Perfect Coherence

What this figure is for

This is the reference case. The inhomogeneity slider starts at zero, so every location in the sample sees exactly the same B₀, every moment precesses at exactly the same rate, and the phase fan never opens. The sum stays at full length and simply rotates.

• Signal scales with coherence, not with the number of spins alone. Add spins with the lattice slider: the sum does not grow relative to its maximum, because it is an average. What grows in a real scanner is the absolute voltage, but only while the phases agree.
• The only decay left is T₂. With a uniform field there is nothing to refocus, so the signal falls at the true spin-spin rate and a 180° pulse buys you nothing. Drop T₂ to 100 ms and watch the envelope tighten.
• Now break it. Nudge the inhomogeneity slider off zero and the same demo becomes the dephasing demo. That single slider is the difference between T₂ and T₂*, and between a spin echo and a gradient echo.

Shim coils exist to push the inhomogeneity slider back toward zero over the volume you are imaging. A good shim is worth more signal than almost anything else you can do at the console.