Clinical Applications of Magnetic Resonance Spectroscopy

Abstract

Summary: Magnetic resonance spectroscopy (MRS) is a new tool for evaluation of patients with epilepsy, demonstrating abnormalities of energy and lipid metabolism ictally and, more recently, interictally. These metabolic abnormalities include increased inorganic phosphate, pH, and decreased phosphomonoesters as determined by "P MRS, as well as decreased Nacetylaspartate determined by 'H MRS. Furthermore, increased lactic acid has been detected postictally. These metabolic changes appear to be confined to the region of seizure origination and can be detected interictally. Therefore, they can be used for lateralization of the epileptogenic focus. Ongoing research suggests that these abnormalities may also be useful in localization of the focus, demonstrating metabolic alterations in temporal lobe epilepsy (TLE) similar to those in neocortical epilepsy. However, further technical development will be required before the goal of using these techniques for localization of the epileptogenic focus can be realized. For TLE lobe epilepsy at least, the clinical utility of 'H MRS to lateralize the seizure focus has clearly been demonstrated by several centers. The consistent findings in TLE suggest that 'H MRS is ready to become part of the evaluation process of patients with medically refractory epilepsy being evaluated for seizure surgery. Key Words: Neurological diagnosis-Magnetic resonance spectroscopy-Frontal lobe epilepsy-Temporal lobe epilepsy-Partial seizures-Phosphorous-N-acetylaspartate.Magnetic resonance spectroscopy (MRS) has been used extensively over the past 30 years in molecular physics, but only recently has it been applied to medicine and, in particular, to the study of epilepsy. MRS is the only technique that can noninvasively measure the chemical constituents of tissue in vivo. When placed in a magnetic field, chemically distinct nuclei in a compound resonate at slightly differing frequencies. This resonance depends not only on the strength of the externally applied magnetic field but also on the local magnetic environment due to the surrounding electron cloud. The separation of compounds based on these resonant frequency shifts (chemical shift) is the basis of MRS [see Matson and Weiner (1) for review]. The chemical shift allows, for example, the detection of a wide variety of proton signals on a given protein or the different phosphates in ATP. In addition, the MRS signals from many compounds can be measured simultaneously in a single MRS experiment. Because the same instrument can be used for MRI and MRS, the overwhelming proliferation of MRI units throughout the world will allow this technique to flourish. EARLY STUDIESMany of the metabolites involved in energy metabolism can be measured by 3'P MRS, including phosphocreatine (PCr), ATP, and inorganic phosphate (Pi), which allows the determination of intracellular pH. This technique applied to experimental seizures in animals and human neonatal seizures has demonstrated intraictal depletion of PCr and ATP and increases in Pi and H+ (2,3). Similarly, in...

Role of proton magnetic resonance spectroscopy in preoperative evaluation of patients with mesial temporal lobe epilepsy

“…This might be caused by inclusion of patients with intraaxial brain tumors, since the presence of Lac in gliomas is frequently observed [36,38]. Also, postictal elevation of the metabolite was demonstrated in non-lesional temporal lobe epilepsy [10,29]. In the present series of both non-lesional and lesional MTLE, decrease of NAA content below 0.75 and/or unilateral identifiable Lac peak was observed in 78% of cases, and accuracy of the presence of either factor for lateralization of the epileptogenic zone was 86%.…”

“…Lateralization accuracy of these spectroscopic findings in temporal lobe epilepsy varies from 45% to 100% [8][9][10][11]17,[19][20][21][25][26][27][31][32][33]. Such a wide range seemingly results from differences in patients' characteristics, investigated temporal lobe regions, spectroscopy techniques used, and applied diagnostic criteria [14,26,29].…”

“…In the meta-analysis of 22 reported series, Willmann et al [31] identified ipsilateral decrease of NAA in 72% of patients with temporal lobe epilepsy and good postoperative seizure outcome, whereas the positive predictive value of such a finding for a favorable surgical result was 82%. The presence of bilateral spectroscopic alterations can be detected in 20% to 70% of patients with temporal lobe epilepsy [9][10][11][13][14][15]19,21,25,27], and it was estimated that in such cases the chance of postoperative Engel Class I outcome is 4.9 times worse than in those cases with unilateral metabolic abnormalities on the side of surgery [31].…”

“…Multiple investigations consistently demonstrated decrease of NAA on both ipsi-and contralateral sides in patients with temporal lobe epilepsy [8][9][10][11][12]14,15,17,[19][20][21][22][25][26][27][28][29][31][32][33][34]. NAA is an amino acid with exceptionally high concentrations in the brain.…”

Evaluation of Children for Epilepsy Surgery

“…Similar techniques can be used outside the temporal lobes to look for subtle focal cortical abnormalities such as cortical dysgenesis, heterotopia and vascular abnormalities [47]. Additional MRI techniques such as magnetic resonance spectroscopy and surface coils are useful when there is a candidate region that can be more closely examined for a subtle structural abnormality [48, 49, 50]. …”