Inversion Recovery
The Common Vein Copyright 2010
Introduction
T1 relaxation times can be emphasized, using a technique called inversion recovery. An extra setting, selectable, is added to the pulse sequences, called time of inversion. This is the time that occurs between the radiofrequency pulse that hits the proton at 90° and the pulse that hits it at 180°. When protons transit from one energy level to another, they do not all do it at the same time; there is a difference, in which some are faster than others, also element specific. By using radiofrequency pulses at these angles separated by a given time, it is possible to interact with the nucleus and show the difference between the slow and fast moving protons.
Tissues with short T1 times yield a brighter signal. Short T1 inversion recovery (STIR) sequences thus increase lesion detail. Fat has a short T1. Hence by choosing a short TI of 140 milliseconds, the fat signal can be suppressed. STIR should not be used to suppress fat signal when using gadolinium contrast, because gadolinium–enhanced tissues short T1 and may be erased.
Fluid attenuated inversion-recovery (FLAIR) sequences are used to suppress liquid signals by adjusting the TI. Water has a long T1, and its signal can be erased at TI of 200 milliseconds.