Purpose: To improve the myelin water quantification in the brain in the presence of measurement noise and to increase the visibility of small focal lesions in myelin-waterfraction (MWF) maps.
Materials and Methods:A spatially regularized non-negative least squares (srNNLS) algorithm was developed for robust myelin water quantification in the brain. The regularization for the conventional NNLS algorithm was expanded into the spatial domain in addition to the spectral domain. Synthetic data simulations were performed to study the effectiveness of this new algorithm. Experimental free-induction-decay measurements were obtained using a multi-gradient-echo pulse sequence and MWF maps were estimated using the srNNLS algorithm. The results were compared with other conventional methods.Results: A substantial decrease in MWF variability was observed in both simulations and experimental data when the srNNLS algorithm was applied. As a result, false lesions in the MWF maps disappeared and the visibility of small focal lesions improved greatly. On average, the contrast-tonoise ratio for focal lesions was improved by a factor of 2.
Conclusion:The MWF variability due to the measurement noise can be substantially reduced and the detection of small focal lesions can be improved by using the srNNLS algorithm.Key Words: myelin content; myelin water fraction; multiple sclerosis; non-negative least squares; regularization; multiexponential analysis. QUANTITATIVE MEASUREMENTS of myelin content can substantially improve our understanding of the pathological progress of several white matter (WM) diseases, such as multiple sclerosis (MS) (1-4). A technique that provides specific and sensitive information about the myelin content was developed based on an analysis of T 2 relaxation times (5,6). It has been reported that the T 2 spectrum of WM and several myelinated tissue samples consists of multiple components (7-13) and that the short component with T 2 between 10 ms and 50 ms corresponds to the water pool within the myelin sheath (2-6). Myelin content can therefore be quantitated using the fraction of this short T 2 component. To implement this method, T 2 decay signals are acquired using a 32-echo single-slice Carr-Purcell-Meiboom-Gill (CPMG) sequence with composite 90x-180y-90x refocusing pulses and big crusher gradients around the refocusing pulses (5,6,14,15). A non-negative least squares (NNLS) algorithm can then be used to estimate the T 2 spectrum from the acquired decay signal, and the myelin water fraction (MWF) can be calculated from the ratio of the short T 2 component (10 ms Ͻ T 2 Ͻ 50 ms) to the total (16). A strong correlation was found between the MWF measured using this technique and the myelin distribution using histopathology in fixed brains (4,17). This technique has been used to quantitatively measure the MWF in brains of subjects with MS (3-6,17).T 2 spectrum regularization has been applied to the NNLS algorithm to improve the reliability of fitting in the presence of noise. The regularized non-negative least squ...