What causes T2* decay to occur faster than T2 decay?

T2* decay occurs faster than T2 decay primarily due to inhomogeneities in the magnetic field. In a perfectly homogeneous magnetic field, the decay of transverse magnetization that defines T2 relaxation is determined solely by spin-spin interactions among nearby protons. However, real-world magnetic fields are seldom perfectly uniform; they can have slight variations due to imperfections in the magnet and external magnetic influences.

These inhomogeneities in the magnetic field introduce additional local variations in precession frequencies for the spins. As a result, spins experience different local magnetic environments, which causes them to become dephased more rapidly than they would in a uniform field. This leads to a more pronounced and faster decay of the signal, characterizing T2* relaxation.

While factors like higher spin-spin interactions and higher molecular tumbling rates can influence T2 relaxation times, they do not account for the rapid decay observed in T2*. Lower inherent energy is not a factor in T2* decay relative to T2 since it pertains more to the energy levels of the spins rather than their magnetic field interactions.