Dear Shiliang,
 
signal loss can't be solved, what is lost will stay lost. Forever and
always.
 
However, before you start acquiring data you can tweak you 2DEPI
scanning parameters (directly at your scanner) in order to reduce signal
loss over the cavities. I assume you use 2DEPI? 
Taken short, you have to shorten your echo train as much as possible,
without losing to much SNR, so that during signal readout your protons
do not dephase too much close to susceptibility transition and hence
lose their signal. You might try to increase your bandwith of your
readout gradient (watch out for SNR loss though). You could use some
form of sparse K-space sampling with paralellel imaging techniques like
SENSE (Philips) or the equivalent for Siemens (I believe its called
GRAPPA) to shorten readout and hence signal loss substantially, provided
you have head coils and receivers on your scanner that support paralell
imaging. Most recent brands of scanners support parallel imaging, we do
it a lot on Philips Achieva that has 16 channels. You could try and use
3DEPI or PRESTO, benefiting even more from parallel imaging.

You could use segmented EPI, filling K-space in two shots, thereby
shortening readout but prolonging time/scan (which is also a
disadvantage to take into account). Watch out for possible increased
sensitivity to head motion though!
 
Finally, you could shorten your echo time to something shorter than
optimal to remain some signal in the troubled regions. Some say reducing
voxel size also helps, a bit.

I bet others have more tricks available, but those are the ones we
tried. 

Good luck,
 
Bas
 
--------------------------------------------------
Dr. S.F.W. Neggers
Division of Brain Research
Rudolf Magnus Institute for Neuroscience
Utrecht University Medical Center

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Mail     : Huispost B.01.206, P.O. Box 85500
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Web      : http://www.fmri.nl/people/bas.html
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________________________________

Van: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
Namens shiliang huang
Verzonden: maandag 14 april 2008 15:41
Aan: [log in to unmask]
Onderwerp: [SPM] Which software can solve the problem of
susceptibility-induced signal losses in EPI images



Dear every one

I want to solve the susceptibility-induced signal losses in EPI images.
Who can tell me which software can solve this problem? Thank you.

Best wishes

Shiliang