File:Inversion Recovery Attenuation (expand the description below for a detailed explanation).webm

English: Inversion recovery is primarily concerned with T1 relaxation and is used to nullify signal from a subpopulation of protons. By starting with a 180 degree pulse, the rotated protons have practically zero XY-component; they experience only T1 decay along the Z-axis. The individual protons do indeed precess with an XY-component, but the 180 degree pulse does not necessarily align them. They point in multiple random directions, and so they cancel each other out.

If another 90 degree pulse is delivered just as net magnetic dipole of the targeted subpopulation crosses through the XY-plane, the protons will all rotate into the XY-plane, but they will have an XY-component that will continue to experience T1 decay without producing any measurable signal. Meanwhile, tissues with greater or lesser T1 values will rotate into the XY-plane with a magnitude greater than one, and will precess normally and produce a measurable signal. The result is that the same image can be produced, but the targeted subpopulation will show up as hypointense on the final image. The graphic shows the following three individual sequences: 1. The 90 degree pulse occurs before the Z-component crosses zero. It thus rotates out into the XY-plane and creates a measurable signal. 2. The 90 degree pulse occurse right when the Z-component crosses zero. It thus has no net XY-component after the rotation and creates no signal. 3. The 90 degree pulse occurs after the Z-component crosses zero. It thus rotates back out into the XY-plane and produces measurable signal.

Examples of IR sequences include FLuid Attenuation Inversion Recovery (FLAIR) and Short T1 Inversion recovery (STIR).