Correcting inter-scan motion artefacts in quantitative R1 mapping at 7T

TL;DR

This paper introduces two new methods for correcting inter-scan motion artefacts in R1 mapping at 7T, which do not require a body coil reference and outperform existing correction schemes, especially at high field strengths.

Contribution

The authors propose two novel correction techniques using coil sensitivities that are effective at 7T without needing body coil images, improving R1 map accuracy.

Findings

Proposed methods outperform baseline at 3T.

Artefact reduction achieved at 7T.

Reproducibility improved with position-specific transmit field correction.

Abstract

Purpose: Inter-scan motion is a substantial source of error in $R_1$ estimation, and can be expected to increase at 7T where $B_1$ fields are more inhomogeneous. The established correction scheme does not translate to 7T since it requires a body coil reference. Here we introduce two alternatives that outperform the established method. Since they compute relative sensitivities they do not require body coil images. Theory: The proposed methods use coil-combined magnitude images to obtain the relative coil sensitivities. The first method efficiently computes the relative sensitivities via a simple ratio; the second by fitting a more sophisticated generative model. Methods: $R_1$ maps were computed using the variable flip angle (VFA) approach. Multiple datasets were acquired at 3T and 7T, with and without motion between the acquisition of the VFA volumes. $R_1$ maps were constructed without correction, with the proposed corrections, and (at 3T) with the previously established correction scheme. Results: At 3T, the proposed methods outperform the baseline method. Inter-scan motion artefacts were also reduced at 7T. However, reproducibility only converged on that of the no motion condition if position-specific transmit field effects were also incorporated. Conclusion: The proposed methods simplify inter-scan motion correction of $R_1$ maps and are applicable at both 3T and 7T, where a body coil is typically not available. The open-source code for all methods is made publicly available.