Closed bore XMR (CBXMR) systems for aortic valve replacement: Active magnetic shielding of x‐ray tubes

Abstract

Hybrid closed bore x-ray/MRI systems are being developed to improve the safety and efficacy of percutaneous aortic valve replacement procedures by harnessing the complementary strengths of the x-ray and MRI modalities in a single interventional suite without requiring patient transfer between two rooms. These systems are composed of an x-ray C-arm in close proximity (approximately 1 m) to an MRI scanner. The MRI magnetic fringe field can cause the electron beam in the x-ray tube to deflect. The deflection causes the x-ray field of view to shift position on the detector receptacle. This could result in unnecessary radiation exposure to the patient and the staff in the cardiac catheterization laboratory. Therefore, the electron beam deflection must be corrected. The authors developed an active magnetic shielding system that can correct for electron beam deflection to within an accuracy of 5% without truncating the field of view or increasing exposure to the patient. This system was able to automatically adjust to different field strengths as the external magnetic field acting on the x-ray tube was changed. Although a small torque was observed on the shielding coils of the active shielding system when they were placed in a magnetic field, this torque will not impact their performance if they are securely mounted on the x-ray tube and the C-arm. The heating of the coils of the shielding system for use in the clinic caused by electric current was found to be slow enough not to require a dedicated cooling system for one percutaneous aortic valve replacement procedure. However, a cooling system will be required if multiple procedures are performed in one session.

Navigational Tools for Interventional Radiology and Interventional Oncology Applications

“…Incorporation of a magnetic shielding system can allow for placement of the X-ray beam source within 1 m of the magnet. 63 However, current capabilities preclude routine use, and the challenges of image registration pose an additional challenge.…”

Foxels for high flux, high resolution computed tomography (foxelct) Using broad x-ray focal spots

“…For efforts to reduce that structure see [190]. The space charge of electrons [21,154], magnetic fields [100,191,192], as in dual CT/MRI scanners, and position along the cathode/anode axis [193] and projection direction [194,195], all alter the size and shape of focal spots. While we are assuming that the focal spot does not vary in the course of data collection for a single image, we found no proof of this in the literature.…”

Design optimization of MR‐compatible rotating anode x‐ray tubes for stable operation

“…Existing rotating anode x-ray tubes fail near an MR imaging system, since MR fringe fields alter the electron trajectories in the x-ray tube cathode-anode gap and create eddy currents in the metal rotor which cause a reduction in the rotation speed of the motor. 16 Several solutions for the electron ballistics problem have been proposed to correct for the displacement and change in shape of the focal spot, [17][18][19] and a novel motor prototype has been developed to solve the problem of reduced rotation speed. The proposed x-ray tube motor design utilizes the MR fringe field to gen- erate torque, analogous to the permanent magnet stator field used in DC motors, but large vibrations indicating instability were detected during its operation.…”