Yamin Arefeen^{1}, Borjan Gagoski^{2,3}, Berkin Bilgic^{4,5}, Ellen Grant^{2,3}, and Elfar Adalsteinsson^{1,6,7}

^{1}Massachusetts Institute of Technology, Cambridge, MA, United States, ^{2}Fetal-Neonatal Neuroimaging and Developmental Science Center, Boston Children’s Hospital, Boston, MA, United States, ^{3}Harvard Medical School, Boston, MA, United States, ^{4}Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ^{5}Department of Radiology, Harvard Medical School, Boston, MA, United States, ^{6}Harvard-MIT Health Sciences and Technology, Cambridge, MA, United States, ^{7}Institute for Medical Engineering and Science, Cambridge, MA, United States

Fetal MRI utilizes Half-Fourier-acquisition-single-shot-turbo-spin-echo (HASTE) for motion robust imaging. However, specific-absorption-rate (SAR) constraints from the refocusing pulse train lengthens scan time and increases vulnerability to motion-induced artifacts. Variable refocusing flip angle (VFA) acquisitions improve efficiency, but may suffer from poor contrast-to-noise-ratios (CNR). We propose an optimization technique for VFA that retains ~90% CNR with 2.5x SAR reduction. Furthermore, we demonstrate the first application of wave-encoding in fetal MRI at R_{in-plane} = 3-fold prospectively under-sampled acquisitions. Combining HASTE, VFA, and wave-encoding improves acquisition time with reduced repetition times and shorter echo-trains and could enable simultaneous-multislice acquisitions for further acceleration.

We propose an optimization scheme

We utilize an optimization scheme that designs the VFA control angles to maximize CNR

$$\alpha^* = argmax_{\alpha} ||C_{T_2^A}(\alpha,TE) - C_{T_2^B}(\alpha,TE)||_2^2$$

designs control angles $$$\alpha^*$$$ that maximize contrast between two $$$T_2$$$-values, $$$T_2^A$$$ and $$$T_2^B$$$, at a specified echo-time. We use $$$T_2^A$$$ = 80 ms and $$$T_2^B$$$ = 120 ms and match the clinical HASTE acquisition with $$$TE$$$ = 122 ms. The optimization problem is solved with gradient descent using finite-differences to compute gradients.

Figure 1.A compares constant 160 degree, previous heuristic, and optimized proposed refocusing trains and resultant signal evolutions from an isochromat based Bloch, Carr-Purcell-Meiboom-Gill simulation with slice-profile effects using characteristic fetal $$$T_2$$$ values

Second, standard HASTE data with constant flip angles and optimized VFA was acquired from a pregnant mother (FOV = 320 x 320 mm, RES = 1.25 x 1.25 mm, slice-thickness = 3 mm).

We compare product reconstructions of the above acquisitions, and compare CNR by computing ratios between regions-of-interest in the phantom.

Next, we implemented wave-encoding along the phase-encoded axis in the HASTE sequence

Combining $$$R_{in-plane}$$$ = 3 with VFA improves SAR reduction from 2.5x to 2.75x since the sequence applies fewer refocusing pulses. For some subjects the minimum prescribed TR with VFA operates below the SAR limit, enabling simultaneous-multislice (SMS) encoding to further improve sequence efficiency.

As proof-of-concept, we generated retrospective wave-encoded-SMS k-space ($$$R_{in-plane}$$$ = 3, SMS = 2, slice-gap = 60 mm) from a prospectively wave-encoded acquisition with constant flip angles and optimized VFA on an adult brain ($$$R_{in-plane}$$$ = 3, 77% partial fourier, $$$TE$$$ = 122 ms, FOV = 224 x 204 mm, RES = 1 x 1 mm, slice-thickness = 3 mm, and wave trajectories with max-gradient-amplitude = 14.1 mT/m, max-slew-rate = 180 T/m/s, and 3 cycles). Note that prospectively, SAR constraints preclude SMS acquisitions with constant refocusing trains.

We reconstruct wave-encoded data with the wave-forward model and zero-filled partial fourier using an external GRE scan for coil estimation

Figure 3 compares product HASTE in-pregnancy with constant flip angles and optimized VFA. Optimized VFA achieves similar image quality with 2.5x less SAR.

Figure 4 displays reconstructed prospective wave-encoded-HASTE fetal data with artifact reduction and improved inverse-gfactor maps in comparison to Cartesian acquisitions. VFA with $$$R_{in-plane}$$$ = 3 achieves 2.7x reduced SAR.

Figure 5 (a) illustrates reconstructions and inverse-g-factor maps of prospective brain wave-encoded-HASTE data with constant and optimized refocusing. Wave-encoding with VFA enables the potential SMS reconstructions in Figure 5(b).

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DOI: https://doi.org/10.58530/2022/0740