Bias field correction of MPRAGE by an external reference -- The poor man's MP2RAGE

TL;DR

This paper presents a simple, cost-effective method for bias field correction of MPRAGE images at 7T using an external reference, enabling semi-quantitative T1 mapping comparable to MP2RAGE with reduced scan time.

Contribution

It introduces a non-interleaved normalization approach for MPRAGE that simplifies bias correction and T1 mapping, reducing scan time while maintaining image quality.

Findings

Optimized flip angle at 3° for best contrast and SNR.

Reduced acquisition time by increasing reference voxel size with negligible bias.

Normalized WM segmentation variability decreased from 24% to 9%.

Abstract

Purpose: To implement and evaluate a sequential approach to obtain semi-quantitative T1-weighted MPRAGE images, unbiased by B1 inhomogeneities at 7T. Methods: In the reference gradient echo used for normalization of the MPRAGE image, flip angle (aGE) and acquisition voxel size (Vref) was varied to optimize tissue contrast and acquisition time (Tacq). The finalized protocol was implemented at three different resolutions and the reproducibility was evaluated. Maps of T1 were derived based on the normalized MPRAGE through forward signal modelling. Results: A good compromise between tissue contrast and SNR was reached at aGE=3{\deg}. A reduction of the reference GE Tacq by a factor of 4, at the cost of negligible bias, was obtained by increasing Vref with a factor of 8 relative the MPRAGE resolution. The coefficient-of-variation in segmented WM was 9+/-5% after normalization, compared to 24+/-12% before. The T1 maps showed no obvious bias and had reasonable values with regard to literature, especially after optional B1 correction through separate flip angle mapping. Conclusion: A non-interleaved acquisition for normalization of MPRAGE offers a simple alternative to MP2RAGE to obtain semi-quantitative purely T1-weighted images. These images can be converted to T1 maps analogously to the established MP2RAGE approach. Scan time can be reduced by increasing Vref which has a miniscule effect on image quality.