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Neuroimage. 2018 Jul 15;175:1-11. doi: 10.1016/j.neuroimage.2018.03.066. Epub 2018 Mar 29.

MRI gradient-echo phase contrast of the brain at ultra-short TE with off-resonance saturation.

Author information

1
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, USA.
2
Radiology and Biomedical Imaging, UCSF School of Medicine, San Francisco, CA, USA.
3
Department of Neurology, University of California - San Francisco, San Francisco, CA, USA; UC Berkeley-UCSF Graduate Program in Bioengineering, University of California, Berkeley and University of California, San Francisco, CA, USA.
4
Radiology and Biomedical Imaging, UCSF School of Medicine, San Francisco, CA, USA; UC Berkeley-UCSF Graduate Program in Bioengineering, University of California, Berkeley and University of California, San Francisco, CA, USA.
5
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, USA; Helen Wills Neuroscience Institute, University of California, Berkeley, CA, USA. Electronic address: chunlei.liu@berkeley.edu.

Abstract

Larmor-frequency shift or image phase measured by gradient-echo sequences has provided a new source of MRI contrast. This contrast is being used to study both the structure and function of the brain. So far, phase images of the brain have been largely obtained at long echo times as maximum phase signal-to-noise ratio (SNR) is achieved at TE = T2* (∼40 ms at 3T). The structures of the brain, however, are compartmentalized and complex with a wide range of signal relaxation times. At such long TE, the short-T2 components are largely attenuated and contribute minimally to phase contrast. The purpose of this study was to determine whether proton gradient-echo images of the brain exhibit phase contrast at ultra-short TE (UTE). Our data showed that UTE images acquired at 7 T without off-resonance saturation do not contain significant phase contrast between gray and white matter. However, UTE images of the brain can attain strong phase contrast even at a nominal TE of 106 μs by using off-resonance RF saturation pulses, which provide direct saturation of ultra-short-T2 components and indirect saturation of longer-T2 components via magnetization transfer. In addition, phase contrast between gray and white matter acquired at UTE with off-resonance saturation is reversed compared to that of the long-T2 signals acquired at long TEs. This finding opens up a potential new way to manipulate image phase contrast of the brain. By accessing short and ultra-short-T2 species, MRI phase images may further improve the characterization of tissue microstructure in the brain.

KEYWORDS:

Brain; MRI – magnetic resonance imaging; Off-resonance saturation; Phase contrast; UTE – ultra-short TE

PMID:
29604452
PMCID:
PMC5960631
[Available on 2019-07-15]
DOI:
10.1016/j.neuroimage.2018.03.066

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