 |
 |
I think the echo space for this sequence should be somewhere between 0.5-0.9ms range. The easiest way to measure it would be by looking at the sequence in simulator and just measuring the time between the middle of each of the readout blocks. If I’m not mistaken this can be done on a console of a phillips scanner.
pk
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Matt Glasser
Sent: Thursday, August 05, 2010 9:09 AM
Why isn’t echo spacing simply echo train length (I am assuming you gave it in ms) divided by number of phase encoding steps? That would give 0.5875ms, which is in the range of the echo spacings I usually get from the Siemens scanner. 1.81ms seems high to me for an echo spacing on an EPI diffusion sequence. The echo spacing is used as a scaling factor for applying the field map correction, and with a TE of ~77ms you should have less distortion than one would have with a higher TE e.g. 90-100ms that gives an echo spacing of around 0.7ms.
Peace,
Matt.
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of kochunov
Sent: Thursday, August 05, 2010 9:07 AM
To: [log in to unmask]
Subject: Re: [FSL] Echo Spacing (FUGUE)
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
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