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 × 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.