smaller than T2. The relation between them is: where γ represents gyromagnetic ratio, and ΔB0 the difference in strength of the locally varying field. Unlike T2, T2* is influenced by magnetic field gradient irregularities. The T2* relaxation time is always shorter than the T2 relaxation time and is typically milliseconds for water samples in imaging magnets. Is T1 always longer than T2? In NMR systems, the following relation holds absolute true . In most situations (but not in principle) is greater than . The cases in which are rare, but not impossible. Bloch equations Bloch equations are used to calculate the nuclear magnetization M = (Mx, My, Mz) as a function of time when relaxation times T1 and T2 are present. Bloch equations are phenomenological equations that were introduced by Felix Bloch in 1946. Where is the cross-product, γ is the gyromagnetic ratio and B(t) = (Bx(t), By(t), B0 + Bz(t)) is the magnetic flux density experienced by the nuclei. The z component of the magnetic flux density B is typically composed of two terms: one, B0, is constant in time, the other one, Bz(t), is time dependent. It is present in magnetic resonance imaging and helps with the spatial decoding of the NMR signal. The equation listed above in the section on T1 and T2 relaxation are those in the Bloch equations. Solomon equations Solomon equations are used to calculate the transfer of magnetization as a result of relaxation in a dipolar system. They can be employed to explain the nuclear Overhauser effect, which is an important tool in determining molecular structure. Common relaxation time constants in human tissues Following is a table of the approximate values of the two relaxation time constants for hydrogen nuclear spins in nonpathological human tissues. Following is a table of the approximate values of the two relaxation time constants for chemicals that commonly show up in human brain magnetic resonance spectroscopy (MRS) studies, physiologically or pathologically. Relaxation in the rotating frame, T1ρ The discussion above describes relaxation of nuclear magnetization in the presence of a constant magnetic field B0. This is called relaxation in the laboratory frame. Another technique, called relaxation in the rotating frame, is the relaxation of nuclear magnetization in the presence of the field B0 together with a time-dependent magnetic field B1. The field B1 rotates in the plane perpendicular to B0 at the Larmor frequency of the nuclei in