Magnetization transfer studies of the fast and slow tissue water diffusion components in the human brain (original) (raw)
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Journal of Magnetic Resonance, 1999
The effect of possible susceptibility-induced gradients on measurements of water diffusion along the transverse and longitudinal axes of white matter fibers in the brain was investigated in vivo at 1.5 T. Measurements obtained with sequences sensitive and insensitive, respectively, to susceptibility-induced gradients indicated that these gradients do not contribute significantly to diffusion anisotropy in brain white matter. Furthermore, diffusion measurements were unaffected by the presence of known susceptibilityinduced gradients at the interface between the petrous bone and brain parenchyma. These results agree with those obtained on in vitro samples and appear to support the hypothesis that interactions between the diffusing water molecules and the cellular environment constitute the principal mechanism for diffusion anisotropy in brain white matter at 1.5 T. This, in turn, simplifies the interpretation of diffusion time-dependent measurements in terms of membrane separation and permeability.
2010
Over the last decade, Diffusion Tensor Imaging (DTI) became an MR technique routinely used in clinical environments. The importance of preclinical studies at ultrahigh magnetic field resulted in an increasing number of publications focused on the rodent brain. Several studies have shown that the choice of sequence parameters (diffusion gradient sampling scheme, number of directions, diffusion gradient duration, diffusion time, region of interest) as well as the intrinsic specifications of the system (magnetic field strength) can have a huge impact on the derived tensor quantifications [1-3], but this work has never been done at ultrahigh magnetic field. In this context, the aim of this work was to study the influence of t d and brain microstructures on diffusion tensor derived parameters in the rat brain at 9.4T. Materials and Methods: All experiments were performed on an actively-shielded 9.4T/31cm magnet (Varian/Magnex) equipped with 12-cm gradient coils (400mT/m, 120μs) with a quadrature transmit-receive 18-mm surface RF coil. The rat (n = 4) was lying prone, its head secured via ear bars and continuously anesthetized under a flow of 1.5-2% isoflurane in oxygen. Body temperature was maintained at 37±0.5°C using thermoregulated water circulation. After automatic adjustment of first and second order shims (FASTMAP [4]-water half-height linewidth ranged between 18 and 22 Hz), 3 repeated Diffusion Tensor Echo Planar Images (4 shots) were performed with t d = 10, 25 and 39 ms respectively. A semi-adiabatic double spin echo sequence was used [5] and diffusion gradients were applied around the first 180° with the same polarity for short t d or around the two 180° with inverted polarity for long t d , resulting in a b-value set to 1000 s.mm −2. Diffusion gradients were applied along 42 spatial directions: Icosahedral 21 directions as well as the 21 opposite directions to cancel b-value cross terms [6]. For the three measurements, image parameters were: FOV = 23 × 15 mm 2 , matrix size = 128 × 64 zero-filled to 256 × 168, 8 slices of 0.8 mm thickness in the axial plane, 18 averages with TE/TR = 50/2000 ms. Note that, both TE and the b-value were kept constant (50 ms and 1000 s.mm −2 respectively) for the three different t d , allowing an accurate assessment of the effects of the diffusion time only. Using homemade Matlab (Mathworks, Natick, MA) software, diffusivity values (ADC, D // and D ⊥) as well as fractional anisotropy (FA) was derived from the tensor. On the direction encoded color maps, ROIs were drawn in the corpus callosum and in the cortex for the 8 different slices of the rat brain (fig. 1) in order to evaluate the variation of diffusion tensor derived parameters function of the t d and along the rat brain. Collected data were submitted to a Friedman non-parametric test. Results: Effects of t d : In the cortex FA values remained constant for all the t d (fig. 2), but all diffusivity values (ADC, D // and D ⊥) were significantly lower at t d = 25 and 39 ms compared to t d of 10 ms (fig. 3). In the corpus callosum, on 7 to 8 slices D // remained stable whereas D ⊥ was significantly decreased at longer diffusion times (25 and 39 ms compared with 10 ms-fig. 3), resulting in a significantly larger FA (fig. 2). We measured, in the cortex as well as in the corpus callosum, no significant difference on diffusion tensor derived parameters between the acquisitions performed at the two longer diffusion times (25 and 39 ms-fig. 2 and 3). Effects of the position: At short t d (10 ms) derived tensor parameters remained stable along the cortex, whereas at longer t d (25 and 39 ms) D // appeared significantly different as a function of the slice (essentially increasing from S5 to S8). In the corpus callosum, independently of the diffusion time, D ⊥ values were found to be significantly higher at slices 3 and 4 compared with the others, resulting in a lower FA (fig. 2).
NeuroImage, 2012
Diffusion-weighted MRI of non-human primates revealed that USPIO Bulk Magnetic Susceptibility (BMS) T2' effects of Ultrasmall Superparamagnetic Particles with Iron Oxide (USPIO) in the brain cannot be explained by a single compartment model, as diffusion and T2' effects appear coupled: Apparent Diffusion Coefficient (ADC) values depend on USPIO concentration and relaxivity effects of USPIO decrease with the b value. On the other hand, USPIO and diffusion effects could be well uncoupled using a model consisting in a fast and a slow diffusion pool with different relaxivities. Diffusion-weighting acts as a filter which emphasizes the contribution of the slow pool when increasing b values (apparent decrease in ADC and R2'). Those results have implications for human studies using BMS contrast agents, as well as BOLD and diffusion fMRI.
Magnetic Resonance in Medicine, 2000
In vivo measurements of the human brain tissue water signal decay with b-factor over an extended b-factor range up to 6,000 s/mm 2 reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b-factor signal decay measurements are used to make separate measurements of the spin-lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin-lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b-factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin-lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is "fast" enough to ensure volume averaged T 1 relaxation yet "slow" enough to allow for the observation of biexponential ADC decay curves over an extended b-factor range. Magn Reson Med 44:292-300, 2000.
Magnetic Resonance in Medicine, 1997
Proton magnetization transfer contrast (MTC) imaging, using continuous wave off-resonance irradiation, was performed on the rat brain in vivo at 4.7 Tesla. The observed MTC was studied in three different brain regions: the corpus callosum, the basal ganglia, and the temporal lobe. By systematically varying the offset frequency and the amplitude of the RF irradiation, the observed signal intensities for each region of interest were modeled using a system including free water and a pool of protons with restricted motions (R. M. Henkelman, X. Huang, Q. Xiang, G. J. Stanisz, SD Swanson, M. J. Bronskill, Magn. ). Most of the relaxation parameters of both proton pools remained fairly constant for the three regions of interest, with a T2 value of about 9 ps for the immobilized protons, whereas the rate of exchange increased significantly from the temporal lobe to the corpus callosum. The optimal acquisition parameters for the improved MTC under steady-state saturation were found to be 2-10 kHz offset frequency and 500-800 Hz RF irradiation amplitude. Conversely, an irradiation amplitude of 3 kHz at an offset frequency of 12 kHz is required to minimize the direct effect of off-resonance irradiation. Such an approach could be extended to human brain imaging with the aim of characterizing tissue-specific disease.
Insight into in vivo magnetization exchange in human white matter regions
Magnetic Resonance in Medicine, 2011
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. Magn Reson Med 000:000-000,
Investigation of multicomponent diffusion in cat brain using a combined MTC–DWI approach
Magnetic Resonance Imaging, 2006
In this study, multiple-component water diffusion in the cat brain is investigated using an approach that combines diffusion-weighted images using multiple b values with magnetization transfer contrast (MTC). The MTC allows filter of signal originating from water molecules that rapidly exchange with binding sites on large macromolecular structures, and in brain white matter, it is assumed that a significant portion of the MTC is due to the interaction of water with the extraaxonal myelin sheath. Henceforth, multicomponent analysis of diffusion curves with and without MTC may shed light on the contribution of the extraaxonal water to the diffusion signal and on the relationship between diffusion components and tissue compartments in the brain. When a biexponential model was applied to the data, the volume fractions of the two diffusion components changed significantly in white matter with the application of the MTC. These changes are then discussed in the frame of tissue components and the possible interaction with the myelin sheath. D
Magnetic Resonance in Medicine, 1997
The extraction fraction of vascular water in rat brain is investigated by means of diffusion measurements of arterial spin labeled water at varying cerebral blood flow (CBF) values. The apparent diffusion coefficient (ADC) of the difference of the proton magnetization signal in the brain acquired with and without continuous arterial spin labeling is modeled to provide a measure of the amount of arterial water in tissue and vasculature and thus of the extraction fraction. The tissue and vascular portion of the arterial spin labeled water are differentiated based on their diffusion characteristics in a manner analogous to the intravoxel incoherent motion (IVIM) method. The amount of labeled arterial water that exchanges with tissue water is determined by estimating the fraction of the total signal that is associated with the slow-decaying component of a biexponential fit to the normalized difference signal between the magnetization of brain tissue acquired with and without arterial sp...
Three-dimensional quantitative magnetisation transfer imaging of the human brain
NeuroImage, 2005
Quantitative magnetisation transfer (MT) analysis is based on a twopool model of magnetisation transfer and allows important physical properties of the two proton pools to be assessed. A good signal-to-noise ratio (SNR) for the measured signal is essential in order to estimate reliably the parameters from a small number of samples, thus prompting the use of a sequence with high SNR, such as a threedimensional spoiled gradient acquisition. Here, we show how full brain coverage can be accomplished efficiently, using a three-dimensional acquisition, in a clinically acceptable time, and without the use of large numbers of slice-selective radio-frequency pulses which could otherwise confound analysis. This acquisition was first compared in post mortem human brain tissue to established two-dimensional acquisition protocols with differing SNR levels and then used to collect data from six healthy subjects. Image data were fitted using the two pool model and showed negligible residual deviations. Quantitative results were assessed in several brain locations. Results were consistent with previous single-slice data, and parametric maps were of good quality. Further investigations are needed to interpret the regional variation of quantitative MT quantities. D
Journal of Magnetic Resonance, 1998
Magnetization transfer contrast imaging using turbo spin echo protons of ''free'' water (pool A) and protons with restricted and continuous wave off-resonance irradiation was carried out on motions (pool B) in macromolecules and lipid bilayers ( 10). rat brain in vivo at 4.7 T. By systematically varying the off-reso-In the presence of such an interaction, a selective irradiation nance irradiation power and the offset-frequency, the signal intenof the ''immobile'' proton pool, at an offset frequency of sities obtained under steady-state for both transverse and longituseveral kilohertz with respect to the water resonance, will dinal magnetization were successfully analyzed with a simple cause a partial saturation of the former and a decrease in the binary spin -bath model taking into account a free water comintensity of the latter, thus creating contrast (11). The obpartment and a pool of protons with restricted motions bearing a served MT contrast can be modulated according to experisuper-Lorentzian lineshape. Due to important RF power deposimental constraints, which are the offset frequency of the MT tion, such experimental conditions are not practical for routine pulse along with its amplitude, duration, and shape.