10-06 Fat and Water Suppression
Sometimes it is helpful to dispose of the high-intensity signal of fat or fluids. There are numerous suppression techniques. Two of them are inversion-recovery methods; additional techniques are discussed in Chapter 11.
Images with long inversion times (TI) have little contrast between neighboring tissues, except the one created by differences in proton density, whereas images with short TI can show high contrast. This feature is exploited in a special inversion-recovery pulse sequence, the STIR sequence.
STIR (Short TI Inversion Recovery) sequences are often used when looking for high signal intensity lesions such as contrast-enhancing tumors close to or within fatty tissue because this inversion-recovery sequence facilitates the suppression of the signal stemming from fat. The fat signal reaches zero signal intensity at TR > 3000 ms, TI ~ 200 ms, and TE 20 ms (at 1.5 T); TI is lower at lower field strength (Figure 10-10).
FLAIR (Fluid Attenuated Inversion Recovery) eliminates the signal from cerebrospinal fluid by using very long inversion times (2000–2500 ms). It is especially useful in brain lesions with low contrast (Figure 10-11). As seen in the animation of Figure 10-09, CSF reaches the null point of no signal at an inversion time of ~2000 ms (TR > 8000 ms; TE > 100 ms) [⇒ De Coene].
Both techniques can be combined with RSE sequences; they then become Fast STIR and Fast FLAIR.