Introduction

Echo Planar Spectroscopic Imaging (EPSI) significantly accelerates MRSI by simultaneous encoding of one spatial and the temporal dimension with echo-planar fast gradient oscillation during signal read-out [1]. However, only half spectral bandwidth is used in conventional EPSI, as the even and odd readout echoes are utilized separately during reconstruction. Limited spectral bandwidth is a problem particularly at higher magnetic field strengths (>3 T), as chemical field dispersion effects increase, leading to spectral aliasing [2]. Besides, the splittings because of J couplings result a broader peak along the chemical shift axis, which make coupled compounds such as GABA and glutamate harder to detect and quantify [3, 4]. Recently, cross-term spatiotemporal encoding (xSPEN) [5] is an introduced imaging approach delivering single-scan 2D images with exceptional resilience to chemical shifts and field inhomogeneities. The shot-to-shot distortion-free xSPEN images endow robustness to in-plane motion. Here, we introduced a time increasing (t₁) evolution as chemical shift encoding to fast obtain multiple-voxel spectroscopy. This xSPEN spectroscopy has no limitation of spectral bandwidth while the echo-planar imaging acquisition grants a high sampling efficiency of MRSI. Furthermore, we demonstrate in vitro results with an increasing chemical shift separation and detection by taking advantage of the unique J-decoupled spectrum capability of this technique.

Methods

The xSPEN spectroscopy sequence is shown in Fig. 1c by performing multiple scans with increasing time (t1) evolution while the echo-planar imaging-based acquisition of xSPEN grants a high sampling efficiency of MRSI. The chemical shift spectrum of each voxel is obtained by the Fourier transform along the time-increasing dimension. The resolution and bandwidth of the chemical shift spectrum will be determined by the usual Nyquist criteria of the maximum t₁ and the t₁ step, which can be flexibly adapted. By splitting the t₁ evolution into τ-t₁/2 and t₁/2 on two sides of π pulse, it results a constant-τ J coupling evolution bringing a J-decoupled xSPEN spectroscopy sequence as shown in Fig.1d. This unique J-decoupled xSPEN spectroscopy increases the chemical shift separation and detection of coupled compounds. Besides, the single-shot distortion-free xSPEN images allow a self-navigated in-plane motion correction. Studies were conducted with a 3.0T United Imaging uMR790 system (Shanghai, China). To demonstrate the concept, a phantom of ~200 mM GABA aqueous solution with three different multiplets correspond to the three methylene (CH₂) groups in the GABA molecule. Parameters for the EPSI include TR =2 s, 200×200 mm² FOV, 10 mm slice thickness, 80×80 image matrix size was cropped into 32×32 for the comparison, the echo spacing is 0.53 ms. Parameters for the xSPEN spectroscopy include TR =2 s, 200×200 mm² FOV, 10 mm slice thickness, 32×32 image matrix size, a total of 160 t₁ encodings with two bandwidths of 1000 Hz and 2000 Hz. The constant J coupling evolution τ=120 ms is used for J-decoupled xSPEN spectroscopy. For the robustness to motion study, factitious phantom motions were introduced during the xSPEN spectroscopy scanning with/without water suppression.

Results and Discussion

Figure 2 shows the comparison among EPSI, xSPEN spectroscopy, and J-decoupled xSPEN spectroscopy. The splittings because of J couplings in the EPSI and xSPEN spectroscopy result broader peaks along the chemical shift axis, while high-resolution singlets of J-decoupled xSPEN spectroscopy enhance the chemical shift separation and detection. A larger spectral bandwidth of 2000 Hz can be easily achieved by xSPEN spectroscopy sequences as shown in Fig.2b. Both the field inhomogeneities and J couplings can result spectral broadening, which may lead to inaccurate chemical shift maps as shown in Fig. 3 while the J-decoupled xSPEN spectroscopy can benefit the quantification by simplifying the coupled compounds. Figure 4 demonstrates the robustness of xSPEN spectroscopy to the in-plane motion, shot-to-shot chemical shift encoded xSPEN images allow a self-navigated motion tracking and correction to greatly reduce the spectral distributions.

Conclusions

We propose an alternative fast magnetic spectroscopy imaging termed as xSPEN spectroscopy endowed with flexible spectral bandwidths, a unique J-decoupled spectrum capability and the robustness to in-plane motion.

Acknowledgements

This work is supported by National Science Foundation of China (No. 62001290) and sponsored by Shanghai Sailing Program (20YF1420900).