The performance of interventional loopless MRI antennae at higher magnetic field strengths

Authors

  • El-Sharkawy AbdEl-Monem M.,

    1. Division of MR Research, Department of Radiology, and Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21287
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  • Qian Di,

    1. Division of MR Research, Johns Hopkins University, Baltimore, Maryland 21287
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  • Bottomley Paul A.

    1. Division of MR Research, Department of Radiology, and Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21287
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    • a)

      Author to whom correspondence should be addressed. Present address: Division of MR Research, Department of Radiology, Johns Hopkins University, 601 N. Caroline St, Baltimore, MD 21287-0843. Telephone: 410-955-0366. Electronic mail: bottoml@mri.jhu.edu


Abstract

Interventional, “loopless antenna” MRI detectors are currently limited to 1.5T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7T, immersed in physiologically comparable saline. Gains of 3.8±0.2- and 9.7±0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7T, respectively, compared to 1.5T. Heat testing of a 3T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of 1°C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3T antenna versus an equivalent commercial 1.5T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

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