and methods
Forces acting on 5×5×7mm3 crystals were calculated using magnetic susceptibility theory for realistic fringe-field gradients (5–20T/m) at a magnetic flux density of 1.5T. Magnetic forces were compared with gravitational and static friction forces to assess the likelihood of translational motion. Order-of-magnitude estimates of crystal acceleration and velocity were derived for cm-scale displacements. Qualitative experimental validation was performed in an Elekta Unity by observing the motion of crystals during treatment couch insertion and withdrawal.
Results
Calculated magnetic forces ranged from c.a.0.01 to 0.04N in fringe-field regions, exceeding estimated static friction forces. Observed motion corresponds to centimeter-order displacement along the couch during entry, consistent with the experimentally observed directional migrations. No visually observable displacement occurred in homogeneous magnetic field region near the magnetic isocenter, whereas translational motion was observed in regions with strong magnetic field gradients near the magnet bore entrance. The magnitude of crystal displacement qualitatively correlated with the local magnetic field gradient.
Conclusions
Magnetic-field-induced forces acting on GAGG:Ce scintillation crystals in 1.5T environment are negligible in the homogeneous magnetic field region near the magnetic isocenter but become significant in fringe-field regions characterized by strong magnetic field gradients. Both theoretical analysis and experimental observations demonstrate that these forces may induce translational and, potentially, rotational motion of freely placed crystals during treatment couch translation. Consequently, magnetic pull-in effects must be carefully considered in mechanical design, fixation, handling, and safe clinical operation of MR-compatible scintillation and dosimetric detectors used for MRgRth.