MRI with sub-millisecond temporal resolution over a reduced field of view

Purpose: To demonstrate an MRI pulse sequence-Sub-millisecond Periodic Event Encoded Dynamic Imaging with a reduced field of view (or rFOV-SPEEDI)-for decreasing the scan times while achieving sub-millisecond temporal resolution. Methods: rFOV-SPEEDI was based on a variation of SPEEDI, known as get-SPEEDI, which used each echo in an echo-train to sample a distinct k-space raster by synchronizing with a cyclic event. This can produce a set of time-resolved images of the cyclic event with a temporal resolution determined by the echo spacing (typically < 1 ms). rFOV-SPEEDI incorporated a 2D radiofrequency (RF) pulse into get-SPEEDI to limit the field of view (FOV), leading to reduction in phase-encoding steps and subsequently decreased scan times without compromising the spatial resolution. Two experiments were performed at 3T to illustrate rFOV-SPEEDI's capability of capturing fast-changing electric currents in a phantom and the rapid opening and closing of aortic valve in human subjects over reduced FOVs. The results were compared with those from full FOV get-SPEEDI. Results: In the first experiment, the rapidly varying currents (50-200 Hz) were successfully captured with a temporal resolution of 0.8 ms, and agreed well with the applied currents. In the second experiment, the rapid opening and closing processes of aortic valve were clearly visualized with a temporal resolution of 0.6 ms over a reduced FOV (12 x 12 cm(2)). In both experiments, the acquisition times of rFOV-SPEEDI were decreased by 33%-50% relative to full FOV get-SPEEDI acquisitions and the spatial resolution was maintained. Conclusion: Reducing the FOV is a viable approach to shortening the scan times in SPEEDI, which is expected to help stimulate SPEEDI applications for studying ultrafast, cyclic physiological and biophysical processes over a focal region.

Automated summary

The purpose of this study was to demonstrate a reduced field of view (rFOV-SPEEDI) MRI pulse sequence for achieving sub-millisecond temporal resolution and decreasing scan times. The rFOV-SPEEDI method successfully captured fast-changing electric currents and the rapid opening and closing processes of the aortic valve with reduced FOVs, leading to a significant reduction in acquisition times compared to full FOV get-SPEEDI while maintaining spatial resolution. The results suggest that reducing the FOV can be an effective approach to accelerating scan times in MRI imaging.