Noise robust spatially regularized myelin water fraction mapping with the intrinsic B1-error correction based on the linearized version of the extended phase graph model.

PURPOSE

To improve the quantification accuracy of transverse relaxometry by accounting for B1 -error, after minimizing slice profile imperfections.

MATERIALS AND METHODS

The slice profile of refocusing pulses was optimized by setting refocusing slice thicknesses three times that of the excitation pulse. The first step of data processing combined the L-curve approach with the linearized version of the extended phase graph model to jointly estimate the temporal regularization constant map and the flip angle error (FAE)-map. The second step improved the noise robustness of the reconstruction by imposing a spatial smoothness constraint on T2 -distributions. The proposed method (spatial-regularization-with-FAE-correction) was evaluated against methods without FAE-correction (conventional-regularization-without-FAE-correction, spatial-regularization-without-FAE-correction) and conventional-regularization-with-FAE-correction using relevant statistics (simulated data: mean square myelin reconstruction error [MSMRE] and averaged-symmetric-Kullbeck-Leibler score [SKL] between returned distributions and ground truths; experimental data: median of mean square error [MMSE] of fitting across entire data-set and coefficient of variation [COV] in white-matter [WM] regions of interest [ROIs]).

RESULTS

In simulation, our method resulted in reduced MSMRE (at signal-to-noise ratio [SNR] = 200: MSMRESpatial-regularization-without-FAEC  = 0.057; MSMRESpatial-regularization-with-FAEC  = 0.0107) and reduced SKL scores (at SNR = 200: SKLSpatial-regularization-without-FAEC  = 0.061; SKLSpatial-regularization-with-FAEC  = 0.0143). In human volunteers, our method yielded a reduced MSE of fitting (MMSESpatial-regularization-without-FAEC  = (2.26 ± 0.60) × 10(-3) ; MMSESpatial-regularization-with-FAEC  = (1.57 ± 0.44) × 10(-4) )and also resulted in reduced COV (COVSpatial-regularization-without-FAEC  = 0.08-0.19; COVSpatial-regularization-with-FAEC  = 0.09-0.12). In a water-phantom, a good correlation between the absolute value of measured B1 -map and FAE-map was found (regression analysis: slope = 1.04; R(2)  = 0.66).

CONCLUSION

The proposed method resulted in more accurate and noise robust myelin water fraction maps with improved depiction of subcortical WM structures.