Closed‐bore XMR (CBXMR) systems for aortic valve replacement: X‐ray tube imaging performance

Abstract

A hybrid closed-bore x-ray/MRI system (CBXMR) is proposed to improve the safety and efficacy of percutaneous aortic valve replacement procedures. In this system, an x-ray C-arm will be positioned about 1 m from the entrance of a 1.5 T MRI scanner. The CBXMR system will harness the complementary strengths of both modalities to guide and deploy a bioprosthetic valve into the aortic annulus of the heart without coronary artery obstruction. A major challenge in constructing this system is ensuring proper operation of a rotating-anode x-ray tube in the MRI magnetic fringe field environment. The electron beam in the x-ray tube responsible for producing x rays can be deflected by the fringe field. However, the clinical impact of electron beam deflection in a magnetic field has not yet been studied. Here, the authors investigated changes in focal spot resolving power, field of view shift, and field of view truncation in x-ray images as a result of electron beam deflection. The authors found that in the fringe field acting on the x-ray tube at the clinical location for the x-ray C-arm (4 mT), focal spot size increased by only 2%, so the fringe field did not limit the resolving power of the x-ray system. The magnetic field also caused the field of view to shift by 3 mm. This shift must be corrected to avoid unnecessary primary radiation exposure to the patient and the staff in the cardiac catheterization laboratory. The fringe field was too weak to cause field of view truncation.

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.…”

Interventional Cardiovascular Magnetic Resonance Imaging

“…1) aortic valve implants in swine with immediate evaluation of valve performance and flow. Hybrid imaging guidance for valve placement is also under development with the use of an adjacent “CMR-friendly” X-ray fluoroscope at the edge of the magnet (25). …”

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

“…15 The electron beam deflection causes a shift in the irradiated field of view. 16 This shift can lead to exposure of patient's anatomy that is not imaged and, in some cases, pose a safety hazard to the staff in the cardiac catheterization laboratory. Therefore, field of view shift must be corrected in the clinic.…”

“…In weak magnetic fields ͑several mT or less͒, this correction can be accomplished by moving the blades of the x-ray collimator toward the center axis of the x-ray field by a distance greater than or equal to the shift. 16 The drawback to this approach is that field of view shift is corrected by a reduction in the size of the field of view. This is a problem in strong magnetic fields ͑tens of mT or stronger͒, which can shift the field of view by several centimeters or more.…”

“…This can be accomplished by using a larger detector and opening up the collimator blades to encompass the relevant anatomy. 16 The drawback of this approach is that anatomy that does not require imaging for the clinical task is exposed to radiation. It is possible to reposition the x-ray tube instead of opening up the collimator blades, but this adds additional mechanical complexity to the C-arm to facilitate repositioning.…”