Pulsed magnetization transfer imaging with body coil transmission at 3 Tesla: feasibility and application

Enhanced and robust contrast in CEST MRI: Saturation pulse shape design via optimal control

PURPOSE

To employ optimal control for the numerical design of Chemical Exchange Saturation Transfer (CEST) saturation pulses to maximize contrast and stability againstB 0 $$ {\mathrm{B}}_0 $$ inhomogeneities.

THEORY AND METHODS

We applied an optimal control framework for the design pulse shapes for CEST saturation pulse trains. The cost functional minimized both the pulse energy and the discrepancy between the corresponding CEST spectrum and the target spectrum based on a continuous radiofrequency (RF) pulse. The optimization is subject to hardware limitations. In measurements on a 7 T preclinical scanner, the optimal control pulses were compared to continuous-wave and Gaussian saturation methods. We conducted a comparison of the optimal control pulses with Gaussian, block pulse trains, and adiabatic spin-lock pulses.

RESULTS

The optimal control pulse train demonstrated saturation levels comparable to continuous-wave saturation and surpassed Gaussian saturation by up to 50 % in phantom measurements. In phantom measurements at 3 T the optimized pulses not only showcased the highest CEST contrast, but also the highest stability against field inhomogeneities. In contrast, block pulse saturation resulted in severe artifacts. Dynamic Bloch-McConnell simulations were employed to identify the source of these artifacts, and underscore theB 0 $$ {\mathrm{B}}_0 $$ robustness of the optimized pulses.

CONCLUSION

In this work, it was shown that a substantial improvement in pulsed saturation CEST imaging can be achieved by using Optimal Control design principles. It is possible to overcome the sensitivity of saturation to B0 inhomogeneities while achieving CEST contrast close to continuous wave saturation.

Candidate imaging biomarkers for PMP22-related inherited neuropathies

OBJECTIVE

Charcot-Marie-Tooth type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsy (HNPP) are caused by mutations to the peripheral myelin protein 22 (PMP22) gene. A need exists for sensitive and reliable biomarkers of progression and treatment response. Magnetic resonance imaging (MRI) metrics of nerve pathology and morphology were investigated for this purpose.

METHODS

MRI was performed at 3.0 T in the thigh of CMT1A (N = 11) and HNPP patients (N = 12) and controls (N = 23). Three potential imaging biomarkers of the sciatic nerve were investigated: 1) magnetization transfer ratio (MTR), which assays myelin content, and 2) cross-sectional area (CSA) and 3) circularity, which assay morphological changes. Potential imaging biomarkers were compared across cohorts and assessed for relationships with disability in the legs (CMTESL ), compound motor action potentials (CMAP), and motor conduction velocities (MCV). Inter-rater reliability and test-retest repeatability were established for each imaging metric.

RESULTS

Significant differences in MTR, CSA, and circularity were observed in CMT1A relative to controls (p = 0.02, p < 0.001, and p = 0.003, respectively, via Wilcoxon rank-sum tests). Differences were not observed in the HNPP cohort. Significant relationships were observed between MTR and clinical metrics (CMTESL : p = 0.003, CMAP: p = 0.03, MCV: p = 0.01); and between CSA and electrophysiology (CMAP: p = 0.002, MCV: p < 0.001). All metrics were reliable and repeatable with MTR the most reliable (intraclass correlation coefficient [ICC] >0.999, CV = 0.30%) and repeatable (ICC = 0.84, CV = 3.16%).

INTERPRETATION

MTR, CSA, and circularity showed promise as reliable and sensitive biomarkers of CMT1A, but not HNPP. These warrant longitudinal investigation as response biomarkers in upcoming clinical trials of CMT1A, while other methods should be considered for HNPP.

Review and consensus recommendations on clinical APT-weighted imaging approaches at 3T: Application to brain tumors

Amide proton transfer-weighted (APTw) MR imaging shows promise as a biomarker of brain tumor status. Currently used APTw MRI pulse sequences and protocols vary substantially among different institutes, and there are no agreed-on standards in the imaging community. Therefore, the results acquired from different research centers are difficult to compare, which hampers uniform clinical application and interpretation. This paper reviews current clinical APTw imaging approaches and provides a rationale for optimized APTw brain tumor imaging at 3 T, including specific recommendations for pulse sequences, acquisition protocols, and data processing methods. We expect that these consensus recommendations will become the first broadly accepted guidelines for APTw imaging of brain tumors on 3 T MRI systems from different vendors. This will allow more medical centers to use the same or comparable APTw MRI techniques for the detection, characterization, and monitoring of brain tumors, enabling multi-center trials in larger patient cohorts and, ultimately, routine clinical use.

Quantitative magnetization transfer imaging in relapsing-remitting multiple sclerosis: a systematic review and meta-analysis

Myelin-sensitive MRI such as magnetization transfer imaging has been widely used in multiple sclerosis. The influence of methodology and differences in disease subtype on imaging findings is, however, not well established. Here, we systematically review magnetization transfer brain imaging findings in relapsing-remitting multiple sclerosis. We examine how methodological differences, disease effects and their interaction influence magnetization transfer imaging measures. Articles published before 06/01/2021 were retrieved from online databases (PubMed, EMBASE and Web of Science) with search terms including 'magnetization transfer' and 'brain' for systematic review, according to a pre-defined protocol. Only studies that used human in vivo quantitative magnetization transfer imaging in adults with relapsing-remitting multiple sclerosis (with or without healthy controls) were included. Additional data from relapsing-remitting multiple sclerosis subjects acquired in other studies comprising mixed disease subtypes were included in meta-analyses. Data including sample size, MRI acquisition protocol parameters, treatments and clinical findings were extracted and qualitatively synthesized. Where possible, effect sizes were calculated for meta-analyses to determine magnetization transfer (i) differences between patients and healthy controls; (ii) longitudinal change and (iii) relationships with clinical disability in relapsing-remitting multiple sclerosis. Eighty-six studies met inclusion criteria. MRI acquisition parameters varied widely, and were also underreported. The majority of studies examined the magnetization transfer ratio in white matter, but magnetization transfer metrics, brain regions examined and results were heterogeneous. The analysis demonstrated a risk of bias due to selective reporting and small sample sizes. The pooled random-effects meta-analysis across all brain compartments revealed magnetization transfer ratio was 1.17 per cent units (95% CI -1.42 to -0.91) lower in relapsing-remitting multiple sclerosis than healthy controls (z-value: -8.99, P < 0.001, 46 studies). Linear mixed-model analysis did not show a significant longitudinal change in magnetization transfer ratio across all brain regions [β = 0.12 (-0.56 to 0.80), t-value = 0.35, P = 0.724, 14 studies] or normal-appearing white matter alone [β = 0.037 (-0.14 to 0.22), t-value = 0.41, P = 0.68, eight studies]. There was a significant negative association between the magnetization transfer ratio and clinical disability, as assessed by the Expanded Disability Status Scale [r = -0.32 (95% CI -0.46 to -0.17); z-value = -4.33, P < 0.001, 13 studies]. Evidence suggests that magnetization transfer imaging metrics are sensitive to pathological brain changes in relapsing-remitting multiple sclerosis, although effect sizes were small in comparison to inter-study variability. Recommendations include: better harmonized magnetization transfer acquisition protocols with detailed methodological reporting standards; larger, well-phenotyped cohorts, including healthy controls; and, further exploration of techniques such as magnetization transfer saturation or inhomogeneous magnetization transfer ratio.

Development of Radiofrequency Saturation Amplitude-independent Quantitative Markers for Magnetization Transfer MRI of Prostate Cancer

BACKGROUND

In the United States, prostate cancer has a relatively large impact on men's health. Magnetic resonance imaging (MRI) is useful for the diagnosis and treatment of prostate cancer.

INTRODUCTION

The purpose of this study was to develop a quantitative marker for use in prostate cancer magnetization transfer (MT) magnetic resonance imaging (MRI) studies that is independent of radiofrequency (RF) saturation amplitude.

METHODS

Eighteen patients with biopsy-proven prostate cancer were enrolled in this study. MTMRI images were acquired using four RF saturation amplitudes at 33 frequency offsets. ROIs were delineated for the peripheral zone (PZ), central gland (CG), and tumor. Z-spectral data were collected in each region and fit to a three-parameter equation. The three parameters are: the magnitude of the bulk water pool (Aw), the full width at half maximum of the water pool (Gw), and the magnitude of the bound pool (Ab), while, the slopes from the linear regressions of Gw and Ab on RF saturation amplitude (called kAb and kGw) were used as quantitative markers.

RESULTS

A pairwise statistically significant difference was found between the PZ and tumor regions for the two saturation amplitude-independent quantitative markers. No pairwise statistically significant differences were found between the CG and tumor regions for any quantitative markers.

CONCLUSION

The significant differences between the values of the two RF saturation amplitudeindependent quantitative markers in the PZ and tumor regions reveal that these markers may be capable of distinguishing healthy PZ tissue from prostate cancer.

Impaired biophysical integrity of macromolecular protein pools in the uncinate circuit in late-life depression

Major depressive disorder is a common mood disorder in the elderly. Although the neuroanatomical abnormalities have been identified in patients with late-life depression (LLD), the precise biological basis of LLD remains largely unknown. The purpose of this study was to examine the biophysical integrity of macromolecular protein pools in the nodal regions of the "uncinate circuit," a component of fronto-limbic circuitry that is connected by the uncinate fasciculus and is critical in the regulation of mood and emotions, using novel magnetization transfer (MT) imaging. Twenty-four patients with LLD and 27 non-depressed healthy control subjects (HCs) of comparable age, sex, and race were recruited from the communities of the greater Chicago Area. The nodal regions of the uncinate circuit, i.e., bilateral amygdala, hippocampus, and lateral and medial orbitofrontal cortices (OFCs), were examined. Compared with HCs, patients with LLD had significantly lower magnetization transfer ratio (MTR), a measure of the biophysical integrity of macromolecular protein pools, in bilateral amygdala and hippocampus. The lower MTR was negatively correlated with the depression score. Moreover, the MTR of these regions decreased with age and positively correlated with neuropsychological performance in the LLD group but not in the HC group. These findings suggest that LLD is associated with compromised biophysical integrity of macromolecular protein pools in nodal regions of the uncinate circuit, and that major depression may accentuate age-related attenuation of the biophysical integrity of macromolecular protein pools in this circuit. These findings provide important new insights into the neurobiological mechanisms of the pathophysiology of LLD.

Quantitative magnetization transfer imaging of the human locus coeruleus

The locus coeruleus (LC) is the major origin of norepinephrine in the central nervous system, and is subject to age-related and neurodegenerative changes, especially in disorders such as Parkinson's disease and Alzheimer's disease. Previous studies have shown that neuromelanin (NM)-sensitive MRI can be used to visualize the LC, and it is hypothesized that magnetization transfer (MT) effects are the primary source of LC contrast. The aim of this study was to characterize the MT effects in LC imaging by applying high spatial resolution quantitative MT (qMT) imaging to create parametric maps of the macromolecular content of the LC and surrounding tissues. Healthy volunteers (n = 26; sex = 17 F/9M; age = 41.0 ± 19.1 years) underwent brain MRI on a 3.0 T scanner. qMT data were acquired using a 3D MT-prepared spoiled gradient echo sequence. A traditional NM scan consisting of a T1-weighted turbo spin echo sequence with MT preparation was also acquired. The pool-size ratio (PSR) was estimated for each voxel using a single-point qMT approach. The LC was semi-automatically segmented on the MT-weighted images. The MT-weighted images provided higher contrast-ratio between the LC and surrounding pontine tegmentum (PT) (0.215 ± 0.031) than the reference images without MT-preparation (-0.005 ± 0.026) and the traditional NM images (0.138 ± 0.044). The PSR maps showed significant differences between the LC (0.090 ± 0.009) and PT (0.188 ± 0.025). The largest difference between the PSR values in the LC and PT was observed in the central slices, which also correspond to those with the highest contrast-ratio. These results highlight the role of MT in generating NM-related contrast in the LC, and should serve as a foundation for future studies aiming to quantify pathological changes in the LC and surrounding structures in vivo.

Low duty-cycle pulsed irradiation reduces magnetization transfer and increases the inhomogeneous magnetization transfer effect

Intense off-resonant RF irradiation can lead to saturation of the macromolecular pool magnetization and enhance bound pool dipolar order responsible for the inhomogeneous magnetization transfer (ihMT) effect, but the intensity of RF power in human imaging studies is limited by safety constraints on RF heating. High RF intensities can still be achieved if applied in short pulses with low duty-cycle. Here we investigate the benefits of low duty-cycle irradiation for MT and ihMT studies with both theoretical and experimental methods. Solutions for pulsed irradiation of a two-pool model including dipolar order effects were implemented. Experiments were conducted at 3 T in the brain and through the calf of healthy human subjects. 2D echo planar images were acquired following a preparation of RF irradiation with a 2 s train of 5 ms pulses repeated from between 10 to 100 ms for duty-cycles (DCs) of 50% to 5%, and at varying offset frequencies, and time averaged RF powers. MT and ihMT data were measured in regions of interest within gray matter, white matter and muscle, and fit to the model. RF irradiation effects on signal intensity were reduced at 5% relative to 50% DCs. This reduced RF effect was much larger for single than dual frequency irradiation. 5% DC irradiation reduced single and dual frequency MT ratios but increased ihMT ratios up to 3 fold in brain tissues. Muscle ihMT increased by an even larger factor, depending on the frequency and applied power. The model predicted these changes with duty-cycle. The model fit the data well and constrained model parameters. Low duty-cycle pulsed irradiation reduces MT effects and markedly increases dipolar order effects. This approach is an attractive method to enhance ihMT signal-to-noise ratio and demonstrates a measurable ihMT effect in muscle tissue at 3 T under acceptable specific absorption rates. The effects of duty-cycle changes demonstrated in a separate MT/ihMT preparation provide a route for new applications in magnetization-prepared MRI sequences.

Pool size ratio of the substantia nigra in Parkinson's disease derived from two different quantitative magnetization transfer approaches

PURPOSE

We sought to measure quantitative magnetization transfer (qMT) properties of the substantia nigra pars compacta (SNc) in patients with Parkinson's disease (PD) and healthy controls (HCs) using a full qMT analysis and determine whether a rapid single-point measurement yields equivalent results for pool size ratio (PSR).

METHODS

Sixteen different MT-prepared MRI scans were obtained at 3 T from 16 PD patients and eight HCs, along with B1, B0, and relaxation time maps. Maps of PSR, free and macromolecular pool transverse relaxation times ([Formula: see text], [Formula: see text]) and rate of MT exchange between pools (k _mf ) were generated using a full qMT model. PSR maps were also generated using a single-point qMT model requiring just two MT-prepared images. qMT parameter values of the SNc, red nucleus, cerebral crus, and gray matter were compared between groups and methods.

RESULTS

PSR of the SNc was the only qMT parameter to differ significantly between groups (p < 0.05). PSR measured via single-point analysis was less variable than with the full MT model, provided slightly better differentiation of PD patients from HCs (area under curve 0.77 vs. 0.75) with sensitivity of 0.75 and specificity of 0.87, and was better than transverse relaxation time in distinguishing PD patients from HCs (area under curve 0.71, sensitivity 0.87, and specificity 0.50).

CONCLUSION

The increased PSR observed in the SNc of PD patients may provide a novel biomarker of PD, possibly associated with an increased macromolecular content. Single-point PSR mapping with reduced variability and shorter scan times relative to the full qMT model appears clinically feasible.

Magnetization Transfer Contrast and Chemical Exchange Saturation Transfer MRI. Features and analysis of the field-dependent saturation spectrum

Magnetization Transfer Contrast (MTC) and Chemical Exchange Saturation Transfer (CEST) experiments measure the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss ("saturation"). This allows molecular information to be accessed with the enhanced sensitivity of MRI. In analogy to Magnetic Resonance Spectroscopy (MRS), these saturation data are presented as a function of the chemical shift of participating proton groups, e.g. OH, NH, NH₂, which is called a Z-spectrum. In tissue, these Z-spectra contain the convolution of multiple saturation transfer effects, including nuclear Overhauser enhancements (NOEs) and chemical exchange contributions from protons in semi-solid and mobile macromolecules or tissue metabolites. As a consequence, their appearance depends on the magnetic field strength (B₀) and pulse sequence parameters such as B₁ strength, pulse shape and length, and interpulse delay, which presents a major problem for quantification and reproducibility of MTC and CEST effects. The use of higher B₀ can bring several advantages. In addition to higher detection sensitivity (signal-to-noise ratio, SNR), both MTC and CEST studies benefit from longer water T₁ allowing the saturation transferred to water to be retained longer. While MTC studies are non-specific at any field strength, CEST specificity is expected to increase at higher field because of a larger chemical shift dispersion of the resonances of interest (similar to MRS). In addition, shifting to a slower exchange regime at higher B₀ facilitates improved detection of the guanidinium protons of creatine and the inherently broad resonances of the amine protons in glutamate and the hydroxyl protons in myoinositol, glycogen, and glucosaminoglycans. Finally, due to the higher mobility of the contributing protons in CEST versus MTC, many new pulse sequences can be designed to more specifically edit for CEST signals and to remove MTC contributions.

Metabolic Aberrations Impact Biophysical Integrity of Macromolecular Protein Pools in the Default Mode Network

The brain's default mode network (DMN), having a high rate of basal energy metabolism, is vulnerable to altered glucose metabolism in type 2 diabetes mellitus (T2DM) due to insulin resistance and chronic hyperglycemia. Previous studies showed that functional connectivity and structural connectivity among the DMN nodal regions are compromised in T2DM. We applied magnetization transfer imaging to examine the impact of T2DM on the biophysical integrity of the DMN. The results showed that the biophysical integrity of macromolecular protein pools in the posterior cingulate cortex (PCC), a central DMN hub region, was selectively compromised in T2DM, whereas the other nodal regions of the DMN, including the medial prefrontal cortex, lateral inferior parietal cortex, precuneus, and medial and lateral temporal cortices, were biophysically intact compared with those of control subjects without diabetes. Furthermore, the degree of biophysical impairment of the PCC correlated with both hyperglycemia and vascular compromise, the two physiological hallmarks of diabetes. These new findings demonstrate that the PCC is vulnerable in the DMN and may shed light on the molecular neurobiology of T2DM and help to elucidate the pathophysiology of diabetes-related cognitive comorbidities and increased risk for dementia.

Biophysical changes in subcortical nuclei: the impact of diabetes and major depression

Magnetization transfer (MT) is a neuroimaging technique that is frequently used to characterize the biophysical abnormalities in both gray and white matter regions of the brain. In our study, we used MT to examine the integrity of key nodes in frontal-subcortical circuits in four subject groups: patients diagnosed with type 2 diabetes with and without major depression (MDD), a healthy control group, and a group diagnosed with MDD without diabetes. In the MDD group, MT studies demonstrated lower magnetization transfer ratios (MTR), a marker of abnormalities in the macromolecular protein pool, in the thalami when compared with the control groups. The group with diabetes and MDD showed lower MTR in the globus pallidus when compared with the group with MDD. Biophysical measures, in subcortical nuclei, correlated inversely with measures of glycemic control, cerebrovascular burden and depression scores. These findings have broad implications for the underlying neuronal circuitry and neurobiology of mood disorders.

Development and aging of superficial white matter myelin from young adulthood to old age: Mapping by vertex-based surface statistics (VBSS)

Superficial white matter (SWM) lies immediately beneath cortical gray matter and consists primarily of short association fibers. The characteristics of SWM and its development and aging were seldom examined in the literature and warrant further investigation. Magnetization transfer imaging is sensitive to myelin changes in the white matter. Using an innovative multimodal imaging analysis approach, vertex-based surface statistics (VBSS), the current study vertexwise mapped age-related changes of magnetization transfer ratio (MTR) in SWM from young adulthood to old age (30-85 years, N = 66). Results demonstrated regionally selective and temporally heterochronologic changes of SWM MTR with age, including (1) inverted U-shaped trajectories of SWM MTR in the rostral middle frontal, medial temporal, and temporoparietal regions, suggesting continuing myelination and protracted maturation till age 40-50 years and accelerating demyelination at age 60 and beyond, (2) linear decline of SWM MTR in the middle and superior temporal, and pericalcarine areas, indicating early maturation and less acceleration in age-related degeneration, and (3) no significant changes of SWM MTR in the primary motor, somatosensory and auditory regions, suggesting resistance to age-related deterioration. We did not observe similar patterns of changes in cortical thickness in our sample, suggesting the observed SWM MTR changes are not due to cortical atrophy. Hum Brain Mapp 37:1759-1769, 2016. © 2016 Wiley Periodicals, Inc.

Simultaneous optimization of power and duration of radio-frequency pulse in PARACEST MRI

Chemical exchange saturation transfer (CEST) MRI is increasingly used to probe mobile proteins and microenvironment properties, and shows great promise for tumor and stroke diagnosis. The CEST effect is complex and depends not only on the CEST agent concentration, exchange rates, the characteristic of the magnetization transfer (MT), and the relaxation properties of the tissue, but also varies with the experimental conditions such as radio-frequency (RF) pulse power and duration. The RF pulse is one of the most important factors that promote the CEST effect for biological properties such as pH, temperature and protein content, especially for contrast agents with intermediate to fast exchange rates. The CEST effect is susceptible to the RF duration and power. The present study aims at determining the optimal power and the corresponding optimal duration (that maximize the CEST effect) using an off-resonance scheme through a new definition of the CEST effect. This definition is formulated by solving the Bloch-McConnell equation through the R1ρ method (based on the eigenspace solution) for both of the MT and CEST effects as well as their interactions. The proposed formulations of the optimal RF pulse power and duration are the first formulations in which the MT effect is considered. The extracted optimal RF pulse duration and power are compared with those of the MTR asymmetry model in two- and three-pool systems, using synthetic data that are similar to the muscle tissue. To validate them further, the formulations are compared with the empirical formulation of the CEST effect and other findings of the previous researches. By extending our formulations, the optimal power and the corresponding optimal duration (in the biological systems with many chemical exchange sites) can be determined.

Fast magnetization transfer and apparent T1 imaging using a short saturation pulse with and without inversion preparation

PURPOSE

Measurements of magnetization transfer (MT) metrics, such as the steady-state (SS) MT ratio and apparent longitudinal relaxation rate require multiple MT irradiation durations and a long experimental time. To overcome these problems, we propose a novel method using a short MT off-resonance pulse with and without on-resonance inversion preparation pulse.

THEORY AND METHODS

Computer simulations were performed to examine the accuracy of the proposed method and to find the optimal off-resonance irradiation pulse duration (Tirad) and power level (ω1). Our approach, with echo planar imaging data acquisition, was applied to animals at 9.4 T and humans at 3 T with ω1/2π = 100 Hz and 177 Hz, respectively. Steady-state MT ratio and relaxation rate were obtained from a pair of MT images at a Tirad, with and without inversion.

RESULTS

For Tirad ≥ 0.4 s, steady-state MT ratio, and relaxation rate measured at any single Tirad agreed well with those of the conventional fitting method that uses multiple Tirad. Our simulation indicates that a higher ω1 can use a shorter Tirad.

CONCLUSION

Steady-state MT ratio and relaxation rate can be determined from MT data with only one, short Tirad by incorporation of an inversion prepulse. This MT imaging approach is simple, fast, and easily implementable.

Molecular engineering of nonmetallic biosensors for CEST MRI

Recent advancements in molecular and synthetic biology, combined with synthetic chemistry and biotechnology, have opened up new opportunities to engineer novel platforms that can monitor complex biological processes with various noninvasive imaging modalities. After decades of using gadolinium- or iron-based metallic sensors for MRI, the recently developed chemical exchange saturation transfer (CEST) contrast mechanism has created an opportunity for rational design, in silico, of nonmetallic biosensors for MRI. These biomolecules are either naturally occurring compounds (amino acids, sugars, nucleosides, native proteins) or can be artificially engineered (synthetic probes or recombinant proteins). They can be administered either as exogenous agents or can be genetically (over)expressed. Moreover, they can be precisely engineered to achieve the desired biochemical properties for fine tuning optimized imaging schemes. The availability of these agents marks the dawn of a new scientific era for molecular and cellular MRI.

Impaired macromolecular protein pools in fronto-striato-thalamic circuits in type 2 diabetes revealed by magnetization transfer imaging

Previous research has shown that type 2 diabetes mellitus (T2DM) is associated with white matter microstructural changes, cognitive impairment, and decreased resting-state functional connectivity and spontaneous brain activity. This study used magnetization transfer imaging to examine, for the first time, the integrity of macromolecular protein pools in fronto-striato-thalamic circuits and its clinical and cognitive correlates in patients with T2DM. T2DM patients without mood disorders (n = 20, aged 65.05 ± 11.95 years) and healthy control subjects (HCs; n = 26, aged 62.92 ± 12.71 years) were recruited. Nodes of fronto-striato-thalamic circuits-head of the caudate nucleus (hCaud), putamen, globus pallidus, thalamus-and four cortical regions-rostral and dorsal anterior cingulate cortex, dorsolateral prefrontal cortex, and lateral orbitofrontal cortex-were examined. Compared with HCs, patients with T2DM had significantly lower magnetization transfer ratio (MTR) in bilateral anterior cingulate and hCaud. Reduced MTRs in the above regions showed correlations with T2DM-related clinical measures, including hemoglobin A1c level and vascular risk factors, and neuropsychological task performance in the domains of learning and memory, executive function, and attention and information processing. The impaired biophysical integrity of brain macromolecular protein pools and their local microenvironments in T2DM patients may provide insights into the neurological pathophysiology underlying diabetes-associated clinical and cognitive deficits.

Proximal nerve magnetization transfer MRI relates to disability in Charcot-Marie-Tooth diseases

OBJECTIVE

The objectives of this study were (1) to develop a novel magnetization transfer ratio (MTR) MRI assay of the proximal sciatic nerve (SN), which is inaccessible via current tools for assessing peripheral nerves, and (2) to evaluate the resulting MTR values as a potential biomarker of myelin content changes in patients with Charcot-Marie-Tooth (CMT) diseases.

METHODS

MTR was measured in the SN of patients with CMT type 1A (CMT1A, n = 10), CMT type 2A (CMT2A, n = 3), hereditary neuropathy with liability to pressure palsies (n = 3), and healthy controls (n = 21). Additional patients without a genetically confirmed subtype (n = 4), but whose family histories and electrophysiologic tests were consistent with CMT, were also included. The relationship between MTR and clinical neuropathy scores was assessed, and the interscan and inter-rater reliability of MTR was estimated.

RESULTS

Mean volumetric MTR values were significantly decreased in the SN of patients with CMT1A (33.8 ± 3.3 percent units) and CMT2A (31.5 ± 1.9 percent units) relative to controls (37.2 ± 2.3 percent units). A significant relationship between MTR and disability scores was also detected (p = 0.01 for genetically confirmed patients only, p = 0.04 for all patients). From interscan and inter-rater reliability analyses, proximal nerve MTR values were repeatable at the slicewise and mean volumetric levels.

CONCLUSIONS

MTR measurements may be a viable biomarker of proximal nerve pathology in patients with CMT.

Whole brain myelin mapping using T1- and T2-weighted MR imaging data

Despite recent advancements in MR imaging, non-invasive mapping of myelin in the brain still remains an open issue. Here we attempted to provide a potential solution. Specifically, we developed a processing workflow based on T1-w and T2-w MR data to generate an optimized myelin enhanced contrast image. The workflow allows whole brain mapping using the T1-w/T2-w technique, which was originally introduced as a non-invasive method for assessing cortical myelin content. The hallmark of our approach is a retrospective calibration algorithm, applied to bias-corrected T1-w and T2-w images, that relies on image intensities outside the brain. This permits standardizing the intensity histogram of the ratio image, thereby allowing for across-subject statistical analyses. Quantitative comparisons of image histograms within and across different datasets confirmed the effectiveness of our normalization procedure. Not only did the calibrated T1-w/T2-w images exhibit a comparable intensity range, but also the shape of the intensity histograms was largely corresponding. We also assessed the reliability and specificity of the ratio image compared to other MR-based techniques, such as magnetization transfer ratio (MTR), fractional anisotropy (FA), and fluid-attenuated inversion recovery (FLAIR). With respect to these other techniques, T1-w/T2-w had consistently high values, as well as low inter-subject variability, in brain structures where myelin is most abundant. Overall, our results suggested that the T1-w/T2-w technique may be a valid tool supporting the non-invasive mapping of myelin in the brain. Therefore, it might find important applications in the study of brain development, aging and disease.

Subcortical biophysical abnormalities in patients with mood disorders

Cortical-subcortical circuits have been implicated in the pathophysiology of mood disorders. Structural and biochemical abnormalities have been identified in patients diagnosed with mood disorders using magnetic resonance imaging-related approaches. In this study, we used magnetization transfer (MT), an innovative magnetic resonance approach, to study biophysical changes in both gray and white matter regions in cortical-subcortical circuits implicated in emotional regulation and behavior. Our study samples comprised 28 patients clinically diagnosed with major depressive disorder (MDD) and 31 non-depressed subjects of comparable age and gender. MT ratio (MTR), representing the biophysical integrity of macromolecular proteins within key components of cortical-subcortical circuits-the caudate, thalamic, striatal, orbitofrontal, anterior cingulate and dorsolateral regions-was the primary outcome measure. In our study, the MTR in the head of the right caudate nucleus was significantly lower in the MDD group when compared with the comparison group. MTR values showed an inverse relationship with age in both groups, with more widespread relationships observed in the MDD group. These data indicate that focal biophysical abnormalities in the caudate nucleus may be central to the pathophysiology of depression and critical to the cortical-subcortical abnormalities that underlie mood disorders. Depression may also accentuate age-related changes in the biophysical properties of cortical and subcortical regions. These observations have broad implications for the neuronal circuitry underlying mood disorders across the lifespan.

Magnetization transfer contrast imaging reveals amyloid pathology in Alzheimer's disease transgenic mice

The presence of amyloid plaques in the brain is one of the pathological hallmarks of Alzheimer's disease, which might already be present in the early stage of the disease. Therefore it is important to track amyloid plaques as early as possible. In this paper, we report magnetization transfer contrast magnetic resonance imaging (MTC MRI) as a novel approach to detect amyloid plaques in vivo. Two mice models, APP/PS1 and BRI, developing amyloid pathology were investigated with MTC MRI, T2 relaxation measurements and immunohistochemistry (IHC). MT-ratios of several brain regions were compared to T2-values and correlated with quantitative IHC, revealing amyloid load and gliosis in different brain regions. APP/PS1 mice develop large compact plaques, resembling late stage Alzheimer's disease, while rather small and diffuse plaques are deposited in BRI mice, reflecting early stage of Alzheimer's disease. We found significantly higher MT-ratio's in the brain of APP/PS1 mice as compared to their controls and similar trends in BRI mice. A region based MT-ratio and IHC analysis and correlations between MT-ratios and quantitative IHC indicate amyloid plaques as the main substrate for altered MT-ratios in transgenic animals. We additionally demonstrated the improved sensitivity of MTC MRI to amyloid pathology as compared to traditional T2 relaxation measurements. Our results suggest that MTC MRI reveals extensive, and potentially even early amyloid pathology. Further unraveling the MT-effect of each pathological feature during each stage of AD might indicate MTC MRI as a useful diagnostic technique.

Gender, age-related, and regional differences of the magnetization transfer ratio of the cortical and subcortical brain gray matter

PURPOSE

To explore gender, age-related, and regional differences of magnetization transfer ratio (MTR) of brain cortical and subcortical gray matter (GM).

MATERIALS AND METHODS

In all, 102 healthy subjects (51 women and 51 men; range 25-84 years) were examined with 3-mm thick MT images. We assessed MTR in automatically segmented GM structures including frontal, parietal-insular, temporal, and occipital cortex, caudate, pallidus and putamen, and cerebellar cortex. A general linear model analysis was conducted to ascertain the linear and quadratic relationship among the MTR and gender, age, and anatomical structure.

RESULTS

The effect of gender was borderline (P = 0.07) in all GM structures (with higher MTR values in men), whereas age showed a significant linear as well as quadratic effect in all cortical and subcortical GM structures (P ≤ 0.001). Quadratic age-related decrease in MTR began at about 40 years of age. Mean and standard deviation (SD) of MTR had the following decreasing order: thalamus (58.3 + 0.8), pallidus (56.8 ± 1.3), caudate (55.5 ± 1.6) and putamen (54.6 ± 1.1); temporal (56.8 ± 0.9), parietal-insular (56.8 ± 1.1), frontal (56.5 ± 1.1), occipital (55.4 ± 1.0) and cerebellar (53.2 ± 1.0) cortex. In post-hoc testing, all regional pairwise differences were statistically significant except pallidus vs. temporal or parietal-insular cortex, caudate vs. occipital cortex, frontal vs. parietal-insular or temporal cortex.

CONCLUSION

MTR of the cortical and subcortical brain GM structures decreases quadratically after midlife and shows significant regional differences.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

In-vivo multi-exponential T2, magnetization transfer and quantitative histology in a rat model of intramyelinic edema

Two MRI methods, multi-exponential analysis of transverse relaxation (MET₂) and quantitative magnetization transfer (qMT), were used along with quantitative evaluation of histology in a study of intra-myelinic edema in rat spinal white matter. The results showed a strong linear correlation between a distinct long-T₂ signal from MET₂ analysis and the edema water volume fraction as measured by histology, although this analysis overestimated the edema water content by ≈ 100% relative to quantitative histological measurements. This overestimation was reasoned to result from the effects of inter-compartmental water exchange on observed transverse relaxation. Commonly studied MRI markers for myelin, the myelin water fraction (from MET₂ analysis) and the macromolecular pool size ratio (from qMT analysis) produced results that could not be explained purely by changes in myelin content. The results demonstrate the potential for MET₂ analysis as well as the limits of putative myelin markers for characterizing white matter abnormalities involving intra-myelinic edema.

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We studied a rat model of intra-myelinic edema induced by hexachlorophene ingestion.

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We used multi-exponential T₂ (MET₂) and quantitative magnetization transfer MRI.

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Histology was quantitatively evaluated to measure edema volume and myelin content.

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MET₂ provides a measure that correlates but overestimates with edema volume fraction.

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MET₂ measure of edema is affected by microscopic water dynamics.

The current state-of-the-art of spinal cord imaging: methods

A first-ever spinal cord imaging meeting was sponsored by the International Spinal Research Trust and the Wings for Life Foundation with the aim of identifying the current state-of-the-art of spinal cord imaging, the current greatest challenges, and greatest needs for future development. This meeting was attended by a small group of invited experts spanning all aspects of spinal cord imaging from basic research to clinical practice. The greatest current challenges for spinal cord imaging were identified as arising from the imaging environment itself; difficult imaging environment created by the bone surrounding the spinal canal, physiological motion of the cord and adjacent tissues, and small cross-sectional dimensions of the spinal cord, exacerbated by metallic implants often present in injured patients. Challenges were also identified as a result of a lack of "critical mass" of researchers taking on the development of spinal cord imaging, affecting both the rate of progress in the field, and the demand for equipment and software to manufacturers to produce the necessary tools. Here we define the current state-of-the-art of spinal cord imaging, discuss the underlying theory and challenges, and present the evidence for the current and potential power of these methods. In two review papers (part I and part II), we propose that the challenges can be overcome with advances in methods, improving availability and effectiveness of methods, and linking existing researchers to create the necessary scientific and clinical network to advance the rate of progress and impact of the research.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

In vivo three-dimensional whole-brain pulsed steady-state chemical exchange saturation transfer at 7 T

Chemical exchange saturation transfer (CEST) is a technique to indirectly detect pools of exchangeable protons through the water signal. To increase its applicability to human studies, it is needed to develop sensitive pulse sequences for rapidly acquiring whole-organ images while adhering to stringent amplifier duty cycle limitations and specific absorption rate restrictions. In addition, the interfering effects of direct water saturation and conventional magnetization transfer contrast complicate CEST quantification and need to be reduced as much as possible. It is shown that for protons exchanging with rates of less than 50-100 Hz, such as imaged in amide proton transfer experiments, these problems can be addressed by using a three-dimensional steady state pulsed acquisition of limited B(1) strength (≈ 1 μT). Such an approach exploits the fact that the direct water saturation width, magnetization transfer contrast magnitude, and specific absorption rate increase strongly with B(1) , while the size of the CEST effect for such protons depends minimally on B(1) . A short repetition time (65 ms) steady-state sequence consisting of a brief saturation pulse (25 ms) and a segmented echo-planar imaging train allowed acquisition of a three-dimensional whole-brain volume in approximately 11 s per saturation frequency, while remaining well within specific absorption rate and duty cycle limits. Magnetization transfer contrast was strongly reduced, but substantial saturation effects were found at frequencies upfield from water, which still confound the use of magnetization transfer asymmetry analysis. Fortunately, the limited width of the direct water saturation signal could be exploited to fit it with a Lorentzian function allowing CEST quantification. Amide proton transfer effects ranged between 1.5% and 2.5% in selected white and grey matter regions. This power and time-efficient 3D pulsed CEST acquisition scheme should aid endogenous CEST quantification at both high and low fields.

Simulation and optimization of pulsed radio frequency irradiation scheme for chemical exchange saturation transfer (CEST) MRI-demonstration of pH-weighted pulsed-amide proton CEST MRI in an animal model of acute cerebral ischemia

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is capable of measuring dilute labile protons and microenvironmental properties. However, the CEST contrast is dependent upon experimental conditions-particularly, the radiofrequency (RF) irradiation scheme. Although continuous-wave RF irradiation has been used conventionally, the limited RF pulse duration or duty cycle of most clinical systems requires the use of pulsed RF irradiation. Here, the conventional numerical simulation is extended to describe pulsed-CEST MRI contrast as a function of RF pulse parameters (i.e., RF pulse duration and flip angle) and labile proton properties (i.e., exchange rate and chemical shift). For diamagnetic CEST agents undergoing slow or intermediate chemical exchange, simulation shows a linear regression relationship between the optimal mean RF power of pulsed-CEST MRI and continuous-wave-CEST MRI. The optimized pulsed-CEST contrast is approximately equal to that of continuous-wave-CEST MRI for exchange rates less than 50 s(-1), as confirmed experimentally using a multicompartment pH phantom. In the acute stroke animals, we showed that pulsed- and continuous-wave-amide proton CEST MRI demonstrated similar contrast. In summary, our study elucidated the RF irradiation dependence of pulsed-CEST MRI contrast, providing useful insights to guide its experimental optimization and quantification.

Z-spectroscopy with Alternating-Phase Irradiation

Magnetization transfer (MT) MRI and Z-spectroscopy are tools to study both water-macromolecule interactions and pH-sensitive exchange dynamics between water and the protons of mobile chemical groups within these macromolecules. Both rely on saturation of frequencies offset from water and observation of the on-resonance water signal. In this work, an RF saturation method called Z-spectroscopy with Alternating-Phase Irradiation (ZAPI) is introduced. Based on the T(2)-selectivity of the irradiation pulse, ZAPI can be used to separate the different contributions to a Z-spectrum, as well as to study the T(2) distribution of the macromolecules contributing to the MT signal. ZAPI can be run at resonance for water and with low power, thus minimizing problems with specific absorption rate (SAR) limits in clinical applications. In this paper, physical and practical aspects of ZAPI are discussed and the sequence is applied in vitro to sample systems and in vivo to rat head to demonstrate the method.

Multi-parametric neuroimaging reproducibility: a 3-T resource study

Modern MRI image processing methods have yielded quantitative, morphometric, functional, and structural assessments of the human brain. These analyses typically exploit carefully optimized protocols for specific imaging targets. Algorithm investigators have several excellent public data resources to use to test, develop, and optimize their methods. Recently, there has been an increasing focus on combining MRI protocols in multi-parametric studies. Notably, these have included innovative approaches for fusing connectivity inferences with functional and/or anatomical characterizations. Yet, validation of the reproducibility of these interesting and novel methods has been severely hampered by the limited availability of appropriate multi-parametric data. We present an imaging protocol optimized to include state-of-the-art assessment of brain function, structure, micro-architecture, and quantitative parameters within a clinically feasible 60-min protocol on a 3-T MRI scanner. We present scan-rescan reproducibility of these imaging contrasts based on 21 healthy volunteers (11 M/10 F, 22-61 years old). The cortical gray matter, cortical white matter, ventricular cerebrospinal fluid, thalamus, putamen, caudate, cerebellar gray matter, cerebellar white matter, and brainstem were identified with mean volume-wise reproducibility of 3.5%. We tabulate the mean intensity, variability, and reproducibility of each contrast in a region of interest approach, which is essential for prospective study planning and retrospective power analysis considerations. Anatomy was highly consistent on structural acquisition (~1-5% variability), while variation on diffusion and several other quantitative scans was higher (~<10%). Some sequences are particularly variable in specific structures (ASL exhibited variation of 28% in the cerebral white matter) or in thin structures (quantitative T2 varied by up to 73% in the caudate) due, in large part, to variability in automated ROI placement. The richness of the joint distribution of intensities across imaging methods can be best assessed within the context of a particular analysis approach as opposed to a summary table. As such, all imaging data and analysis routines have been made publicly and freely available. This effort provides the neuroimaging community with a resource for optimization of algorithms that exploit the diversity of modern MRI modalities. Additionally, it establishes a baseline for continuing development and optimization of multi-parametric imaging protocols.

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.

Incidental magnetization transfer contrast by fat saturation preparation pulses in multislice Look-Locker echo planar imaging

In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look-Locker (LL) imaging. It is shown that frequency-selective FS prep pulses can invoke MTC through the exchange between free and motion-restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T(1) measurement bias (up to 20%) when using inversion recovery (IR) LL echo-planar imaging (EPI) sequences. Furthermore, it is shown that water-selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T(1) measurement without fat artifacts in MR images.

Direct saturation MRI: theory and application to imaging brain iron

When applying RF saturation to tissue, MRI signal reductions occur due to magnetization transfer (MT) and direct saturation (DS) effects on water protons. It is shown that the direct effects, often considered a nuisance, can be used to distinguish gray matter (GM) regions with different iron content. DS effects were selected by reducing the magnitude and duration of RF irradiation to minimize confounding MT effects. Contrary to MT saturation spectra, direct water saturation spectra are characterized by a symmetric Lorentzian-shaped frequency dependence that can be described by an exact analytical solution of the Bloch equations. The effect of increased transverse relaxation, e.g., due to the presence of iron, will broaden this saturation spectrum. As a first application, DS ratio (DSR) images were acquired to visualize GM structures in the human brain. Similar to T(2)*-weighted images, the quality of DSR images was affected by local field inhomogeneity, but this could be easily corrected for by centering the saturation spectrum on a voxel-by-voxel basis. The results show that, contrary to commonly used T(2)*-weighted and absolute R(2) images, the DSR images visualize all GM structures, including cortex. A direct correlation between DSR and iron content was confirmed for these structures.

Magnetization transfer enhanced vascular-space-occupancy (MT-VASO) functional MRI

Vascular-space-occupancy (VASO) MRI is a novel technique that uses blood signal nulling to detect blood volume alterations through changes in tissue signal. VASO has relatively low signal to noise ratio (SNR) because only 10-20% of tissue signal remain at the time of blood nulling. Here, it is shown that by adding a magnetization transfer (MT) prepulse it is possible to increase SNR either by attenuating the initial tissue magnetization when the MT pulse is placed before inversion, or, accelerating the recovery process when the pulse is applied after the inversion. To test whether the MT pulse would affect the blood nulling time in VASO, MT effects in blood were measured both ex vivo in a bovine blood phantom and in vivo in human brain. Such effects were found to be sufficiently small (<2.5%) under a saturation power <or= 3 microT, length=500 ms, and frequency offset >or=40 ppm to allow use of the same nulling time. Subsequently, functional MRI experiments using MT-VASO were performed in human visual cortex at 3 Tesla. The relative signal changes in MT-VASO were of the same magnitude as in VASO, while the contrast to noise ratio (CNR) was enhanced by 44+/-12% and 36+/-11% respectively. Therefore, MT-VASO should provide a means for increasing inherently low CNR in VASO experiments while preserving the CBV sensitivity.

Direct quantitative comparison between cross-relaxation imaging and diffusion tensor imaging of the human brain at 3.0 T

Cross-relaxation imaging (CRI) describes the magnetization transfer within tissues between mobile water protons and macromolecular protons. Whole-brain parametric maps of the principle kinetic components of magnetization transfer, the fraction of macromolecular protons (f) and the rate constant (k), revealed detailed anatomy of white matter (WM) fiber tracts at 1.5 T. In this study, CRI was first adapted to 3.0 T, and constraints for transverse relaxation times of water and macromolecular protons were identified to enable unbiased f and k estimation. Subsequently, whole-brain CRI and diffusion tensor imaging (DTI) were performed in five healthy subjects. The parameters f and k were compared to DTI indices (fractional anisotropy (FA), apparent diffusion coefficient (ADC), radial diffusivity (RD), and axial diffusivity (AD)) across a range of anatomic regions. In WM, neither f nor k was significantly correlated to FA, RD, and AD. In contrast, both f (r=0.90 and r=-0.80) and k (r=0.92 and r=-0.89) in gray matter (GM) were strongly correlated to FA and RD, respectively. A moderate correlation between ADC and k (r=0.48) was identified in WM, while an inverse correlation was identified in GM (r=-0.72). The lack of association between CRI and FA in WM is consistent with differences in the underlying physical principles between techniques - fiber density vs. directionality, respectively. The association in GM may be attributable to variable axonal density unique to each structure. Our findings suggest that whole-brain CRI provides distinct quantitative information compared to DTI, and CRI parameters may prove constructive as biomarkers in neurological diseases.

Quantitative magnetization transfer characteristics of the human cervical spinal cord in vivo: application to adrenomyeloneuropathy

Magnetization transfer (MT) imaging has assessed myelin integrity in the brain and spinal cord; however, quantitative MT (qMT) has been confined to the brain or excised tissue. We characterized spinal cord tissue with qMT in vivo, and as a first application, qMT-derived metrics were examined in adults with the genetic disorder Adrenomyeloneuropathy (AMN). AMN is a progressive disease marked by demyelination of the white matter tracts of the cervical spinal cord, and a disease in which conventional MRI has been limited. MT data were acquired at 1.5 Tesla using 10 radiofrequency offsets at one power in the cervical cord at C2 in 6 healthy volunteers and 9 AMN patients. The data were fit to a two-pool MT model and the macromolecular fraction (M(ob)), macromolecular transverse relaxation time (T(2b)) and the rate of MT exchange (R) for lateral and dorsal column white matter and gray matter were calculated. M(ob) for healthy volunteers was: WM = 13.9 +/- 2.3%, GM = 7.9 +/- 1.5%. In AMN, dorsal column M(ob) was significantly decreased (P < 0.03). T(2b) for volunteers was: 9 +/- 2 micros and the rate of MT exchange (R) was: WM = 56 +/- 11 Hz, GM = 67 +/- 12 Hz. Neither T(2b) nor R showed significant differences between healthy and diseased cords. Comparisons are made between qMT, and conventional MT acquisitions.

Measurement of T1 and T2 in the cervical spinal cord at 3 tesla

T(1) and T(2) were measured for white matter (WM) and gray matter (GM) in the human cervical spinal cord at 3T. T(1) values were calculated using an inversion-recovery (IR) and B(1)-corrected double flip angle gradient echo (GRE) and show significant differences (p = 0.002) between WM (IR = 876 +/- 27 ms, GRE = 838 +/- 54 ms) and GM (IR = 973 +/- 33 ms, GRE = 994 +/- 54 ms). IR showed significant difference between lateral and dorsal column WM (863 +/- 23 ms and 899 +/- 18 ms, respectively, p = 0.01) but GRE did not (p = 0.40). There was no significant difference (p = 0.31) in T(2) between WM (73 +/- 6 ms) and GM (76 +/- 3 ms) or between lateral and dorsal columns (lateral: 73 +/- 6 ms, dorsal: 72 +/- 7 ms, p = 0.59). WM relaxation times were similar to brain structures with very dense fiber packing (e.g., corpus callosum), while GM values resembled deep GM in brain. Optimized sequence parameters for maximal contrast between WM and GM, and between WM and cerebrospinal fluid (CSF) were derived. Since the spinal cord has rostral-caudal symmetry, we expect these findings to be applicable to the whole cord.

MRI in multiple sclerosis: current status and future prospects

Many promising MRI approaches for research or clinical management of multiple sclerosis (MS) have recently emerged, or are under development or refinement. Advanced MRI methods need to be assessed to determine whether they allow earlier diagnosis or better identification of phenotypes. Improved post-processing should allow more efficient and complete extraction of information from images. Magnetic resonance spectroscopy should improve in sensitivity and specificity with higher field strengths and should enable the detection of a wider array of metabolites. Diffusion imaging is moving closer to the goal of defining structural connectivity and, thereby, determining the functional significance of lesions at specific locations. Cell-specific imaging now seems feasible with new magnetic resonance contrast agents. The imaging of myelin water fraction brings the hope of providing a specific measure of myelin content. Ultra-high-field MRI increases sensitivity, but also presents new technical challenges. Here, we review these recent developments in MRI for MS, and also look forward to refinements in spinal-cord imaging, optic-nerve imaging, perfusion MRI, and functional MRI. Advances in MRI should improve our ability to diagnose, monitor, and understand the pathophysiology of MS.

Quantitative description of the asymmetry in magnetization transfer effects around the water resonance in the human brain

Magnetization transfer (MT) imaging provides a unique method of tissue characterization by capitalizing on the interaction between solid-like tissue components and bulk water. We used a continuous-wave (CW) MT pulse sequence with low irradiation power to study healthy human brains in vivo at 3 T and quantified the asymmetry of the MT effects with respect to the water proton frequency. This asymmetry was found to be a difference of approximately a few percent from the water signal intensity, depending on both the RF irradiation power and the frequency offset. The experimental results could be quantitatively described by a modified two-pool MT model extended with a shift contribution for the semisolid pool with respect to water. For white matter, this shift was fitted to be 2.34 +/- 0.17 ppm (N = 5) upfield from the water signal.

MRI in multiple sclerosis: what's inside the toolbox?

Magnetic resonance imaging (MRI) has played a central role in the diagnosis and management of multiple sclerosis (MS). In addition, MRI metrics have become key supportive outcome measures to explore drug efficacy in clinical trials. Conventional MRI measures have contributed to the understanding of MS pathophysiology at the macroscopic level yet have failed to provide a complete picture of underlying MS pathology. They also show relatively weak relationships to clinical status such as predictive strength for clinical progression. Advanced quantitative MRI measures such as magnetization transfer, spectroscopy, diffusion imaging, and relaxometry techniques are somewhat more specific and sensitive for underlying pathology. These measures are particularly useful in revealing diffuse damage in cerebral white and gray matter and therefore may help resolve the dissociation between clinical and conventional MRI findings. In this article, we provide an overview of the array of tools available with brain and spinal cord MRI technology as it is applied to MS. We review the most recent data regarding the role of conventional and advanced MRI techniques in the assessment of MS. We focus on the most relevant pathologic and clinical correlation studies relevant to these measures.

Multiparametric magnetic resonance imaging analysis of the corticospinal tract in multiple sclerosis

BACKGROUND/PURPOSE

Muscle weakness is an important feature of multiple sclerosis and is responsible for much of the disability associated with that condition. Here, we describe the quantitative magnetic resonance imaging (MRI) attributes of the major intracerebral motor pathway--the corticospinal tract--in multiple sclerosis. To do so, we develop an intuitive method for creating and displaying spatially normalized tract-specific imaging data.

METHODS

In 75 individuals with multiple sclerosis and 29 healthy controls, the corticospinal tracts were reconstructed from diffusion tensor imaging at 3 T. Multiple MRI indices--T2 relaxation time; fractional anisotropy; mean, longitudinal, and transverse diffusivity; and magnetization transfer ratio--were examined within the reconstructed tracts. Spatially normalized tract profiles were created to compare, across subjects, the variation in MRI index as a function of tract position.

RESULTS

Each index's tract profile had a characteristic shape. Individual subjects had markedly abnormal tract profiles, particularly at lesion sites. On average, tract profiles were different between patients and controls, particularly in the subcortical white matter and corona radiata, for all indices examined except for fractional anisotropy. Magnetization transfer ratio was further decreased in subjects with secondary progressive disease. Tract asymmetry was increased in multiple sclerosis compared to controls.

CONCLUSION

Multiparametric MRI allows rapid detection, localization, and characterization of tract-specific abnormalities in multiple sclerosis. Tract profiles bridge the gap between whole-brain imaging of neurological disease and the interrogation of individual, functionally relevant subsystems.