Slice-Selective Transmit Array Pulses for Improvement in Excitation Uniformity and Reduction of SAR

BuSik Park, Kyunghyun Sung, John McGarrity, Suk-hoon Oh, Zhipeng Cao, Zhangwei Wang, Christopher M. Collins

1Department of Bioengineering, The Pennsylvania State University College of Medicine, Hershey, USA.
2Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, USA.
3Department of Radiology, Stanford University, Stanford, USA.
Department of Bioengineering, The Pennsylvania State University College of Medicine, Hershey, USA.
GE Healthcare, Aurora, USA.

DOI: 10.4236/jemaa.2013.55033                

Abstract

To overcome challenges of inhomogeneous transmit B₁ distribution and high specific energy absorption rate (SAR) in MRI, we compare slice-selective array-optimized composite pulse and RF shimming designed to both improve B₁ uniformity and reduce SAR using an 8-channel transmit head array loaded with a head model at various RF pulse excitation times, and compare results with standard quadrature voltage distribution at 3T (128 MHz) and 7T (300 MHz). The excitation uniformity was estimated throughout the 3D brain region and SAR was calculated for the whole head. The optimized composite pulse could produce significantly better homogeneity and significantly better homogeneity when SAR was not constrained, and both significantly better homogeneity and lower SAR when the pulse duration was allowed to be twice that of the quadrature or RF shimmed pulse. When the total pulse durations were constrained to the same length, the relative advantages of the optimized composite pulse for producing better homogeneity and lower SAR simultaneously were diminished. Using the optimization results, the slice-selective composite pulse sequence was implemented on a 3D MRI simulator currently under development, and showed both effective slice selection and improvement in excitation uniformity compared to a conventional quadrature driving method.

Keywords

MRI; Composite Pulse; RF Shimming; SAR; MRI Simulator

Conflicts of Interest

The authors declare no conflicts of interest.

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