In vivo Assessment of Focal Brain Lactate Alterations with NMR Proton Spectroscopy

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Abstract
Spectral editing techniques and localization of 1H signals were applied to monitor lactate accumulation in a circumscribed region of brain damage. The experiments were performed at 2.35 T (100 MHz) in a 40-cm bore magnet. Following unilateral craniectomy in anesthetized adult cats, a two-turn surface coil was positioned over the dural surface. Proton spectra were obtained before and 1–5 h after production of a cortical cold lesion from three curved shells of brain tissue, each ∼3 mm thick. The localized spectrum was obtained from each region with and without spectral difference editing for the lactate CH3 protons, but always with the maximum excitation produced by the semiselective binomial pulse centered on the lactate CH3 resonance. Region 1 represented the damaged area, Region 2 was located immediately below Region 1, and Region 3 was immediately below Region 2. Spin-echo magnetic resonance imaging was used to confirm the relationship between the location of the lesion and the regions from which the spectra were obtained. Spectra obtained without lactate editing showed, in addition to the N-acetylaspartate peak, a large lactate peak in Region 1 after production of the cold lesion. In Regions 2 and 3, changes in lactate were more difficult to assess owing to the presence of a lipid peak at a similar frequency that results from incomplete suppression by the spin-echo pulse sequence alone. Spectra acquired using lactate editing did not contain the lipid peak and clearly showed relatively small lactate accumulations in Regions 2 and 3. Examination of signal intensity profiles through brain images upon which the region of spectral acquisition was highlighted revealed a good consistency between the position of the spectral acquisition region and the appearance of a lactate peak in the spectra. Our results demonstrate that spectrally selective, localized 1H spectra can be obtained for the in vivo evaluation of changes in a single resonance, in this case lactate, which are associated with focal surface lesions of the brain. Moreover, by utilizing a combination of localized spectroscopy and imaging, the anatomical content of the region of spectral acquisition is known.