Introduction:

Cartesian HASTE, a single-shot technique, uses a very long echo train (e.g., 128) to acquire all k-space data following a single RF excitation.^1,2 Partial-Fourier (PF) sampling³ and parallel imaging (e.g., GRAPPA⁴) are normally introduced to reduce image blurring and artifacts from T₂ decay. However, 2D-Cartesian sampling remains time-consuming because of its relatively inefficient k-space coverage when high-spatial-resolution and large-FOV are needed. This becomes more severe in abdominal imaging where different kinds of motion (respiratory, peristatic) exist, and affects the generation of high-quality images.

Spiral acquisitions cover k-space more efficiently than Cartesian sampling and are attractive in fast/dynamic imaging, where very short overall scan times are preferred.^5-7 Researchers have developed sampling strategies including spiral TSE for fast T₂-weighted imaging. Hennig et al.⁵, for example, has proposed single-shot spiral TSE with annular-ring segments, enabling highly efficient acquisition of high-resolution brain-images in less than 200 ms per slice.

In this work, we developed and optimized a 2D single-shot TSE sequence using annular spiral-ring trajectories for abdominal imaging at 3T. We use a variable-flip-angle approach to flatten the signal evolution as well as maintain a relatively high signal intensity at later echoes, and compressed sensing to further accelerate the data acquisition. Additionally, to achieve images with different effective TEs, we utilize different echo-ordering schemes and compare the performance of spiral-rings with different shifts along the echo-train.

Methods:

A schematic of the proposed spiral HASTE sequence is shown in Fig.1A. The preparation module consisted of saturation bands along the head-foot direction to reduce flow artifacts from the abdominal aorta and vena cava, and a spectral attenuated inversion recovery (SPAIR) pulse to suppress fat, each followed by large-gradients applied immediately to spoil unwanted transverse-magnetization. A single-shot TSE module with linear-variable-density spiral-ring readouts was used for data acquisition. The central spiral-in-out ring was placed at the echo with the effective TE (TE_eff), while other echoes were filled with outer spiral-out rings. Specifically, two echo-ordering schemes are shown in Fig.1B.

The variable-flip-angle RF series shown in Fig.1C (red line) was calculated for liver specifically at 3T (T1: 800 ms, T2: 35 ms)⁸ using the EPG algorithm^9,10, with a maximum 140^o flip angle for SAR reduction. The contrast-equivalent TE (TE_equiv) was calculated using the acquired signal and the transverse coherence component of the signal where relaxation effects were ignored.¹⁰ A constant-flip-angle scheme (blue line) was implemented for comparison.

Experiments were performed on a 3T scanner (MAGNETOM Vida, Siemens Healthcare, Erlangen, Germany) using an 18-channel Ultraflex large coil array. For image reconstruction, L1-ESPIRiT¹¹ was performed on all undersampled datasets. For healthy volunteer studies, images were acquired using spiral HASTE (7s/25slices) and Cartesian HASTE (two versions: 8.5s/ and 10s/25slices) as a reference. Other sequence parameters are given in Table-1.

Results and Discussion:

Fig.2A shows the signal evolutions for liver, kidney, spleen, and muscle, using the constant-flip-angle (top) and variable-flip-angle (bottom) schemes. Note that all signals from the variable-flip-angle approach are much higher than those for the constant-flip-angle counterpart at middle and late echoes, and the signal pathways at the bottom are relatively flattened with a smooth plateau. The performance of using the variable-flip-angle approach can be seen in Fig.2B, where images from constant-flip-angle (top) show strong blurring at early-TE and contrast loss at late-TE because of substantial T₂-decay signal variation, while images from variable-flip-angle (bottom) depict reasonable image contrast and sharpness.

Fig.3 illustrates the comparison of spiral HASTE with two proposed echo-ordering schemes. Simulation results of center lines of 2D PSFs are shown in Fig.3A for each echo-ordering and each tissue. Note that the PSF was generated based on the designed undersampled k-space and was then normalized to [0, 1] by dividing by its own peak. The echo-ordering #2 produces a smoother frequency response, yielding a narrower main lobe of the PSF for all tissues compared to #1, especially in tissues with short T2 values (liver and muscle). However, the sidelobe energy is increased in echo-ordering #2 relative to that for #1; this can be mitigated by using L1-ESPIRiT. Fig.3B shows one example of in-vivo abdomen-images, demonstrating the performance of improved image sharpness when using echo-ordering #2 rather than #1.

Fig.4 shows abdomen-images of spiral HASTE using L1-ESPIRiT (right) with a 7-s scan time, compared to Cartesian HASTE images using GRAPPA and PF with 8.5-s scan time (left) and 10-s scan time (middle). Images from Cartesian HASTE show obvious blurring along the phase-encoding direction, especially when using shorter scan time, while spiral HASTE yields the best image sharpness and shortest scan time, as well as a 20% reduced power deposition (1.51 versus 1.94 W/kg).

Acknowledgements

No acknowledgement found.

References

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