Magnetic Resonance Spectroscopy of the hippocampus Marton A 1, Bús L 2, Juhos V 3, Rudas G 2, Barsi P 2 1 University of Pécs, 2 SU MR Research Centre 3 St. Stephen Hospital Dept. of Neurology Budapest, Hungary

Why hippocampus? Common focuse region, mostly when complex partial seizures are seen. Only certain structural lesions are MRI-visible. Correct lateralisation is crucial to obtain favorable postoperative outcome

Why MR spectroscopy? Noninvasive measurement of molecular content in vivo. Early detection of metabolic alterations, even in the silent intervall and prior to structural changes. Reveals exact spatial extent and the severity of the disease may predict outcome. When EEG and MRI findings are not concordant further invasive methods are needed for preoperative evaluation of intractable epilepsy.

Clinical relevance Underlying mechanism Metabolite change Clinical sign Neuron mitochondrium N-acetyl-aspartate Neuron dysfunction/-death Active cell proliferation Choline Gliosis, malignancies Energy usage/-storage Creatine Hipo-/hipermetabolic states Ischaemia Lactate Stroke, seizures Necrosis Lipids Inside tumors Neurotransmitter production GABA, glutamate, acetilcholine Epilepsy, demencies

Taking up the challenge Small and elongated shape Heterogeneous histology, different subregions Water signal from adjacent cerebrospinal fluid Field distortions due to skull base Hippocampal sclerosis on the right side.

Patients and method 18 TLE patients. 13 unilateral hippocampal sclerosis proved by MRI, 5 without visible abnormality 3T magnet (Philips Achieva X-series), SENSE 8-channel phased array head coil Single voxel technique, PRESS localisation, CHESS water suppression, short echo time (TE) 35 ms, long TE 288 ms, repetition time 2000 ms, 128 of averages Comparison between two sides was done by calculating asymmetry coefficient: Results were compared with MRI and EEG findings.

Voxel positioning 30 (AP) x 8 (RL) x 6 (FH) mm

Results MRI shows hippocampal sclerosis on the right side. TE 35 ms right side TE 288 ms right side TE 35 ms left side TE 288 ms left side

Results Means of NAA/Cho and NAA/Cr were lower, Cho/Cr was higher ipsilaterally. The degree of metabolic asymmetry was consistently higher in unilateral cases. Correct lateralization rate was 50-84% depending on the applied parameters. TE1 35 msTE2 288 ms EEGMREEGMR NAA/Cr67% (4/6)67% (8/12)67% (4/6)50% (6/12) NAA/Cho50% (3/6)75% (9/12)84% (5/6)67% (8/12) Cho/Cr50% (3/6)84% (10/12)50% (3/6)58% (7/12)

Discussion Where lateralisation by MRS was mistaken, EEG and MRI findings were also contradictory or dual pathology (tumor, malrotation) was associated with TLE Presumably there are underlying metabolic changes contralaterally as well which explains the uncertainty of the asymmetry coefficient. Even if the hippocampus is considered as normal it is still possible that a lesion has evolved in an early stage causing mild asymmetry.

Conclusions A small and elongated voxel is preferable to cover most of the hippocampus, while excluding surrounding structures. Ratios indicate the affected side in unilateral cases. Short TE is more precise for quantification at higher fields. Additional metabolites and neurotransmitters provides more detailed information on pathophysiological processes.

Further aims Involving a healthy control group is needed for validation issues. Enlarge patient group to determine cut off value. Automation and visualisation tools would be useful in clinical practice. Absolute quantification. Correction of partial volume effects could improve sensitivity.

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