Hi,

Given that your bandwidth per pixel is over 3Khz e.g. It just takes 0.3 ms to read a line in a k-space, I’d say your echo spacing should be smaller than 1.8.

Cheers

pk

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From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Michael Roche
Sent: Thursday, August 05, 2010 8:46 AM
To: [log in to unmask]
Subject: Re: [FSL] Echo Spacing (FUGUE)

Hi,

Thanks for the quick reply. Well the sequence I am using is definitely a single shot EPI sequence. The acquired matrix is 80x63 and there is an acceleration factor making the echo train length equal 47. I have tried the formula ES = TR/Number of Slices/Number of Phase Encoding Steps, did you mean ES = TR/(Number of Slices*Number of Phase encoding steps)? Using the version of the formula I have assumed I calculated an ES of 1.81ms. Which is a value which makes sense to me. Would I be correct in assuming this value is the ES?

TR = 849ms

Number of Slices = 10

echo train length = 47

Regards

Michael

On Thu, Aug 5, 2010 at 1:23 PM, Auer, Tibor M.D. Ph.D. <[log in to unmask]> wrote:

 Hello,

Echo spacing is the time btw. the phase encoding steps (i.e. btw. the K-space lines). You case seems to be a bit more complicated b/c it seems to be a multi-echo sequence, but not an EPI.

In EPI:
    - Number of Phase Encoding Steps = Echo Train Length
    - 1 excitation pulse/slice
    - TR = time btw. volumes
   The basic equation is:
       ES = TR/Number of Slices/Number of Phase Encoding Steps

So I suppose, your case is:
   - TR = time btw. excitation pulses
   - Echo Train Length = Number of Phase Encoding Steps btw. two excitation pulses
   The equation may be:
       ES = Min TR/Echo Train Length

Does that sounds correct?

Auer, Tibor M.D. Ph.D.
Biomedizinische NMR Forschungs GmbH
am Max-Planck Institut für Biophysikalische Chemie
Am Fassberg 11
37077 Göttingen
Germany
Phone/Work: +49-(0)551-201-1725
Phone/Home: +49-(0)551-387-0076
Mobile: +49-(0)176-8012-7921
Mail: [log in to unmask]



2010.08.05. 13:59 keltezéssel, Michael Roche írta:

Hello,
My query is in relation to calculating the echo spacing (ES) for use in distortion correction of DWI images using FUGUE. I am scanning with a 3T Philips Achieva which does not specify the ES and neither does the DICOM header file. I have asked the local Physicist and he is unsure on how to calculate the ES. The following are my acquisition parameters:
Scanning Sequence: SE
Slice thickness: 3mm
TE: 76.56ms
Number of Phase Encoding Steps: 80
Echo Train Length: 47
Pixel Bandwidth: 3295.2Hz
Min TR: 849ms
Calculating the ES has been causing me major problems so any help on this topic would be greatly appreciated. If anyone could outline the steps involved in calculating the ES that would be great. Feel free to suggest any parameters i may need to calculate the ES.
Thanks
Michael