X-ray tube in parallel magnetic fields

Our hybrid x-ray/MR system (a fixed-anode x-ray fluoroscopic system in an interventional MR system) provides the high spatial and temporal resolution of x-ray fluoroscopy with the soft-tissue contrast, 3D visualization and physiological information of MRI. X-rays are produced in an x-ray tube by bombarding a target with high-energy electrons, ionized from the cathode, then accelerated by the electric field between the cathode and anode. In the hybrid system, the x-ray tube is placed in a high magnetic field, aligned to be parallel to the cathode-anode axis of the tube. A finite-element program was used to simulate the electron trajectories in a geometry similar to our fixed anode tube. External magnetic fields parallel to the cathode-anode axis, ranging from 0 to 0.5T, were simulated. Experimentally, focal spot images were acquired using a 30μm pinhole at a magnification of 8.9 in magnetic fields ranging from 0 to 0.5T. No lateral deflection of the focal spot was observed in either the simulation or the experiment, if the magnetic field and the cathode-anode axis were aligned, regardless of the field strength. However, the field strength affected the size and the current density distribution of the focal spot. We conclude that fixed anode x-ray tubes can be used in a magnetic field although its desired electron optics must be fairly "straight" and the cathode-anode axis must be well aligned with the field. Further issues arising from the focal spot size change, such as overheating of the target, and to a lesser extent the system spatial resolution, should be carefully considered.