In vitro experiments on 15 white matter samples from five bovine brains were performed on a 1 H-NMR spectrometer at 24°C and 37°C. The average myelin water fractions (MWFs) were 10.9% and 11.8% for samples at 24°C and 37°C, respectively. The T 1 relaxation time at 37°C was found to be 830 ms, exhibiting monoexponential behavior. A four-pool model including intra/extracellular (IE) water, myelin water, nonmyelin tissue, and myelin tissue was proposed to simulate the NMR behavior of bovine white matter. A cross-relaxation correction was introduced to compensate for shifting of the measured data points and T 2 times over the duration of the Carr-Purcell-Meiboom-Gill (CPMG) measurement due to cross relaxation. This correction was found to be slight, providing evidence that MWFs measured using a multiecho technique are near physical values. At 24°C the cross-relaxation times between myelin tissue and myelin water, myelin water and IE water, and IE water and nonmyelin tissue were found to be approximately 227, 2064, and 402 ms, respectively. At 37°C these same cross-relaxation times were 158, 1021, and 170 ms, respectively. The exchange rate between myelin water and myelin was found to be 11.8 s -1 at 37°C, while the exchange rate between IE water and nonmyelin tissue was found to be 6.8 s -1 . These exchange rates are of similar magnitude, which indicates that the interaction between IE water and nonmyelin tissue cannot be ignored. Magn Reson Med 54:1072-1081, 2005.

Purpose: To evaluate the reproducibility of multicomponent quantitative T 2 (QT2) measurements, in particular the myelin water fraction (MWF), to determine the sensitivity of this method for monitoring myelin changes in longitudinal studies and to provide a basis for correctly powering such studies. Materials and Methods:The de facto standard 32-echo spin-echo imaging sequence was used throughout, and data were analyzed using regularized non-negative least squares (NNLS) to produce T 2 distributions. Three studies were conducted in healthy subjects. First, two acquisition protocols were compared in 10 subjects. Second, variability of QT2 was evaluated over same-day scan-rescan experiments in 6 subjects. Finally, variability was quantified in a longitudinal study of 5 subjects.Results: A within-subject coefficient of variation (CoV) of 12% (range 4-25%) was observed for the MWF in brain white matter (WM) regions of interest (ROIs). The geometric mean T 2 was more stable, with a longitudinal CoV of 4% (range 1-6%). The choice of the geometry and repetition time of the acquisition protocol influenced the estimates of the MWF and T 2 values. The choice of integration range for the short-T 2 component had a significant effect on MWF estimates, but not on reproducibility. Conclusion:The reproducibility of QT2 measurements using existing methods is moderate and the method can be used in longitudinal studies, with careful consideration of the methodologic variability and an appropriate group size.Key Words: multicomponent T 2 ; myelin water fraction; reproducibility; scan-rescan; longitudinal; intra/extracellular T 2 ; regularized NNLS J. Magn. Reson. Imaging 2010; 32:60-68. V C 2010 Wiley-Liss, Inc.QUANTITATIVE MAGNETIC RESONANCE IMAGING (MRI) techniques sensitive to specific tissue constituents are becoming increasingly important, and therefore a technique that could reliably map myelin content and status in vivo would be of significant value. One proposed method of myelin imaging is based on multiexponential analysis of multiecho spin-echo data, often referred to as quantitative T 2 relaxometry (QT2). In these imaging experiments, the T 2 decay is sampled with at least 32 closely spaced echoes, usually with echo spacing around 10 msec. Data analysis is most often performed with a modified version of the non-negative least-squares (NNLS) fitting technique (1-3), which transforms the discretely sampled T 2 decay curve into a distribution of signal amplitudes as a function of T 2 . To deal with the presence of measurement noise and with fitting more T 2 values than available data points, the NNLS distribution can be regularized to yield a more continuous model and avoid spurious peaks (2). Regularization is commonly performed by including an energy or curvature term in the minimization cost function (4), with iterative adjustment of the weight of the regularization (5). Water in different microenvironments can be identified by distinct T 2 relaxation times, reflected in the T 2 distribution (3). In brain white matter (W...

The NMR behaviour of normal and psoriatic stratum corneum (SC) was investigated as a function of hydration with the aim of obtaining a better understanding of the role of water in the SC structure. Time domain NMR techniques were employed to identify the signal from water and that from nonaqueous components of the SC, such as lipids and proteins. The signals were investigated as a function of water content. The free induction decay was separated into mobile signal (from water and mobile lipids) and solid signal (from protein and 'solid' lipids). Spin-spin relaxation (T(2)) measurements further separated the mobile domains within the SC. The results suggested that, when water is added to dry SC, it first enters the corneocytes; then, at a hydration of 0.24-0.33 g H(2)O/g SC (normal SC) or 0.12-0.24 g H(2)O/g SC (psoriatic SC), water begins to accumulate in hydrated lipid regions. Water was found to exchange between these two domains on the time scale of a few hundred milliseconds. When compared with normal SC, psoriatic SC had a looser corneocyte structure, a larger mobile lipid component at low hydration and a smaller capacity for corneocyte water.