Study of MS Tissue Injury and Repair with In Vivo MR Spectroscopy of Lipids and Proteins - First Results
Christoph Juchem1, Hetty Prinsen1, Kevin L Behar1, Robert K Fulbright1, and Douglas L Rothman1

1Yale University, New Haven, CT, United States

Synopsis

Altered lipid and protein contents have been detected with MR spectroscopy in acute multiple sclerosis lesions and speculated to reflect de- and remyelination processes. To date, however, neither the metabolites involved nor their pathological relevance have been satisfactorily described. Hofmann et al. introduced an elegant concept for the separation of biochemicals of varying metabolic weight based on differences in T1 relaxation behavior (MRM 46, 2001). Here, we describe T1-encoded MR spectroscopy at 7 Tesla, the extraction of individual, spectroscopically overlapping lipid/protein signals by two-dimensional processing and initial applications to the study of the pathological processes associated to multiple sclerosis tissue injury and repair.

Introduction

Multiple sclerosis (MS) is a chronic disorder of the central nervous system that leads to demyelination and neurodegeneration. Magnetic resonance spectroscopy (MRS) allows the assessment of the pathological changes and brain damage from the earliest stage of the disease1-4. Altered lipid and protein contents have been detected with MRS in acute MS lesions and speculated to reflect de- and remyelination processes5-9. Lipids/proteins have been characterized in detail with MRS in the healthy human brain10 and assigned based on measurements in rat brain extracts11. In MS, however, neither the particular lipids/proteins involved nor their pathological relevance have been satisfactorily described, at least in part due to methodological obstacles to quantify them in vivo. The purpose of this study was the establishment of dedicated lipid/protein MRS as a tool for clinical MS research.

Methods

Signals from lipids/proteins, so-called macromolecules (MM, 10-100 kD), overlap with signals from low molecular weight (LMW) neurochemicals including NAA, choline, or glutamate in 1H MRS of the in vivo human brain (Fig. 1E, gray) which fundamentally limits reliable lipid/protein quantification. Commonly used approaches to disentangle the MM baseline from the LMW signals rely on inversion-recovery preparation to "null" the LMW contribution. These techniques, however, are limited by the substance-specific spread of T1 relaxation times of both MM and LMW metabolites. Hofmann et al. introduced an elegant way to encode metabolite T1’s with an array of saturation-recovery prepared MRS acquisitions12. This technique employs the systematically shorter and longer T1 times of MMs compared to LMW substances, respectively, to separate both groups based on a T1 threshold. Their work was primarily aimed at improving the quantification of LMW metabolites. Here, the quantification of the lipid/protein profile, i.e. the MM baseline, was the primary goal. Lipid/protein concentrations and composition are thought to reflect processes involved in MS tissue injury and repair. The level of lipid elevation is hypothesized to correlate with the active inflammation process, e.g. in new plaques, based on the macrophage lipid signal similar to stroke tissue13,14. T1-decomposition was employed with optimized 7 Tesla technology for gains in sensitivity and spectral dispersion. Non-selective saturation was followed by short echo-time STEAM15 (TE 10 ms) for an array of logarithmically scaled recovery delays ranging from 0.2 to 10/14 sec. Third order B0 shimming and an 8-channel RF phased-array coil were used to achieve optimized B0 homogeneity and B1 sensitivity, respectively16. In addition, two-dimensional processing (similar to 2D NMR) was applied to disentangle not only the MM baseline from LMW neurochemicals, but also individual, spectroscopically overlapping lipid/protein signals.

Results

T1-encoded MRS at 7 Tesla obtained from the healthy human brain allowed the separation of the short-T1 MM baseline from LMW metabolites, in line with Hofmann et al. (Fig. 1). The lipid/protein signals are consistent with the MM resonances reported by Behar et al.10, including the superposition of long-T1 glutamate and short-T1 'M6' macromolecule signals at 2.3 ppm. The integration of T1-frequency regions (as opposed to T1 thresholding) furthermore allowed the separation of individual lipid/protein signal components, which is a requirement for their independent quantification (Fig. 2). While the measurement of lipids/proteins was the primary goal, the complementary quantification of 10+ additional substances including glutamate and established biomarkers of MS1-4 is crucial for the comprehensive study of MS pathological processes.

A clinical research study to validate the method as a tool for diagnostics and monitoring is currently under way that includes a cross-sectional comparison of the lipid/protein profile in MS lesions and of contra-lateral, normal appearing control areas in the same patient and in healthy controls (Figs. 3/4, >10 subjects completed).

Discussion / Outlook

T1-decomposed MRS with improved sensitivity and spectral dispersion at 7 Tesla is presented for the quantification of lipids/proteins associated with MS pathology. An increase of the combined valine+leucine+isoleucine signals from proteins around 0.9 ppm and threonine+alanine signals around 1.3 ppm in lesions in MS subjects compared to healthy controls has been described previously6. Once fully validated, the method is expected to promote the assignment of specific spectral components to specific protein/lipid changes characteristic of different stages of the pathological process and to thereby shed light on the role of individual compounds in MS tissue and repair processes. We note that such in vivo direct knowledge of the de- and remyelination processes is not accessible otherwise. T1-encoded MRS at 7 Tesla is expected to allow 1) the assessment of the pathobiochemistry of lipids/proteins in MS lesions, 2) the development of sensitive lipid/protein-based MRS biomarkers and 3) the correlation of clinical prognosis to injury severity and potential for repair, e.g. remyelination, in future studies.

Acknowledgements

Research Participants:

We are very grateful to the MS patients and healthy controls who participated in this research.

Clinical Support:

We are very grateful for the continuous support from the neurologists and staff of the Yale Multiple Sclerosis Center

Grant Support:

1) National Multiple Sclerosis Society (Research Grant RG-4319, Pilot Grant PP-3356)

2) Nancy Davis Foundation (Pilot Grant, ”Race-to-Erase”)

3) Yale Center for Clinical Investigation (CTSA Scholar Award, UL1 TR 000142)

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Figures

T1-encoded MRS of the MS brain. Variable saturation recovery delays lead to an array of T1-weighted spectra (A/B). Multi-exponential T1 decomposition allows for disentangling of overlapping signals of low and high molecular weight (C/D). Summation of short and long T1 signals leads to lipid/protein (E:red) and LMW (F) spectra, respectively.


Extraction of individual lipid/protein signals by regional T1-frequency integration. The assignment of specific spectral components to specific MM changes characteristic of different stages of MS lesion pathology is expected to shed light on the role of individual compounds in tissue injury and repair processes.

The brain of a 52-year old female MS patient shows multiple lesions (MPRAGE, A: axial, B: sagittal, post-Gadavist). In this research, the lipid/protein profile of active, enhancing lesions was assessed with single voxel MRS (red box).

Assessment of the lipid/profile of an active, enhancing MS lesion with regular (A,black), double-inversion recovery (A,red) and saturation-recovery prepared (B) MRS. T1-parametrization at 7 Tesla along with two-dimensional integration of T1-frequency signals is expected to enable the quantification of individual lipids/proteins involved in MS pathology of tissue injury and repair.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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