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Electromagnetic fields of surface coil in vivo NMR at high frequencies†
Article first published online: 18 NOV 2005
Copyright © 1991 Wiley-Liss, Inc., A Wiley Company
Magnetic Resonance in Medicine
Volume 22, Issue 2, pages 467–480, December 1991
How to Cite
Keltner, J. R., Carlson, J. W., Roos, M. S., Wong, S. T. S., Wong, T. L. and Budinger, T. F. (1991), Electromagnetic fields of surface coil in vivo NMR at high frequencies. Magn Reson Med, 22: 467–480. doi: 10.1002/mrm.1910220254
- Issue published online: 18 NOV 2005
- Article first published online: 18 NOV 2005
- Manuscript Revised: 27 NOV 1990
- Manuscript Received: 9 AUG 1990
- Director, Office of Energy Research, of the U.S. Department of Energy. Grant Number: DE-AC03-76SF00098
- National Heart, Lung and Blood Institute. Grant Numbers: HL 25840, HL 07367
- International Machines Corporation
A high frequency solution of the electromagnetic field produced by a circular surface coil adjacend to a homogenous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo1H NMR spec troscopy experiments from 200 to 430 MHz (4.7–10T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experments. The power radited by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axix and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verfy the presence of dielectric resonances at frequencies above 200 MHz. © 1991 Academic Press, Inc.