Spectral Fat Sat = Fast, frequency-selective, best in homogeneous B₀. 3ms Sinc IR Pulse = T1-based, field-robust, requires correct TI selection.
Spectral Fat Suppression (also known as CHESS on real life scanners):
This method uses a frequency-selective RF pulse that targets the resonance frequency of fat. It works by saturating fat protons, followed by a spoiler gradient that eliminates their signal before imaging. It's fast and commonly used in clinical settings, especially with gradient echo sequences. However, it is sensitive to magnetic field inhomogeneities (B₀), which may reduce its effectiveness near air/tissue interfaces or metallic implants.
3ms Sinc IR Pulse – TI Selection (STIR-type on real life scanners):
This is an inversion recovery method. It begins with a 3 ms sinc-shaped 180° RF pulse that inverts all spins. After this, a time delay known as the inversion time (TI) allows different tissues to recover their magnetization at different rates. When TI is chosen correctly (typically to match the zero-crossing point of fat), the fat signal is effectively nulled. This method is more robust in challenging areas, such as around implants or where the B₀ field is inhomogeneous, but it is slightly slower and can also suppress other short-T1 tissues (such as contrast-enhanced lesions).