Respiration resolved imaging with continuous stable state 2D acquisition using linear frequency SWEEP

• Published in: Magnetic resonance in medicine Vol. 82; no. 5; pp. 1631 - 1645
• Main Authors: Jackson, L. H., Price, A. N., Hutter, J., Ho, A., Roberts, T. A., Slator, P. J., Clough, J. R., Deprez, M., McCabe, L., Malik, S. J., Chappell, L., Rutherford, M. A., Hajnal, J. V.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01-11-2019; John Wiley and Sons Inc
• Subjects: angiography; Brain Mapping - methods; Computer Simulation; Data acquisition; Female; fetal; Fetuses; free‐breathing; Full Papers—Imaging Methodology; Humans; Image Enhancement - methods; Image Processing, Computer-Assisted - methods; Imaging; Magnetic Resonance Angiography; Magnetic Resonance Imaging - methods; Magnetization; Placenta; Placenta - blood supply; Placenta - diagnostic imaging; placental; Pregnancy; Radio frequency; Respiration; Sequences; steady state; 4D; steady state; RF; placental; free-breathing; fetal; angiography
• DOI: 10.1002/mrm.27834
• EISSN: 1522-2594
• ISSN: 0740-3194

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). 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.
Results
The 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.
Conclusions
The 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.