Zhilang Qiu^{1}, Sen Jia^{1}, Haifeng Wang^{1}, Xin Liu^{1}, Hairong Zheng^{1}, and Dong Liang^{1,2}

Three-dimensional (3D) SPACE (sampling perfection with application optimized contrast using different flip angle evolutions) sequences are the workhorse for volume imaging with isotropic spatial resolution. However, spatial resolution is often scarified to achieve clinically acceptable scan time. Conventional one- and two-dimensional parallel imaging techniques could help reducing the scan time but would lead to deteriorated signal-to-noise (SNR) performance at submillimeter spatial resolutions. In this study, three-dimensional parallel imaging technique-Wave-CAIPI is utilized to improve the SNR performance for whole brain SPACE imaging with isotropic 0.6 mm resolution. In vivo results demonstrated that Wave-CAIPI could improve the SNR at 5x acceleration.

**Pulse Sequence:**
The sequence diagram of developed
Wave-SPACE is shown in Figure 1. Wave gradients of sinusoidal waveform are
played simultaneously along the phase (Gy) and partition (Gz) encoding
dimensions during the readout. A π/2 phase shift is imposed between the two
waveforms^{4}. Large wave gradient amplitude is desired to achieve effective
geometry factor reduction by Wave-CAIPI^{6}. However, high-resolution
SPACE sequence requires high bandwidth and small echo spacing for optimized T1
contrast and SNR^{1}. To obtain the maximal wave gradient amplitude
while not exceeding the gradient slew-rate limitation, a slight modification is
made to the wave gradient on the partition encoding dimension (Gz).
Specifically, 1/4 cycle in the head and 3/4 cycle at the tail (i.e. one cycle
in total) are truncated in the sinusoidal Gz wave gradient. The wave gradient
along Gz is then allowed to moderately ramp up from zero point and ramp down to
zero point and leads to reduced slew rates.

**In Vivo Experiments:**
The
IRB approved study was performed on a 3T Siemens Tim Trio MRI system with a
commercial 32-channel head coil. T1 weighted SPACE was used for whole brain
structure imaging with an isotropic resolution of 0.6 mm. A 25 years-old
healthy volunteer was prospectively recruited with informed consent being
obtained. Three scans using different subsampling schemes were performed after
localization: (1) 2x2 Uniform, (2) 2x2 Wave-CAIPI, and (3) 2x2 CAIPI. Common
imaging parameters were: TE/TR = 8.5/850 ms, ETL = 42, matrix size = 304x304x256,
FOV = 180x180x154 mm^{3}, bandwidth = 567 Hz/pixel. Each scan took 4:51 minutes to be completed. Both
uniform and CAIPI accelerations embedded a 24x24 calibration region in the
k-space center for coil sensitivity estimation. Separate ACS data with k-space
size of 304x24x24 was acquired immediately before Wave-CAIPI scan to estimate
the coil sensitivity map. Four calibration scans taking single slice projection
data were also acquired before imaging to characterize the Point Spread
Function (PSF) for wave-encoding model. These PSF calibration scans used the
same wave amplitude and cycle as the imaging scan, but used longer TR and shorter
ETL to guarantee the SNR in estimated PSF.

**Image Reconstruction:**
All
datasets were reconstructed offline in MATLAB (Mathworks, Natick, MA, USA). ESPIRiT
algorithm was firstly utilized for estimating the three-dimensional coil
sensitivity map. Then, 3D SENSE
reconstruction model was iteratively solved by LSQR algorithm to reconstruct Uniform
and CAIPI accelerated datasets. For Wave-CAIPI accelerated dataset, PSF model
characterizing the wave encoding was firstly estimated and integrated into the 3D
SENSE model. Then the parallel imaging model was solved by the LSQR iteration. Finally,
geometry factor maps showing the spatial distribution of noise amplification
were derived using theoretical analysis.

In this study, 3D parallel imaging method Wave-CAIPI is successfully integrated into the SPACE sequence. The geometry factor at 5x acceleration is improved comparing with 2D parallel imaging methods. The SNR performance improvement of wave-CAIPI is not significant at present. One reason may be only small wave gradient amplitude was applied currently. The second reason is the challenging PSF estimation, which is susceptible to heavy noise at high-resolution imaging. Further work will focus on improving G-factor reduction via increasing the amplitude of wave gradient. The signal-to-noise of projection scan will also be improved to achieve accurate PSF calibration for wave-CAIPI accelerated high-resolution T1 SPACE.

[1] Mugler III JP. Optimized Three-Dimensional Fast-Spin-Echo MRI. J Magn Reson Imaging 2014; 39:745-767.

[2] Blaimer M, Breuer FA, Mueller M, et al. 2D-GRAPPA-Operator for Faster 3D Parallel MRI. Magn Reson Med 2006; 56:1359-1364.

[3] Breuer FA, Blaimer M, Mueller MF, et al. Controlled Aliasing in Volumetric Parallel Imaging (2D CAIPIRINHA). Magn Reson Med 2006; 55:549-556.

[4] Bilgic B, Gagoski BA, Cauley SF, et al. Wave-CAIPI for Highly Accelerated 3D Imaging. Magn Reson Med 2015; 73:2152-2162.

[5] Park J, Mugler III JP, Horger W, Kiefer B. Optimized T1-weighted contrast for single-slab 3D Turbo Spin-Echo Imaging with long echo trains: application to whole-brain imaging.

[6] Qiu Z, Wang JJ, Ying L, Liu X, Liang D. Parameter Optimization of Wave-CAIPI Based on Theoretical Analysis. In Proceedings of the 26th Annual Meeting of ISMRM, Paris, France, 2018. p. 3506.

Figure 1. Sequence diagram
of the proposed wave-SPACE. Wave gradients of sinusoidal waveforms are played
simultaneously during the readout (with a π/2 phase shift between the two
waveforms). 1/4 cycle in the head and 3/4 cycle at the tail (i.e. one cycle in
total) are truncated in the sinusoidal wave gradient playing on the partition
encoding dimension (Gz).

Figure 2. R=2x2-fold
prospectively accelerated imaging at 3 T. Two sagittal slices of reconstructed
3D images for the three acquisition schemes: uniform, 2D CAIPIRINHA and Wave-CAIPI, and their
zoomed images are shown.

Figure
3. G-factor maps corresponding to the two sagittal slices of reconstructed 3D
images (as shown in figure 2) for the three acquisition schemes: uniform, 2D CAIPIRINHA and Wave-CAIPI.