Impact of number of channels on RF shimming at 3T

Alexander S. Childs,Declan P. O’Regan,Joseph V. Hajnal,Shaihan J. Malik

doi:10.1007/s10334-012-0360-5

Abstract

ObjectAt high-field strengths (≥3T) inhomogeneity of the radio frequency (RF) field and RF power deposition become increasingly problematic. Parallel Transmission (PTx)—the use of segmented transmission arrays with independently driven elements—affords the ability to combat both of these issues. There are a variety of existing designs for PTx coils, ranging from systems with two channels to systems with eight or more. In this work, we have investigated the impact of the number of independent channels on the achievable results for both homogeneity improvement and power reduction in vivo.Materials and methodsA 3T Philips Achieva MRI system fitted with an 8-channel PTx body coil was driven so as to emulate configurations with 1, 2 4 and 8 independent channels. RF shimming was used in two different anatomies in order to assess improvements in RF homogeneity.ResultsSignificant homogeneity improvements were observed when increasing from 1 to 2, 2 to 4, and 4 to 8 channel configurations. Reductions in RF power requirements and local SAR were predicted for increasing numbers of channels.ConclusionIncreasing the number of RF transmit channels adds extra degrees of freedom which can be used to benefit homogeneity improvement or power reduction for body imaging at 3T.

Highlights

Object At high-field strengths (C3T) inhomogeneity of the radio frequency (RF) field and RF power deposition become increasingly problematic
Reductions in RF power requirements and local SAR were predicted for increasing numbers of channels
At high field strengths (3T and greater), the excitation radio-frequency (RF) wavelength becomes comparable to the size of the human body, causing strong coil–tissue interactions which are subject-specific, depending on geometry and tissue electrical properties

Summary

Introduction

Object At high-field strengths (C3T) inhomogeneity of the radio frequency (RF) field and RF power deposition become increasingly problematic. Parallel Transmission (PTx)—the use of segmented transmission arrays with independently driven elements—affords the ability to combat both of these issues. There are a variety of existing designs for PTx coils, ranging from systems with two channels to systems with eight or more. We have investigated the impact of the number of independent channels on the achievable results for both homogeneity improvement and power reduction in vivo. Materials and methods A 3T Philips Achieva MRI system fitted with an 8-channel PTx body coil was driven so as to emulate configurations with 1, 2 4 and 8 independent channels. RF shimming was used in two different anatomies in order to assess improvements in RF homogeneity

Methods

Results

Discussion

Conclusion