Frequency-Selective Fat Suppression Radiofrequency Pulse Train to Remove Olefinic Fats
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CHESS pulse can suppress the signal originating from aliphatic fat protons but cannot suppress the signal from olefinic fat protons, which is near the resonance frequency of water protons. Adipose tissue contains various fat species; aliphatic fat comprises about 90 % and olefinic fat about 10 % of adipose tissue. Thus, CHESS pulse cannot be used to suppress the signal from adipose tissue completely. The purpose of this study was to find a method to suppress the signal from adipose tissue completely. The Fatsat train pulse, created with an arbitrary flip angle and insensitive to B1 inhomogeneity, was used. Because B1 inhomogeneity is larger on higher field magnetic resonance imaging, the fat suppression radiofrequency pulse needs to be B1-insensitive. To investigate a percentage of olefinic fat in adipose tissues, the excitation frequency of the Fatsat train pulse was varied from −240 to +400 Hz and the images and fat-suppressed images were obtained. The presence of olefinic fat comprising about 10 % of abdominal adipose tissue was identified. The result agreed with some previous papers. Complete fat suppression could be achieved by partial (10 %) inversion of longitudinal aliphatic fat magnetization and by canceling out the two fat magnetizations. The flip angle was identified to about 95°. In conclusion, the cause that the signal from adipose tissues cannot be suppressed completely has been found. Improved images that signals from adipose tissues were suppressed completely have been demonstrated. This technique can also be applied to several pulse sequences.
KeywordsAdipose Tissue Flip Angle Fast Spin Echo High Field Magnetic Resonance Imaging Adiabatic Inversion Pulse
Fat suppression is used in routine clinical magnetic resonance imaging (MRI). Adipose tissue appears bright in T1- and T2-weighted imaging, and a chemical shift artifact occurs in diffusion-weighted imaging (DWI). These signals can interfere with the diagnosis of certain diseases. Thus, a method is needed to suppress completely the signal from adipose tissue in these applications. A chemical shift-selective (CHESS) radiofrequency (RF) pulse is commonly used for fat saturation (Fatsat) [1, 2]. To suppress the signal from lipid (aliphatic fat), which precesses at a lower frequency of 3.5 ppm than water, the CHESS pulse nutates the lipid magnetization to the transverse plane, while leaving the water magnetization unperturbed along the longitudinal axis. A spoiler gradient pulse is subsequently applied to diphase the excited lipid signal. Adipose tissue has a complex chemical spectrum that contains a number of different spectral components. Generally, there are two components of fatty tissues, lipid (aliphatic fat) and olefinic fat, which precesses at the same frequency as water [3, 4]. Although the CHESS RF pulse can suppress the signal from aliphatic fat, it cannot suppress the signal from olefinic fat. Therefore, the conventional CHESS RF pulse does not completely suppress the signal from adipose tissue, and a residual signal from olefinic fat remains.
Fat-suppressed images using different τ 2 values were obtained in studies of adipose tissue to identify the optimal FA that achieved complete fat suppression. Active shimming was performed and the same conditions were maintained in studies of adipose tissue in human volunteers.
The Fatsat train pulse with an optimized FA was applied to a 3D T1-weighted gradient echo sequence (TIGRE) and a 2D fast spin echo (FSE) sequence. MRI scans of the liver and breasts of healthy volunteers were conducted after permission was obtained with informed consent. The scan parameters were as follows: 3D TIGRE: TR = 4.7 ms, TE = 1.7 ms, FA = 12°, thickness = 6 mm, acquisition matrix = 224 × 224 × 32, SENSE factor = 1.8, and scan time = 23 s; 2D FSE: TR/TE/FA = 5,500 ms/93 ms/90°, thickness = 8 mm, number of slices = 18, acquisition matrix = 256 × 192, interecho time = 8 ms, echo train length = 30, and scan time = 23 s. A Fatsat pulse and the Fatsat train were used for the comparison. Imaging was performed on a 1.5-T MRI scanner (Echelon, Hitachi Medical Corporation). The RF transmitter coil was a quadrature volume coil, and the RF receiver coil was either an eight-element torso coil or a seven-element breast coil.
3.1 Fraction of Remaining Adipose Tissues by Varying Offset Frequency
Figure 3a(b–j) shows abdominal images at varying offset frequencies of the Fatsat train. Figure 3a(f) shows the relationship between the offset frequency and the frequency profile. Figure 3b shows a plot of the fraction of remaining adipose tissues normalized by the signal without the Fatsat pulse as a function of the offset frequency for the five areas shown in Fig. 3a(a). The adipose tissue was suppressed almost completely at −224 Hz for the offset frequency, but the percentages of remaining adipose tissues for the five areas were 10–15 %. The percentages at −100 Hz of the offset frequency were 10 %. The signal from renal and other tissues was suppressed almost completely at −100 Hz of the offset frequency. The percentages at −60 Hz of the offset frequency were <5 %, and the signals from adipose tissue were suppressed completely. The bright areas in the subcutaneous tissues represent the distribution of the receiver coil sensitivity. The differences in the percentages between −220 and −60 Hz were about 10 %.
3.2 Optimization of the FA of the Fatsat Pulse
3.3 Demonstration of Complete Fat Suppression in Human Volunteers
The focus of this research was to identify a method to suppress completely the signal from olefinic fat in adipose tissue. To accomplish this objective, a spectrally selective fat suppression RF pulse was used to measure the percentage of olefinic fat in adipose tissue in human volunteers. This study produced two major findings.
First, the reason why the signal from adipose tissue cannot be suppressed completely by a CHESS RF pulse with an offset frequency of −224 Hz was identified. The cause is the presence of olefinic fat, which precesses near the water resonance frequency. Olefinic fat comprises about 10 % of adipose tissue. This result is consistent with previous reports [3, 4]. Although this paper shows the results from adipose tissue in only one volunteer, the results obtained from other volunteers were the same as those shown in Fig. 3.
Second, to suppress the signal from adipose tissue completely, the longitudinal fat magnetization was inverted to −0.1 M0 (10 %), which was identical to an FA of 95°. This was achieved using a B1-insensitive Fatsat train pulse and an arbitrary FA. It is found that the inversion of longitudinal aliphatic fat magnetization by 10 % could cancel out the olefinic fat magnetization by 10 %, as shown in Fig. 1, and that complete fat suppression was demonstrated in this study. In other words, we can suppress the signals from olefinic fat, which precesses at the same frequency as water if we use the Fatsat train pulse with an FA of 95°. The Fatsat train pulse can be applied to several pulse sequences. If the conventional CHESS RF pulse is used, the signal from fat will remain because of B1 inhomogeneity. In contrast, if an adiabatic inversion pulse is used, the inversion time (TI) must be adjusted [7, 8, 9, 10, 11], and TI can represent large amount of extra time. Improved Dixon method can separate the signal from several fats, respectively , but the reconstruction of the method is time-consuming compared to the frequency-selective fat suppression RF pulse.
The limitation of this study is that the fraction of the olefinic fat may vary between individuals. Future work should include more subjects with a range of fat levels to confirm whether this method can be applied to all people.
In conclusion, the reason why the signal from adipose tissues cannot be suppressed completely by CHESS RF pulse with an offset frequency of –224 Hz is the presence of olefinic fat, which comprises about 10 % of adipose tissue. To suppress the signal from adipose tissues completely, longitudinal fat magnetization should be inverted to −0.1 M0 (10 %), which is identical to an FA of 95°. This was realized using a Fatsat train pulse with B1 insensitivity and producing an arbitrary FA.
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