Introduction

Here we explore using “Controlled aliasing in parallel imaging results in higher acceleration” (CAIPIRINHA, or CAIPI)(1) as a respiratory gating method. With two slices simultaneously excited, one slice is used for respiratory monitoring while the other slice is the actual target for imaging. CAIPI acquires multiple slices simultaneously, with different RF excitation phases for each slice. The data is then reconstructed using parallel imaging. Radial is a promising trajectory for CAIPI (2,3). Importantly, for retrospective respiratory-gating, radial CAIPI with golden angles (4) allows flexible retrospective reconstruction, generating a uniform distribution of k-space data.

Methods

Methods: All imaging was performed on a 3.0T Siemens scanner using a 32 channel cardiac coil (Invivo, Pewaukee, WI), in subjects providing written informed consent. The overall acquisition scheme is shown in Figure 1. CAIPI was implemented within a 2D GRE radial cine sequence. To reduce eddy current effects from rapidly changing spoke angles while also preserving the even coverage of golden angle acquisition the cine data was acquired using a reordered golden angle acquisition. A series of N_pall golden angles were calculated. These angles were divided into N_sets2π successive sets which were reordered from 0 to 2π. These segments were acquired in N_pall /N_sets2π heart-beats, at multiple cardiac phases. ECG-gating and free-breathing were used. The RF pulse was modified to excite two slices. Within each heart-beat the RF excitation phase was alternated (0,π,0…) for the 2nd slice. Two types of images were generated (Figure 2). The respiratory-slices were reconstructed with Np=N_sets2π projections, to obtain an image of the diaphragm at any point chronological time during scanning, and to monitor respiration. From these images, the heart-beats which were timed to end-expiration were selected. The target slice was then reconstructed using Np=N_sel· N_sets2π/2 data, where N_sel indicates the number of selected heart-beats timed to the end-expiratory phase, and only the first half (0-π), or 2nd half (π -2π) of the projections in a single segment were used, for a reduced acquisition window. All radial CAIPI data was reconstructed with parallel imaging using Algebraic Reconstruction Technique (ART) (6) (9 iterations, λ=0.1), an iterative method similar to projection onto convex sets (POCS). The coil-maps estimates were generated from all acquired projections. Typical scan parameters were: TR/TE/θ=5.5ms/2.0ms/12◦, FOV=36cm, Hz/pix=400Hz, slice=10mm, slice gap ~ 10cm, N_pall=512, N_sets2π=32, 88ms per phase.

Results

Figure 3 shows phantom data using golden angle radial CAIPI, scanned with a 32 channel head coil, showing reasonable quality. Figure 4 shows an undersampled respiratory slice (32 Np), the target slice (16Np) from a single heart-beat, the target-slice from all heart-beats (which is blurry), and finally the targeted slice reconstructed with selected projection sets (here using 4 end-expiratory heart-beats, i.e. 64 Np). This data is from a healthy volunteer.

Discussion

We present a novel method for CAIPI respiratory gating, which uses CAIPI to excite simultaneously one slice for monitoring respiration, and another for imaging. We demonstrate its feasibility in initial experiments. Respiratory compensation is still a key challenge in cardiac imaging, especially for high resolution applications, and for poor breath-holders. Many novel solutions exist beyond traditional breath-holding, bellows-gating, and navigator-gating: e.g. self-gating and image-based gating (6-8) methods, among others. CAIPI-gating provides an almost-ideal respiratory monitoring technique.

Acknowledgements

No acknowledgement found.

References

1) Beuer FA et al., MRM 2005. 2) Wang H, et al., MRI 2016. 3) Yutzy SR et al., MRM 2011. 4) Winkelman S et al., IEEE TMI 2007. 5) Li S et al. MRM 2014. 6)Larson AC et al., MRM 2005. 7) Stehning C et al., MRM 2005. 8) Henningsson M et al., MRM 2015.