In vivo 17O NMR approaches for brain study at high field

170 is the only stable oxygen isotope that can be detected by NMR. The quadrupolar moment of O-17 spin (I = 5/2) can interact with local electric field gradients, resulting in extremely short T-1 and T-2 relaxation times which are in the range of several milliseconds. One unique NMR property of O-17 spin is the independence of O-17 relaxation times on the magnetic field strength, and this makes it possible to achieve a large sensitivity gain for in vivo O-17 NMR applications at high fields. In vivo O-17 NMR has two major applications for studying brain function and cerebral bioenergetics. The first application is to measure the cerebral blood flow (CBF) through monitoring the washout of inert (H2O)-O-17 tracer in the brain tissue following an intravascular bolus injection of the O-17-labeled water. The second application, perhaps the most important one, is to determine the cerebral metabolic rate of oxygen utilization (CMRO2) through monitoring the dynamic changes of metabolically generated (H2O)-O-17 from inhaled O-17-labeled oxygen gas in the brain tissue. One great merit of in vivo O-17 NMR for the determination of CMRO2 is that only the metabolic (H2O)-O-17 is detectable. This merit dramatically simplifies both CMRO2 measurement and quantification compared to other established methods. There are two major NMR approaches for monitoring H-2 O-17 in vivo, namely direct approach by using O-17 NMR detection (referred as direct in vivo O-17 NMR approach) and indirect approach by using H-1 NMR detection for measuring the changes in T-2- or T-1 rho-weighted proton NMR signals caused by the O-17-H-1 scalar coupling and proton chemical exchange (referred as indirect in Vivo O-17 NMR approach). Both approaches are suitable for CBF measurements. However, recent studies indicated that the direct in vivo O-17 NMR approach at high/ultrahigh fields appears to offer significant advantages for quantifying and imaging CMRO2. New developments have further demonstrated the feasibility for establishing a completely noninvasive in vivo O-17 NMR approach for imaging CMRO2 in a rat brain during a brief O-17(2) inhalation. This approach should be promising for studying the central role of oxidative metabolism in brain function and neurological diseases. Finally, the similar approach could potentially be applied to image CMRO2 noninvasively in human brain. Copyright (c) 2005 John Wiley & Sons, Ltd.