The goal of this research is to develop advanced magnetic resonance spectroscopy (MRS) and imaging techniques and to apply them and other complementary methods to studying brain metabolism, neurotransmission, and enzyme activity. MRS allows measurement of the neurotransmission of glutamate and GABA in vivo, which plays important roles in many major psychiatric diseases, including depression and schizophrenia. During 2017-2018, by overcoming daunting technical challenges we successfully performed the worlds first in vivo measurement of carbonic anhydrase activity in the human brain (Li et al, Sci. Rep., 8:2328:1-8 (2018)). Carbonic anhydrase plays an important role in life. Recent proteomic studies of brain disorders such as schizophrenia and major depression have revealed significant alterations in carbonic anhydrase expression. After oral administration of uniformly 13C-labeled glucose, we performed 13C saturation transfer experiments (a technique pioneered by our group) with interleaved control spectra and carbon dioxide saturation spectra. Our results showed that the 13C signal of bicarbonate was reduced by 72% upon saturating carbon dioxide. The unidirectional dehydration rate constant of the carbonic anhydrase reaction was determined to be 0.3sec1 in the human brain. Our finding of a very large and quantifiable 13C saturation transfer effect in the human brain makes it possible to characterize this important enzyme in patients with brain disorders. We have made significant progress in the development and application of novel spectroscopic and imaging techniques for studying metabolism and neurotransmission in vivo in the brain. We invented and validated a highly novel spectral editing technique that can simultaneously measure glutamate, glutamine, GABA and glutathione without using conventional subtraction which is sensitive to motion (An et al, Magn. Reson. Med., 2018, doi: 10.1002/mrm.27172). We discovered that, at the presence of a weak radiofrequency field, the unique evolution pattern of strongly coupled spins systems (e.g., glutamate) can be leveraged to make sharp singlets out of stronly coupled multiplet signals. As such, multiple otherwise strongly overlapped signals can now be well determined at 7 Tesla. Since this mechanism of spectral resolution does not involve motion-sensitive subtraction it represents a significant advance in spectral editing methodology. We further validated this new technique using human studies and applied it to the frontal lobe where structural and functional abnormalities are strongly associated with psychiatric symptoms. A subsequent Monte Carlo studies have also been performed to quantify the effectiveness of compensating radiofrequency drifts during spectral editing. We have successfully demonstrated that our retrospective frequency drift correction strategy can eliminate the detrimental effects of frequency drifts on accurate quantitative editing of brain chemicals. For characterizing cell-specific microenvironments we invented a unique echo-time independent technique for measuring chemical relaxation in vivo (Li et al, Magn. Reson. Med., 79(5):2491-2499 (2018)). Unlike all other methods, our radiofrequency-driven longitudinal steady state method does not require changing echo time to encode transverse relaxation of chemicals. Instead we used a novel radiofrequency pulse train interspersed by magnetic field gradient pulses to elicit relaxation weighting. By targeting glutamate, which is predominantly located in glutamatergic neurons, we quantified its relaxation in the prefrontal cortex using a fixed echo time that is optimized for glutamate detection at 7 Tesla. Significant progress has also been made in designing novel strategies for studying brain metabolism and neurotransmission in both human and animal experiments.
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