Quantitative in vivo magnetic resonance imaging of multiple sclerosis at 7 Tesla with sensitivity to iron

Improved in vivo detection of cortical lesions in multiple sclerosis using double inversion recovery MR imaging at 3 Tesla

Objective

To investigate the impact of a higher magnetic field strength of 3 Tesla (T) on the detection rate of cortical lesions in multiple sclerosis (MS) patients, in particular using a dedicated double inversion recovery (DIR) pulse sequence.

Methods

Thirty-four patients with clinically isolated syndromes or definite MS were included. All patients underwent magnetic resonance imaging (MRI) at 1.5 T and 3 T, including T2-weighted turbo spin echo (TSE), fluid-attenuated inversion recovery (FLAIR) and DIR sequences. All images were analysed for focal lesions categorised according to their anatomical location.

Results

The total number of detected lesions was higher at 3 T across all pulse sequences. We observed significantly higher numbers of lesions involving the cortex at 3 T using a DIR sequence. DIR at 3 T showed 192% more pure intracortical (p < 0.001) and 30% more mixed grey matter-white matter lesions (p = 0.008). No significant increase in cortical lesions could be detected on the FLAIR and T2-weighted images. Using the T2-weighted and FLAIR sequences, significantly more lesions could be detected at 3 T in the infratentorial, periventricular and juxtacortical white matter.

Conclusion

DIR brain MR imaging at 3 T substantially improves the sensitivity of the detection of cortical lesions compared with the standard magnetic field strength of 1.5 T.

Detecting lesions in multiple sclerosis at 4.7 tesla using phase susceptibility-weighting and T2-weighting

PURPOSE

To demonstrate 4.7 Tesla (T) imaging methods for visualizing lesions in multiple sclerosis in the human brain using phase susceptibility-weighting and T2 weighting.

MATERIALS AND METHODS

Seven patients with relapsing-remitting multiple sclerosis were imaged at 4.7T using three-dimensional (3D) susceptibility-weighted imaging (SWI) with 0.90 mm(3) voxel volumes, and with 2D T2-weighted fast spin echo (T2WFSE) with 0.34 mm(3) voxels and 1.84 mm(3) voxels. The visibility of MS lesions at 4.7T with phase SWI and T2WFSE was assessed by independent lesion counts made by an experienced neuroradiologist, and by quantitative measures.

RESULTS

High resolution T2WFSE at 4.7T provided excellent depiction of hyperintense lesions. When combined with phase SWI, 124 total lesions were identified of which 18% were only visible on phase SWI and not on T2WFSE. The phase lesions had a mean phase shift relative to local background of -11.15 +/- 5.97 parts per billion.

CONCLUSION

Imaging at 4.7T can provide both high quality, high resolution T2WFSE and SWI for visualization of lesions in multiple sclerosis. Phase susceptibility-weighting can identify additional lesions that are not visible with high resolution T2WFSE.

Multiple sclerosis lesions: insights from imaging techniques

The hallmark of multiple sclerosis (MS) pathology is the presence of inflammatory demyelinated lesions distributed throughout the CNS. Along with more diffuse tissue abnormalities, it is considered one of the major determinants of neurological deficit in MS. Conventional MRI has contributed to improve our understanding of MS pathology and has provided objective and reliable measures to monitor the effect of treatments. Advanced MRI techniques have offered the opportunity to quantify pathological changes in lesions, as well as in normal-appearing brain tissue and to characterize their dynamics. This review will discuss the characteristics and development of MS lesions and the contribution of conventional and quantitative MRI techniques to understanding pathological changes associated with them.

Anomalous venous blood flow and iron deposition in multiple sclerosis

Multiple sclerosis (MS) is primarily an autoimmune disorder of unknown origin. This review focuses iron overload and oxidative stress as surrounding cause that leads to immunomodulation in chronic MS. Iron overload has been demonstrated in MS lesions, as a feature common with other neurodegenerative disorders. However, the recent description of chronic cerebrospinal venous insufficiency (CCSVI) associated to MS, with significant anomalies in cerebral venous outflow hemodynamics, permit to propose a parallel with chronic venous disorders (CVDs) in the mechanism of iron deposition. Abnormal cerebral venous reflux is peculiar to MS, and was not found in a miscellaneous of patients affected by other neurodegenerative disorders characterized by iron stores, such as Parkinson's, Alzheimer's, amyotrophic lateral sclerosis. Several recently published studies support the hypothesis that MS progresses along the venous vasculature. The peculiarity of CCSVI-related cerebral venous blood flow disturbances, together with the histology of the perivenous spaces and recent findings from advanced magnetic resonance imaging techniques, support the hypothesis that iron deposits in MS are a consequence of altered cerebral venous return and chronic insufficient venous drainage.

Deep gray matter T2 hypointensity is present in patients with clinically isolated syndromes suggestive of multiple sclerosis

Gray matter (GM) magnetic resonance imaging (MRI) T2 hypointensity, a putative marker of iron deposition, is a frequent finding in patients with clinically definite (CD) multiple sclerosis (MS). The objective of this study was to assess: (a) how early deep GM T2 hypointensity occurs in MS, by studying patients with clinically isolated syndromes (CIS) suggestive of MS, and (b) whether they contribute to predict subsequent evolution to CDMS. Dual-echo scans using two different acquisition protocols were acquired from 47 CIS patients and 13 healthy controls (HC). Normalized T2-intensity of the basal ganglia and thalamus was quantified. Patients were assessed clinically at the time of MRI acquisition and after three years. During the observation period, 18 patients (38%) evolved to CDMS. At the baseline, only the GM T2-intensity of the left caudate nucleus was significantly reduced in CIS patients in comparison with the HC (p = 0.04). At the baseline, the T2 intensity of the left caudate nucleus was significantly lower (p = 0.01) in CIS patients with disease dissemination in space (DIS), but not in those without DIS, compared to the HC. The baseline T2 lesion volume, but not GM T2 hypointensity, was associated with evolution to CDMS (hazard ratio = 1.60, 95% confidence interval (CI) = 1.05-2.42; p = 0.02). In CIS patients, deep GM is not spared, suggesting that iron-related changes and neurodegeneration occurs early. The magnitude of such damage is only minor and not associated with an increased risk of evolution to CDMS.

Biophysical mechanisms of phase contrast in gradient echo MRI

Recently reported contrast in phase images of human and animal brains obtained with gradient-recalled echo MRI holds great promise for the in vivo study of biological tissue structure with substantially improved resolution. Herein we investigate the origins of this contrast and demonstrate that it depends on the tissue "magnetic architecture" at the subcellular and cellular levels. This architecture is mostly determined by the structural arrangements of proteins, lipids, non-heme tissue iron, deoxyhemoglobin, and their magnetic susceptibilities. Such magnetic environment affects/shifts magnetic resonance (MR) frequencies of the water molecules moving/diffusing in the tissue. A theoretical framework allowing quantitative evaluation of the corresponding frequency shifts is developed based on the introduced concept of a generalized Lorentzian approximation. It takes into account both tissue architecture and its orientation with respect to the external magnetic field. Theoretical results quantitatively explain frequency contrast between GM, WM, and CSF previously reported in motor cortex area, including the absence of the contrast between WM and CSF. Comparison of theory and experiment also suggests that in a normal human brain, proteins, lipids, and non-heme iron provide comparable contributions to tissue phase contrast; however, the sign of iron and lipid contributions is opposite to the sign of contribution from proteins. These effects of cellular composition and architecture are important for quantification of tissue microstructure based on MRI phase measurements. Also theory predicts the dependence of the signal phase on the orientation of WM fibers, holding promise as additional information for fiber tracking applications.

In vivo imaging of cortical pathology in multiple sclerosis using ultra-high field MRI

OBJECTIVE

We used ultra-high field MRI to visualize cortical lesion types described by neuropathology in 16 patients with multiple sclerosis (MS) compared with 8 age-matched controls; to characterize the contrast properties of cortical lesions including T2*, T2, T1, and phase images; and to investigate the relationship between cortical lesion types and clinical data.

METHODS

We collected, on a 7-T scanner, 2-dimensional fast low-angle shot (FLASH)-T2*-weighted spoiled gradient-echo, T2-weighted turbo spin-echo (TSE) images (0.33 x 033 x 1 mm(3)), and a 3-dimensional magnetization-prepared rapid gradient echo.

RESULTS

Overall, 199 cortical lesions were detected in patients on both FLASH-T2* and T2-TSE scans. Seven-tesla MRI allowed for characterization of cortical plaques into type I (leukocortical), type II (intracortical), and type III/IV (subpial extending partly or completely through the cortical width) lesions as described histopathologically. Types III and IV were the most frequent type of cortical plaques (50.2%), followed by type I (36.2%) and type II (13.6%) lesions. Each lesion type was more frequent in secondary progressive than in relapsing-remitting MS. This difference, however, was significant only for type III/IV lesions. T2*-weighted images showed the highest, while phase images showed the lowest, contrast-to-noise ratio for all cortical lesion types. In patients, the number of type III/IV lesions was associated with greater disability (p < 0.02 by Spearman test) and older age (p < 0.04 by Spearman test).

CONCLUSIONS

Seven-tesla MRI detected different histologic cortical lesion types in our small multiple sclerosis (MS) sample, suggesting, if validated in a larger population, that it may prove a valuable tool to assess the contribution of cortical MS pathology to clinical disability.

Quantitative assessment of brain iron by R2* relaxometry in patients with clinically isolated syndrome and relapsing–remitting multiple sclerosis

Background

Increased iron deposition has been implicated in the pathophysiology of multiple sclerosis (MS), based on visual analysis of signal reduction on T2-weighted images. R2* relaxometry allows to assess brain iron accumulation quantitatively.

Objective

To investigate regional brain iron deposition in patients with a clinically isolated syndrome (CIS) or relapsing–remitting MS (RRMS) and its associations with demographical, clinical, and conventional magnetic resonance imaging (MRI) parameters.

Methods

We studied 69 patients (CIS, n = 32; RRMS, n = 37) with 3T MRI and analyzed regional R2* relaxation rates and their correlations with age, disease duration, disability, T2 lesion load, and normalized brain volumes.

Results

Basal ganglia R2* relaxation rates increased in parallel with age ( r = 0.3–0.6; P < 0.01) and were significantly higher in RRMS than in CIS ( P < 0.05). Using multivariate linear regression analysis, the rate of putaminal iron deposition was independently predicted by the patients’ age, disease duration, and gray matter atrophy.

Conclusions

Quantitative assessment by R2* relaxometry suggests increased iron deposition in the basal ganglia of MS patients, which is associated with disease duration and brain atrophy. This technique together with long-term follow-up thus appears suited to clarify whether regional iron accumulation contributes to MS morbidity or merely reflects an epiphenomenon.

Using magnetic field simulation to study susceptibility-related phase contrast in gradient echo MRI

Using a Fourier-based method for rapidly calculating the spatially varying magnetic field perturbation generated by a general susceptibility distribution, chi(r)<<1, maps of the NMR frequency variation in a structured phantom, the human cortex and mid-brain structures have been simulated and compared with experimental data acquired at 7 T. The results from the phantom indicate that the Fourier method can generate an accurate estimate of the NMR frequency variation in a complex structure where the magnitude of susceptibility variation is similar to that in the human brain. Simulations based on a 1-mm-resolution model of the human head show that susceptibility variation can give rise to sharp boundaries between grey and white matter regions. The frequency difference in the simulated data was found to be significantly less than the volume susceptibility difference scaled by three. Simulations based on a simple model of the substantia nigra, red nuclei and surrounding white matter, made up of three homogeneous compartments, showed patterns of field perturbation that were very similar to those seen in experimental data. These results indicate that care must be exercised in interpreting phase or susceptibility weighted images, since the simulations show that features such as rings and graded phase variation can easily be generated by simple homogeneous structures. The results also indicate that even after spatial filtering, frequency variation can be projected outside of localised structures. k-space analysis provides some insight into the conditions needed to ensure that the measured frequency perturbation directly represents the anatomical features.

T2 hypointensity in the deep gray matter of patients with benign multiple sclerosis

Background

Gray matter (GM) magnetic resonance imaging (MRI) T2 hypointensity, a putative marker of iron deposition, commonly occurs in multiple sclerosis (MS). However, GM T2 hypointensity in benign MS (BMS) has not yet been characterized.

Objective

To determine the presence of deep GM T2 hypointensity in BMS, compare it to secondary progressive (SP) MS and assess its association with clinical and diffusion tensor (DT) MRI measures.

Methods

Thirty-five cognitively unimpaired BMS, 26 SPMS patients, and 25 healthy controls were analyzed for normalized T2-intensity in the basal ganglia and thalamus, global T2 hyperintense lesion volume, global atrophy, and white matter and GM DT metrics.

Results

BMS and SPMS patients showed deep GM T2 hypointensity compared with controls. T2 hypointensity was similar in both MS subgroups and moderately correlated ( r = −0.45 to 0.42) with DT MRI metrics. GM T2 hypointensity in BMS showed a weak to moderate correlation ( r = −0.44 to −0.35) with disability.

Conclusions

GM in BMS is not spared from structural change including iron deposition. However, while T2 hypointensity is related to global tissue disruption reflected in DT MRI, the expression of benign versus non-benign MS is likely related to other factors.

High field MRI in the diagnosis of multiple sclerosis: high field-high yield?

Following the approval of the U.S. Food and Drug Administration (FDA), high field magnetic resonance imaging (MRI) has been increasingly incorporated into the clinical setting. Especially in the field of neuroimaging, the number of high field MRI applications has been increased dramatically. Taking advantage on increased signal-to-noise ratio (SNR) and chemical shift, higher magnetic field strengths offer new perspectives particularly in brain imaging and also challenges in terms of several technical and physical consequences. Over the past few years, many applications of high field MRI in patients with suspected and definite multiple sclerosis (MS) have been reported including conventional and quantitative MRI methods. Conventional pulse sequences at 3 T offers higher lesion detection rates when compared to 1.5 T, particularly in anatomic regions which are important for the diagnosis of patients with MS. MR spectroscopy at 3 T is characterized by an improved spectral resolution due to increased chemical shift allowing a better quantification of metabolites. It detects significant axonal damage already in patients presenting with clinically isolated syndromes and can quantify metabolites of special interest such as glutamate which is technically difficult to quantify at lower field strengths. Furthermore, the higher susceptibility and SNR offer advantages in the field of functional MRI and diffusion tensor imaging. The recently introduced new generation of ultra-high field systems beyond 3 T allows scanning in submillimeter resolution and gives new insights into in vivo MS pathology on MRI. The objectives of this article are to review the current knowledge and level of evidence concerning the application of high field MRI in MS and to give some ideas of research perspectives in the future.