Purpose: Ultrafast imaging techniques based on spatiotemporal encoding (SPEN), such as RASER (rapid acquisition with sequential excitation and refocusing), is a promising new class of sequences since they are largely insensitive to magnetic field variations which cause signal loss and geometric distortion in EPI. So far, attempts to theoretically describe the point-spread-function (PSF) for the original SPEN-imaging techniques have yielded limited success. To fill this gap a novel definition for an apparent PSF is proposed. Theory: Spatial resolution in SPEN-imaging is determined by the spatial phase dispersion imprinted on the acquired signal by a frequency-swept excitation or refocusing pulse. The resulting signal attenuation increases with larger distance from the vertex of the quadratic phase profile. Methods: Bloch simulations and experiments were performed to validate theoretical derivations. Results: The apparent PSF quantifies the fractional contribution of magnetization to a voxel's signal as a function of distance to the voxel. In contrast, the conventional PSF represents the signal intensity at various locations. Conclusion: The definition of the conventional PSF fails for SPEN-imaging since only the phase of isochromats, but not the amplitude of the signal varies. The concept of the apparent PSF is shown to be generalizable to conventional Fourier-imaging techniques.
Bibliographical noteFunding Information:
Financial support by the NIH -grants P41 RR008079 (NCRR), P41 EB015894 (NIBIB), P30 NS057091 (BTRC), S10 RR026783 and R01 EB000331 and the WM KECK Foundation is acknowledged. The author thanks Drs. Michael Garwood and Kamil Ugurbil (CMRR, University of Minnesota) for insightful discussions.
Financial support by the NIH-grants P41 RR008079 (NCRR), P41 EB015894 (NIBIB), P30 NS057091 (BTRC), S10 RR026783 and R01 EB000331 and the WM KECK Foundation is acknowledged. The author thanks Drs. Michael Garwood and Kamil Ugurbil (CMRR, University of Minnesota) for insightful discussions.
© 2016 Elsevier Inc.
Copyright 2017 Elsevier B.V., All rights reserved.
- Frequency-swept pulse
- Magnetic resonance imaging (MRI)
- Spatiotemporal encoding
- Ultrahigh magnetic field