Application of Spectroscopy to Epilepsy

Abstract

A variety of imaging techniques have been used to characterise the ictal and inter-ictal metabolic abnormalities associated with focal epilepsy. Position emission tomography (PET) and SPECT scanning have demonstrated the inter-ictal focus to be hypoperfused and hypometabolic. Magnetic resonance spectroscopy (MRS) allows the non-invasive measuring of chemicals within the body and can be performed on many conventional MRI systems. MRS exploits the principle that every chemically distinct nucleus in a compound resonates at a slightly different frequency. Nuclear magnetic resonance (NMR) signals from many compounds can be detected simultaneously in one MRS experiment and magnetic resonance spectroscopic imaging (MRSI) with phase encoding has the ability to obtain MRS signals from multiple regions simultaneously. ¹H MRS detects N-acetylaspartate (NAA), lactate, choline, creatine/phosphocreatine, and amino acids including glutamate, glutamine, aspartate and taurine. ³¹P MRS detects phosphocreatine (PCr), ATP, inorganic phosphate (Pi), pH (from the chemical shift of Pi), free Mg²⁺ (from the chemical shift of ATP), phosphomonoesters (PME), and phosphodiesters (PDE). PCr, ATP, Pi, pH and lactate provide information concerning bioenergetics. PDE, PME and choline provide information regarding lipid metabolism (Matson and Weiner, 1992). MRS studies in animals and in human neonates during seizures have confirmed previously reported alterations in energy metabolism including the depletion of PCr, ATP, and increased Pi, lactate and H⁺ (Young et al., 1985; Younkin et al., 1986). With the recognised metabolic abnormalities detected by PET and SPECT scanning inter-ictally, we questioned whether or not ¹H and ³¹P MRS could document focal metabolic changes localised to the seizure focus which led to the following pilot studies.