With age, many aspects of the brain structure undergo a pronounced decline, yet individuals generally function well until advanced old age. There appear to be several compensatory mechanisms in brain aging, but their precise nature is not well characterized. Here we provide evidence that the brain of older adults expends more energy when compared to younger adults, as manifested by an age-related increase (P=0.03) in cerebral metabolic rate of oxygen (CMRO2) (N=118, men=56, ages 18 to 74). We further showed that, before the mean menopausal age of 51 years old, female and male groups have similar rates of CMRO2 increase (P=0.015) and there was no interaction between age and sex effects (P=0.85). However, when using data from the entire age range, women have a slower rate of CMRO2 change when compared to men (P<0.001 for age × sex interaction term). Thus, menopause and estrogen level may have played a role in this sex difference. Our data also revealed a possible circadian rhythm of CMRO2 in that brain metabolic rate is greater at noon than in the morning (P=0.02). This study reveals a potential neurobiological mechanism for age-related compensation in brain function and also suggests a sex-difference in its temporal pattern.

Purpose Knowledge of blood T1 and T2 is of major importance in many applications of MRI in neonates. However, to date, there has not been a systematic study to examine neonatal blood T1/T2 relaxometry. This present study aims to investigate this topic. Methods Using freshly collected blood samples from human umbilical cord, we performed in vitro experiments under controlled physiological conditions to measure blood T1 and T2 at 3T and their dependence on several factors, including hematocrit (Hct), oxygenation (Y) and temperature. Results The arterial T1 in neonates was 1825±184ms (Hct=0.42±0.08), longer than that of adult blood. Neonatal blood T1 was strongly dependent on Hct (p<0.001) and Y (p=0.005), and the dependence of T1 on Y was more prominent at higher Hct. The arterial T2 of neonatal blood was 191ms at an Hct of 0.42, which was also longer than adult blood. Neonatal blood T2 was positively associated with blood oxygenation and negatively associated with hematocrit level, and can be characterized by an exchange model. Neonatal blood T1 was also positively associated with temperature (p<0.001). Conclusion The values provided in this report may provide important reference and calibration information for sequence optimization and quantification of in vivo neonatal MRI studies.