After the 3 tesla MRS came into use, we are able to measure main metabolites more accurately, also with current softwares we are able to measure more metabolites. So with all those new developments, differential diagnosis of metabolical diseases, follow up of demyelinating diseases, differentiation of malignant and benign tumours and staging of tumours are much easier. The most important part of MRS is, it has ability to measure chemical component of the area that we study, non invasively (Figure 1).

Generally metabolites are distributed between water and lipid peaks. There are different techniques to obtain metabolite information but this is not a subject for this review. But at least we can say that there are mainly two

methods in use and with PRESS (point resolved spectroscopy) method signal-noise ratio is higher [2] . In STEAM (stimulated echo acquisition mode) method we could work with shorter TE values and we could get more detailed table of metabolites [3] .

During examination, voxel of interest should not include especially scalp, sinuses, air or haemorrhagic foci. Normal CSF may dilute concentration but does not cause significant degradation of spectrum content. Also jewellery and ventriculoperitoneal shunts could disturb optimal settings of shimming and could change spectrum. Gadolinium could decrease the cho peak by % 12 - 15, because of that MRS should be done before administration of contrast material. Because there is no certain normal international reference values for MRS metabolites, measurements could be compared with examined cases’ own normal and abnormal regions. Also, some determined ratios of well known ratios can help in differential diagnosis as defined in Table 1 and Table 2.

On starting to examination, single or multi voxel protocol must be selected considering the purpose of the study. In searching of brain tumour which has got a heterogeneous or infiltrative pattern we should use multi-voxel protocol [4] . But in small lesions, posterior fossa tumors, or if there is a non removable metallic implant near the VOI, we should use single voxel protocol [1] . Especially for searching metabolic diseases, to achieve resonances of minor metabolites short TE values must be selected. Distribution of metabolites in resonance spectrum peaks could change due to: the device we use, the power of magnetic field. Also, shimming parameters can lead to different ppm values thorough the baseline but these changes are minor and do not cause significant diagnostic difficulties. In short TE examinations, Cr shows faster decay then other metabolites cause of that Cho/Cr ratio is higher at short examinations [5] .

In grey matter NAA, Cr and glx are higher quantities, but in white matter Cho is relatively high [6] [7] Both hemispheres nearly have the same amount of metabolites but from occipital to frontal direction Cho decreases little bit. In basal ganglia, NAA and myo-inositol decrease. In pons, Cho and Cr are high but NAA and myo-in- ositol are low. In cerebellum, Cho and Cr are high but NAA is low [8] .

NAA is the major neuronal metabolite and Cho, Cr, mIns, glutamate are the other ones. Whereas, the brain normally doesn’t include high quantity of some metabolites (like lactate, hydroxy butyrate, acetone, glycine, succinate, pyruvate, alanine, lipid, phenylalanine, mannitol, ethanol, etc.) to definable on normal resonance line.

Diagnosis could be made by determining such abnormal metabolites or with determining changes of normal metabolites. Also with using a special technique, NAA’s amid component’s changes could be determined by administering histidine and homocarnosine. And so, brain PH levels could be measured by determination of NAA level changes. Also with whole brain spectroscopy, quantitative values of all metabolites could be measured.

Different components of metabolites could be useful for defining the different peak values of the same product. For example: NAA 2.02 and 2.6 ppm, Cr at 3.02 and 3.9 ppm, Glx between 2.05 - 2.4 and 3.65 - 3.8 ppm, glucose at 3.43 and 3.48 ppm, myo-inositol between 3.56 and 4.06. But, generally, only the established dominant ones to be considered for diagnosis.

Main metabolites: Cho (choline, 3.2 ppm), Cr (creatine, 3.0 ppm), NAA (n-acetile aspartic acid, 2.02 ppm), Mi (myo-inositole, 3.56 ppm) Lip (Lipid, 0.8 - 1.5 ppm), Lac ( lactate, 1.3 ppm) . The other metabolites will be explained later by shortly. Water peak visualised at 4.7 ppm and it is used as reference value. All these metabolites that rises before water peak could be identified by spectral editing.

Cho: Marker of membrane proliferation, above normal values usually gives warning about malignity [9] .

Cr: Peaks at 3.2 ppm. Useful as an internal reference [10] .

NAA: Neuronal marker. The only situation causes increased NAA is Canavan disease. Generally all other diseases cause reduced NAA levels [11] .

mI: As a glial proliferation marker, it increases in diabetes, Alzheimer disease, PMLE, Down syndrome; and decreases in stroke, tumours and encephalopathies. While mI is increases in Alzheimer and Down syndrome, it decreases in other cases of dementia [12] [13] .

mI and glycine’s peak values are so close to each other, they both resonate at 3.5 - 3.6 ppms (gly: 3.55-singlet, mIns: 3.52 - 3.61-multiplet). Myo-inositol makes multiplet at short TE’s. It’s strength decreases at long TE values. Contrary to this, glycine resonates as a singlet peak and its visualisation enhances at long TE values. Gly increases in stroke, encephalitis, Huntington, Parkinson, Alzheimer and Van der Knaap diseases [14] .

In infantile periods, mI is the dominant peak at first months and Cho is also high at these times. But then mI’s and Cho’s peak values decreases as NAA, Cr increases. After the end of age 2, MRS patterns of metabolites becomes similar with adult level [15] .

Glutamate/glutamine: Some times these couple of metabolites are called as Glx. Glx peaks mainly between 2.1 and 2.45. In MS plaques, hepatic encephalopathy, and Reyes syndrome, increased ammonia level causes increase in Glx peak values [16] [17] .

GABA: It is visualised at 1.9, 2.3, 3.0 ppm’s, but 2.3 ppm should be first choice because at 1.9 and 3.0 ppm values it overlaps by other dominant metabolites. Simply, for GABA 2.31 ppm and for Glutamine 2.62 pm values could be used. As a new method, MegaPRESS sequence could be used for quantitative differentiation of GABA [18] .

Lactate (resonates at 1.3 ppm) is more prominent at longs TE values. In 1.5 tesla 150 ms, in 3 tesla 140 ms is the most suitable values. Inversion of lactate at short TE values could be incomplete at 3 Tesla systems. But generally, in shorts TE’s lactate is inverted but lipid is more prominent.

2-Hydroxyglutarate: gioma cases that have isocitrate dehydrogenase mutations. So, 2-hydroxyglutarate increases and visualised especially with TE: 90 - 100 ms at 2.25 ppm. Glioma cases that have high rate of this metabolite have good response to angiogenic therapy [19] [20] .

Lipid-macromolecular peaks resonates at 0.8, 1.2 1.5, 6.0 ppms. 0.8 - 1.2 ppm values usually called as lipid-macromolecular non-specific peak. If there is too much lipid we should use long TE for decrease lipids that have shorter TE values to get clear resonance graphics (It increases in tumours, metastasis and post-radiotherapy changes) [11] [21] .

Pyruvate: it peaks at 2.37 ppm. Normally couldn't detected but increase and could be defined in pyruvate dehydrogenase enzyme deficiency [22] .

Syllo-inositole: peaks at 3.35 ppm, increases in alcoholism and glioneuronal tumours. It can be rise in early infancy especially at examinations obtained in 3 Tesla systems. It can be superposed by taurine but taurine has simple peak while sylloinositole has a more complex one [23] .

Guanidinoacetate: Increases at 3.78 ppm. It is lower in anaplastic astrocytomas then other gliomas [24] .

Citrate: Increases at 2.6 ppm in some brain tumours like PNET and high grade glial tumours [25] .

Melanin: Peaks at 6.72 ppm. It can be detected in lesions that contain melanin.

Glucose: Peaks at 3.43 and 3.80 ppm. Increases in DM and hyperglycemia crisis.

Glutathione: Detected at 2.95 ppm. Increases in GBM, meningioma, and ischemic oxidative stress.

Additional to searching all these metabolites on resonance line, also there are some useful metabolite ratios to increase the strength of differential diagnosis (Table 1 and Table 2).

It’s very hard to keep all those values defined on Table 1 and Table 2 in mind so we should check on tables when we need to. But with practical approach we can use these following simplified ratios especially when searching for gliomas.

NORMAL: NAA/Cr > 2.0, NAA/Cho > 1.6, Cho/Cr < 1.2

ABNORMAL: NAA/Cr < 1.6, NAA/Cho < 1.2, Cho/Cr > 1.5

Generally, instead of using MRS alone, we should use other conventional and multi parametric methods together for diagnosis. For example in high grade tumours; Cho and rCBV is high, in low grade tumours myo-in- ositol and glycine is high but rCBV is low. In PNET, Cho is high but NAA is low and taurine can also be high.

In conclusion, with this condensed review, despite a high technical-physical burden about MRS which is highly complicated and intense, we provide much practical information. We also believe to ensure knowing basic informations from performing protocol to permutation of metabolites as well as essential data about rare metabolites and differences on spectrum made by several diseases. With development of software and hardware, utilisation of MRS in diagnostic radiology will expand beyond today.