Print

Print
Hi,

Just to add to this - the term "Effective echo spacing" is a term that we made up because there was no existing term, that we were aware of, that covered what we needed.  This term is trying to describe the echo spacing that would exist for a non-accelerated sequence with matching distortions.  In this way we can use the number of reconstructed voxels in the phase encode direction along with this effective echo spacing, to calculate the distortion.  It is not the true echo spacing when the sequence is accelerated (where "true echo spacing" here is the time between echoes formed by crossing the phase-encode axis in k-space).  The effective echo spacing is just N times less than the true echo spacing for an accelerated sequence where the acceleration factor is N, as long as this is in-plane.  For accelerated sequences like multi-band, where multiple slices are excited simultaneously, but the in-plane readout is not changed, then no factor need be applied.

I have tried to make this clearer on the wiki how to calculate the "effective echo spacing" from the real echo spacing and the in-plane acceleration factor.  If you had any suggestions (e.g. from your physicist) about how this could be made even clearer then we would welcome feedback on this.

All the best,
Mark



On 28 Apr 2014, at 15:12, Andreas Bartsch <[log in to unmask]> wrote:

Ups – sorry: I did not read that Michael had just suggested the same thing in parenthesis…

Von: "Harms, Michael" <[log in to unmask]>
Antworten an: FSL - FMRIB's Software Library <[log in to unmask]>
Datum: Montag, 28. April 2014 15:06
An: <[log in to unmask]>
Betreff: Re: [FSL] Clarification of 'Effective Echo Spacing'


In that case, you'll have to check with your local GE engineering/physicist as to how GE reports ES.  (Or, just check if the reported value changes as you change the phase acceleration factor).

You can see the tooltip in FEAT for more on "Effective echo spacing".

cheers,
-MH

-- 
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO  63110 Email: [log in to unmask]

From: Andreas Lidström <[log in to unmask]>
Reply-To: FSL - FMRIB's Software Library <[log in to unmask]>
Date: Monday, April 28, 2014 7:59 AM
To: FSL - FMRIB's Software Library <[log in to unmask]>
Subject: Re: [FSL] Clarification of 'Effective Echo Spacing'

No, this is a GE Discovery MR750 scanner.

In that case it sounds like we should divide by 2,
I would just like to get get an explanation as to why. What is the meaning of 'Effective echo spacing' in that case?


2014-04-28 14:10 GMT+02:00 Harms, Michael <[log in to unmask]>:

Is this a Siemen's scanner? The actual echo spacing reported by Siemens is indeed unchanged by the use of iPAT.  But FSL wants the "effective echo spacing", which is the actual ES divided by the iPAT factor.  This assumes you are talking about iPAT/GRAPPA acceleration (which it sounds like is the case) and not "multiband" (slice) acceleration.

cheers,
-MH

-- 
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO  63110 Email: [log in to unmask]

From: Andreas Lidström <[log in to unmask]>
Reply-To: FSL - FMRIB's Software Library <[log in to unmask]>
Date: Monday, April 28, 2014 6:08 AM
To: FSL - FMRIB's Software Library <[log in to unmask]>
Subject: [FSL] Clarification of 'Effective Echo Spacing'

Hello,

I am running RS fMRI with an esp of 0.464ms and acceleration factor of 2.

In the instructions for B0 unwrapping it says that you should divide the esp by the acceleration factor but our MR physician is perplexed by this. He says that since we are accelerating our pulse-sequence in the phase direction(y) we are simply cutting the echo train in half leaving the echo-spacing unchanged. Does this mean we should not divide our esp by 2 or is this division still correct based on something else hidden in the program?

//Andreas

 
The materials in this message are private and may contain Protected Healthcare Information or other information of a sensitive nature. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail.

 
The materials in this message are private and may contain Protected Healthcare Information or other information of a sensitive nature. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail.