Physics, Techniques and ProceduresIsochromat
or spin isochromat, is a physical construct, which consists of a very small imagined volume containing an ensemble of many spins. Isochromats are small enough that microscopic field inhomogeneities have spatial dimensions much larger than the isochromat dimension, but they are much smaller than a
voxel. The construct of an isochromat is useful, because one can only assign a classical physical magnetization vector to a volume containing a large number of spins. The same holds for other classical physical properties, thus making it possible to use the simple concepts of classical magnetism together with the
Bloch equations rather than
quantum theory to understand spin behaviour . With a single spin or an ensemble of only a few spins this is not permitted, because the quantum mechanics of a spin system with a few spins does not support the simple concept of magnetization vectors being tilted into the xy-plane (
plane perpendicular to the
B0 field) in a continuous way.
An imaging voxel is considered to contain a large number of such isochromat volumes, and it is these isochromats which we loosely call spins which are looked at in a classical physical manner, can be described then as having a magnetization vector which is tilted in any direction when applying radiofrequency pulses and is precessing in the xy-plane.
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