Fat suppression is commonly used in magnetic resonance imaging (MRI) 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 MRI. This high, easily recognised signal 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. 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.
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Fat saturation (fat-sat, FS)
These technique uses the difference in resonance frequency between fat and water by means of frequency selective pulses (CHESS).
This is the most widely used method for fat suppression in MRI, it involves applying a radiofrequency (RF) pulse at the resonance frequency of fat protons, which saturates and dephases the fat signal, the remaining water protons produce the desired signal, resulting in fat suppression. CHESS can be added to any pulse sequence and is versatile in its application.
Inversion recovery
This technique uses short T1 relaxation time of fat by means of inversion recovery sequences (STIR technique)
This technique exploits the difference in T1 relaxation times between fat and other tissues. By applying an inversion pulse followed by a delay, the fat signal is suppressed while the signal from other tissues is preserved. STIR is commonly used for whole-body imaging and provides homogeneous fat suppression.
Dixon method
Techniques based on the Dixon method, such as IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation), are used for fat suppression in MRI. These techniques exploit the phase differences between fat and water signals to separate and suppress the fat signal. By acquiring multiple echoes and using mathematical algorithms, the fat and water signals can be separated, allowing for effective fat suppression.
Spectral Presaturation with Inversion Recovery (SPIR)
SPIR is a hybrid technique that combines fat saturation and inversion recovery methods.It uses a combination of RF pulses and inversion recovery to suppress the fat signal while preserving the signal from other tissues.
Water excitation
The water excitation technique is used to selectively excite the water signal while suppressing the fat signal. It involves applying a specific RF pulse that excites the water protons and suppresses the fat protons. This technique is particularly useful when imaging structures with a high fat content, such as the breast or abdomen.
Magnetisation Transfer Contrast (MTC)
Magnetisation Transfer Contrast (MTC) is a technique used to suppress the fat signal by selectively saturating the magnetisation of fat protons. It involves applying an off-resonance RF pulse that transfers magnetisation from fat to the surrounding water protons.
Saturation recovery
Saturation recovery techniques, such as Spectral Attenuated Inversion Recovery (SPAIR), are used for fat suppression in MRI. These techniques use a combination of RF pulses and inversion recovery to selectively suppress the fat signal.
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