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. 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.
Distribution of Boltzmann
Nup / Ndown = Proportion of parallel / antiparallel spins
γ = Gyromagnetic ratio
h = Planck's constant / 2π
k = Boltzmann's constant
T = Temperature (in Kelvins)
B = Intensity of static magnetic field (Teslas)