A novel electron gun for inline MRI-linac configurations.

Area of Science:

• Medical Physics
• Accelerator Physics
• Biomedical Engineering

Background:

• Integrating linear accelerators (linacs) with magnetic resonance imaging (MRI) systems offers combined diagnostic and therapeutic capabilities.
• Conventional MRI-linac designs require magnetic shielding to protect the linac from strong MRI magnetic fields, complicating integration and reducing magnet homogeneity.
• A key challenge is developing electron gun systems that can function reliably within the strong, axisymmetric fringe fields of MRI magnets.

Purpose of the Study:

• To introduce a new electron gun geometry for robust operation within high-strength external magnetic fields in MRI-linac configurations.
• To enable inline MRI-linac operation without magnetic shielding, preserving magnet homogeneity and allowing linac repositioning.
• To optimize the electron gun design for efficient beam injection and acceleration within MRI fringe fields.

Main Methods:

• Investigated electron gun geometry modifications of a Varian 600 C electron gun under external magnetic fields to establish design principles.
• Proposed and optimized a new electron gun geometry within the fringe field of a 0.5 T open bore MRI magnet.
• Utilized computer modeling of a 6 MeV Varian 600 C linac to assess capture efficiency in MRI fringe fields (0.5 T, 1.0 T, 1.5 T).

Main Results:

• A new geometry featuring a smaller transverse cathode diameter, flat cathode surface, and larger anode diameter was identified.
• Optimized parameters include a 5 mm cathode-anode distance, 5° focusing electrode angle, and 17.1 mm anode drift tube length.
• The system demonstrated robust performance in fringe fields (>0.11 T), with a 20% increase in target current at 0.5 T and <16% loss in other fields compared to zero-field conditions.

Conclusions:

• The specially designed electron gun operates effectively in strong axisymmetric MRI fringe fields, producing high-quality beams for linac injection and acceleration.
• This configuration allows linac displacement along the magnet axis without significant capture efficiency degradation.
• The system is viable for MRI-linac integration, functioning reliably in fringe fields above 0.11 T with minimal efficiency loss.