Compartmental study of diffusion and relaxation measured in vivo in normal and ischemic rat brain and trigeminal nerve.

The correlation between the apparent diffusion coefficient (ADC) and T(2) of water in rat brain and trigeminal nerve was investigated using a hybrid diffusion-weighted-CPMG imaging sequence. Little dependence of ADC on T(2) was found in brain regions of interest, which is postulated to be due to rapid exchange between intra- and extracellular water. Conversely, the ADC of water in trigeminal nerve was found to change significantly with echo time (TE). Parallel to the nerve and with a constant diffusion time (t(diff) = 10.8 ms), the ADC increased by approximately 30% between TEs of 25 ms and 185 ms; perpendicular to the nerve, the ADC decreased by a similar amount over the same range of TE. Measurements made following the onset of global ischemia yielded lower ADCs, with similar dependence on TE. Observations that transverse relaxation of water in nerves is multiexponential have previously been interpreted in terms of microanatomical compartments in slow exchange. In the context of this interpretation, our data suggest that diffusional anisotropy is greater outside than within the myelinated axons. Further, data following the onset of global ischemia suggest that the mechanism(s) by which ADC is reduced affect most or all microanatomical environments of nerve, at least insofar as they are represented over the TE domain investigated. Magn Reson Med 43:837-844, 2000.     

Bayesian algorithm using spatial priors for multiexponential T(2) relaxometry from multiecho spin echo MRI

Multiexponential T₂ relaxometry is a powerful research tool for detecting brain structural changes due to demyelinating diseases such as multiple sclerosis. However, because of unusually high signal‐to‐noise ratio requirement compared with other MR modalities and ill‐posedness of the underlying inverse problem, the T₂ distributions obtained with conventional approaches are frequently prone to noise effects. In this article, a novel multivoxel Bayesian algorithm using spatial prior information is proposed. This prior takes into account the expectation that volume fractions and T₂ relaxation times of tissue compartments change smoothly within coherent brain regions. Three‐dimensional multiecho spin echo MRI data were collected from five healthy volunteers at 1.5 T and myelin water fraction maps were obtained using the conventional and proposed algorithms. Compared with the conventional method, the proposed method provides myelin water fraction maps with improved depiction of brain structures and significantly lower coefficients of variance in white matter. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Compartment‐specific enhancement of white matter and nerve ex vivo using chromium

Chromium—Cr(VI) in the form of potassium dichromate—has been shown to specifically enhance white matter signal. The proposed mechanism for this enhancement is reduction of diamagnetic Cr(VI) to paramagnetic chromium species by oxidizable myelin lipids. The purpose of the study herein was to better understand the microanatomical basis of this enhancement (i.e., the relative enhancement of myelin, intra‐axonal, and extra‐axonal water). Toward this end, integrated T₁‐T₂ measurements were performed in potassium dichromate loaded (hereafter referred to as chromated) rat brains, rat optic nerve samples, and frog sciatic nerve samples ex vivo. In control optic nerve and white matter, two T₁‐T₂ components were resolved, representing myelin and nonmyelin water (intra‐ and extra‐axonal water). Following chromation, three T₁‐T₂ components were resolved in these same tissues. Results from similar measurements in sciatic nerve—all three components are resolvable in control and chromated samples—and quantitative histologic analysis suggest that this additional T₁‐T₂ component is due to a splitting of the nonmyelin water component into intra‐ and extra‐axonal water components. This compartment‐specific enhancement may provide unique contrast for MR histology, as well as allow one to probe the compartmental basis of various contrast mechanisms in neural tissue. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo measurement of T1rho dispersion in the human brain at 1.5 tesla

Purpose

To measure T_1ρ relaxation times and T_1ρ dispersion in the human brain in vivo.

Materials and Methods

Magnetic resonance imaging (MRI) was performed on a 1.5‐T GE Signa clinical scanner using the standard GE head coil. A fast spin‐echo (FSE)‐based T_1ρ‐weighted MR pulse sequence was employed to obtain images from five healthy male volunteers. Optimal imaging parameters were determined while considering both the objective of the study and the guarantee that radio‐frequency (RF) power deposition during MR did not exceed Food and Drug Administration (FDA)‐mandated safety levels.

Results

T_1ρ‐weighted MR images showed excellent contrast between different brain tissues. These images were less blurred than corresponding T₂‐weighted images obtained with similar contrast, especially in regions between brain parenchyma and cerebrospinal fluid (CSF). Average T_1ρ values for white matter (WM), gray matter (GM), and CSF were 85 ± 3, 99 ± 1, and 637 ± 78 msec, respectively, at a spin‐locking field of 500 Hz. T_1ρ is 30% higher in the parenchyma and 78% higher in CSF compared to the corresponding T₂ values. T_1ρ dispersion was observed between spin‐locking frequencies 0 and 500 Hz.

Conclusion

T_1ρ‐weighted MRI provides images of the brain with superb contrast and detail. T_1ρ values measured in the different brain tissues will serve as useful baseline values for analysis of T_1ρ changes associated with pathology. J. Magn. Reson. Imaging 2004;19:403–409. © 2004 Wiley‐Liss, Inc.

     

Measurement of short and ultrashort T2 components using progressive binomial RF saturation.

A new magnetic resonance (MR) method for measuring T2 relaxation times in tissues is proposed. The method is based on a T2 selective saturation period followed by sampling of the remaining longitudinal magnetization. Saturation of the longitudinal magnetization is accomplished by a single binomial RF pulse of zeroth order with a constant flip angle. The T2 selectivity is controlled by the RF pulse duration. A full T2 spectrum can be obtained by performing a series of measurements with varying RF pulse duration. On a conventional 1.5 T system this approach allows detection of T2 components as short as several hundred microseconds. A major limitation is the method's susceptibility to resonance offsets. At typical offsets of 0.1-0.2 ppm the sensitivity of the method is limited to a T2 range below 20 ms, which corresponds to an RF pulse duration shorter than 50 ms. The new method was used to acquire T2 spectra from the liver of pigs in vitro on a conventional 1.5 T system. We observed a short T2 component around 17 ms and an ultrashort T2 component in the range of 0.9-1.1 ms. Numerical simulations and in vitro measurements suggest that resonance offsets have effects that require further investigation.     

Fast T(1) mapping with volume coverage.

Four different sequences which enable high-resolution, multislice T(1) relaxation-time mapping are presented. All these sequences are based on the Look-Locker method with differences arising from the use of either a saturation-recovery or inversion-recovery module prior to data acquisition with a full k-space or banded k-space acquisition scheme. The methods were implemented on a standard clinical scanner and the accuracy of the T(1) results was evaluated against spectroscopic measurements. The accuracy of the T(1) maps validated by phantom imaging measurements is around 1% for species which relax with T(1) times that mimic gray/white matter (T(1) < or = 1000 ms). Additionally, the inherent multislice, multipoint capability of the methods is demonstrated. Finally, in vivo results of the human brain obtained using the faster method are presented. The fastest data acquisition was achieved with a saturation-recovery, banded k-space method where k-space was divided into three segments; an overall acquisition time of around 5 min (for species with T(1) < or = 1 sec) was achieved for a T(1) map which can, in principle, provide whole-brain coverage with a matrix size of 256 x 256 at multiple time-points. Magn Reson Med 46:131-140, 2001.     

Characterization of T2* heterogeneity in human brain white matter

Recent in vivo MRI studies at 7.0 T have demonstrated extensive heterogeneity of T₂* relaxation in white matter of the human brain. In order to study the origin of this heterogeneity, we performed T₂* measurements at 1.5, 3.0, and 7.0 T in normal volunteers. Formalin‐fixed brain tissue specimens were also studied using T₂*‐weighted MRI, histologic staining, chemical analysis, and electron microscopy. We found that T₂* relaxation rate (R₂* = 1/T₂*) in white matter in living human brain is linearly dependent on the main magnetic field strength, and the T₂* heterogeneity in white matter observed at 7.0 T can also be detected, albeit more weakly, at 1.5 and 3.0 T. The T₂* heterogeneity exists also in white matter of the formalin‐fixed brain tissue specimens, with prominent differences between the major fiber bundles such as the cingulum (CG) and the superior corona radiata. The white matter specimen with substantial difference in T₂* has no significant difference in the total iron content, as determined by chemical analysis. On the other hand, evidence from histologic staining and electron microscopy demonstrates these tissue specimens have apparent difference in myelin content and microstructure. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo.

Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.     

In vivo measurement of T2 distributions and water contents in normal human brain

Using a 32-echo imaging pulse sequence, T₂ relaxation decay curves were acquired from five white- and six gray-matter brain structures outlined in 12 normal volunteers. The water contents of white and gray matter were 0.71 (0.01) and 0.83 (0.03) g/ml, respectively. All white-matter structures had significantly higher myelin water percentages (signal percentage with T₂ between 10 and 50 ms) than all gray-matter structures. The range in geometric mean T₂ of the main peak for both white and gray matter was from 70 to 86 ms. T₂ distributions from the posterior internal capsules and splenium of the corpus callosum were significantly wider (width is related to water environment inhomogeneity) than those from any other white- or gray-matter structures. Thus, quantitative measurement and analysis of T₂ relaxation reveals differences in brain tissue water environments not discernible on conventional MR images. These differences may make short T₂ components reliable markers for normal myelin.