82 / 2021-10-24 22:26:15

MRI Phantom Study of implementation of Lipid Insensitive Binomial Off-Resonant RF Excitation (LIBRE) pulses in SPACE sequence at 3T

MRI,SPACE,fat suppression,LIBRE

Abstract—LIBRE is a new fat suppression technology. The LIBRE method was shown to be robust to magnetic field inhomogeneities in GRE sequence at 3T. This work is to do a preliminary test of the fat suppression effect of LIBRE in the SPACE sequence by phantom experiments.

Keywords—MRI; fat suppression; LIBRE; SPACE

• I. Purpose

In the SPACE sequence, the existing fat suppression methods still have the problem of being sensitive to the inhomogeneity of the magnetic field. And  LIBRE is a new fat suppression technology that can effectively against the inhomogeneity of the magnetic field. The purpose of this article is to do a preliminary test of the fat suppression effect of LIBRE in the SPACE sequence.

• II. Methods

The LIBRE pulse consists of two rectangular sub-pulses. Each rectangular sub-pulse has the same offset frequency f, duration  τ, RF excitation angle α, yet an unequal phase φ. Here, the phase offset of the first RF sub-pulse is zero (φ₁₌₀ ), while the relative phase offset of the second RF sub-pulse is determined by the frequency and duration of the first RF sub-pulse, given by φ₂=φ₁+2πfτ[1].

First, find out the LIBRE parameters that can suppress fat well  through numerical simulation. At the same time, the effective flip angle of the LIBRE parameter needs to be close to 90°, which can replace the original excitation pulse of SPACE.

Use MATLAB  for simulation. The inhomogeneities of the B0 and B1 fields are assumed to be zero.Under a fixed pulse excitation angle, the duration of a single LIBRE pulse is 0.05-2 ms, and the frequency deviation is -2000-2000 Hz. The excitation angle of LIBRE is respectively 50°, 60°, 70°, 80°, 90°. The T1 of muscle was assumed to be 1500 ms, T2 50 ms , repetition time 700 ms, number of RF excitations 3, and echo train length 35. Simulate the signal of the third echo. The difference between water and fat signals (M_XY,water– M_XY,fat) is numerically simulated and a two-dimensional color image is made. At the same time mark the point where the final effective flip angle in the figure is 85-95°. The parameter corresponding to the point where the difference between the water and fat signals is greater than 60% of the original magnetization vector is the LIBRE parameter we are looking for.



B.MRI Phantom Study

Phantoms (baby oil and mixed solutions of NiSO4 and NaCl) used in this study were designed to mimic the fat and water of the human body. Use the LIBRE parameters obtained by numerical simulation to scan the water model and baby oil on the 3T MR. Then use WE 1-1 , FS , SPAIR and STIR (TI=200 ms) method  to compare the fat  suppression effect of LIBRE. The phantom measurements were  with  a field of view (FOV) of  253×330 ×144 , a matrix size  of  196×256 ×256 , TR=700 ms,  TE=12 ms, turbo factor = 35, using a radial reordering approach, using RF spoiling. After scanning, calculate the signal-to-noise ratio of water model and baby oil under different scanning methods. The signal-to-noise ratio (SNR) was computed as the ratio of the average signal and the standard deviation of the background noise. Find the best LIBRE parameters  under the same excitation angle.

• III. Results and discussion

Phantom experiments showed that the best LIBRE parameters are 60°,  300 Hz, 1.35 ms; 70°,  430 Hz, 1.2 ms; 80°,  480 Hz, 1.05 ms; 90°,  520 Hz, 1.05 ms. From the image point of view, the fat signal at the inhomogeneous magnetic field in the image of the LIBRE method  is lower than that of FS, WE 1-1, and SPAIR. However, the fat can be clearly seen in the image of the STIR method. From the point of view of SNR, the water signal obtained by the LIBRE method is lower than that of WE 1-1, FS and SPAIR. And the larger the RF angle of LIBRE, the lower the SNR, which is largely caused by the inhomogeneities of the B1 field.

• IV. Conclusion

In the phantom experiments, LIBRE is still more effective against the magnetic field inhomogeneity than WE 1-1, FS, and STIR in the SPACE sequence at 3T.



References

1. Bastiaansen JAM, Stuber M. Flexible water excitation for fat-free MRI at 3T using lipid insensitive binomial off-resonant RF excitation (LIBRE) pulses. Magn Reson Med. 2018; 79:3007-3017.