Fat suppressed imaging

Fat suppression is commonly used in magnetic resonance (MR) imaging to suppress the signal from adipose tissue or detect adipose tissue ¹. It can be applied to both T1 and T2 weighted sequences. 

Due to short relaxation times, fat has a high signal on magnetic resonance images (MRI). This high signal, easily recognised on MRI, may be useful to characterise a lesion.² 

However, small amounts of lipids are more difficult to detect on conventional MRI. In addition, the high signal due to fat may be responsible for artifacts such as ghosting and chemical shift. The high signal can also mask subtle contrast difference in non-fatty tissue by filling the dynamic range of the receiver with mostly fat signal. Lastly, a contrast enhancing tumour may be hidden by the surrounding fat. These problems have prompted development of fat suppression techniques in MRI ³.

Fat suppression can be achieved in a number of different ways ^3,5-6:  

1. difference in resonance frequency with water by means of frequency selective pulses (CHESS)
2. phase contrast techniques (by same mechanism as black boundary or india ink artifacts)
3. short T1 relaxation time by means of inversion recovery sequences (STIR technique)
4. Dixon method
5. hybrid techniques combining several of these fat suppression techniques such as SPIR (spectral presaturation with inversion recovery)

Selection of a fat suppression technique should depend on the purpose of the fat suppression (contrast enhancement vs. tissue characterisation) and the amount of fat in the tissue being studied, the field strength of the magnet and the homogeneity of the main magnetic field.