Dosimetry evaluation of an MRI guided small animal irradiator

OBJECTIVE: Magnetic Resonance guided Radiation Therapy (MRgRT) integrates magnetic resonance imaging (MRI) with radiation delivery, providing superior soft-tissue contrast and real-time treatment guidance. For preclinical MRgRT irradiators, a critical concern is whether the ambient magnetic fields compromise the radiation dose distribution. This study aims to investigate the influence of magnetic fields on X-ray tube electron trajectories and dosimetric outcomes. APPROACH: The system integrates a 0.5 T MRI scanner and a 225 kVp X-ray tube. The localized magnetic field distribution was characterized via finite element analysis. Electron trajectories were simulated across various magnetic field strengths and alignment angles to quantify the X-ray focal-spot displacement. Monte Carlo simulation was then performed to evaluate the dose distribution within homogeneous and heterogeneous tissue-equivalent phantoms. MAIN RESULTS: In the proposed system, the fringe magnetic field across the X-ray tube electrodes measures 40.4 G and aligns nearly parallel to the electric field. This configuration restricts the X-ray focal-spot displacement to 0.15 mm, resulting in negligible dosimetric deviations. There is good agreement between the dose distributions in the phantoms with and without magnetic fields, maintaining ≧99.9% gamma passing rates at 0.5 mm / 1% criteria even under a main magnetic field of up to 10 T. SIGNIFICANCE: This study confirms that, for the small animal MRgRT system, main magnetic field as low as 0.5 T exerts no significant dosimetric impact, provided the magnet and the X-ray tube are well separated, and it may be unnecessary to perform dosimetric corrections.