Which quantity is determined by the number of nuclei per unit volume and their orientation with respect to relaxation?

• A. phase coherence
• B. relaxation
• C. spin density
• D. strength of the MR signal

Answer: (D)

The key idea is that the measurable MR signal strength depends on both how many nuclei are present and how those spins behave during relaxation. The signal you detect comes from the transverse magnetization, which scales with the number of hydrogen nuclei per unit volume—more spins mean a larger potential signal. But what you actually observe is shaped by relaxation processes: T1 governs how quickly spins recover along the magnetic field, and T2 (and related dephasing) governs how quickly they lose coherence in the transverse plane. The combination of spin density and how the spins orient and evolve during relaxation determines the observed signal magnitude. That makes the strength of the MR signal the quantity that best reflects both factors.