Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters.

To help understand which tissue parameters best account for the water proton NMR relaxation times, the longitudinal relaxation time (T1), the transverse relaxation time (T2), and the water content of 16 tissues from normal adult rats were measured at 10.7 MHz and 29 degrees C. Regression analyses between the above and other tissue parameters were performed. These other tissue parameters included: the amounts of various organic and inorganic components, protein synthetic rate, oxygen consumption rate, and morphological composition. In addition, the differences in T1, T2, and water content values between normal liver and malignant tumor (Morris #7777 a transplantable hepatoma) were studied to help understand how a disease state can be detected and characterized by NMR spectroscopy. The results of this study and information from the literature allow the following generalizations to be made about tissue T1 and T2 values: (1) Each normal tissue has rather consistent and characteristic T1 and T2 relaxation times which are always shorter than the T1 and T2 of bulk water; (2) tissues with higher water content tend to have longer T1 relaxation times; (3) tissue T2 values are not, however, as well correlated with water content as T1 values; (4) tissues with shorter T1 values have higher calculated hydration fractions, greater amounts of rough endoplasmic reticulum, and a greater rate of protein synthetic activity; (5) tissues with higher lipid content, associated with intracellular non-membrane bounded lipid droplets, tend to have longer T2 values; (6) tissues with greater overall surface area, whether in the form of cellular membranes or intracellular or extracellular fibrillar macromolecules, tend to have shorter T2 values; (7) the differences between T1 and T2 values between tumor and normal tissues correlated with differences in the volume fraction (amounts) of extracellular fluid volumes and in the amounts of membrane and fibrillar surface area in the cells. The above generalizations should be useful in predicting T1 and T2 changes associated with specific tissue pathologies.