Multi-Exponential Relaxometry Using $\ell_{{1}}$ -Regularized Iterative NNLS (MERLIN) With Application to Myelin Water Fraction Imaging
Markus Zimmermann, Ana‐Maria Oros‐Peusquens, Elene Iordanishvili, Seonyeong Shin, Seong Dae Yun, Zaheer Abbas, Nadim Jon Shah · IEEE Transactions on Medical Imaging · 2019
A new parameter estimation algorithm, MERLIN, is presented for accurate and robust multiexponential relaxometry using magnetic resonance imaging, a tool that can provide valuable insight into the tissue microstructure of the brain. Multi-exponential relaxometry is used to analyze the myelin water fraction and can help to detect related diseases. However, the underlying problem is ill-conditioned, and as such, is extremely sensitive to noise and measurement imperfections, which can lead to less precise and more biased parameter estimates. MERLIN is a fully automated, multi-voxel approach that incorporates state-of-the-art ℓ1-regularization to enforce sparsity and spatial consistency of the estimated distributions. The proposed method is validated in simulations and in vivo experiments, using a multi-echo gradient-echo (MEGE) sequence at 3 T. MERLIN is compared to the conventional single-voxel ℓ2-regularized NNLS (rNNLS) and a multivoxel extension with spatial priors (rNNLS + SP), where it consistently showed lower root mean squared errors of up to 70 percent for all parameters of interest in these simulations.