Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate, tissue temperature, and thermal dose

Thomas M. Fiedler, Stephan Orzada, Martina Flöser, Stefan H.G. Rietsch, Simon Schmidt, Jonathan K. Stelter, Marco Wittrich, Harald H. Quick, Andreas K. Bitz, Mark E. Ladd

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

Abstract

In this study, the performance of an integrated body-imaging array for 7 T with 32 radiofrequency (RF) channels under consideration of local specific absorption rate (SAR), tissue temperature, and thermal dose limits was evaluated and the imaging performance was compared with a clinical 3 T body coil. Thirty-two transmit elements were placed in three rings between the bore liner and RF shield of the gradient coil. Slice-selective RF pulse optimizations for B1 shimming and spokes were performed for differently oriented slices in the body under consideration of realistic constraints for power and local SAR. To improve the B1+ homogeneity, safety assessments based on temperature and thermal dose were performed to possibly allow for higher input power for the pulse optimization than permissible with SAR limits. The results showed that using two spokes, the 7 T array outperformed the 3 T birdcage in all the considered regions of interest. However, a significantly higher SAR or lower duty cycle at 7 T is necessary in some cases to achieve similar B1+ homogeneity as at 3 T. The homogeneity in up to 50 cm-long coronal slices can particularly benefit from the high RF shim performance provided by the 32 RF channels. The thermal dose approach increases the allowable input power and the corresponding local SAR, in one example up to 100 W/kg, without limiting the exposure time necessary for an MR examination. In conclusion, the integrated antenna array at 7 T enables a clinical workflow for body imaging and comparable imaging performance to a conventional 3 T clinical body coil.

Original languageEnglish (US)
Article numbere4656
JournalNMR in biomedicine
Volume35
Issue number5
DOIs
StatePublished - May 2022
Externally publishedYes

Bibliographical note

Funding Information:
We thank REINHAUSEN POWER COMPOSITES (Regensburg, Germany) for providing a sample of the bore liner material. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007‐2013)/ERC Grant Agreement no. 291903 MRexcite.

Publisher Copyright:
© 2021 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Keywords

  • body imaging at 7 T MRI
  • thermal dose
  • tissue temperature
  • transmit antenna arrays

PubMed: MeSH publication types

  • Journal Article
  • Research Support, Non-U.S. Gov't

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