Local SAR compression with overestimation control to reduce maximum relative SAR overestimation and improve multi-channe
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RESEARCH ARTICLE
Local SAR compression with overestimation control to reduce maximum relative SAR overestimation and improve multi‑channel RF array performance Stephan Orzada1,2 · Thomas M. Fiedler3 · Andreas K. Bitz4 · Mark E. Ladd1,3,5,6 · Harald H. Quick1,2 Received: 23 April 2020 / Revised: 31 August 2020 / Accepted: 11 September 2020 © The Author(s) 2020
Abstract Purpose In local SAR compression algorithms, the overestimation is generally not linearly dependent on actual local SAR. This can lead to large relative overestimation at low actual SAR values, unnecessarily constraining transmit array performance. Method Two strategies are proposed to reduce maximum relative overestimation for a given number of VOPs. The first strategy uses an overestimation matrix that roughly approximates actual local SAR; the second strategy uses a small set of pre-calculated VOPs as the overestimation term for the compression. Result Comparison with a previous method shows that for a given maximum relative overestimation the number of VOPs can be reduced by around 20% at the cost of a higher absolute overestimation at high actual local SAR values. Conclusion The proposed strategies outperform a previously published strategy and can improve the SAR compression where maximum relative overestimation constrains the performance of parallel transmission. Keywords SAR · Vops · VOP compression · MRI · Local SAR
Introduction
* Stephan Orzada Stephan.orzada@uni‑due.de 1
Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Kokereiallee 7, 45141 Essen, Germany
2
High‑Field and Hybrid MR Imaging, University Hospital Essen, 45147 Essen, Germany
3
Medical Physics in Radiology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
4
Electromagnetic Theory and Applied Mathematics, Faculty of Electrical Engineering and Information Technology, FH Aachen, University of Applied Sciences, 52066 Aachen, Germany
5
Faculty of Physics and Astronomy, University of Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
6
Faculty of Medicine, University of Heidelberg, Im Neuenheimer Feld 672, 69120 Heidelberg, Germany
While single-channel and dual-channel transmit systems are still standard in clinical MRI systems, multi-channel parallel transmit (pTx) radiofrequency (RF) systems are often used at ultra-high field (UHF). Not only are these systems necessary to cope with the inhomogeneity introduced by the short wavelength of the RF signals [1, 2], these systems also offer more flexibility in excitation, especially at UHF [3, 4]. Examples of the techniques utilizing pTx systems are RF shimming [5, 6], kT-points [7], 2D spokes [8], 3D tailored radiofrequency pulses [9], Transmit SENSE [10, 11], and TIAMO [12]. Common among all these techniques is the use of arbitrary amplitudes and phases on the different transmit channels (excitation vector), whereby they differ in how rapidly the vectors are changed over time. Altering the excitation vector changes
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