Nuclear Magnetic Relaxation in Blood

The nuclear magnetic relaxation time T1 of protons in human blood has been measured as a function of frequency, pH, and hematocrit. For whole blood at 25°C, T1 is approximately 0.1 s at 20 kHz, increasing to approximately 1 s at 50 MHz. T1 of whole blood is analyzed in terms of the exchange of water molecules between plasma and erythrocyte cytoplasm. A cellular residence time of 19 ms provides the best fit to the data. The T1values for plasma and cytoplasm are explained in terms of their protein content, using the well-established theory of nuclear relaxation in macromolecular solutions. The plasma and cytoplasm data are compared with previous T1 results for apotransferrin and hemoglobin solutions, respectively, and qualitative agreement is found. The T1 values increased with decreasing pH, as is expected from existing data on hemoglobin solutions.