Improving EPI - Revision history

imported>Knutson at 22:58, 28 June 2011

2011-06-28T22:58:17Z

← Older revision		Revision as of 22:58, 28 June 2011
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	== Reducing the echo time ==		== Reducing the echo time ==
	Shortening the echo time (TE) will also reduce EPI distortions and signal drop-out. However, BOLD contrast increases over several tens of milliseconds and is ideally sampled at a TE of 30ms ''(add references and signal curves)''. Even so, a small reduction in the TE can improve EPI image quality ~~wile~~ retaining adequate BOLD contrast.		Shortening the echo time (TE) will also reduce EPI distortions and signal drop-out. However, BOLD contrast increases over several tens of milliseconds and is ideally sampled at a TE of 30ms ''(add references and signal curves)''. Even so, a small reduction in the TE can improve EPI image quality while retaining adequate BOLD contrast.

imported>Bobd at 17:22, 7 April 2011

2011-04-07T17:22:13Z

← Older revision		Revision as of 17:22, 7 April 2011
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	Echo-planar imaging (EPI) is commonly used for both BOLD imaging (gradient-echo EPI) and diffusion imaging (spin-echo EPI) because it acquires images very quickly. However, the price that we pay for such fast acquisitions are various image artifacts. Most of these are well-know and various methods have been devised to improve EPI images. This page summarizes work at the CNI to improve various EPI imaging problems.		Echo-planar imaging (EPI) is commonly used for both BOLD imaging (gradient-echo EPI) and diffusion imaging (spin-echo EPI) because it acquires images very quickly. However, the price that we pay for such fast acquisitions are various image artifacts. Most of these are well-know and various methods have been devised to improve EPI images. This page summarizes work at the CNI to improve various EPI imaging problems.

			[[File:faces_TE25_zscore.png\|thumb\|BOLD contrast (in z-scores) overlaid on the mean EPI image from the timeseries. (TR=3sec, TE=25ms, 2mm^3, one 6-minute event-related run)]]
			[[File:faces_TE25_tsplot.png\|thumb\|TIme-series plot for a voxel from the same data.]]

	= Signal drop-out =		= Signal drop-out =
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	== Reducing the echo time ==		== Reducing the echo time ==
	~~[[File:faces_TE25_zscore.png\|thumb\|BOLD contrast (in z-scores) overlaid on the mean EPI image from the timeseries. (TR=3sec, TE=25ms, 2mm^3, one 6-minute event-related run)]]~~
	~~[[File:faces_TE25_tsplot.png\|thumb\|TIme-series plot for a voxel from the same data.]]~~
	Shortening the echo time (TE) will also reduce EPI distortions and signal drop-out. However, BOLD contrast increases over several tens of milliseconds and is ideally sampled at a TE of 30ms ''(add references and signal curves)''. Even so, a small reduction in the TE can improve EPI image quality wile retaining adequate BOLD contrast.		Shortening the echo time (TE) will also reduce EPI distortions and signal drop-out. However, BOLD contrast increases over several tens of milliseconds and is ideally sampled at a TE of 30ms ''(add references and signal curves)''. Even so, a small reduction in the TE can improve EPI image quality wile retaining adequate BOLD contrast.

imported>Bobd: Created page with "Echo-planar imaging (EPI) is commonly used for both BOLD imaging (gradient-echo EPI) and diffusion imaging (spin-echo EPI) because it acquires images very quickly. However, the p..."

2011-04-07T17:19:49Z

Created page with "Echo-planar imaging (EPI) is commonly used for both BOLD imaging (gradient-echo EPI) and diffusion imaging (spin-echo EPI) because it acquires images very quickly. However, the p..."

New page

Echo-planar imaging (EPI) is commonly used for both BOLD imaging (gradient-echo EPI) and diffusion imaging (spin-echo EPI) because it acquires images very quickly. However, the price that we pay for such fast acquisitions are various image artifacts. Most of these are well-know and various methods have been devised to improve EPI images. This page summarizes work at the CNI to improve various EPI imaging problems.

= Signal drop-out =
Because of the long read-outs involved, EPI is very sensitive to field inhomogeneity. Sticking a person in the scanner is guaranteed to create some fairly substantial inhomogeneities in the MR field. These field distortions are particularly problematic near air-tissue interfaces, such as the regions around the nasal sinuses and the auditory canals. Unfortunately for those interested in imaging brain regions near these structures, standard EPI images will have significant geometric distortions and signal drop-out in these regions.

== Reducing the read-out duration ==
Shortening the read-out duration with partial k-space methods (such as ASSET acceleration) can help. At the CNI, 2x acceleration works well with our 8-channel coil. The 32-channel coil produces reasonable EPI data with 2.5x or even 3x acceleration. Note, however, that more acceleration means less data acquired per TR, and thus lower SNR.

== Reducing the echo time ==
[[File:faces_TE25_zscore.png|thumb|BOLD contrast (in z-scores) overlaid on the mean EPI image from the timeseries. (TR=3sec, TE=25ms, 2mm^3, one 6-minute event-related run)]]
[[File:faces_TE25_tsplot.png|thumb|TIme-series plot for a voxel from the same data.]]
Shortening the echo time (TE) will also reduce EPI distortions and signal drop-out. However, BOLD contrast increases over several tens of milliseconds and is ideally sampled at a TE of 30ms ''(add references and signal curves)''. Even so, a small reduction in the TE can improve EPI image quality wile retaining adequate BOLD contrast.