The magnetic vector of spinning protons can be broken down into two orthogonal components: a longitudinal or Z component, and a transverse component, lying on the XY plane.

 

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Precession corresponds to rotation of the transverse component about the longitudinal axis.

Within the B0 magnetic field, there are more spins aligned with the field (parallel - low energy state) than spins aligned against the field (anti-parallel - high energy state). Due to this slight excess of parallel spins, net magnetization (macroscopic magnetization) has a longitudinal component (along the Z axis) aligned with B0
As spins do not rotate in phase, the sum of all the microscopic transverse magnetizations of each spin is a null transverse macroscopic magnetization.

 

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