Synopsis

The optimal SNR efficiency of 3D multi-slab multi-shot DWI acquisition can be enabled by using a TR range of 1-2s. However, due to the low feasible slab thickness, the multi-slab acquisition can only achieve limited brain coverage when using a short TR. In this study, we first develop a 3D multi-band (MB) iDW-EPI sequence with MB-EVI-based navigator to increase brain coverage when using optimal TR. Second, we extend 3D-MUSER algorithm, which is proposed in another study, to reconstruct 3D-MB-iDW-EPI data with 3D phase correction. Our preliminary result demonstrates the feasibility of 3D MB DWI with 3D phase correction.

Introduction

Methods

Pulse sequence design: Figure 1 shows a 3D-MB-iDW-EPI sequence using two consecutive RF pulses to simultaneously excite two slabs⁷. Alternating polarity of second excitation RF pulse between each excitation was used to enable controlled aliasing⁸. The technical consideration of MB-EVI-based navigator echo is to separate overlapped EVI-based navigator echoes (with R=4) from different slab locations. In order to refrain from using SENSE-based reconstruction of MB-EVI-based navigator echo, we adopted a split-echo scheme to acquire the MB-EVI-based navigator echo by adding a small G_x gradient between two consecutive RF pulses of second refocusing (red triangle in Figure 1). The area of fly-back gradient (orange triangle in Figure 1) was adjusted for aligining the k-space center of navigator echo for each slab, as shown as in FIgure 1.

Reconstruction of MB-EVI-based navigator: With implementation of split-echo scheme during acquisition, the k-space signal of MB-EVI-based navigator data can be directly divided into two portions and then reconstructed using the flowchart shown in Figure 2.

Data reconstruction with 3D-MB-MUSER: The 3D-MUSER proposed in another study is a hybrid k-space based reconstruction algorithm with a plane-by-plane reconstruction. The signal equation of a single slab in x-k_y-k_z space after 1D-FT along x direction can be written as:

$$$S_{\gamma,k_{y|j},k_{z}}=\mathbf{F}_{k_{y|j},k_{z}}\mathbf{\phi}_{k_{y|j},k_{z}}\mathbf{C}_{\gamma}\overline{\rho}$$$ (1).

where $$$\mathbf{F}_{k_{y|j},k_{z}}$$$ denotes an operator of 2D FT along k_y|j and k_z (k_y-k_z plane at each x’), $$$\mathbf{\phi}_{k_{y|j},k_{z}}$$$ represents 3D phase variation measured from navigator echo at specific k_y|j and k_z, C_γ represents the 3D coil sensitivity map of γ^th coil element, and $$$\overline{\rho}$$$ represents image intensity. With a 2-band excitation, the k-space signal is theoretically a sum of signals from two simultaneously excited slabs, which can be written as:

$$$S_{\gamma,k_{y|j},k_{z}}=\mathbf{F}_{k_{y|j},k_{z}}[\mathbf{\phi}_{k_{y|j},k_{z}}^{z1}\mathbf{C}_{\gamma}^{z1}\overline{\rho}_{z1}+\mathbf{\phi}_{k_{y|j},k_{z}}^{z2}\mathbf{C}_{\gamma}^{z2}\overline{\rho}_{z2}]$$$ (2).

Solving the linear system of 3D-MB-MUSER (Equation 2) by concatenating all signals equations for different coil elements, k_y segments, and k_z encoding steps can obtain the 3D image volumes of two overlapped slabs. In addition, 3D-MB-MUSER algorithm takes 3D inter-shot phase variations of two overlapped slabs into account, thereby eliminating aliasing artifacts in both of them.

Preliminary test: One set of 3D-MB-iDW-EPI data with 2-band excitation was acquired from a healthy subject at 1.5T MRI scanner (GE HDxt) using an 8-channel head coil. The scan parameters included: slab thickness = 20mm, number of k_y segment = 4, number of k_z encoding steps = 10, x-y matrix size = 128, TR = 1250 ms, b-value = 800 s/mm²).

Results

Discussion and Conclusion

Acknowledgements

References

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