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).

Figure 10-10: STIR. In all images, TR = 4000 ms, TE = 10 ms; (a) TI = 50 ms, (b) TI = 240 ms, (c) TI = 450 ms. The fatty tissue close to the optic nerve disappears with a TI of ap­pro­xi­ma­te­ly 240 ms.

 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].

Figure 10-11: FLAIR. TR = 8000 ms, TE = 120 ms, TI = 2000 ms. The brain lesions caused by the pressure of huge meningioma pe­ri­ven­tri­cu­lar­ly and in other parts of the brain are well seen on the FLAIR image.

Both techniques can be combined with RSE sequences; they then become Fast STIR and Fast FLAIR.