Proton MRS in Neuroradiology

Proton magnetic resonance spectroscopy of the brain reveals specific biochemical information about cerebral metabolites, which may support clinical diagnosis and enhance the understanding of neurological disorders. Analysis of the resonance signals of low-molecular weight brain metabolites (concentrations in mmol) provides information on metabolite concentrations and makes it possible to correlate their modifications with various pathological conditions. The high diagnostic specificity of MRS enables the biochemical changes that accompany various diseases to be detected, as well as disease characterization, sometimes diagnosis, and monitoring. At 1.5 T the main metabolites detected vary according to the acquisition parameters (TR, TE) and type of pulse sequence adopted (STEAM, PRESS). 1.5 T brain MRS currently has a number of clinical applications, including the characterization of cerebral tumours and the monitoring of their treatment (e.g., radiation necrosis versus recurrence tumour), epilepsy, infection, stroke, multiple sclerosis (MS), trauma, neurodegenerative processes, such as Alzheimer's and Parkinson's diseases [2-4], and allows to diagnose several hereditary and acquired brain metabolic disorders such as Canavan's disease [5], brain creatine deficiency syndromes [6,7], adrenoleukodystrophy [8] and hepatic encephalopathy [9].

However, despite the demonstrated ability of MRS to detect neurochemical changes and to be technically feasible to study the brain in vivo, there are no standardized techniques for acquiring and interpreting MRS spectra, and little high-quality direct evidence of its influence on diagnosis and therapeutic decision-making is available. Its specificity, diagnostic and prog-

nostic value needs to be improved, and especially its sensitivity to disease markers, all of which can be achieved at higher magnetic field.

In the recent past, high magnetic field MR systems, particularly 3 T instruments, have proliferated with FDA „non-significant risk" clearance [10] and are expected to replace 1.5 T in many clinical and research applications now performed with these magnets [11]. 1.5 T fields have long been seen as the standard, but the development of 3 T and higher field technology suggests that the concept of "high field" maybe a moving target. Indeed, NMR spectrographs operating at magnetic fields of 14-21 T, are routinely used for in vitro structural studies of complex molecules. The development of in vivo high-field MRS has however been delayed by safety considerations, hardware limitations (such as the availability of wide-bore magnets), high-performance gradients and methods to correct magnetic field inho-mogeneity [11,13]. MRS like other advanced MR techniques to study the brain (e. g. angiography, diffusion, perfusion and functional imaging) should considerably benefit from the greater SNR and contrast/noise ratio and the increased spatial and temporal resolution provided by high-field systems [10-12].

Several studies comparing brain *H-MRS at different field strengths in the same subjects using the same experimental parameters, and have demonstrated the usefulness of high-field *H-MRS [13-19]. Its advantages rest on greater SNR and spectral resolution, which afford greater spatial and temporal resolution and enable the acquisition of high-quality, easily quantifiable spectra in acceptable acquisition times. In addition to improved measurement precision of common metabolites, such as N-acetylaspartate (NAA), choline (Cho), creatine/phosphocreatine (Cr/PCr), myoinositol (mI) and when present lactic acid (Lac) and lipids (Lip), high-field systems allow the high-resolution measurement of other metabolites, such as glutamate (Glu), glutamine (Gln), glutathione (GSH), y-amino-butyric acid (GABA), scyllo-inositol (ScyI), aspartate (Asp), taurine (Tau), N-acetylaspartylglutamate (NAAG) and glucose (Glc), thus extending the range of metabolic information. However, these advantages may be hampered by intrinsic field-dependent technical difficulties, such as increased T2 signal decay, chemical shift dispersion errors, J-modulation anomalies, increased magnetic susceptibility, eddy current artefacts, limitations in the design of homogeneous and sensitive radiofrequency coils, magnetic field instabilities and safety issues. Several studies have demonstrated that these limitations can be overcome, suggesting that optimization of high-field 1H-MRS can lead to its broader application in clinical research and diagnosis.

Table 6.1 summarizes several metabolites involved in brain biochemistry detectable with 1H-MRS. Beside

Table 6.1. Some of the primary resonances found in 'H brain spectroscopy and corresponding chemical shifts (in ppm)

Brain metabolites detected on 1H MRS Compound Abbreviation

Frequency (ppm)

Alanine

Ala

1.48,3.78

Aspartate

Asp

3.9, 2.69, 2.82

Choline

Cho

3.22 (4.05, 3.54)

Creatine/phosphocreatine

Cr/PCr

3.03, 3.95

y-Aminobutyric acid

GABA

2.31, 1.91,3.01

Glucose

Glc

3.43, 3.84 (... )

Glutamate

Glu

3.77, 2.06, 2.38

Glutamine

Gln

3.71,2.15,2.46

Glycine

Gly

3.56

Lactate

Lac

1.33

Myo-Inositol

mI

3.56, 4.06

N-Acetylaspartate

NAA

2.02

Scyllo-inositol

ScyI

3.35

Taurine

Tau

3.44,3.38,3.32,3.27

the most prominent resonances of NAA, choline and creatine, a variety of other resonances might or might not be present in a spectrum depending on its type and quality as well as disease. This list is not complete, in that several metabolites are only observed in the rare cases when their concentrations are several times higher than normal, while metabolites such as alcohol and propylene glycol are not present in normal brain metabolism but maybe found in certain patients. For a full list of detectable metabolites the reader required to interpret spectra with unusual resonances is referred to the literature.

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