(R1/R2) The longitudinal relaxation rate is equal to reciprocal of relaxation time.

The T1 time constant, which determines the rate at which excited protons return to equilibrium within the lattice. The longitudinal relaxation time is a measure of the time taken for spinning protons to realign with the external magnetic field. The magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1.

See also T1 Time.

(T1) The spin lattice relaxation time (also called longitudinal relaxation time and T1 Time) is a spin property, whereby the value changes between different tissues. By the spin lattice relaxation process, the longitudinal magnetization Mz achieve the equilibrium value Mz0. The T1 time constant is an exponential approach toward Mz0.
The equation for the magnetization at a time t will be (if at t=0 the longitudinal magnetization is Mz0):
Mz(t) = M0+(Mz (0) - Mz0) exp(t/T1)

(FRFSE, FR-FSE) The fast relaxation fast spin echo sequence provides high signal intensity of fluids even with short repetition times, and can be used with parallel imaging techniques for short breath hold imaging or respiratory gating for free-breathing, high isotropic resolution MR imaging. After signal decay at the end of the echo train, a negative 90° pulse align spins with long T2 from the transverse plane to the longitudinal plane, leading to a much faster recovery of tissues with long T2 time to the equilibrium and thus better contrast between tissues with long and short T2.
Fast relaxation FSE has advantages also for volumetric imaging as the TR can be substantially reduced and thus the scan time. The sequence can be post processed with maximum intensity projection, surface or volume rendering algorithms to visualize anatomical details in brain or spine MRI. Cerebro spinal fluid pulsation artifacts, often problematic in the cervical or thoracic spine may be reduced by radial sampling, in particular when combined with acquisitions of the PROPELLER type.

See also Fast spin echo, Driven Equilibrium.

Protons in -CH2- groups, e.g., contained in fatty emulsions, mineral or vegetable oil or sucrose polyester, have a fast relaxation and short T1 time. These agents with short T1-relaxation, if used in gastrointestinal imaging, produce bright signal intensities in the intestine on T1 weighted sequences. Palatable oil emulsions can produce appropriate contrast opacification of the stomach as well as the small bowel, but caused by absorption in the distal small bowel these materials are not suitable for use in MRI colonoscopy.

See also Positive Oral Contrast Agents and Gastrointestinal Paramagnetic Contrast Agents.