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TEM Fields and SAR

Published in 2011

  1. Jinfeng Tian

Published Online: 15 MAR 2011

DOI: 10.1002/9780470034590.emrstm1128

eMagRes

eMagRes

How to Cite

Tian, J. 2011. TEM Fields and SAR. eMagRes. .

Author Information

  1. University of Minnesota, Minneapolis, Minnesota, USA

Publication History

  1. Published Online: 15 MAR 2011

Abstract

RF coils for high-frequency magnetic resonance imaging (MRI) are difficult to design, owing to the shortened wavelength and increased power losses. RF coils with transmission line elements employ distributed circuit theory, support transverse electromagnetic (TEM) or quasi-TEM modes, and are the most widely used RF coils for high-frequency MRI ( ≥ 128 MHz).

Basic TEM coil structures are introduced at the beginning of this article to give readers an idea of what a TEM coil looks like, followed by B1 fields of head and body TEM coils. The article concludes with a discussion of specific absorption rate (SAR) within head and body TEM coils at 170 and 300 MHz.

For head TEM coils, B1 distributions over central transverse, sagittal, and coronal slices at 170 and 300 MHz are presented. “Center brightness,” a commonly seen B1 phenomenon within the human head at 300 MHz or above, is discussed. B1 distribution within the human body with a TEM body coil is much less uniform and results in severe image artifacts. At least one sharp line of low-level B1 is predicted to run longitudinally through the body at 300 MHz. Calculations of SAR show that the maximum permitted SAR will be reached in 1 g and 10 g volumes before the limit for the globally averaged SAR is breached.

Keywords:

  • high-frequency MRI;
  • RF coil;
  • TEM;
  • B1;
  • center brightness;
  • SAR