1.6.1 - Frequency-selective Fat Saturation (Fat SaT)

Frequency-selective Fat Sat is the most commonly used technique for fat suppression, and relies on fat and water having different resonant frequencies (i.e. chemical shifts). Traditionally, this method approximated fat and water as two resonant peaks, separated by 3.5ppm (220 Hz at 1.5 Tesla). In this method, the fat signal is excited using a spectrally selective RF pulse tuned to the frequency of fat (a Chemical Shift Selective, or, CHESS pulse). (See Figure 1.28) This is followed by a large amplitude, pulsed gradient that "crushes" the transverse magnetization from fat. The imaging pulse sequence is then played out after the signal from fat is suppressed.

Figure 1.28
Pulse sequence timing diagram illustrating the use of a CHESS pulse for frequency-selective fat saturation.

An improved Fat Sat technique is included in SE and FSE pulse sequences on 5.8, 8.2.5 and later operating software revisions (i.e. it is not included in 5.7, nor in 8.2). This technique incorporates two major modifications. First, instead of approximating the fat resonance as a single peak at 3.5 ppm from water, the method models the fat peak as a series of peaks with varying chemical shifts and relative amplitudes (See Figure 1.29).

Figure 1.29
Schematic diagram showing the position, and relative amplitudes of the nine resonance peaks used to model fat in the new, improved Fat sat technique.

Secondly, the improved Fat Sat method tailors the CHESS pulse flip angles on-the-fly for the user's specific choice of number of slices and TE/TR. Fat suppression is optimized for all image slices in the acquisition by taking into account and compensating for the timing differences between multiple interleaved slices. Each time the user prescribes an FSE- or SE- based sequence with improved Fat Sat, their choice of timing parameters and number of slices are entered into a mathematical model. This model calculates what the intensity of signal from fat will be for this particular pulse sequence set-up, based on the component fat peaks (See Figure 1.30).

Figure 1.30 An example of an optimization performed with a particular choice of timing parameters to determine the best flip angle for the CHESS pulses to produce a minimum signal from fat on the images.

The signal intensity from fat is calculated for a range of possible flip angles for the CHESS pulses, and the flip angle for which the signal from fat will be minimized is selected. This flip angle is then passed to the pulse sequence to provide on-the-fly optimization of fat suppression.

In orthopedic imaging, achieving reliable and uniform Fat Sat is a notoriously difficult problem for any clinical MRI system. The Fat Sat technique relies on the ability to completely separate the peaks corresponding to fat and water. This technique is therefore only feasible for use at high field strengths, where the peaks are well separated*. In addition, this technique requires a very homogeneous main magnetic field to ensure that the peaks for fat and water are at the same frequency for the entire FOV. Any imperfections in the main magnetic field homogeneity due to the superconducting magnet itself, to interfaces with air, or to the presence of metal objects, can cause the partial or complete failure of Fat Sat.

For all cylindrical superconducting magnets used in high-field MRI, the homogeneity of the magnetic field created within the bore drops off as one approaches the outer edges of the bore. For larger patients, the shoulder, elbow and wrist are unavoidably located in this region of decreasing magnetic field homogeneity. For these off-center FOVs, the center frequencies of water and fat are different at different locations within the FOV. It is therefore not possible to saturate fat while not affecting the water signal for all locations within the FOV. This is the cause of the inhomogeneous fat suppression often seen for off-center FOVs. The presence of metal hardware can also have an adverse effect on the reliability of Fat Sat. When a metal object is placed in an MR scanner, a magnetic field is set up around the object that is determined by the shape of the object, and its orientation with respect to B0. The strength of the magnetic field created depends on the metallic composition of the object. Ferromagnetic objects can create very strong magnetic fields. For cases where extreme off-center FOVs must be used, or where metal is present, the STIR technique is a more reliable alternative to frequency-selective Fat Sat.