INVERSION RECOVERY SEQUENCE
Contrast can be enhanced by using first a 180° RF pulse, followed by a basic SE sequence (90° and 180° RF pulses to obtain an echo). The period between the first 180° and the 90° RF pulses is known as inversion time (IT).
The hydrogen protons of fatty tissues have a short relaxation time. If a 90° RF pulse is applied when the longitudinal magnetization of these protons reaches the zero (0) point of the vertical axes, no magnetization will be flipped to a transverse plane and there will be no signal from fat. Only the magnetization of other tissues will contribute to the signal. The same principle can be applied to CSF: the hydrogen protons of the CSF have a longer relaxation time so if the TI is longer, there will be no CSF signal.
Short time inversion recovery sequence (STIR)
In STIR imaging, a short TI is applied before the 90° RF pulse to suppress the fat signal. The sequence has to be weighted in T1.
Fluid attenuated inversion recovery sequence (FLAIR)
In FLAIR imaging, a longer TI is used before the 90° RF pulse. It coincides with the null point of the CSF. The sequence has to be weighted in T2. The TI of the CSF is the same for the fluid of necrosis as for the content of certain cysts. Areas of edema, tumors, and other pathology will retain the signal.