Accelerated calibrationless parallel transmit mapping using joint transmit and receive low-rank tensor completion.

Aaron T Hess,Iulius Dragonu,Mark Chiew

doi:10.1002/mrm.28880

Abstract

PurposeTo evaluate an algorithm for calibrationless parallel imaging to reconstruct undersampled parallel transmit field maps for the body and brain.MethodsUsing a combination of synthetic data and in vivo measurements from brain and body, 3 different approaches to a joint transmit and receive low-rank tensor completion algorithm are evaluated. These methods included: 1) virtual coils using the product of receive and transmit sensitivities, 2) joint-receiver coils that enforces a low rank structure across receive coils of all transmit modes, and 3) transmit low rank that uses a low rank structure for both receive and transmit modes simultaneously. The performance of each is investigated for different noise levels and different acceleration rates on an 8-channel parallel transmit 7 Tesla system.ResultsThe virtual coils method broke down with increasing noise levels or acceleration rates greater than 2, producing normalized RMS error greater than 0.1. The joint receiver coils method worked well up to acceleration factors of 4, beyond which the normalized RMS error exceeded 0.1. Transmit low rank enabled an eightfold acceleration, with most normalized RMS errors remaining below 0.1.ConclusionThis work demonstrates that undersampling factors of up to eightfold are feasible for transmit array mapping and can be reconstructed using calibrationless parallel imaging methods.

Highlights

Parallel transmit technology mitigates transmit field heterogeneity[1,2], accelerates spatial RF pulses[3,4], and lowers SAR deposition[5,6,7], which is achieved by driving multiple transmit coils with subject, target and pulse specific amplitudes and phases
To map parallel transmit fields both absolute maps of B1+ and relative magnitude and phase can be measured
In this work we investigate the extension of these calibrationless parallel imaging approaches to recover undersampled relative transmit field maps using a structured low rank tensor completion

Summary

Introduction

Parallel transmit (pTx) technology mitigates transmit field heterogeneity[1,2], accelerates spatial RF pulses[3,4], and lowers SAR deposition[5,6,7], which is achieved by driving multiple (parallel) transmit coils with subject, target and pulse specific amplitudes and phases. RF pulses are designed by optimising a cost function using measured transmit fields. This work investigates the acceleration of pTx field mapping, with the goal of streamlining the use of pTx. Universal pulses[8] have been demonstrated as an effective way of streamlining the use of pTx, for specific targets such as single voxel spectroscopy or anatomy that cannot be generalised including the heart, liver, prostate and spinal cord, subject specific field mapping with personalised RF pulse design is still required. To map parallel transmit fields both absolute maps of B1+ and relative magnitude and phase can be measured. Relative maps are measured by transmitting in different transmit configurations, usually transmitting on one channel at a time while receiving on all receive channels all the time

Methods

Results

Conclusion