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Water exchange can play an important role in interpreting compartment-specific magnetic resonance imaging data in brain. For example, an MR method of myelin measurement, known as myelin water fraction imaging, assumes that water exchange processes are slow compared with the measurement time scale. In this article, we examined whether water exchange processes have an effect on myelin water fraction values. A previously established four pool model of white matter was used to simulate the interactions between two aqueous compartments (myelin water and intra/extracellular water) and nonaqueous compartments (myelin and nonmyelin tissues). To extract the water exchange cross relaxation times, the Bloch equations were solved analytically. As the water exchange time scales are dependent on the spin-lattice T(1) relaxation of each of these four pools and due to the current uncertainties regarding the T(1) associated with each pool, exchange cross relaxation times for three different T(1) scenarios were calculated. The corrections that need to be considered in order for myelin water fraction to be an accurate marker for myelin were found to be less than 15%. This work indicates that regional variations in white matter myelin water fraction values are most likely due to variations in myelin content rather than regional differences in exchange rates., (Copyright © 2011 Wiley-Liss, Inc.)

T2 was used in this study to assess tendon microstructure. Two unloaded digital extensor tendons were bent such that their long axes were imaged throughout 180° with respect to B0. T2-weighted images reveal periodic banding (∼200 μm) when tendons were oriented at ±55° with respect to B0. Five pairs of tendons were used to study the influence of load on T2W MRI: one tendon of each pair was loaded with a 7.8-N mass, and both tendons were fixed in formalin then imaged at 55° to B0. MRI banding was present in the unloaded, but not loaded, tendons. In unloaded tendons, polarized-light microscopy revealed collagen crimp with a periodicity similar to MRI. In loaded tendons, there was a strain-induced extinction of periodicity on both MRI and polarized-light microscopy. These studies confirm that crimp is detectable by high-field MRI and could serve as an in vivo index of physiological strains in collagenous tissues., (Copyright © 2011 Wiley-Liss, Inc.)

Typical quantitative T2 (qT2) analysis involves creating T2 distributions using a regularized algorithm from region-of-interest averaged decay data. This study uses qT2 analysis of simulated and experimental decay signals to determine how (a) noise-type, (b) regularization, and (c) region-of-interest versus multivoxel analyses affect T2 distributions. Our simulations indicate that regularization causes myelin water fraction and intra/extracellular water geometric mean T2 underestimation that worsens as the signal-to-noise ratio decreases. The underestimation was greater for intra/extracellular water geometric mean T2 measures using Rician noise. Simulations showed significant differences between myelin water fractions determined using region-of-interest and multivoxel approaches compared to the true value. The nonregularized voxel-based approach gave the most accurate measure of myelin water fraction and intra/extracellular water geometric mean T2 for a given signal-to-noise ratio and noise type. Additionally, multivoxel analysis provides important information about the variability of the analysis. Results obtained from in vivo rat data were similar to our simulation results. In each case, a nonregularized, multivoxel analysis provided myelin water fractions significantly different from the regularized approaches and obtained the largest myelin water fraction. We conclude that quantitative T2 analysis is best performed using a nonregularized, multivoxel approach., (Copyright (c) 2009 Wiley-Liss, Inc.)

The desire to monitor the spatial-temporal characteristics of myelination in the spinal cord (SC), in the context of pathological change in demyelinating diseases or proposed neuroregenerative protocols, has led to an interest in noninvasive image-based myelin measurement methods. We present one strategy: a magnetic resonance-based measure that capitalizes on the characteristics of T(2) relaxation of water compartmentalized within tissue. In this study, 32-echo relaxation studies for measuring the myelin water fraction (MWF) were applied in healthy control SC in vivo using a sagittal inversion recovery multiecho sequence, and findings were supported with supplemental studies in bovine SC samples in vitro. Mean human MWF varied according the level of the SC examined: cervical, thoracic, and lumbar MWF was found to be 21.8 (SD=2.1)%, 24.3 (3.6)%, and 11.4 (6.4)%, respectively. Noteworthy reductions were observed in areas consistent with the expected locations of the cervical and lumbar enlargements. Average bovine MWF was 30.0 (2.7)% in white matter and 8.2 (0.4)% in gray matter. The potential applications of T(2) measurement in SC, both in characterizing disease processes like multiple sclerosis and in monitoring neuroregenerative therapies, should encourage future research in this area.

In vitro experiments on 15 white matter samples from five bovine brains were performed on a 1H-NMR spectrometer at 24 degrees C and 37 degrees C. The average myelin water fractions (MWFs) were 10.9% and 11.8% for samples at 24 degrees C and 37 degrees C, respectively. The T1 relaxation time at 37 degrees 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 T2 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 degrees 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 degrees 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 degrees 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., ((c) 2005 Wiley-Liss, Inc.)