DETERMINATION OF RCMRO(2) AND RCBF BY NONINVASIVE O-17 IN-VIVO NMR AND MRI .3. DETERMINATION OF REGIONAL CEREBRAL OXYGEN-CONSUMPTION IN THEHUMAN - O-17 NATURAL-ABUNDANCE CEREBRAL MAGNETIC-RESONANCE-IMAGING AND SPECTROSCOPY IN A WHOLE-BODY SYSTEM

O-17 natural abundance imaging in a whole body imager is demonstrated using standard MRI spectrometer and H-1 imaging methods. A novel desig n of a highly sensitive O-17/H-1 doubly tuned surface head coil is sho wn. The head probe allows simultaneous acquisition of O-17 and H-1 ima ges using a single coil. The relatively low O-17 signal intensity due to the low natural abundance of O-17 (0.037 atom percent) is partially compensated by fast repetition of the pulse sequence, achievable due to the short spin lattice relaxation time, T1. A small number of signa l averages (e.g, NEX = 50) is sufficient for obtaining images having s ignal to noise of about 5:1. Due to the short longitudinal relaxation time of O-17, i.e, 2-5 msec, short TR values can be used. 128 phase en coding steps with TR = 10-25 msec correspond to total acquisition time of 1 to 2.5 min. Due to the small gyromagnetic ratio of O-17 and the relatively small gradients in a standard whole body system, i.e. 0.5 G /cm, the image in-plane resolution is about 3 mm and a slice thickness of 15 mm. In vivo O-17 MRS and MRI natural abundance spectroscopic si gnals and images of human brain have been observed. The transverse rel axation time, T2 was found to be 2.00 +/- 0.17 msec at 1.5 T. MRS O-17 measurements of signal intensity in the occipital cortex during inhal ation of oxygen gas, 21.8% O-17 enriched, showed a maximum signal enha ncement of 25% within the inhalation period. The rate of the metabolis m of oxygen (CMRO2) in the occipital cortex was found to be 1.5 mumole /(g tissue) in good agreement with the value of 1.435 mumole/(g tissue ) given in the literature. Current measurements using higher O-17 enri chments and larger quantities of O-17 enriched oxygen gas will enhance resolution and provide more accurate determination of the rate of oxy gen metabolism rate and blood flow. The potential of O-17 imaging is t hus demonstrated in physiological in vivo studies of cerebral metaboli sm of oxygen and blood flow.