A multicenter measurement of magnetization transfer ratio in normal white matter

Lesional magnetization transfer ratio: a feasible outcome for remyelinating treatment trials in multiple sclerosis

Magnetization transfer ratio (MTR) is a sensitive parameter to quantify the integrity of myelinated white matter in patients with multiple sclerosis. Lesional MTR decreases in the acute phase due to demyelination, and subsequently shows recovery depending on the degree of remyelination in the absence of axonal loss. Recovery of average lesion MTR therefore might prove a viable outcome measure to assess the effect of remyelinating agents. Our objective was to determine the required sample size for phase II multicentre clinical trials using the recovery of average lesion MTR as primary outcome measure. With 7-monthly MRI scans, the MTR evolution of 349 new enhancing lesions before and after enhancement was assessed in 32 MS patients from 5 centres. Multilevel models were fitted to the data yielding estimates for the variance components, which were applied in power calculations. Sample sizes were determined for placebo-controlled, multicentre trials using lesional MTR recovery post-enhancement as primary outcome measure. Average lesion MTR decreased slightly in the build-up to enhancement, decreased dramatically during enhancement and showed recovery in the period after cessation. The power calculations showed that for a power of 80%, approximately 136 patients per trial (mean number of 6 lesions per patient) are required to detect a 30% increase in lesional MTR post-enhancement compared with placebo, whereas 48 subjects are required to detect a 50% increase in lesional MTR compared with placebo. Recovery of lesion MTR is a feasible outcome measure for future multicentre clinical trials measuring the effect of remyelinating agents.

A fast B1-mapping method for the correction and normalization of magnetization transfer ratio maps at 3 T

In neuroimaging, there is increasing interest in magnetization transfer (MT) techniques which yield information about bound water protons. One of the main applications is the investigation of the myelin integrity in the central nervous system (CNS). However, several problems may arise, in particular at high magnetic field strengths: B1 inhomogeneities may yield deviations of the MT saturation angle and thus non-uniformities of the measured MT ratio (MTR). This effect can be corrected for but requires in general additional time consuming B1 mapping. Furthermore, increased values of the specific absorption rate (SAR) may require a reduction of the saturation angle for individual subjects, impairing comparability of results. In this work, a B1 mapping method based on magnetization-prepared FLASH with slice selective preparation and excitation pulses and correction for relaxation effects is presented, yielding B1 maps with whole brain coverage, an in-plane resolution of 4 mm, a slice thickness of 3 mm, and a clinically acceptable duration of 46 s. The method is tested both in vitro and in vivo and applied in a subsequent in vivo study to show that MTR values in human brain tissue depend approximately linearly on the preparation angle, with a slope similar to values reported for 1.5 T. Calibration data and B1 maps are applied to B1 inhomogeneity corrections of MTR maps. Subsequently, it is shown that B1-corrected MTR maps acquired at reduced preparation angles due to individual SAR restrictions can be normalized, allowing for a direct comparison with maps acquired at the full angle.

Investigation of quantitative magnetisation transfer parameters of lesions and normal appearing white matter in multiple sclerosis

The aim of this study was to use quantitative magnetisation transfer (MT) imaging to assess the different pathological substrates of tissue damage in multiple sclerosis (MS) and examine whether the MT parameters may be used to explain the disability in relapsing remitting (RR) MS. Thirteen patients with RRMS and 14 healthy controls were prescribed conventional MRI and quantitative MT imaging at 3.0 T. A two-pool model of MT (where A refers to the free pool and B to the macromolecular pool) was fitted to the data yielding a longitudinal relaxation rate R(A), a relative size F of macromolecular pool, transverse relaxation times T(2) (A) and T(2) (B) for the two pools and a forward exchange rate RM(0) (B). The MT ratio (MTR) was also computed. The mean MT parameters of the normal appearing white matter (NAWM) and of lesions in patients, and of white matter in controls were estimated. MT parameters were significantly different between lesions and NAWM in patients, and between the NAWM and the white matter of controls (with the exception of T(2) (B) and the MTR). Two models were investigated using ordered logistic regression, with the expanded disability status scale (EDSS) as the dependent variable. In the first one, mean NAWM MT parameters and lesion load were entered as explanatory variables; in the second one, mean MT variables within lesions and lesion load were entered as explanatory variables. Unexpectedly, T(2) (B) was the parameter most significantly associated with EDSS in NAWM. This parameter might represent a weighted average of the relaxation times of spins with different molecular environments, and therefore its variation could indicate a change in the balance between subpopulations of macromolecular spins. Conversely, in lesions, RM(0) (B), T(2) (B), F, R(A), and lesion load significantly predicted disability only when combined together. This might reflect the complex interaction between demyelination, remyelination, gliosis, inflammation and axonal loss taking place within lesions.

[Magnetic resonance imaging of central nervous system inflammation]

Magnetic resonance imaging (MRI) is widely used to explore central nervous system inflammatory disorders, especially multiple sclerosis (MS). Advanced MRI methods are bringing more sensitive and specific tools for each step of the inflammatory process. In this review, we discuss the different MRI approaches for inflammatory disorders exploration, especially MS. We give particular emphasize on sensibility and specificity of each MRI approach and we also discuss the current knowledge concerning biological and histopathological substratum that could explain MRI signal with each modality.

The MT pool size ratio and the DTI radial diffusivity may reflect the myelination in shiverer and control mice

A quantitative magnetization transfer (qMT) technique was employed to quantify the ratio of the sizes of the bound and free water proton pools in ex vivo mouse brains. The goal was to determine the pool size ratio sensitivity to myelin. Fixed brains from both shiverer mice and control littermates were imaged. The pool size ratio in the corpus callosum of shiverer mice was substantially lower than that in the control mice, while there was no distinguishable difference in the pool size ratio in the gray matter. These results correlate with diffusion tensor imaging (DTI) derived radial diffusivity which previously was shown to reflect myelin integrity in this animal model. Histological study reveals the presence of myelin in control mice white matter and the absence of myelin in shiverer mice white matter, supporting the qMT and DTI results. Our findings support the view that qMT may be used for estimating myelin integrity.

Quantitation of brain tissue changes associated with white matter hyperintensities by diffusion-weighted and magnetization transfer imaging: the LADIS (Leukoaraiosis and Disability in the Elderly) study

PURPOSE

To explore the value of diffusion-weighted imaging (DWI) and magnetization transfer imaging (MTI) for the improved detection and quantification of cerebral tissue changes associated with ageing and white matter hyperintensities (WMH).

MATERIALS AND METHODS

DWI (n = 340) and MTI (n = 177) were performed in nine centers of the multinational Leukoaraiosis And DISability (LADIS) study investigating the impact of WMH on 65- to 85-year-old individuals without prior disability. We assessed the apparent diffusion coefficient (ADC) and magnetization transfer ratio (MTR) of normal appearing brain tissue (NABT) and within WMH and related them to subjects' age and WHM severity according to the Fazekas score.

RESULTS

ADC and MTR values showed a significant inter-site variation, which was stronger for the MTR. After z-transformation multiple regression analysis revealed WMH severity and age as significant predictors of global ADC and MTR changes. Only lesional ADC, but not MTR was related to WMH severity.

CONCLUSION

ADC and MTR are both sensitive for age and WMH related changes in NABT. The ADC is more sensitive for tissue changes within WMH and appears to be more robust for multicenter settings.

Characterization of white matter damage in animal models of multiple sclerosis by magnetization transfer ratio and quantitative mapping of the apparent bound proton fraction f*

Quantitative magnetization transfer magnetic resonance imaging (qMT-MRI) can be used to improve detection of white matter tissue damage in multiple sclerosis (MS) and animal models thereof. To study the correlation between MT parameters and tissue damage, the magnetization transfer ratio (MTR), the parameter f* (closely related to the bound proton fraction) and the bound proton transverse relaxation time T2B of lesions in a model of focal experimental autoimmune encephalomyelitis (EAE) were measured on a 7T animal scanner and data were compared with histological markers indicative for demyelination, axonal density, and tissue damage. A clear spatial correspondence was observed between reduced values of MTR and demyelination in this animal model. We observed two different levels of MTR and f* reduction for these lesions. One was characterized by a pronounced demyelination and the other corresponded to a more severe loss of the cellular matrix. Changes in f* were generally more pronounced than those of MTR in areas of demyelination. Moreover, a reduction of f* was already observed for tissue where MTR was virtually normal. No changes in T2B were observed for the lesions. We conclude that MTR and qMT mapping are efficient and reliable readouts for studying demyelination in animal models of MS, and that the analysis of regional f* might be even superior to the analysis of MTR values. Therefore, quantitative mapping of f* from human brains might also improve the detection of white matter damage in MS.

Quantitative magnetic resonance of postmortem multiple sclerosis brain before and after fixation

Unfixed and fixed postmortem multiple sclerosis (MS) brain is being used to probe pathology underlying quantitative MR (qMR) changes. Effects of fixation on qMR indices in MS brain are unknown. In 15 postmortem MS brain slices T(1), T(2), MT ratio (MTR), macromolecular proton fraction (f(B)), fractional anisotropy (FA), and mean, axial, and radial diffusivity (MD, D(ax), and D(rad)) were assessed in white matter (WM) lesions (WML) and normal appearing WM (NAWM) before and after fixation in formalin. Myelin content, axonal count, and gliosis were quantified histologically. Student's t-test and regression were used for analysis. T(1), T(2), MTR, and f(B) obtained in unfixed MS brain were similar to published values obtained in patients with MS in vivo. Following fixation T(1), T(2) (NAWM, WML) and MTR (NAWM) dropped, whereas f(B) (NAWM, WML) increased. Compared to published in vivo data all diffusivity measures were lower in unfixed MS brain, and dropped further following fixation (except for FA). MTR was the best predictor of T(myelin) (inversely related to myelin) in unfixed MS brain (r = -0.83; P < 0.01) whereas postfixation T(2) (r = 0.92; P < 0.01), T(1) (r = 0.89; P < 0.01), and f(B) (r = -0.86; P < 0.01) were superior. All diffusivity measures (except for D(ax) in unfixed tissue) were predictors of myelin content.

Quantitative magnetization transfer imaging in postmortem multiple sclerosis brain

PURPOSE

To investigate the relationship of myelin content, axonal density, and gliosis with the fraction of macromolecular protons (fB) and T2 relaxation of the macromolecular pool (T2B) acquired using quantitative magnetization transfer (qMT) MRI in postmortem brains of subjects with multiple sclerosis (MS).

MATERIALS AND METHODS

fB and T2B were acquired in unfixed postmortem brain slices of 20 subjects with MS. The myelin content, axonal count, and severity of gliosis were all quantified histologically. t-Tests and multiple regression were used for analysis.

RESULTS

MR indices obtained in unfixed postmortem MS brains were consistent with in vivo values reported in the literature. A significant correlation was detected between Tr(myelin) (inversely proportional to myelin content) and 1) fB (r = -0.80, P < 0.001) and 2) axonal count (r = -0.79, P < 0.001). fB differed between 1) normal-appearing white matter (NAWM) and remyelinated WM lesions (rWMLs) (mean: fB 6.9 [SD 2] vs. 4.0 [1.8], P = 0.01), and 2) rWMLs and demyelinated WMLs (mean: 4.2 [2.2] vs. 2.5 [1.3], P = 0.016). No association was detected between T2B and any of the histological measures.

CONCLUSION

fB in MS WM is dependent on myelin content and may be a tool to monitor patients with this condition.

Quantitative magnetization transfer imaging in Alzheimer disease

PURPOSE

To prospectively measure magnetization transfer (MT) parameters, along with established atrophy parameters, in patients with Alzheimer disease (AD) and in age- and sex-matched control subjects.

MATERIALS AND METHODS

Participants provided informed consent, and additional assent was obtained from next of kin of all patients with AD. The study was approved by the local ethics committee. Fourteen patients with AD (seven men; mean age, 67.2 years+/-6.5 [standard deviation]) and 14 control subjects (nine men; mean age, 65.5 years+/-9.4) underwent volumetric T1-weighted magnetic resonance and MT imaging. Whole-brain and total hippocampal volumes were adjusted for total intracranial volume. MT images were processed to derive four fundamental parameters in the hippocampal region by using the two-pool model of the MT phenomenon. Pearson correlation coefficients were used to assess the association between volumetric and MT parameters and Mini-Mental State Examination (MMSE) results. Logistic regression models were used to investigate whether combinations of parameters associated with MMSE could help provide better group discrimination.

RESULTS

Patients with AD had significantly reduced whole-brain (P=.001) and total hippocampal (P<.001) volumes compared with those of control subjects. Two MT parameters were significantly reduced in the hippocampal region of patients: 1/(RAT2A)--that is, ratio of relaxation times of free proton pool, where RA equals 1/T1A and is the inverse of the longitudinal relaxation time of the free proton pool (P=.01)--and f*b, which equals fb/[RA(1-fb)], where fb is the restricted proton fraction (P<.001). Among patients with AD, whole-brain volume and hippocampal were correlated with MMSE results. When both parameters were included in a logistic regression model, only hippocampal was significantly associated with case-control status (P=.03).

CONCLUSION

Certain MT parameters may serve as useful biomarkers of AD.

Magnetization transfer ratio measurement in multiple sclerosis normal-appearing brain tissue: limited differences with controls but relationships with clinical and MR measures of disease

We investigated the magnetization transfer ratio (MTR) of normal-appearing white (NAWM) and grey matter (NAGM) in a relatively large group of multiple sclerosis (MS) patients, and the relations of MTR changes with clinical disability. MTR was measured in 66 MS patients (12 PP, 35 RR, 19 SP) and 23 healthy controls, using a whole-brain 3D-FLASH technique corrected post-hoc for B1-induced variation. Histogram parameters of conservatively selected NAWM and cortical NAGM were analysed using Bonferroni-corrected ANOVA with age as covariate. Additionally, manually outlined regions of interest were analysed using a multilevel method. Lesions had low MTR (mean 22.7±6.9%), but NAWM exhibited limited changes: MTR histogram peak position was 32.8±1.0% in controls and 32.4±0.9% in MS patients, with a significant decrease compared to controls only in SPMS patients (31.9±1.1%, p=0.045). Cortical NAGM histograms did not differ significantly between patients and controls. In SPMS, regional mean MTR was significantly decreased in corpus callosum and hippocampus. MTR histogram parameters of NAGM and NAWM were correlated with EDSS and MSFC scores, with lesion volume and with normalized brain volume. We conclude that disease-induced MTR changes were small in MS NAWM and NAGM, but did correlate with clinical decline, lesion volume and overall cerebral atrophy. Multiple Sclerosis 2007; 13: 708-716. http://msj.sagepub.com

Reducing inter-scanner variability of activation in a multicenter fMRI study: Role of smoothness equalization

Scanner-to-scanner variability of activation in multicenter fMRI studies is often considered undesirable. The purpose of this investigation was to evaluate the effect of a new procedure, "smoothness equalization", on reducing scanner differences in activation effect size as part of a multicenter fMRI project (FIRST BIRN). Five subjects were sent to 9 centers (10 scanners) and scanned on 2 consecutive days using a sensorimotor fMRI protocol. High-field (4 T and 3 T) and low-field (1.5 T) scanners from three vendors (GE, Siemens, and Picker) were included. The activation effect size of the scanners for the detection of neural activation during a sensorimotor task was evaluated as the percent of temporal variance accounted for by our model (percent of variance accounted for or PVAF). Marked scanner effects were noted for both PVAF as well as the degree of smoothness of the raw and processed images. After smoothness equalization, there was a dramatic (low field) or consistent (high-field) reduction in scanner-to-scanner variation of activation. It was shown that the likely basis of the scanner differences in smoothness was differences in k-space filtering algorithms. This work highlights the need to account for differences in smoothness when comparing scanners on activation effect size in multicenter fMRI studies.

Sources of variation in multi-centre brain MTR histogram studies: body-coil transmission eliminates inter-centre differences

OBJECT

1. Identify sources of variation affecting Magnetisation Transfer Ratio (MTR) histogram reproducibility between-centres. 2. Demonstrate complete elimination of inter-centre difference.

MATERIALS AND METHODS

Six principle sources of variation were summarised and analysed. These are: the imager coil used for radiofrequency (RF) transmission, imager stability, the shape and other parameters describing the Magnetisation Transfer (MT) pulse, the MT sequence used (including its parameters), the image segmentation methodology, and the histogram generation technique. Transmit field nonuniformity and B1 errors are often the largest factors. PLUMB (Peak Location Uniformity in MTR histograms of the Brain) plots are a convenient way of visualising differences. Five multi-centres studies were undertaken to investigate and minimise differences.

RESULTS

Transmission using a body coil, with a close-fitting array of surface coils for reception, gave the best uniformity. Differences between two centres, having MR imagers from different manufacturers, were completely eliminated by using body coil excitation, making a small adjustment to the MT pulse flip angle, and carrying out segmentation at a single centre. Histograms and their peak location and height values were indistinguishable.

CONCLUSIONS

Body coil excitation is preferred for multi-centre studies. Analysis (segmentation and histogram generation) should ideally be carried out at a single site.

Reproducibility and reliability of MR measurements in white matter: clinical implications

The purpose of this study was to determine the reproducibility and reliability of five MRI-derived measurements, namely, total water content (WC), myelin water content (MWC), mean T2 relaxation time (GMT2), T1 relaxation time (T1) and magnetization transfer ratio (MTR). Five controls were scanned 5 times over 1 year. The five MR measurements were made for 5 white matter regions. All measurements were found to be highly reproducible. MTR had a low reliability coefficient because all individual values were similar. Therefore, MTR would be most sensitive in detecting changes from normal. WC had a high reliability coefficient in all regions. For MWC, GMT2 and T1, the overall reliability coefficients were high but for some individual regions were low. The high coefficients suggest that these measurements, although different between normal subjects, are consistent over time. They could be used to explore natural differences in the normal population, but due to the large spread in normal values, larger sample sizes are needed to detect pathological changes.

A simple correction for B1 field errors in magnetization transfer ratio measurements

B1 errors are a problem in magnetization transfer ratio (MTR) measurements because the MTR value is dependent on the amplitude of the magnetization transfer (MT) pulse. B1 errors can arise from radiofrequency (RF) nonuniformity (caused by the RF coil, or skin effect and dielectric resonance in the subject's head) and also from inaccurate setting of the transmitter output when compensating for varying amounts of loading of the RF coil. B1 errors, and hence MTR errors, may be up to 5-10%, a large source of error in quantitative MR measurements. Radiofrequency nonuniformity may cause MTR histograms to be broadened. The dependence of MTR on B1 was modeled using binary spin bath theory, with a continuous wave (CW) approximation. For B1 reductions of up to 20%, normalized plots for different brain tissue types could be approximated by a single line, indicating that a systematic correction could be applied to MTR measurements with a known B1 error, regardless of tissue type. On a 1.5-T scanner with a birdcage coil, MTR was measured in 18 tissue types in five controls. The MT pulse amplitude was reduced in steps from its nominal value by up to 20%. Averaging data over all controls and tissue types resulted in a line fitting mtr(normalized)=0.812b(1normalized)+0.193, where mtr(normalized) is the normalized value of MTR (relative to its value at the nominal B1) and b(1normalized) is the normalized value of B1 (relative to its nominal value). For a 20% reduction in MT pulse amplitude (i.e., b(1normalized)=0.80), the mean MTR value for the 18 tissue types was 7.0 percent units (pu) below the correct value. After correction using the single equation above for all tissue types, all MTR values were within 1.5 pu of their correct value [root mean square (rms) error=0.7 pu]. Magnetization transfer ratio values tended to be slightly overcorrected because the simple linear correction scheme is only an approximation to the true MTR dependence on B1. A B1 field mapping technique was implemented, based on the double angle method (DAM), with fast spin-echo (FSE) readout, and TR=15 s; this took a total of 6 min of imaging time. This was used to quantify B(1) errors and correct MTR maps and histograms. However, the cerebrospinal fluid (CSF) T1 is very long (approximately 4.2 s); thus, to achieve complete longitudinal relaxation (a requirement of the DAM B1 mapping method), an increase in TR and, hence, acquisition time would be required. In general, however, we are not interested in calculating the B1 in the CSF, although it is important that the B1 is determined in partial volume voxels around the CSF. Using our birdcage head coil, whole-brain B1 histograms were found to have full-width half maximums (FWHMs) ranging from just 6.8% to 11.5% of the nominal B1 value. The FSE DAM B1 field mapping technique was shown to be robust, although a longer TR time may be desirable to ensure complete elimination of CSF partial volume errors. The procedure can be applied on any scanner where the Euro-MT sequence is available, or alternatively, where the amplitude of B1 or of the MT pulse can be manually reduced in order to perform this type of "calibration" experiment for the particular MTR sequence used. The MTR is known to be highly dependent on the parameters of the sequence used, in particular, the MT pulse shape, flip angle, duration, and offset frequency, and the repetition time TR' between successive MT pulses. Therefore, correction schemes will differ for different MTR sequences, and new data sets would be required to calculate these different correction schemes.

Whole-body magnetization transfer contrast imaging

PURPOSE

To demonstrate the feasibility of whole-body magnetization transfer (MT) contrast imaging.

MATERIALS AND METHODS

Whole-body MT imaging was performed on eight healthy volunteers and five patients (mean age=40.5+/-17.8 years) with diagnoses of dermatomyositis (N=1), B-symptoms with suspicion of paraneoplastic disease (N=1), metastatic malignant melanoma (N=1), and multiple sclerosis (MS) (N=2). Measurements were carried out on a 1.5-Tesla whole-body MR scanner capable of parallel signal reception. A three-dimensional (3D) gradient-echo sequence (TR=17 msec, TE=4.8 msec, flip angle=10 degrees) was applied in combination with a Gaussian off-resonance MT preparation pulse acting at an off-resonance of 1.500 Hz with a 500 degrees effective flip angle. Whole-body images were constructed from five different body regions.

RESULTS

In all subjects, whole-body MT contrast images were obtained within less than 20 minutes of measuring time. The images showed sufficient diagnostic image quality to assess the patients' pathologies. The MT ratios (MTRs, in percent units) for the volunteers were as follows: white matter (WM) 51.1+/-1.0, gray matter (GM) 42.2+/-1.3, skeletal muscle (mean value of four muscle groups) 50.3+/-2.1, liver 39.4+/-3.2, spleen 31.8+/-2.6, renal cortex 30.4+/-1.9, and renal medulla 25.6+/-1.3. The MTRs for the pathologies were as follows: skeletal muscle in dermatomyositis approximately 30, metastases in malignant melanoma 30.7-36.0, uterus myoma 49.3, and MS lesions 30-40.

CONCLUSION

Our preliminary data indicate that MT contrast in whole-body MRI is feasible, and may be useful for rapid whole-body assessment of diseases that exhibit high contrast in MT imaging, such as MS and muscular disorders.

Fast high-resolution T1 mapping of the human brain

A sequence for the acquisition of high-resolution T1 maps, based on magnetization-prepared multislice fast low-angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice-selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal-to-noise ratio (SNR). The method generates quantitative T1 maps with an in-plane resolution of 1 mm, slice thickness of 4 mm, and whole-brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off-center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully.

A standardised method for measuring magnetisation transfer ratio on MR imagers from different manufacturers--the EuroMT sequence

Magnetisation transfer ratio (MTR) is increasingly used to evaluate neurological disorders, especially those involving demyelination. It shows promise as a surrogate marker of disease progression in treatment trials in multiple sclerosis (MS) but the value measured is highly dependent on pulse sequence parameters, making it hard to include the technique in large multi-centre clinical trials. The variations can be reduced by a normalisation procedure based on the flip angle and timing of the presaturation pulse, but correction for parameters such as saturation pulse shape, amplitude, duration and offset frequency remains problematic. We have defined a standard pulse sequence, to include a standard presaturation pulse and set of parameters, which can be implemented on scanners from both General Electric and Siemens, and has also been used on Phillips scanners. To validate the sequence and parameters, six European centres measured MTR in the frontal white matter of normal volunteers. It was possible to measure MTR values in controls which were consistent to within approximately +/-2.5 percentage units across sites. This degree of precision may be adequate in many situations. The remaining differences between sites and manufacturers are probably caused by B1 errors.

Simultaneous measurement of saturation and relaxation in human brain by repetitive magnetization transfer pulses

Magnetization transfer (MT) by equidistant pulse trains can be described as being analogous to progressive partial saturation, where 'direct' saturation of water is amplified by MT contributions that are dependent on macromolecular content and differential saturation. This concept was applied to study the transition to steady state in the human brain using similar MT-pulses as in imaging. Up to 41 bell-shaped MT-pulses of 12 ms duration were applied at frequency offsets between 0.5 and 15 kHz with flip angles between 1080 and 1440 degrees . Central white and parietal gray matter was studied in human subjects using STEAM for localized read-out (TE = 30 ms, TM = 13.7 ms). The apparent degree of saturation, delta(app), and the longitudinal relaxation of the water pool during the pulse repetition period (PR) were fitted to the transient behavior after signal correction for cerebro-spinal fluid. PR was varied between 15 and 100 ms to assess the PR-dependence of the fitted parameters. The MT-term in delta(app) exceeded the direct saturation and attained its maximum at PR > or = 100 ms. The macromolecular pool was only partially saturated by a single MT-pulse. The offset may be increased to 2.5 kHz to reduce direct saturation without sacrificing MT in white matter. The estimated relaxation rates (1.04 +/- 0.11 s(-1) in WM; 0.76 +/- 0.13 s(-1) in GM) were faster than are commonly observed at 1.5 T. The apparent saturation is a measure for MT that is not confounded by relaxation. To maximize MT in brain tissue, MT-pulses should be applied at PR = 100 ms or longer. At shorter PR, a larger steady state saturation is obtained at the cost of increased contributions from direct saturation. Since this accelerates the convergence, PR should be decreased to reach the steady state within a specified time. A faster transition can always be achieved at a reduced frequency offset via increased direct saturation.

Correlation of apparent myelin measures obtained in multiple sclerosis patients and controls from magnetization transfer and multicompartmentalT2 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.

Diffusion-weighted EPI with magnetization transfer contrast

Capabilities of diffusion-weighted (DW) and magnetization transfer (MT) imaging are well established for tissue characterization in various pathologies individually. However, the effect of suppression of macromolecules on applying MT pulse on signals associated with DW imaging and resulting change in the apparent diffusion coefficient (ADC) of water molecules has not been demonstrated previously. In the present study, we have performed DW echo planar imaging (EPI) with and without MT preparation pulse to see the effect of macromolecular signal suppression on ADC. A total of 10 normal volunteers and 20 patients with different intracranial cystic lesions [abscesses (n=10), cystic tumors (n=5), arachnoid cysts (n=5)] were subjected to DW imaging (b=0 and 1000 s/mm(2)) with and without MT saturation pulse. Analysis of region of interest (ROI) from different areas of white matter in normal volunteers and in the wall and cavity of cystic lesions in patients was carried out for calculating the ADC values. We found a significant increase (P<.05) in the ADC values in brain parenchyma and cavity of those intracranial cystic lesions having considerable amount of proteins after the application of MT preparation pulse except for arachnoid cysts. This is due to the size of the macromolecules present in the normal and abnormal tissue. Our studies suggest that this technique is likely to give a novel image contrast and may be of value in improving the tissue specificity in pathologies associated with variable macromolecular size.

3D DT-MRI using a reduced-FOV approach and saturation pulses

Diffusion tensor imaging (DTI) can provide vital insights into brain connectivity, and may become an important tool for the diagnosis and treatment of neurological disease. However, DTI's intrinsic low signal-to-noise ratio (SNR) and vulnerability to ghosting artifacts can result in poor image quality with low spatial resolution, which limits its clinical applications. In this study, a new double-shot EPI sequence (half-FOV EPI) with high spatial resolution was developed. This method enables DT measurements to be obtained with high isotropic spatial resolution and whole-brain coverage. To avoid ghosting artifacts, the data are combined in image space rather than in k-space.

Magnetization transfer, HASTE, and FLAIR imaging

Continuous technologic developments and research have increased the clinical applications of MT, HASTE, and FLAIR imaging in neuroradiology. HASTE has become the MR imaging sequence of choice for fetal neuroimaging. Other promising uses, such as for diffusion-weighted imaging, have not been fully exploited. FLAIR has been firmly established as one of the cornerstones of brain imaging; however, post-contrast FLAIR images have not offered a clear advantage over standard T1-weighted images as suggested by early studies. FLAIR imaging with echoplanar acquisition is not considered advantageous, because the decreased imaging times are obtained at the expense of lower sensitivity. For a number of applications, diffusion-weighted imaging has surpassed FLAIR. Nevertheless, FLAIR images may be more sensitive for the detection of acute brain infarction. Recently described methods for the elimination of CSF flow artifacts may lead to improved quality and reliability of FLAIR images for subarachnoid space disease. MT preparation is now routinely incorporated in time-of-flight MR angiography and gradient-echo T2*-weighted spine imaging sequences and provides increased sensitivity for postcontrast MR imaging. These applications may not be advantageous in all clinical settings. MTR analysis offers valuable information for an increasing number of pathologic processes but has not yet gained wide clinical acceptance owing to sophisticated postprocessing and significant intercenter variations. Different modifications of these techniques are being evaluated, and further developments are expected.

Optimisation of the 3D MDEFT sequence for anatomical brain imaging: technical implications at 1.5 and 3 T

An algorithm for the optimisation of 3D Modified Driven Equilibrium Fourier Transform (MDEFT) sequences for T1-weighted anatomical brain imaging is presented. Imaging parameters are optimised for a clinical whole body scanner and a clinical head scanner operating at 1.5 and 3 T, respectively. In vivo studies show that the resulting sequences allow for the whole brain acquisition of anatomical scans with an isotropic resolution of 1 mm and high contrast-to-noise ratio (CNR) in an acceptable scan time of 12 min. Typical problems related to the scanner-specific hardware configurations are discussed in detail, especially the occurrence of flow artefacts in images acquired with head transmit coils and the enhancement of scalp intensities in images acquired with phased array receive coils. It is shown both theoretically and experimentally that these problems can be avoided by using spin tagging and fat saturation.

Assessment and correction ofB1-induced errors in magnetization transfer ratio measurements

The magnetization transfer ratio (MTR) is strongly related to the field strength (B(1)) of the saturation pulse. B(1) variations therefore can result in significant MTR variations and can affect histogram analysis, particularly if data from a large volume of interest are included. A multicenter study was performed to determine the typical range of B(1) errors and the corresponding MTR variations in brain tissue of healthy volunteers. Seven subjects were included at each center resulting in a total cohort of 28 subjects. Additionally, numerical simulations were done to study this relationship more generally for pulsed saturation. It could be demonstrated, both theoretically and empirically, that for typical B(1) errors there is a linear relationship between B(1) error and the corresponding MTR change. In addition, for proton density-weighted sequences, this relationship seems to be largely independent of the underlying relaxation properties. Mean B(1) errors in the entire brain were typically in the range between -3% and -7%. Due to different coil characteristics, significant MTR differences between different scanners and sites were observed. Using a simple correction scheme that is based on a linear regression analysis between MTR and B(1) data it was possible to reduce the intersubject variation by approximately 50%. Furthermore, interscanner variation could be reduced such that no significant differences between scanners could be detected. The correction scheme may be useful when investigating MTR as an outcome measure in single or multicenter studies.

Quantitative magnetization transfer mapping of bound protons in multiple sclerosis

Quantitative analysis of magnetization transfer images has the potential to allow a more thorough characterization of the protons, both bound and free, in a tissue by extracting a number of parameters relating to the NMR properties of the protons and their local environment. This work develops previously presented techniques to produce estimates of parameters such as the bound proton fraction, f, and the transverse relaxation time of the bound pool, T(2B), for the whole brain in a clinically acceptable imaging time. This is achieved by limiting the number of data collected (typically to 10); to collect 28 5-mm slices with a reconstructed resolution of 0.94 x 0.94 mm. The protocol takes 82 sec per data point. The fitting technique is assessed against previous work and for fitting failures. Maps and analysis are presented from a group of seven controls and 20 multiple sclerosis patients. The maps show that the parameters are sensitive to tissue-specific differences and can detect pathological change within lesions. Statistically significant differences in parameters such as T(2B) and f are seen between normal-appearing white matter, multiple sclerosis lesions, and control white matter. Whole-brain histograms of these parameters are also presented, showing differences between patients and controls.

Preliminary magnetic resonance study of the macromolecular proton fraction in white matter: a potential marker of myelin?

We report on a new quantitative magnetization transfer (MT) technique that allows for the in vivo estimation of the macromolecular proton fraction (f) and the bound pool T2 relaxation time (T2b), whilst permitting whole brain coverage. In this pilot study, five subjects with multiple sclerosis (MS) and five healthy controls were studied. Both f and T2b were significantly different between MS lesions and normal control white matter (WM). Relationships between f and T1 relaxation time [Spearmans rank correlation coefficient (r(s)) = -0.97, P < 0.001] and f and the magnetization transfer ratio (MTR; r(s) = 0.80, P < 0.001) were observed. Compared with MTR, f and T2b have the potential advantage of relative independence from MT acquisition protocol while offering more pathologically specific information. In particular, f may provide a more direct indication of myelin content in WM.

Guidelines for using quantitative magnetization transfer magnetic resonance imaging for monitoring treatment of multiple sclerosis

Quantitative evaluation of brain magnetic resonance imaging (MRI) scans is now an accepted part of the trial of new putative treatments for multiple sclerosis (MS). However, conventional MRI is not pathologically specific, and it does not reveal the details of the pathological processes that underlie the progression of the disease. Magnetization transfer (MT) imaging is a relatively new quantitative technique that appears to offer some pathological specificity, and can be used to monitor the changes over time in both individual lesions and the central nervous system as a whole. This paper considers the case for incorporating MT imaging into new clinical trials, so that the utility of MT for monitoring the modification of MS progression by treatment can be assessed. Specific guidelines for implementing MT imaging as part of a large multicenter clinical trial are given, and practical considerations when planning such a trial are detailed. It is anticipated that MT imaging will be incorporated into many new trials in the near future.

Precise estimate of fundamental in-vivo MT parameters in human brain in clinically feasible times

A methodology is presented for extracting precise quantitative MT parameters using a magnetisation-prepared spoiled gradient echo sequence. This method, based on a new mathematical model, provides relaxation parameters for human brain in-vitro and in-vivo. The in-vivo parameters have been obtained from three different regions of normal white matter: occipital white matter, frontal white matter and centrum semiovale; two regions of normal grey matter: cerebral cortex and cerebellum, and from five regions with MS lesions. All this has been achieved using MT images collected within a timeframe that is clinically feasible. We hope that this new technique will shed light on the properties and dynamics of water compartments within the brain.

Role of magnetic resonance imaging in the diagnosis and monitoring of multiple sclerosis: consensus report of the White Matter Study Group

On June 24-26, 2001, the first meeting of the White Matter Study Group (WMSG) of the International Society for Magnetic Resonance in Medicine (ISMRM) was held in Bordeaux, France. This paper is the report of the consensus reached among the delegates of the meeting on how to use magnetic resonance imaging (MRI) to make an early diagnosis of multiple sclerosis (MS), to measure MS activity accurately and reliably, and to monitor the effect of treatment on disease evolution.

MRI techniques to monitor MS evolution: the present and the future

Conventional MRI (cMRI) is limited in its ability to provide specific information about pathology in MS. Measures commonly derived from cMRI include T2 lesions, T1-enhanced lesions, atrophy, and possibly T1-hypointense lesions, which have been extensively investigated in many clinical trials. Better MRI measures are needed to advance our understanding of MS and design ideal clinical trials. This article reviews the strengths and weaknesses of the major MRI-based methods used to monitor MS evolution and submits that 1) metrics derived from magnetization transfer MRI, diffusion-weighted MRI, and proton MRS should be implemented to achieve reliable specific in vivo quantification of MS pathology; 2) targeted multiparametric MRI protocols rather than generic application of cMRI should be used in all possible clinical circumstances and trials; and 3) reproducible quantitative MR measures should ideally be used for the assessment of patients but are essential for clinical trials.

Quantitative imaging of magnetization transfer exchange and relaxation properties in vivo using MRI

We describe a novel imaging technique that yields all of the observable properties of the binary spin-bath model for magnetization transfer (MT) and demonstrate this method for in vivo studies of the human head. Based on a new model of the steady-state behavior of the magnetization during a pulsed MT-weighted imaging sequence, this approach yields parametric images of the fractional size of the restricted pool, the magnetization exchange rate, the T(2) of the restricted pool, as well as the relaxation times in the free pool. Validated experimentally on agar gels and samples of uncooked beef, we demonstrate the method's application on two normal subjects and a patient with multiple sclerosis.

Magnetization transfer of water T(2) relaxation components in human brain: implications for T(2)-based segmentation of spectroscopic volumes

Biexponential T(2) relaxation of the localized water signal can be used for segmentation of spectroscopic volumes. To assess the specificity of the components an iterative relaxation measurement of the localized water signal (STEAM, 12 echo times, geometric spacing from 30 ms to 2000 ms) was combined with magnetization transfer (MT) saturation (40 single lobe pulses, 12 ms duration, 1440 degrees nominal flip angle, 1 kHz offset, repeated every 30 ms). Voxels including CSF were examined in parietal cortex and periventricular parietal white matter (10 each), as well as 13 voxels in central white matter and 16 T(1)-hypointense non-enhancing multiple sclerosis lesions without CSF inclusion. Biexponential models (excluding myelin water) were fitted to the relaxation data. In periventricular VOIs the component of long T(2) (1736 +/- 168 ms) that is attributed to CSF was not affected by MT. In cortical VOIs this component had markedly shorter T(2)'s (961 +/- 239 ms) and showed both attenuation and prolongation with MT, indicating contributions from tissue. MS lesions and central WM showed a second tissue component of intermediate T(2) (160-410 ms). In white matter similar MT attenuation indicated strong exchange between the two tissue components, prohibiting segmentation. In MS lesions, however, markedly less MT of the intermediate component was found, which is consistent with decreased cellularity and exchange in a region that is large compared to diffusion motion.

In vivo magnetization transfer measurements of experimental spinal cord injury in the rat

Magnetization transfer (MT) imaging techniques were implemented to study a clip compression model of spinal cord injury (SCI) in the rat. The purpose of this study was to determine if the magnetization transfer ratio (MTR) could be used to classify the stage and severity of SCI. Two clip compression injuries were studied: mild SCI and severe SCI. MTRs were determined for gray matter (GM) and white matter (WM) regions and the GM-WM contrast was determined on days 1 and 7 following surgery. Despite differences in pathologic features of mild and severe SCI, the GM-WM contrast did not allow discrimination between the two degrees of severity of SCI. WM MTR allowed differentiation of mild and severe SCI on day 1. These preliminary results suggest that WM MTR may provide an indication of the severity of injury in SCI. Magn Reson Med 45:159-163, 2001.

The future of magnetic resonance-based techniques in neurology

Magnetic resonance techniques have become increasingly important in neurology for defining: 1. brain, spinal cord and peripheral nerve or muscle structure; 2. pathological changes in tissue structures and properties; and 3. dynamic patterns of functional activation of the brain. New applications have been driven in part by advances in hardware, particularly improvements in magnet and gradient coil design. New imaging strategies allow novel approaches to contrast with, for example, diffusion imaging, magnetization transfer imaging, perfusion imaging and functional magnetic resonance imaging. In parallel with developments in hardware and image acquisition have been new approaches to image analysis. These have allowed quantitative descriptions of the image changes to be used for a precise, non-invasive definition of pathology. With the increasing capabilities and specificity of magnetic resonance techniques it is becoming more important that the neurologist is intimately involved in both the selection of magnetic resonance studies for patients and their interpretation. There is a need for considerably improved access to magnetic resonance technology, particularly in the acute or intensive care ward and in the neurosurgical theatre. This report illustrates several key developments. The task force concludes that magnetic resonance imaging is a major clinical tool of growing significance and offers recommendations for maximizing the potential future for magnetic resonance techniques in neurology.

Quantitative interpretation of magnetization transfer in spoiled gradient echo MRI sequences

A method for analyzing general pulsed magnetization transfer (MT) experiments in which off-resonance saturation pulses are interleaved with on-resonance excitation pulses is presented. We apply this method to develop a steady-state signal equation for MT-weighted spoiled gradient echo sequences and consider approximations that facilitate its rapid computation. Using this equation, we assess various experimental designs for quantitatively imaging the fractional size of the restricted pool, cross-relaxation rate, and T(1) and T(2) relaxation times of the two pools in a binary spin bath system. From experiments on agar gel, this method is shown to reliably and accurately estimate the exchange and relaxation properties of a material in an imaging context, suggesting the feasibility of using this technique in vivo.

Magnetization transfer and multicomponent T2 relaxation measurements with histopathologic correlation in an experimental model of MS

Magnetization transfer and multicomponent T2 imaging techniques were implemented to study guinea pig in vivo. A chronic-progressive model of experimental allergic encephalomyelitis (EAE) was produced, and the inflammatory component of the disease was manipulated using antibodies against integrin. The magnetization transfer ratio (MTR) and T2 relaxation properties were measured in normal-appearing white matter (NAWM) with histological comparisons. Significant reductions in both the mean MTR and the myelin water percentage were measured in NAWM of EAE guinea pig brain. However, the MTR and myelin water percentage appear to measure different aspects of pathology in NAWM in EAE. Reductions in the MTR were prevented or reversed with suppression of inflammation. However, modulation of inflammatory activity was not reflected in the measurement of the myelin water percentage. Since the amount of myelin is not expected to vary with inflammatory-related changes, these observations support our hypothesis that the MTR is sensitive to physiological changes to myelin induced by inflammation, while the short T2 component is a more specific indicator of myelin content in tissue. Pathologic features other than demyelination may be important in the determination of the MTR.

Magnetization transfer ratio of the spinal cord in multiple sclerosis: relationship to atrophy and neurologic disability

The authors compare the spinal cord magnetization transfer ratio (MTR) of multiple sclerosis (MS) patients to healthy volunteers, relate MTR to spinal cord atrophy, and relate these and other magnetic resonance (MR) imaging parameters to disability. Sixty-five patients with MS (14 relapsing remitting [RR], 34 secondary progressive [SP], and 17 primary progressive [PP] MS), and 9 healthy volunteers were studied using MR at 1.0 T. Disability of the patients was assessed using the expanded disability status scale (EDSS). Magnetic resonance parameters were upper spinal cord MTR, number of focal spinal lesions, presence of diffuse abnormalities, and spinal cord cross-sectional area (CSA). Correlations were assessed using Spearman's rank correlation coefficient (r). Magnetization transfer ratio was higher in the controls (median, 33%; range, 30%-38%) than in patients with MS (median, 30%; range, 16-36; p < 0.05). In patients with MS EDSS correlated with spinal cord MTR, albeit weakly (r = -0.25, p < 0.05). Correlation between EDSS and spinal cord CSA was better (SRCC = -0.40, p < 0.01). No correlation was found between MTR and CSA (r = 0.1, p = 0.4). Combining MTR with spinal cord CSA improved correlation with EDSS (r = -0.46, p < 0.001), suggesting an independent correlation between disability and these 2 MR parameters. Expanded disability status scale scores were higher in patients who had diffuse spinal cord abnormality regardless of focal lesions (median, 6; range, 1.5-7.5) than in patients without diffuse abnormalities (median, 3.5; range, 0-8; p < 0.01). CSA was lower in patients with diffuse spinal cord abnormality (median, 62; range, 46-89 mm2) than in patients without diffuse abnormalities (median, 73; range, 47-89 mm2; p < 0.01). MTR was slightly lower in patients with diffuse spinal cord abnormalities (median, 29; range, 21%-33%) than in patients without diffuse abnormalities (median, 31; range, 16-36; t-test, p < 0.05).