2009 Max Halvorson
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Differences Between Spin-Echo and Gradient-Echo Imaging
Spin-Echo and Gradient-Echo imaging are two popular methods of obtaining fMRI data that vary along various dimensions: the pulse sequence used to generate and obtain the signal, signal-to-noise ratio, and sensitivity to large blood vessels, to name a few. This study examines the differences between the two by looking at a data set from the VISTA lab. Overall differences in signal-to-noise, areas of high and low signal, and distortions are examined. Furthermore, hV4 and other areas of visual cortex are considered and compared between the two types of scans.
Background
Gradient-Echo Imaging
Gradient-Echo images are generated by an applied gradient-followed by an RF pulse sequence to excite slices one by one and collect data from each. Gradient-Echo images have a stronger overall signal than Spin-Echo images and the overall signal-to-noise ratio is higher. However, Gradient-Echo images are prone to large distortions from large blood vessels, sinuses, and other inhomogeneities.
Spin-Echo
Spin-Echo images are generated by a 180-degree pulse following the applied gradient. This pulse realigns the dephasing spins and gives another shot at capturing data. Spin-Echo images are weaker in signal and signal-to-noise ratio than Gradient-Echo images. However, Spin-Echo images should be less sensitive to distortions from large blood vessels and sinuses.
Vessel Distortions
Spin-Echo imaging can be used to refocus the loss of phase coherence and eliminate the large-vessel signal. For smaller vessels, the gradient changes rapidly over space relative…Loss of phase coherence cannot be recovered by Spin-Echo imaging (from Huettel et al. text).
Purpose
Although Gradient-Echo images have a stronger overall signal and signal-to-noise ratio and generally show more in most brain regions, Spin-Echo images are theorized to avoid distortions due to large blood vessels and inhomogeneities. The hope of this study was to find areas in which Spin-Echo images could reveal things in visual cortex that Gradient-Echo could not. For example, areas like hV4...
Methods
The data were obtained from Jon Winawer through VISTA lab.
MR Analysis
The MR data were analyzed using mrVista software tools. Features used included correlation analyses, traveling wave analyses, mean maps, phase-projected coherence maps, and time series plots.
Pre-processing
All data were slice-time corrected, motion corrected, and repeated scans were averaged together to create a single average scan for each subject. Pre-processing was done by Jon.
Results
Conclusions
Software: mrVista