Fat
suppression is the process of utilizing specific
MRI parameters to remove the deleterious effects of fat from the resulting images , e.g. with
STIR, FAT SAT
sequences, water selective (PROSET WATS - water only selection, also FATS - fat only selection possible)
excitation techniques, or pulse
sequences based on the
Dixon method.
Spin magnetization can be modulated by using special RF pulses.
CHESS or its variations like
SPIR, SPAIR (
Spectral Selection Attenuated Inversion Recovery) and FAT SAT use
frequency selective excitation pulses, which produce
fat saturation.
Fat
suppression techniques are nearly used in all body parts and belong to every standard
MRI protocol of
joints like
knee,
shoulder,
hips, etc.
Image Guidance
Imaging of, e.g. the
foot can induce bad fat
suppression with SPIR/FAT SAT due to the asymmetric volume of this body part. The volume of the
foot alters the
magnetic field to a different degree than the smaller volume of the lower leg affecting the protons there. There is only a small band of tissue where the fat protons are precessing at the
frequency expected, resulting in
frequency selective
fat saturation working only in that area. This can be corrected by volume
shimming or creating a more symmetrical volume being imaged with water bags.
Even with their longer
scan time and motion sensitivity,
STIR (short
T1/tau
inversion recovery)
sequences are often the better choice to suppress fat.
STIR images are also preferred because of the decreased sensitivity to field inhomogeneities, permitting larger fields of views when compared to fat suppressed images and the ability to image away from the
isocenter.
See also
Knee MRI.
Sequences based on
Dixon turbo spin echo (
fast spin echo) can deliver a significant better fat suppression than conventional
TSE/FSE imaging.
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