Synopsis

2D T2-weighted turbo spin-echo (T2w-TSE), fluid attenuated inversion recovery (FLAIR) with and without fat suppression are widely used in the clinical brain and spine protocols to improve diagnosis. However, FLAIR suffers from low SNR and long scan times. In this work, we developed a dual-acquisition 3D TSE sequence combined with dual-echo Dixon based approach to generate T2-weighted and FLAIR images of the spine with and without fat suppression in a single acquisition using the similar acquisition time as 2D FLAIR. Uniform fat/water separation was achieved using a shared-field-map and complex subtraction was used to generate FLAIR-like images without artifacts.

Introduction

Theory

The proposed dual-acquisition 3D TSE sequence with dual-echo Dixon generates 4 sets of echoes, including two gradient echoes (in-phase, IP; out-of-phase, OP) at both shorter and longer TEs (TE1/TE2) in a single repetition (Fig.1). The four signals can be modeled as: $$S_1=(W_{TE1}+c_1F_{TE1})e^{i\phi_1}~\\S_2=(W_{TE1}+c_2F_{TE1})e^{i\phi_2}~\\S_3=(W_{TE2}+c_1F_{TE2})e^{i\phi_3}~\\S_4=(W_{TE2}+c_2F_{TE2})e^{i\phi_4}$$with$$c_n=\sum_mw_me^{i(2\pi\Delta{f_m}+iR_m)\delta{t_n}}$$where the index m indicates spectral peaks of fat with off-resonance frequency $$$\Delta{f_m}$$$ and transverse relaxation rates $$$R_m$$$. $$$\delta{t_n}$$$ is the echo time shift with respect to Hahn echo. $$$W$$$ and $$$F$$$ are considered as complex, $$$W'=We^{i\psi_1};~F'=Fe^{i\psi_2}$$$. With this approach, the two echoes at the shorter TE can undergo standard Dixon reconstruction using the estimated $$$ \Delta{\Phi_{TE1}}=e^{i(\phi_2-\phi_1)} $$$. This generates $$$W_{TE1}$$$ and $$$F_{TE1}$$$ as⁴:$$\left(\begin{array}{c}W_{TE1}'\\F_{TE1}'\end{array}\right)=\frac{1}{c_2-c_1}\begin{bmatrix}c_2&c_1\\-1&1\end{bmatrix}\left(\begin{array}{c}S_1\\S_2\Delta\Phi_{TE1}^*\end{array}\right)$$Since the majority of the tissue signals decay at longer TE (TE2), the signal to noise ratio (SNR) will be poor for proper fat/water separation at TE2. However, the B0 field should remain the same between TE1 and TE2 and hence, $$$\Delta{\Phi_{TE2}}=e^{i(\phi_4-\phi_3)}$$$ is identical to $$$\Delta{\Phi_{TE1}}$$$. Using this shared B0 field map between shorter TE and longer TE, $$$W_{TE2}'$$$ and $$$F_{TE2}'$$$ can be calculated as:$$\left(\begin{array}{c}W_{TE2}'\\F_{TE2}'\end{array}\right)=\frac{1}{c_2-c_1}\begin{bmatrix}c_2&c_1\\-1&1\end{bmatrix}\left(\begin{array}{c}S_3\\S_4\Delta\Phi_{TE1}^*\end{array}\right)$$Using these two reconstructed water-only images, the fat suppressed FLAIR-like images can be calculated as:$$I=Real[(W_{TE1}'-W_{TE2}')e^{-i\psi_1}]$$with$$\psi_1=\angle{W_{TE1}'}$$.

Methods

Results

Conclusion

Acknowledgements

References

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