Synopsis

Keywords: Data Acquisition, Data Acquisition

This study provides an alternative approach to conventional diffusion-weighted (DW) EPI-based sequences. A 2D single-shot (SS) diffusion-prepared (DP) turbo-spin-echo (TSE) sequence, combined with spiral-ring trajectories and magnitude stabilizers, dubbed “SS-DP-SPRING TSE”, was developed for distortion- and motion artifact-free diffusion imaging. Compared to a SS-DW-EPI sequence, this method is less sensitive to B₀-inhomogeneity and thus provides DW-images with improved geometric fidelity.

Introduction:

Diffusion-weighted pulse sequences are routine MRI protocols for neurological and oncological imaging, including, but not limited to, acute stroke and tumors.^1,2 The combination of single-shot spin-echo acquisition and an EPI-readout enables fast diffusion-weighted imaging with minimal motion artifacts. However, the long EPI-readout duration presents challenges for imperfections, such as B₀-inhomogenity induced geometric distortions and ghosting artifacts, thus limiting its use for some clinical applications (e.g., tumor delineation for radiation therapy). Multi-shot (MS) DW-EPI is a means to improve geometric fidelity³, yet shot-to-shot phase inconsistency caused by physiological motion (e.g., cardiac pulsation) and system imperfections can lead to substantial signal dropouts and artifacts, which require additional motion information to suppress and cannot always be easily corrected by a phase-correction method.^4,5

TSE acquisition⁶ uses a series of refocusing RF pulses that split long sampling trajectories (e.g., a spiral readout) into small portions of k-space coverage to suppress effects of B₀-inhomogenity. Researchers have developed sampling strategies including spiral-ring TSE for fast PD/T₂-weighted imaging.^7-8 However, incorporating diffusion gradients into a TSE acquisition is not straightforward because of the Carr-Purcell-Meiboom-Gill (CPMG) condition.⁹ In this work, we utilize a diffusion-prepared approach^10,11 combined with spiral-ring TSE imaging for single-shot diffusion imaging. Additionally, we include gradient stabilizers to correct for magnitude inconsistency associated with the preparation module.

Methods:

A schematic of the proposed SS-DP-SPRING TSE sequence is depicted in Figure 1. The diffusion-prepared (DP) module consisted of Stejskal–Tanner monopolar diffusion-gradients and one magnitude stabilizer (~4π dephasing) along the slice-select direction, followed by a 90° tip-up RF pulse that flips the diffusion-encoded signal back to the longitudinal axis. Large spoiler gradients (~2X larger than stabilizers) were added immediately after the diffusion-prepared module and before the following TSE acquisition. Rephasing and dephasing gradients were inserted before and after each echo during the readout to form the echo and distribute the magnetization, respectively. The added stabilizers offered the benefit of converting the magnitude-modulated signal loss into a phase-modulated problem⁹, at the cost of 50% SNR loss. We further adopted a single-shot acquisition approach, so there is no need of additional phase-correction which may not completely resolve phase inconsistencies among shots.

The central spiral-in-out ring was placed at the first echo of the TSE acquisition to obtain a short TE (~11 ms). Ten spiral-rings with linearly decreasing sampling density, from 1 to 0.2, were designed for 1.25 mm² isotropic in-plane resolution, with a total TSE acquisition time of 110 ms. The duration of diffusion gradients was 12 ms with 71 mT/m amplitude for an estimated b-value of 750 s/mm², yielding a total duration of 35 ms for the DP module. The slice-thickness of the sinc RF pulses used in the DP module was 1.5X larger than that of the TSE acquisition to mitigate flow-related and cross-talk artifacts. Other sequence parameters are given in Table 1.

To demonstrate the efficiency of stabilizers combined with the single-shot theme, we tested SS-DP-SPRING TSE without stabilizers and with stabilizers, and compared it to 2-shot DP-SPRING TSE with stabilizers. For image reconstruction, NUFFT¹² and L1-ESPIRiT¹³ were performed on undersampled datasets.

Experiments were performed on a 3T scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with a 64-channel head coil array. For both phantom and healthy volunteer studies, multiple slices with 4-mm thickness (2-mm gap) were acquired using the proposed methods and SS-DW-EPI.

Results and Discussion:

Figure 2 shows how the slice-thickness of RF pulses in the preparation module, and the dephasing range of stabilizers, affect image quality. The diffusion gradient was off for all phantom images. Compared to the reference (without preparation module), image quality degrades when the slice-thickness is small or stabilizers are not large enough to uniformly distribute the phase, most likely due to the slice-overlap artifacts, although there are still minor residual artifacts for the proposed method.

Figure 3 demonstrates the performance improvement for the proposed SS-DP-SPRING TSE with magnitude stabilizers (top) compared to that without stabilizers (middle) and for 2-shot DP-SPRING TSE with stabilizers (bottom). All images were reconstructed using NUFFT. Unpredictable severe bands of signal loss can be seen in Figure 3F-G from SS-DP-SPRING TSE without stabilizers when the diffusion gradient is on. When using 2-shot acquisition, artifacts and signal cancellation are obvious as shown in Figure 3L, because of cardiac pulsation, when the diffusion gradient is along head-foot direction. Image degradation attributed to magnitude or phase modulation is substantially reduced when applying stabilizers along with the single-shot theme.

Figure 4 shows examples of estimated ADC maps along three main diffusion directions and the mean ADC, acquired by SS-DP-SPRING TSE with magnitude stabilizers and L1-ESPIRiT reconstruction (top) and SS-DW-EPI (bottom). A T2-weighted Cartesian TSE image is used for anatomical reference. The proposed method shows similar ADC maps compared to the SS-DW-EPI counterpart, while distortion and signal pile-up artifacts are clearly seen in EPI images (e.g., red arrow). However, some slices from SS-DP-SPRING TSE show residual image artifacts (not shown here), likely due to the imperfect slice profile of RF pulses used in the DP module; future work is warranted to investigate this issue.

Conclusion:

We demonstrated a 2D single-shot diffusion-prepared TSE acquisition that utilized variable-density spiral-ring trajectories and magnitude stabilizers for distortion- and motion artifact-free diffusion-weighted brain imaging.

Acknowledgements

No acknowledgement found.

References

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