Inversion recovery pulse sequences are used to give heavy T1-weighting. The basic part of an inversion recovery sequence is a 180 degree RF pulse that inverts the magnetization followed by a 90 degree RF pulse that brings the residual longitudinal magnetization into the x-y or transverse plane where it can be detected by an RF coil.
In imaging, the signal is usually refocused with a 180-degree pulse as in a spin-echo sequence. The time between the initial 180-degree pulse and the 90-degree pulse is the inversion time (TI). A diagram of the sequence is shown to the right.
see FGATIR