Frequency dependence of magnetic resonance spin-lattice relaxation of protons in biological materials.

The frequency dependence of spin-lattice (T1) relaxation times of protons in tissues and other macromolecular solutions was investigated. This dependence can be principally assigned to reduced mobility of water molecules "hydrated" on the surface of macromolecular structures. The T1 relaxation time of "hydration water" was found to increase linearly with frequency in the range from 5 to 100 MHz (T1 = 1.83 f + 25.0). It is assumed that the remainder of water in the tissue has a relaxation rate that is independent of frequency, as is characteristic of bulk tap water. Variations occur in the fraction of water hydrated or bound from one organ to the next. As the observed relaxation rate is a weighted average of the two rates as described by the fast exchange model, the above empirical relationship can be used to take a tissue T1 relaxation time measured at one frequency and then calculate the tissue T1 relaxation time that would be expected at another frequency. Good agreements were obtained between such calculated tissue T1 values and the T1 values actually measured at the second frequency. Variations between the calculated and measured T1 values in some classes of tissues indicate that there are also less important secondary factors such as lipid content.