T1, T2 relaxation and magnetization transfer in tissue at 3T.

T1, T2, and magnetization transfer (MT) measurements were performed in vitro at 3 T and 37 degrees C on a variety of tissues: mouse liver, muscle, and heart; rat spinal cord and kidney; bovine optic nerve, cartilage, and white and gray matter; and human blood. The MR parameters were compared to those at 1.5 T. As expected, the T2 relaxation time constants and quantitative MT parameters (MT exchange rate, R, macromolecular pool fraction, M0B, and macromolecular T2 relaxation time, T2B) at 3 T were similar to those at 1.5 T. The T1 relaxation time values, however, for all measured tissues increased significantly with field strength. Consequently, the phenomenological MT parameter, magnetization transfer ratio, MTR, was lower by approximately 2 to 10%. Collectively, these results provide a useful reference for optimization of pulse sequence parameters for MRI at 3 T.     

Characterizing healthy and diseased white matter using quantitative magnetization transfer and multicomponent T2 relaxometry: A unified view via a four‐pool model

Nuclear magnetic resonance relaxation and magnetization transfer in cerebral white matter can be described using a four‐pool model: two for water protons (in separate myelin and intra/extracellular compartments) and two for protons associated with the lipids and proteins of biologic membranes (of myelin and nonmyelin semisolids). This model was used to gain insight into the observations from multicomponent quantitative T₂ relaxometry and quantitative magnetization transfer imaging, both based on simplified white matter models and experimentally feasible in vivo. Using a set of coupled Bloch equations describing the behavior of the magnetization in a four‐pool model of white matter, simulations of the quantitative T₂ relaxometry and quantitative magnetization transfer imaging techniques were performed. Pathology‐inspired modifications were made to the four‐pool model to gauge their impact on quantitative T₂ relaxometry and quantitative magnetization transfer imaging observations. Our results show that changes in the rate of water movement between microanatomic compartments may impact otherwise stable quantitative T₂ relaxometry observations; that the measure of the quantitative magnetization transfer imaging–based semisolid pool population is robust, despite the presence of two distinct semisolid components; and that quantitative magnetization transfer imaging compartment size estimates are not influenced by changes in the T₂ of the intra/extracellular water pool. The four‐pool model, while impractical for in vivo characterization, yields important insight into the interpretation of changes observed with these quantitative MRI methods based on simplified models of white matter. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Regional variations in normal brain shown by quantitative magnetization transfer imaging.

A quantitative magnetization transfer imaging (qMTI) study, based on a two-pool model of magnetization transfer, was performed on seven normal subjects to determine, on a regional basis, normal values for the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon. Regions were identified on high-resolution anatomical scans using a combination of manual and automatic methods. Only voxels identified as pure tissue at the resolution of the quantitative scans were considered for analysis. While no left/right differences were observed, significant differences were found among white-matter regions and gray-matter regions. These regional differences were compared with existing cytoarchitectural data. In addition, the pattern and magnitude of the regional differences observed in white matter was found to be different from that reported previously for an alternative putative MRI measure of myelination, the 10-50-ms T2 component described as myelin water.     

Correlation of apparent myelin measures obtained in multiple sclerosis patients and controls from magnetization transfer and multicompartmental T2 analysis.

Two relatively new techniques purport to give measures of the myelin content of brain tissue. These measures are the myelin water fraction from multicompartmental T(2) analysis, and the semisolid proton fraction from analysis of magnetization transfer (MT). The myelin water fraction is the fraction of signal with a T(2) of less than 50 ms measured from a 32-echo sequence. It is believed to originate from water trapped between the myelin bilayers. The semisolid proton fraction is thought to include protons within phospholipid bilayers and macromolecular protons, and may also be a measure of myelin content. Multicompartmental T(2) and MT imaging were carried out on controls and patients with multiple sclerosis (MS), and estimates of the semisolid proton and myelin water fractions were obtained from white matter (WM), gray matter (GM), and MS lesions. These were then correlated for each tissue and subject group. Positive correlations were seen for MS lesions (r approximately 0.2) and in WM in patients (r = 0.6). A negative correlation (r approximately -0.3) was seen for GM. These results indicate that the two techniques measure, to some extent, the same thing (most likely myelin content), but that other factors, such as inflammation, mean they may provide complementary information.     

Magnetization transfer contrast and T2 mapping in the evaluation of cartilage repair tissue with 3T MRI

PURPOSE: To use magnetization transfer (MT) imaging in the visualization of healthy articular cartilage and cartilage repair tissue after different cartilage repair procedures, and to assess global as well as zonal values and compare the results to T2-relaxation. MATERIALS AND METHODS: Thirty-four patients (17 after microfracture [MFX] and 17 after matrix-associated autologous cartilage transplantation [MACT]) were examined with 3T MRI. The MT ratio (MTR) was calculated from measurements with and without MT contrast. T2-values were evaluated using a multiecho, spin-echo approach. Global (full thickness of cartilage) and zonal (deep and superficial aspect) region-of-interest assessment of cartilage repair tissue and normal-appearing cartilage was performed. RESULTS: In patients after MFX and MACT, the global MTR of cartilage repair tissue was significantly lower compared to healthy cartilage. In contrast, using T2, cartilage repair tissue showed significantly lower T2 values only after MFX, whereas after MACT, global T2 values were comparable to healthy cartilage. For zonal evaluation, MTR and T2 showed a significant stratification within healthy cartilage, and T2 additionally within cartilage repair tissue after MACT. CONCLUSION: MT imaging is capable and sensitive in the detection of differences between healthy cartilage and areas of cartilage repair and might be an additional tool in biochemical cartilage imaging. For both MTR and T2 mapping, zonal assessment is desirable.     

NMR properties of human median nerve at 3 T: Proton density,T1, T2, and magnetization transfer

Purpose

To measure the proton density (PD), the T1 and T2 relaxation time, and magnetization transfer (MT) effects in human median nerve at 3 T and to compare them with the corresponding values in muscle.

Materials and Methods

Measurements of the T1 and T2 relaxation time were performed with an inversion recovery and a Carr‐Purcell‐Meiboom‐Gill (CPMG) imaging sequence, respectively. The MT ratio was measured by acquiring two sets of 3D spoiled gradient‐echo images, with and without a Gaussian saturation pulse.

Results

The median nerve T1 was 1410 ± 70 msec. The T2 decay consisted of two components, with average T2 values of 26 ± 2 msec and 96 ± 3 msec and normalized amplitudes of 78 ± 4% and 22 ± 4%, respectively. The dominant component is likely to reflect myelin water and connective tissue, and the less abundant component originates possibly from intra‐axonal water protons. The value of proton density of MRI‐visible protons in median nerve was 81 ± 3% that of muscle. The MT ratio in median nerve (40.3 ± 2.0%) was smaller than in muscle (45.4 ± 0.5%).

Conclusion

MRI‐relevant properties, such as PD, T1 and T2 relaxation time, and MT ratio were measured in human median nerve at 3 T and were in many respects similar to those of muscle. J. Magn. Reson. Imaging 2009;29:982–986. © 2009 Wiley‐Liss, Inc.     

Effect of multislice acquisition on T1 and T2 measurements of articular cartilage at 3T

PURPOSE: The aim of this study was to investigate the effect of magnetization transfer on multislice T1 and T2 measurements of articular cartilage. MATERIALS AND METHODS: A set of phantoms with different concentrations of collagen and contrast agent (Gd-DTPA2-) were used for the in vitro study. A total of 20 healthy knees were used for the in vivo study. T1 and T2 measurements were performed using fast-spin-echo inversion-recovery (FSE-IR) sequence and multi-spin-echo (MSE) sequence, respectively, in both in vitro and in vivo studies. We investigated the difference in T1 and T2 values between that measured by single-slice acquisition and that measured by multislice acquisition. RESULTS: Regarding T1 measurement, a large drop of T1 in all slices and also a large interslice variation in T1 were observed when multislice acquisition was used. Regarding T2 measurement, a substantial drop of T2 in all slices was observed; however, there was no apparent interslice variation when multislice acquisition was used. CONCLUSION: This study demonstrated that the adaptation of multislice acquisition technique for T1 measurement using FSE-IR methodology is difficult and its use for clinical evaluation is problematic. In contrast, multislice acquisition for T2 measurement using MSE was clinically applicable if inaccuracies caused by multislice acquisition were taken into account.     

Quantitative T2 analysis: The effects of noise,regularization, and multivoxel approaches

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. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Analysis of discrete T2 components of NMR relaxation for aqueous solutions in hollow fiber capillaries

An analysis is presented of proton NMR T₂ relaxation times measured for aqueous solutions In simple bundles of hollow fibers. The relaxation times are calculated with a two-compartment diffusive exchange model using the known relaxation times of the aqueous solutions and the fiber geometry. When the relaxation time outside the fibers is short (～1 ms), three or more relaxation components are observed from this two compartment system, in agreement with the calculation. The amplitude and relaxation times of the third component are consistent with those of a diffusion-mediated mode, as suggested theoretically by Brownstein and Tarr (Phys. Rev. A 19, 2446 (1979)). The possible contribution of such modes to the multicomponent relaxation observed in tissues is discussed.     

Nonexponential T2* decay in white matter

Visualizing myelin in human brain may help the study of diseases such as multiple sclerosis. Previous studies based on T₁ and T₂ relaxation contrast have suggested the presence of a distinct water pool that may report directly on local myelin content. Recent work indicates that T₂* contrast may offer particular advantages over T₁ and T₂ contrast, especially at high field. However, the complex mechanism underlying T₂* relaxation may render interpretation difficult. To address this issue, T₂* relaxation behavior in human brain was studied at 3 and 7 T. Multiple gradient echoes covering most of the decay curve were analyzed for deviations from mono‐exponential behavior. The data confirm the previous finding of a distinct rapidly relaxing signal component (T₂* ～ 6 ms), tentatively attributed to myelin water. However, in extension to previous findings, this rapidly relaxing component displayed a substantial resonance frequency shift, reaching 36 Hz in the corpus callosum at 7 T. The component's fractional amplitude and frequency shift appeared to depend on both field strength and fiber orientation, consistent with a mechanism originating from magnetic susceptibility effects. The findings suggest that T₂* contrast at high field may be uniquely sensitive to tissue myelin content and that proper interpretation will require modeling of susceptibility‐induced resonance frequency shifts. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

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.     

脑铁含量和T2弛豫时间的关系

探讨脑铁含量与T2弛豫时间的关系。材料与方法选择15例意外死亡尸体脑,除外脑外伤、脑梗塞、脑肿 瘤、神经和精神系统疾病,测量双侧苍白球、尾状核、丘脑、黑质、红核、齿状核及双侧额、枕、颞叶白质T2弛豫时间(SE序 列T1WITR/TE500/40ms;T2WITR/TE2700/80ms)和铁含量,对脑铁含量与T2弛豫时间行相关性分析。结果灰质核团 T2弛豫时间随脑铁含量增加而缩短,脑白质T2弛豫时间无变化趋势。灰质核团(除苍白球)r值在-0.6928～-0.9440之 间,苍白球和白质区r值在-0.0418～-0.3722之间。结论(1)苍白球区T2弛豫时间易偏移不宜作为脑铁MRI研究区 域;(2)脑灰质核团的T2弛豫时间下降主要与铁沉积有关,脑白质T2弛豫时间与多种因素有关。     

A Model for Magnetization Transfer in Tissues

Magnetization transfer in several tissues is measured and successfully modeled using a two-pool model of exchange. The line shape for the semi-solid pool is characterized by a superLorentzian and the liquid pool by a Lorentzian. The tissues investigated were white and gray matter, optic nerve, muscle, and liver. All tissues the authors studied are characterized by the same model but differ in the parameter values of the model. Blood and cerebral spinal fluid (CSF) were also investigated. The two-pool model with a Lorentzian line shape for both the semi-solid and liquid pools modeled the magnetization transfer in blood. In CSF, as expected, there is no measurable exchange of magnetization. The T_2B associated with the semi-solid pool was short (?10 μ) for all tissues indicating a fairly rigid semi-solid pool. In addition characterization of the line shape as superLorentzian indicates molecules such as integral membrane proteins or lipids in membranes are likely molecules participating in the exchange. Conversely, in blood large globular proteins are indicated due to the Lorentzian nature of the semi-solid pool and a T_2B ≈ 300 μs.     

Normal-appearing white matter in multiple sclerosis has heterogeneous, diffusely prolonged T(2).

T(2) relaxation in normal-appearing white matter (NAWM) of multiple sclerosis (MS) patients was reexamined using more complete sampling and analysis of decay curves, and to assess focal vs. diffuse abnormalities. Nine MS patients and 10 controls were scanned using a single-slice 32-echo pulse sequence with a 10-ms echo spacing. Decay curves from outlined white and gray matter structures were analyzed using non-negative least-squares (NNLS). Resulting T(2) distributions were each summarized by the geometric mean T(2), T(2). Different white matter structures had different mean (over the subjects in a group) T(2). Mean T(2) in NAWM was always greater than that of controls. Differences were not caused by a few voxels with extreme T(2) (i.e., focal lesions), but rather by shifts of the entire T(2) distribution (diffuse prolongation). This T(2) increase suggests diffuse myelin or axonal pathology.     

MR evidence of long T2 water in pathological white matter

PURPOSE: To describe what, if any, specific long T(2)-related abnormalities occur in the white matter of subjects with either phenylketonuria (PKU) or multiple sclerosis (MS). MATERIALS AND METHODS: The 48-echo T(2) relaxation data (maximum TE = 1.12 sec) were acquired from 15 PKU subjects, 20 MS subjects, and 15 healthy volunteers. Regions of interest were drawn in diffuse white matter hyperintensities (DiffWM), lesions, normal-appearing white matter (NAWM), and normal white matter. Long T(2) maps (200 msec < T(2) < 800 msec) were created for each subject. RESULTS: A new water reservoir with a markedly prolonged T(2) peak was identified in DiffWM and NAWM in 12 out of 15 subjects with PKU and a long T(2) signal was also seen in 23/97 lesions in 50% of subjects with MS. Additionally, a long T(2) component was observed in the corticospinal tracts of 10 healthy volunteers. The characteristics of the long T(2) signal were unique for each subject group. Potential sources of this signal include vacuolation and increases in extracellular water. CONCLUSION: This study supports the usefulness of increasing the data acquisition window of the multiecho T(2) relaxation sequence to better characterize the T(2) decay from pathological brain.     

Magnetization transfer,cross-relaxation,and chemical exchange in rotationally immobilized protein gels

Water proton spin-lattice relaxation rates are reported as a function of the magnetic field strength for cross-linked bovine Serum albumin samples. The relaxation dispersion profile is analyzed using a relaxation model where the solid components have the magnetic field dependence proportional to v^?0.5 which may result from a defect diffusion model with two degrees of freedom. If the cross-linking agent concentration is not sufficiently high, the relaxation dispersion curve may have significant contributions from freely rotating protein. The magnetic field dependence of the relaxation rates studied as a function of the proton mole fraction in the sample show that approximately 30% of the magnetization transfer rate is directly proportional to the proton mole fraction. This contribution is identified with the magnetization transfer from exchange of whole water molecules with buried binding sites on the protein. The second order magnetization transfer rate constant is 388 s^?1 assuming unit water spin concentration. The solid component relaxation obeys an Arrhenius activation law, but the overall temperature dependence of the crossrelaxation is complicated by chemical exchange processes which enter with opposite sign.