Spectral Fat Suppression
Spectral fat suppression involves using an RF preparation pulse which, instead of rotating the magnetisation of everything, is carefully tuned to the frequency of fat only. This is possible because the different molecular environment of 1H proton magnetisations in fat means they precess at a slightly different frequency. (This is chemical shift.) Good magnet homogeneity is required to make this spectral presaturation effective. Then the rotated magnetisation is spoiled, which does not affect the unpertubed non-fat longitudinal magnetisation. Then a fat suppressed image may be acquired.
The RF preparation pulse may be a 90° RF pulse, to rotate the magnetisations of fat into the xy plane. The imaging sequence follows immediately so that fat has no time to recover its Mz. This preparation pulse is called CHESS.
Frequency selective fat suppression using a 90° preparation pulse.
Alternatively, the RF preparation pulse may be a 180° pulse, to completely invert the magnetisation of fat. Then, much like the STIR pulse sequence, an inversion time is chosen carefully so that the fat Mz is recovering through the zero point (the null point) at the time of imaging. This sequence is called SPIR or SPECIAL.
Frequency selective fat suppression. Notice the suppression of subcutaneous fat.
Note: In addition, slice-selective gradient reversal techniques (SSGR) become feasible for fat suppression at 3T in spin-echo based sequences. SSGR relies on through-plane chemical shift being in opposite directions for the 90 and the 180 pulses, meaning the shifted fat doesn't receive both RF pulses and therefore no echo is formed from the fat.