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FSL  March 2012

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Subject:

Re: Field map unwarping and SENSE

From:

Mark Jenkinson <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Mon, 5 Mar 2012 13:24:11 +0000

Content-Type:

text/plain

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text/plain (85 lines)

Hi Michael,

I have to admit that I am not an MR physicist and I initially used the term
based on what a proper MR physicist told me.  

I agree that "dwell time" seems to be more confusing that "echo spacing" 
and so I prefer to use the term "echo spacing" - although the command 
line option is stuck with --dwell for backward compatibility.  It may be the 
case that you can consider a dwell time in either the readout or phase 
encode directions for an EPI sequence.  It is definitely *not* the readout 
sampling rate that should be used - it is all about the speed in the *phase encode*
direction.  Hopefully "echo spacing" makes it clearer.

All the best,
	Mark



On 5 Mar 2012, at 12:55, Michael Harms wrote:

> Hi Mark,
> Technically, isn't dwell time the inverse of the read-frequency sampling
> rate?  Just pointing this out because I had someone think that was the
> "echo spacing" once.
> 
> cheers,
> -MH
> 
>> Hi,
>> 
>> Yes, in the FEAT GUI (the pre-processing section, which is what we
>> recommend you use, with the b=0 image as the "functional" input) we
>> need the "effective echo spacing" which is the actual echo spacing
>> (or dwell time) _divided_ by the acceleration factor (SENSE factor here).
>> 
>> All the best,
>> 	Mark
>> 
>> 
>> On 5 Mar 2012, at 08:15, Jürgen Hänggi wrote:
>> 
>>> Dear FSL experts
>>> 
>>> We try to unwarp distortions in DTI data using field maps, but he DTI
>>> data
>>> were acquired with the SENSE technique (factor 2.0).
>>> 
>>> Does the dwell time used in fugue have to be adjusted for the SENSE
>>> factor?
>>> 
>>> If yes, do I have to multiply or rather divide the dwell time by the
>>> SENSE
>>> factor?
>>> 
>>> Thanks in advance
>>> Best regards
>>> Jürgen Hänggi
>>> 
>>> 
>>> ----------------------------------------------------------------------------
>>> Jürgen Hänggi, Ph.D.
>>> Division Neuropsychology
>>> Institute of Psychology
>>> University of Zurich
>>> Binzmuehlestrasse 14, PO Box 25
>>> 8050 Zurich, Switzerland
>>> 0041 44 635 73 97 (phone office)
>>> 0041 76 445 86 84 (phone mobile)
>>> 0041 44 635 74 09 (fax office)
>>> BIN 4.D.04 (office room number)
>>> j.haenggi[at]psychologie.uzh.ch (email)
>>> http://www.psychologie.uzh.ch/neuropsy/ (website)
>>> http://www.juergenhaenggi.ch (private website)
>>> 
>>> This e-mail (and any attachment/s) contains confidential and/or
>>> privileged
>>> information. If you are not the intended recipient (or have received
>>> this
>>> e-mail in error) please notify the sender immediately and destroy this
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>> 
> 

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