Abstract
PurposeTo investigate the potential of continuous radiofrequency (RF) shifting (SWEEP) as a technique for creating densely sampled data while maintaining a stable signal state for dynamic imaging.MethodsWe present a method where a continuous stable state of magnetization is swept smoothly across the anatomy of interest, creating an efficient approach to dense multiple 2D slice imaging. This is achieved by introducing a linear frequency offset to successive RF pulses shifting the excited slice by a fraction of the slice thickness with each successive repeat times (TR). Simulations and in vivo imaging were performed to assess how this affects the measured signal. Free breathing, respiration resolved 4D volumes in fetal/placental imaging is explored as potential application of this method.ResultsThe SWEEP method maintained a stable signal state over a full acquisition reducing artifacts from unstable magnetization. Simulations demonstrated that the effects of SWEEP on slice profiles was of the same order as that produced by physiological motion observed with conventional methods. Respiration resolved 4D data acquired with this method shows reduced respiration artifacts and resilience to non‐rigid and non‐cyclic motion.ConclusionsThe SWEEP method is presented as a technique for improved acquisition efficiency of densely sampled short‐TR 2D sequences. Using conventional slice excitation the number of RF pulses required to enter a true steady state is excessively high when using short‐TR 2D acquisitions, SWEEP circumvents this limitation by creating a stable signal state that is preserved between slices.
Highlights
Free‐breathing three‐dimensional imaging of abdominal organs is a challenging application of MRI that requires the spatial encoding of large volumes in the presence of complex, non‐rigid motion induced by quasi‐periodic respiration[1,2] and other processes
We propose an excitation scheme that provides multiple 2D slice acquisitions (M2D) volumetric coverage using a slice excitation that is continuously moving across the volume of interest, we term this method “SWEEP.” The SWEEP method shares some similarities with continuous moving table (CMT) acquisition used for extended field of view (FOV) imaging.[12,13]
For SWEEP the rate (Rs) at which the excited profile moves across the imaging volume is conveniently defined as the percentage of the full width at half maximum slice thickness (Δz) that is shifted with each consecutive pulse Rs = × 100
Summary
Purpose: To investigate the potential of continuous radiofrequency (RF) shifting (SWEEP) as a technique for creating densely sampled data while maintaining a stable signal state for dynamic imaging. Methods: We present a method where a continuous stable state of magnetization is swept smoothly across the anatomy of interest, creating an efficient approach to dense multiple 2D slice imaging. This is achieved by introducing a linear frequency offset to successive RF pulses shifting the excited slice by a fraction of the slice thickness with each successive repeat times (TR). Results: The SWEEP method maintained a stable signal state over a full acquisition reducing artifacts from unstable magnetization. Using conventional slice excitation the number of RF pulses required to enter a true steady state is excessively high when using short‐TR 2D acquisitions, SWEEP circumvents this limitation by creating a stable signal state that is preserved between slices
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