These spin manipulations purely involve fat; the water resonance is unaffected. After a suitable inversion time to null residual fat signal, any pulse sequence can be used to image the remaining water. Typically a T1-weighted sequence is employed.
Unlike CHESS, where a 90°-pulse is considered ideal, the RF-pulse in SPIR is intended to partially or completely invert the fat magnetization. The RF-pulse flip angle in SPIR is therefore set in the 100°-180° range. Any transverse magnetization components of fat created by this RF-pulse are dephased by the spoiler gradient. The residual (inverted) longitudinal magnetization of fat is then allowed to regrow via T1 relaxation over the next 100-200 msec. MR signal generation begins when the Mz for fat is zero, so fat does not contribute to the final image.
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The reason flip angles just slightly greater than 90º are used in SPIR (instead of 180º) is so that the longitudinal magnetization of fat is not driven too far in the negative z-direction by the inverting pulse. T1-recovery takes much less time, so that the inversion time (TI) can be shortened down to the order of 30-60 msec (instead of ~150 msec which would be required if a 180º-pulse were used). The shorter TI means there will be a much smaller time and slice penalty for using the sequence.
Foo TKF, Sawyer AM, Faulkner WH, Mills DG. Inversion in the steady state: contrast optimization and reduced imaging time with fast three-dimensional inversion-recovery-prepared GRE pulse sequences. Radiology 1994; 191:85-90.
Kaldoudi E, Williams SC, Barker GJ, Tofts PS. A chemical shift selective inversion recovery sequence for fat-suppressed MRI: theory and experimental validation. Magn Reson Imaging 1993; 11:341-355.
Visser F. Application Tip. Optimizing SPIR and SPAIR fat suppression. Philips NetForum Community (http://clinical.netforum.healthcare.philips.com), 2004.
Zee CS, Segall HD, Terk MR, et al. SPIR MRI in spinal diseases. J Comput Assist Tomogr 1992;16:356-360.
What is SPAIR? How is it different than SPIR?