When you apply a magnetic field to the protons in your body, there are two populations of protons: those with z-spin aligned and those with it anti-aligned. Boltzmann statistics determine the relative concentration of the two populations.

My question is: these spins are naturally flipping back and forth all the time, even before you stimulate spin-flips with radio emission. Does this cause a background MRI signal that you can detect due to those spins that are flipping from the high energy state to the low energy state? Why do you even need to stimulate the spin flip process?