Interventional MRI: tapering improves the distal sensitivity of the loopless antenna

Date
2010
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Source Title
Magnetic Resonance in Medicine
Print ISSN
0740-3194
Electronic ISSN
Publisher
Wiley
Volume
63
Issue
3
Pages
797 - 802
Language
English
Type
Article
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Abstract

The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an ∼400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-tonoise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.

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Keywords
Interventional MRI, Intravascular MRI, Loopless antenna, MRI detectors, Signal-to-noise ratio, accuracy, aorta, Dielectric constant, Electromagnetic field, Surgery, Computer-Assisted
Citation
Published Version (Please cite this version)