Proton NMR relaxation times of human blood samples at 1.5 T and implications for functional MRI.

To further investigate the dependency of fMRI signal changes on echo time TE, we measured T2 and T2* values, obtained from human blood samples at various oxygenation levels and used them in a simple model to calculated signal enhancement in fMRI. In addition, the longitudinal relaxation time T1 of human blood was determined for reference. All measurements were performed at 23 degrees C to reduce blood cell metabolism during the measurement procedure. At 23 degrees C T1 values of 1434 +/- 48 ms for arterial human blood were obtained after correcting for hematocrit content, as hematocrit values ranged fro 28% to 34% only. The T2 relaxation times obtained are 181 +/- 23 ms for venous and 254 +/- 26 ms for arterial human blood, T2* relaxation times corrected for inhomogeneities of the static magnetic field (B0) are 42 +/- 2.8 ms and 254 +/- 32 ms, respectively. Furthermore, absolute and relative signal changes in fMRI experiments are calculated. The results from these model calculations reveal that contrast in fMRI can be optimised by choosing an appropriate echo time.