¹⁷O relaxation times in the rat brain at 16.4 tesla

Purpose Measurement of the cerebral metabolic rate of oxygen (CMRO₂) by means of direct imaging of the ¹⁷O signal can be a valuable tool in neuroscientific research. However, knowledge of the longitudinal and transverse relaxation times of different brain tissue types is required, which is difficult to obtain because of the low sensitivity of natural abundance H₂¹⁷O measurements. Methods Using the improved sensitivity at a field strength of 16.4 Tesla, relaxation time measurements in the rat brain were performed in vivo and postmortem with relatively high spatial resolutions, using a chemical shift imaging sequence. Results In vivo relaxation times of rat brain were found to be T₁ = 6.84 ± 0.67 ms and T₂∗ = 1.77 ± 0.04 ms. Postmortem H₂¹⁷O relaxometry at enriched concentrations after inhalation of ¹⁷O₂ showed similar T₂∗ values for gray matter (1.87 ± 0.04 ms) and white matter, significantly longer than muscle (1.27 ± 0.05 ms) and shorter than cerebrospinal fluid (2.30 ± 0.16 ms). Conclusion Relaxation times of brain H₂¹⁷O were measured for the first time in vivo in different types of tissues with high spatial resolution. Because the relaxation times of H₂¹⁷O are expected to be independent of field strength, our results should help in optimizing the acquisition parameters for experiments also at other MRI field strengths.