Which factor most directly contributes to the strength of the MR signal among the following?

• A. phase coherence
• B. signal strength
• C. spin density
• D. mass attenuation

Answer: (C)

The strength of the MR signal is determined by the net transverse magnetization that results after the radiofrequency pulse. This net transverse magnetization is proportional to how many hydrogen protons are present in the voxel—spin density. Each proton contributes a tiny magnetic moment, and in a region with more protons, these moments add up to a larger transverse component. That larger transverse magnetization induces a stronger signal in the receive coil, so tissues with higher spin density produce a stronger MR signal.

Phase coherence affects how the spins add up over time, but the initial signal magnitude is set by how many spins there are to begin with. Even with perfect coherence, fewer spins yield a weaker signal. Mass attenuation is a concept from X-ray imaging and doesn’t influence MR signal generation.