Format

Send to

Choose Destination
Magn Reson Med. 2016 Aug;76(2):540-54. doi: 10.1002/mrm.25902. Epub 2015 Oct 7.

Robust time-shifted spoke pulse design in the presence of large B0 variations with simultaneous reduction of through-plane dephasing, B1+ effects, and the specific absorption rate using parallel transmission.

Author information

1
Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, USA.
2
Physics Department, Harvard University, Cambridge, Massachusetts, USA.
3
John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii, USA.
4
Medical Image Analysis and Biometry Laboratory, University Rey Juan Carlos, Mostoles Spain.
5
Madrid-MIT M+ Vision Consortium, Madrid, Spain.
6
Harvard-MIT Division of Health Sciences Technology, Cambridge, Massachusetts, USA.

Abstract

PURPOSE:

To design parallel transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the specific absorption rate (SAR) at 7T.

METHODS:

We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian.

RESULTS:

Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications.

CONCLUSIONS:

The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med 76:540-554, 2016. © 2015 Wiley Periodicals, Inc.

KEYWORDS:

B0 artefact; B1+ homogeneity; parallel transmission; specific absorption rate; spokes

PMID:
26444717
PMCID:
PMC4824674
DOI:
10.1002/mrm.25902
[Indexed for MEDLINE]
Free PMC Article

Supplemental Content

Full text links

Icon for Wiley Icon for PubMed Central
Loading ...
Support Center