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Title: 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 ({approx_equal}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 notmore » 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.« less

Authors:
; ; ; ; ;  [1];  [2];  [3]
  1. Department of Medical Biophysics and Sunnybrook Health Sciences Center, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5 (Canada)
  2. (United States)
  3. (Canada)
Publication Date:
OSTI Identifier:
22100505
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 36; Journal Issue: 5; Other Information: (c) 2009 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; BLOOD VESSELS; COOLING SYSTEMS; ELECTRIC CURRENTS; ELECTRON BEAMS; MAGNETIC FIELDS; MAGNETIC SHIELDING; NMR IMAGING; PATIENTS; SAFETY; SURGERY; X RADIATION; X-RAY TUBES

Citation Formats

Bracken, John A., DeCrescenzo, Giovanni, Komljenovic, Philip, Lillaney, Prasheel V., Fahrig, Rebecca, Rowlands, J. A., Department of Radiology, Stanford University, Stanford, California 94305, and Department of Medical Biophysics and Sunnybrook Health Sciences Center, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5. Closed bore XMR (CBXMR) systems for aortic valve replacement: Active magnetic shielding of x-ray tubes. United States: N. p., 2009. Web. doi:10.1118/1.3116363.
Bracken, John A., DeCrescenzo, Giovanni, Komljenovic, Philip, Lillaney, Prasheel V., Fahrig, Rebecca, Rowlands, J. A., Department of Radiology, Stanford University, Stanford, California 94305, & Department of Medical Biophysics and Sunnybrook Health Sciences Center, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5. Closed bore XMR (CBXMR) systems for aortic valve replacement: Active magnetic shielding of x-ray tubes. United States. doi:10.1118/1.3116363.
Bracken, John A., DeCrescenzo, Giovanni, Komljenovic, Philip, Lillaney, Prasheel V., Fahrig, Rebecca, Rowlands, J. A., Department of Radiology, Stanford University, Stanford, California 94305, and Department of Medical Biophysics and Sunnybrook Health Sciences Center, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5. 2009. "Closed bore XMR (CBXMR) systems for aortic valve replacement: Active magnetic shielding of x-ray tubes". United States. doi:10.1118/1.3116363.
@article{osti_22100505,
title = {Closed bore XMR (CBXMR) systems for aortic valve replacement: Active magnetic shielding of x-ray tubes},
author = {Bracken, John A. and DeCrescenzo, Giovanni and Komljenovic, Philip and Lillaney, Prasheel V. and Fahrig, Rebecca and Rowlands, J. A. and Department of Radiology, Stanford University, Stanford, California 94305 and Department of Medical Biophysics and Sunnybrook Health Sciences Center, University of Toronto, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5},
abstractNote = {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 ({approx_equal}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.},
doi = {10.1118/1.3116363},
journal = {Medical Physics},
number = 5,
volume = 36,
place = {United States},
year = 2009,
month = 5
}
  • In order to improve the safety and efficacy of percutaneous aortic valve replacement procedures, a closed bore hybrid x-ray/MRI (CBXMR) system is proposed in which an x-ray C-arm will be positioned with its isocenter {approx_equal}1 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 externalmore » 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{sub anode} was unaffected when the external magnetic field B{sub b} was parallel to the axis of rotation of the anode but decreased when B{sub b} was perpendicular to the axis of rotation. The experimental f{sub anode} values agreed with predicted values to within {+-}3% over a B{sub b} range of 0-30 mT. The MRI fringe field at the proposed location of the x-ray tube mounted on the C-arm ({approx_equal}4 mT) reduced f{sub 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.« less
  • 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 tubemore » 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.« less
  • A next-generation interventional guidance system is proposed that will enable intraprocedural access to both x-ray and magnetic resonance imaging (MRI) modalities. This closed bore XMR (CBXMR) system is comprised of a conventional radiographic rotating anode x-ray tube and a direct conversion flat panel detector on a rotating gantry positioned adjacent to the bore of a 1.5 T MRI. To assess the feasibility of such a system, we have investigated the degree of compatibility between the x-ray components and the MRI. For vertical bar B-vector{sub ext} vertical bar <200 G the effect on the radiographic tube motor was negligible regardless ofmore » the orientation of B-vector{sub ext} with respect to the motor axis of rotation--the frequency of anode rotation remained within 6% of the 3400 rpm frequency measured at 0 G. For vertical bar B-vector{sub ext} vertical bar >2400 G the anode slowed down to below 2400 rpm at all orientations. At intermediate B-vector{sub ext}, the frequency of rotation varied between 2400 and 3200 rpm, showing a strong dependence on orientation, with B-vector{sub ext} perpendicular to the tube axis having a much stronger effect on the rotation frequency than B-vector{sub ext} parallel to the tube axis. In contrast to the effect of B-vector{sub ext} on the induction motor, parallel B-vector{sub ext} had a stronger detrimental effect on the cathode-anode electron beam, whose path was at 16 deg. to the tube axis, than the perpendicular B-vector{sub ext}. Parallel B-vector{sub ext} of several hundred Gauss had a defocusing effect on the x-ray focal spot. B-vector{sub ext} perpendicular to the electron beam shifted the beam without significant defocusing. We have determined that the direct conversion flat panel detector (FPD) technology is not intrinsically sensitive to B(vector sign){sub ext}, and that the modifications required to make the proposed FPDs MRI compatible are minimal. The homogeneity of the MRI signal in the normal field of view was not significantly degraded by the presence of these x-ray components in the vicinity of the MRI bore entrance.« less
  • Congestive heart failure in patients surviving aortic valve replacement has been associated with a high late mortality. To determine whether myocardial dysfunction in these patients occurred preoperatively, perioperatively, or during the early postoperative period, 19 consecutive patients undergoing aortic valve replacement using cardioplegia and hypothermia were studied by multiple-gated cardiac blood pool imaging. The resting ejection fractions for 8 patients with aortic stenosis did not show significant changes following operation. The 11 patients with aortic insufficiency has resting preoperative values of 58 +/- 15%, which fell to 38 +/- 18% immediately postoperatively (p less than 0.01), with the late valuesmore » being 51 +/- 16%. Eight of 18 patients (44%) showed deterioration of regional wall motion immediately after operation, which persisted in 3 during the late evaluation. The occurrence of new perioperative regional wall motion abnormalities and persistent perioperative depression in left ventricular function in some patients suggest the need for further improvement in myocardial protection during cardiopulmonary bypass for aortic valve replacement.« less