System and SAR characterization in parallel RF transmission

The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E‐field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject‐dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel‐by‐channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction‐guided optimization, proactively managed. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

[1]  Gilbert Strang,et al.  Introduction to applied mathematics , 1988 .

[2]  Klaas P Pruessmann,et al.  Optimal design of multiple‐channel RF pulses under strict power and SAR constraints , 2010, Magnetic resonance in medicine.

[3]  D. Sodickson,et al.  Electrodynamic constraints on homogeneity and radiofrequency power deposition in multiple coil excitations , 2009, Magnetic resonance in medicine.

[4]  K. Pruessmann,et al.  B  1+ interferometry for the calibration of RF transmitter arrays , 2009, Magnetic resonance in medicine.

[5]  Alexis Amadon,et al.  Local SAR reduction in parallel excitation based on channel‐dependent Tikhonov parameters , 2010, Journal of magnetic resonance imaging : JMRI.

[6]  M. Zanchi,et al.  RF Sensor Considerations for Input Predistortion Correction of Transmit Arrays , 2009 .

[7]  I. Graesslin,et al.  Real-time SAR Monitoring to ensure Patient Safety for Parallel Transmission Systems , 2007 .

[8]  Daniel K. Sodickson,et al.  Electrodynamic constraints on minimum SAR in parallel excitation , 2007 .

[9]  V. K. Goyal,et al.  PULSES WITH OPTIMAL SPECIFIC ABSORPTION RATE ( SAR ) CHARACTERISTICS AND EXPLORING EXCITATION FIDELITY , SAR AND PULSE DURATION TRADEOFFS , 2007 .

[10]  Yudong Zhu,et al.  Parallel excitation with an array of transmit coils , 2004, Magnetic resonance in medicine.

[11]  Douglas C Noll,et al.  Spatial domain method for the design of RF pulses in multicoil parallel excitation , 2006, Magnetic resonance in medicine.

[12]  Y. Zhu,et al.  Real Time RF Power Prediction of Parallel Transmission RF Pulse Design at 7 T , 2009 .

[13]  J W Carlson,et al.  Electromagnetic fields of surface coil in vivo NMR at high frequencies , 1991, Magnetic resonance in medicine.

[14]  Michael B. Smith,et al.  SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil , 1998, Magnetic resonance in medicine.

[15]  P. Börnert,et al.  A Minimum SAR RF Pulse Design Approach for Parallel Tx with Local Hot Spot Suppression and Exact Fidelity Constraint , 2007 .

[16]  Peter Börnert,et al.  Eigenmode analysis of transmit coil array for tailored B1 mapping , 2010, Magnetic resonance in medicine.

[17]  Y. Zhu.  RF Power Reduction with Parallel Excitation , 2002 .

[18]  Ferdinand Schweser,et al.  SAR in Parallel Transmission , 2008 .

[19]  P. Börnert,et al.  Transmit SENSE , 2003, Magnetic resonance in medicine.

[20]  E. Adalsteinsson,et al.  Magnitude least squares optimization for parallel radio frequency excitation design demonstrated at 7 Tesla with eight channels , 2008, Magnetic resonance in medicine.

[21]  P. Börnert,et al.  SAR Hotspot Reduction by Temporal Averaging in Parallel Transmission , 2008 .

[22]  I. Graesslin,et al.  Comprehensive RF Safety Concept for Parallel Transmission Systems , 2007 .

[23]  A. Macovski,et al.  Variable-rate selective excitation , 1988 .

[24]  Y. Zhu.  In Vivo RF Power and SAR Calibration for Multi-Port RF Transmission , 2008 .

[25]  Y. Zhu,et al.  Electrodynamic constraints on homogeneity and RF power deposition in multiple coil excitations , 2007 .