T1, or longitudinal relaxation, describes the tendency of the net magnetization to realign itself along the direction of the main magnetic field. T1 processes are similar to T2 processes in that they arise from the magnetic fields that are created at an individual proton by all of its neighboring protons and other magnetic nuclei. For T1 processes, however, the magnetic fields of importance occur at frequencies equal to (or approximately equal to) the proton's precession frequency. The random tumbling motion of the neighboring protons and other magnetic nuclei can create magnetic fields that fluctuate in time, with a component of transverse magnetization fluctuating at the proton's precession frequency. The proton can be stimulated by these fluctuating transverse fields to give up energy, and relax to its lower energy state. This interaction can be seen as the conversion of energy stored in the individual proton's spin states to motional (kinetic) energy of the other protons (often referred to as "the lattice" in this context).

The efficiency of T1 relaxation is determined by the strength of the interaction between the tumbling lattice protons and the relaxing proton. The freedom of the lattice protons to move around in space and tumble determines how much transverse magnetization with the correct frequency of fluctuation will be created at the location of the relaxing proton. T1 relaxation therefore, is dependent both on the main magnetic field strength, and also on the motional characteristics of the lattice protons. Figure 1.13 is a schematic showing the dependence of T2 and T1 on proton mobility in tissue. In general, the higher the water content of a tissue, the longer the T1. For human tissues (other than fluids such as CSF), T1 is typically eight to ten times longer than T2. At 1.5 Tesla, T1 can vary from 0.2s - 4.0s in the human body.

Figure 1.13 Schematic illustrating the efficiency of T1 and T2 relaxation for (A) bound water molecules, (B) less tightly bound water molecules, and (C) free water.