Closed bore XMR (CBXMR) systems for aortic valve replacement: Investigation of rotating‐anode x‐ray tube heat loadability

Abstract

In order to improve the safety and efficacy of percutaneous aortic valve replacement procedures, a closed bore hybrid x-ray/MRI ͑CBXMR͒ system is proposed in which an x-ray C-arm will be positioned with its isocenter Ϸ1 m from the entrance of a clinical MRI scanner. This system will harness the complementary strengths of both modalities to improve clinical outcome. A key component of the CBXMR system will be a rotating anode x-ray tube to produce high-quality x-ray images. There are challenges in positioning an x-ray tube in the magnetic fringe field of the MRI magnet. Here, the effects of an external magnetic field on x-ray tube induction motors of radiography x-ray tubes and the corresponding reduction of x-ray tube heat loadability are investigated. Anode rotation frequency f anode was unaffected when the external magnetic field B b was parallel to the axis of rotation of the anode but decreased when B b was perpendicular to the axis of rotation. The experimental f anode values agreed with predicted values to within Ϯ3% over a B b range of 0 -30 mT. The MRI fringe field at the proposed location of the x-ray tube mounted on the C-arm ͑Ϸ4 mT͒ reduced f anode by only 1%, so x-ray tube heat loadability will not be compromised when using CBXMR systems for percutaneous aortic valve replacement procedures. Eddy current heating power in the rotor due to an MRI fringe field was found to be two orders of magnitude weaker than the heating power produced on the anode due to a fluoroscopic exposure, so eddy current heating had no effect on x-ray tube heat loadability.

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

“…An air-cored electromagnet ͑Stangenes Industries, Palo Alto, CA͒ was used to produce B and is described in detail elsewhere. 12 The electromagnet was able to produce a maximum B of 23 mT at its center along its central axis and field uniformity was within Ϯ5% over a cubic region with 12 cm sides around its center. This magnet was able to provide a magnetic field component perpendicular to the electron beam of comparable strength to the fringe field components B r and B z of the MRI scanner, both of which can cause electron beam deflection depending on x-ray tube orientation.…”

“…11͒ in which a C-arm x-ray system is placed close to ͑Ϸ1 m͒ the entrance of a 1.5 T MRI scanner. 12 This system will harness the complementary strengths of both modalities by providing high quality fluoroscopy and angiography using x-ray imaging and cardiac anatomical imaging using MRI without the risks associated with patient transfers over long distances ͑several meters͒. 12 The challenges in the construction of this system have been outlined in previous publica-tions, most of which have been overcome.…”

“…12 This system will harness the complementary strengths of both modalities by providing high quality fluoroscopy and angiography using x-ray imaging and cardiac anatomical imaging using MRI without the risks associated with patient transfers over long distances ͑several meters͒. 12 The challenges in the construction of this system have been outlined in previous publica-tions, most of which have been overcome. [11][12][13][14][15] The remaining challenge is to overcome x-ray tube electron beam deflection in a magnetic field perpendicular to the electron beam.…”

“…12 The challenges in the construction of this system have been outlined in previous publica-tions, most of which have been overcome. [11][12][13][14][15] The remaining challenge is to overcome x-ray tube electron beam deflection in a magnetic field perpendicular to the electron beam.…”

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

“…A flat-panel detector and a rotating-anode x-ray tube are mounted on the C-arm. 11 Previous work showed that the electron beam in the x-ray tube used to produce x rays 12 at the focal spot on the anode can be deflected 13,14 by the magnetic fringe field of the MRI scanner. 15 The electron beam deflection causes a shift in the irradiated field of view.…”