NMR relaxation times of blood: dependence on field strength, oxidation state, and cell integrity.
The variation with field strength or interecho interval of the T1 and T2 relaxation times of oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and methemoglobin (MHb) in either intact or lysed red blood cells was studied with a variable field (0.19-1.4 T) nuclear magnetic resonance spectroscopy unit. The T2 relaxation time of intracellular HbO2 decreased slightly with increasing field strength and interecho interval. The T2 relaxation times of intracellular Hb and MHb decreased markedly with increasing field strength and interecho interval. This T2 proton relaxation enhancement increased as the square of the applied field strength and was 1.6 times stronger for intracellular MHb than for intracellular Hb. The T2 relaxation enhancement is secondary to the loss of transverse phase coherence of water protons that diffuse across cellular magnetic field gradients. These field gradients occur when an external field is applied to a region with gradients of magnetic susceptibility. The heterogeneity of magnetic susceptibility is caused by the heterogeneous distribution (only intracellular) of the paramagnetic molecules (Hb or MHb). The T2 relaxation times of red cell lysates (homogeneous magnetic susceptibility) were independent of field strength or interecho interval. There was a decrease in the T1 relaxation times when the red cells were lysed. This may be due to an increase in the slow motional components of water molecules, because of the decrease in the average distance between water and hemoglobin molecules in the lysate. The T1 relaxation times of all the MHb samples were shortened because of proton-electron dipolar-dipolar relaxation enhancement. All the T1 relaxation times increased with increasing field strength.