In a magnetic resonance machine, scientists use radio waves to excite atoms in a sample. These excited atoms absorb the radio energy and flip into a higher energy state. Once the radio pulse stops, the excited atoms slowly release this extra energy.
They transfer this stored energy out to their surrounding chemical environment. Chemists traditionally refer to this surrounding molecular environment as the "lattice." Because of this history, longitudinal relaxation time is often called spin-lattice relaxation. The speed of this energy transfer depends on several different physical factors.
The tumbling speed of the molecules and the thickness of the liquid both matter. The overall strength of the machine’s main magnet also changes the relaxation speed. Shorter T1 times mean that the atoms relax and return to normal very quickly.
A common misconception is that all atoms in a single molecule relax at the same speed. In reality, different parts of a molecule will have completely different T1 times. Knowing these exact times helps scientists set the perfect timing for their machine pulses.