Introduction

A recently reported technique – Sub-millisecond Periodic Event Encoded Dynamic Imaging or SPEEDI (also known as SMILE^1,2) – has demonstrated sub-millisecond temporal resolution in MRI using an FID or spin-echo acquisition. However, the total scan time is long, which prevents SPEEDI from being practically used in a wide range of potential applications. Decreasing the matrix size can reduce the scan times, but compromises the spatial resolution. One possible approach is to simultaneously reduce the field-of-view (FOV) by focusing on a region of interest^3–5 so that the spatial resolution can be maintained. In addition, the scan times of SPEEDI can be further shortened by replacing FID-based or spin-echo-based acquisition with echo-train-based acquisition. In this study, we report a variation of the SPEEDI sequence, which we call re-SPEEDI, by incorporating the two aforementioned strategies – reduced FOV and echo-train acquisition.

Methods

re-SPEEDI:
To reduce the FOV, re-SPEEDI employed a 2D RF excitation pulse, which was designed by tilting 2D excitation k-space^3,4. To incorporate echo-train acquisition so that frequency-encoding can be applied along one spatial direction to reduce the scan time, an FID signal in the original SPEEDI sequence was replaced with a non-phase-encoded gradient-echo train (Fig. 1). Each echo in the echo train was positioned in an individual k-space raster, and all echoes in the echo train were spread across a series of time-resolved k-space rasters (k-space 1, k-space 2, … k-space n), as shown in Fig. 1. This process was repeated with different phase-encoding values until all 2D k-space rasters were adequately sampled. After a 2D Fourier transform, a collection of time-resolved images was obtained with a temporal resolution determined by echo spacing (esp). Compared to SPEEDI, re-SPEEDI considerably reduces the total scan times because of a smaller matrix size and the use of frequency-encoded echo train signals.
Data Acquisition:
A re-SPEEDI sequence (Fig. 1) was implemented on a 3T GE MR750 scanner. Experimental demonstration was carried out using a phantom that contained a rectangular wire loop submerged in water. A pulse generator was used to supply a rapidly time-varying current to the wire loop, which was synchronized with the RF unblanking signal from the scanner. The same signal was also used to trigger the re-SPEEDI sequence in order to capture the dynamics of the phase change produced by the time-varying current. The experiments were performed three times, each with a different FOV: (a) full FOV that covered the entire phantom (FOV=12cm×12cm, matrix=64×64, scan time=2:08, esp=0.6ms), (b) a reduced FOV in the phase-encoding direction only (FOV=12cm×3cm, matrix=64×16, scan time=32s, esp=0.6ms), and (c) reduced FOV in both frequency-encoding and phase-encoding directions (FOV=4cm×4cm, matrix=32×32, scan time=1:04, esp=0.8ms). For each experiment, a scan was first performed without current in the wire loop to establish a reference, followed by another scan after applying a 3mA peak current with a duration of 35ms and a shape that imitated an action potential. The other acquisition parameters were: TR=2000ms, TE=2.8ms, and slice thickness=5mm.
Image Analysis:
After acquisition of time-resolved images using re-SPEEDI, the phase maps of these images were produced at each time point. The phase evolution from a specific point in the water phantom was extracted and compared with the simulated phase change calculated using the following equation: $$\phi(t)=\gamma\int_{0}^{t} B(t^{'})dt^{'}, [1]$$ where B(t) is the magnetic field produced by the time-varying current in the wire loop.

Results

The dipole pattern produced by the current can be clearly seen on the phase maps from all three experiments, as shown in Figs. 2-4, respectively. The fast-changing phase evolution was captured with a 0.6ms or 0.8ms temporal resolution, and agreed well with the simulation results using Eq. [1]. Compared to conventional SPEEDI, the scan times were shortened by a factor of 4, 16, and 8 in the three experiments, respectively. In addition, the approach with reduced FOV in both in-plane directions improved the spatial resolution from 1.8×1.8mm² in full FOV to 1.2×1.2mm².

Discussion and Conclusion

We have demonstrated the ability of re-SPEEDI for substantially reducing the overall scan times while offering sub-millisecond temporal resolution for capturing the dynamic current change in a wire loop. The rFOV approach, which has been used to decrease the acquisition times in previous studies^6–8, was successfully incorporated into SPEEDI. Imaging with an rFOV can also improve the spatial resolution without suffering from aliasing artifacts. The echo-train acquisition method was a variable alternative to the FID-based acquisition and balanced the conflicting requirements between the total scan time and the temporal resolution. By combining rFOV and echo-train acquisition, re-SPEEDI is expected to be an important contributor to broadening the applications based on SPEEDI-type sequences.

Acknowledgements

This work was supported in part by NIH 1S10RR028898.