Fat Suppression

How and why is fat suppression performed in an MRS study?  

Adipose tissue is predominantly composed of long-chain triglycerides and free fatty acids, whose prinicipal resonances lie between δ = 0.9 and 1.4 ppm. Signal from extraneous fat can "bleed into" and contaminate the spectra from within the defined volume of interest. Lactate at δ = 1.3 is especially affected. 

Lipid contamination is particularly problematic in breast MRS (since the normal breast contains large amount of fat) and for prostate MRS (since the prostate is a small organ surrounded in pelvic fat). In brain MRS, scalp and marrow fat can affect the spectra from voxels obtained near the brain surface.

The most common method to eliminate unwanted fat signal is to place multiple saturation bands over lipid-containing regions. This method is known as outer volume suppression (OVS).

OVS bands are spatially but not frequency specific, reducing or eliminating signals from all tissues (not just lipids). They can be even thicker than the ones shown in the figure and brought down close to the edges of a single voxel even at the center of the brain, completely surrounding it. They are also commonly placed in the planes above and below the volume of interest (not illustrated).

Outer volume suppression bands

Inversion recovery (IR) method for fat suppression

OVS bands are usually sufficient for brain spectroscopy, but for body applications (especially prostate, breast and skeletal muscle MRS) additional fat suppression methods may be necessary. Inversion recovery methods widely used in conventional MR imaging may be employed, including STIR (Short TI Inversion Recovery) and SPIR (Spectral Presaturation with Inversion Recovery). A current popular choice is SPAIR (SPectral Attenuated Inversion Recovery), described more completely in a prior Q&A.

SPAIR begins with a frequency-selective, 180°-adiabatic pulse that inverts the longitudinal magnetization of fat. A time delay (~165 ms at 1.5T, ~200 ms at 3.0T) is interposed so that fat magnetization partially recovers to pass through zero at the time of spectroscopic excitation. The frequency band of the SPAIR pulse must include principal fat resonances at 0.9 and 1.3 ppm, and thus will obscure metabolites in this region of the spectrum, including lactate at δ = 1.33.

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Advanced Discussion (show/hide)»

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BASING module inserted in middle of PRESS sequence
for fat (or water) suppression

Although not intended as a standalone method, a BASING (BAnd Selective Inversion with gradient dephasiNG) module may be placed in the middle of a spectroscopy sequence to improve fat suppression. An example of BASING used with PRESS is shown in the diagram right.

BASING adds a 180°-frequency-selective pulse flanked by spoiler gradients of opposite polarity placed between the two 180°-pulses of PRESS. The 180º-BASING pulse is typically long (40-50 ms) and asymmetric, designed to stimulate a narrow band of frequencies around the fat resonance with minimal ripples outside this band. The first spoiler dephases all spins in the volume. The BASING 180°-pulse inverts only fat spins, while the second spoiler rephases everything except the inverted spins which continue to dephase and generate no signal. 

The insertion of the BASING module increases the minimum TE of the sequence and is hence best suited for medium or long TE scans. The BASING 180°-pulse can be tuned either to water, fat, or both, and hence can be used as a supplmentary method for water and fat suppression. 

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References
     Felmlee JP, Ehman RL. Spatial presaturation: a method for suppressing flow artifacts and improving depiction of vascular anatomy in MR imaging. Radiology 1987;164:559 –564.     
     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.       
     Star-Lack J, Nelson SJ, Kurhanewicz J, et al. Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING). Magn Reson Med. 1997; 38:311–321.

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Related Questions
     What makes fat and water behave so differently on MRI?  
​     There seem to be many methods available for fat suppression. Which one is the best? 
     What is SPAIR? How is it different than SPIR?
     Why and how do you suppress water signal in MRS?

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