The vive artefact is still there under VB15 and actually related to the gradients. So far the best you can do to minimize it is to put a water cushion under the head and possibly remove the side restraints.
The MGH sequence by Thomas is cool but also has limitations (TE, max ADC...). Nothing is perfect;)
Cheers-
Andreas
-----Ursprüngliche Nachricht-----
Von: FSL - FMRIB's Software Library im Auftrag von David Gutman
Gesendet: Di 03.06.2008 20:18
An: [log in to unmask]
Betreff: Re: [FSL] Optimizing DTI sequences
Yeah I'll have to look into that as well. I've gotten a strong sense the
vibration artifact didn't seem to be unique to our system and was known
about-- just wasn't sure how people had worked around it. Appreciate the
advice!
dg
On Tue, Jun 3, 2008 at 2:14 PM, Markus Gschwind <
[log in to unmask]> wrote:
> Hi!
>
>> However when it is present, it just looks like a signal void,
>> particularly in the axial, towards the parietal/occipital junction--- based
>> on where it is I think it may partially related to the person's head
>> actually shaking around a bit.
>>
> Please see our several mails about the "vibration artifact" on Siemens
> TrioTIM system in the Siemens own DTI sequence on the list.
> We finally changed the Sequence to the Stejskal-Tanner Sequence (ask your
> Siemens Physicist about it). With a good effect so far.
> But I downt know how things are with the Siemens sequence under the VB15
> software update.
>
> Interested in your experience!
> Cheers,
> Markus
>
>
> Quoting Peter Kochunov <[log in to unmask]>:
>
> Well, everytime the baseline software is upgraded it removes all the
>> sequences from the /MriCustomer/seq directory.
>> pk
>> ----- Original Message -----
>> From: David Gutman
>> To: [log in to unmask]
>> Sent: Tuesday, June 03, 2008 12:44 PM
>> Subject: Re: [FSL] Optimizing DTI sequences
>>
>>
>> I'll have to check. In the past we had used the MGH sequence, however
>> after a scanner "upgrade" it went away, and we started using the 64
>> direction sequence built into the scanner. However I've never been super
>> happy with the actual built in sequence-- we appear to get an artifact in
>> the temporal/parietal region in some, but not all of the subjects and on
>> some, but not all gradient directions. Obvioulsy this type of thing is a
>> real joy to sit down and troubleshoot, since it's only there sometimes and
>> only in some patients. However when it is present, it just looks like a
>> signal void, particularly in the axial, towards the parietal/occipital
>> junction--- based on where it is I think it may partially related to the
>> person's head actually shaking around a bit.
>>
>> Anyone else have a similar issue with the built in Siemens sequence?
>>
>>
>>
>>
>> On Tue, Jun 3, 2008 at 1:37 PM, Peter Kochunov <[log in to unmask]>
>> wrote:
>>
>> My first suggestion would be to upgrade siemens product sequence to
>> the MGH sequence. MGH sequence signficantly extends the capabilities in
>> terms of number of directions, etc. You can contact Thomas Benner to get
>> this sequence. [log in to unmask]
>> ----- Original Message -----
>> From: David Gutman
>> To: [log in to unmask]
>> Sent: Tuesday, June 03, 2008 12:33 PM
>> Subject: [FSL] Optimizing DTI sequences
>>
>>
>> I am currently helping optimize a DTI sequence on a 3T siemens
>> scanner for human work, and also beginning a foray into small animal DTI
>> sequences. I was wondering if anyone could give me some pointers or point
>> me towards some good papers on optimizing sequence parameters (# of echoes,
>> # of averages, # of B0's to collect, # of directions, etc, etc..)
>>
>> Obviously we want the best resolution and best image quality we can
>> get in the shortest amount of time while getting good single to noise and
>> contrast to noise (we're quite ambitious). Just eyeballing the images and
>> saying "looks good" lacks a certain amount of academic rigor, and based on
>> the recent discussion on the list about measuring PHI angles,etc.,etc.
>> simply eyeballing it won't do it. Also the question of what images to even
>> look at-- analyzing all 65+ images by eye is obviously not accurate.
>>
>>
>>
>> Is there a particular way of measuring signal to noise and also
>> contrast to noise, in particular as it applies to tractography? Would
>> looking at the standard deviation of a region outside the brian, and then
>> comparing it to a relatively homologous intracranial region (pick some big
>> hunk of gray matter) be a useful metric? Also do you take a sampling of a
>> certain # of gradients to generate an average SNR across your entire DTI
>> acquisition.
>>
>>
>> Sorry I am relatively naive to all of this, it's just hard to get
>> the sense of what's "good enough".
>>
>>
>> Also can anyone comment on the use of a 30 gradient sequence vs a 64
>> gradient sequence. Some of my colleagues are trying to integrate DTI in
>> one of their existing protocols and would like it to be as brief as
>> possible, since it's not their main focus. Obviously 30 gradients would
>> take about half as much time as 64 gradients; however I've been using 60+
>> gradient data now and feel quite compelled to strongly suggest not getting
>> any less than this.
>>
>> Any comments or pointers would be greatly appreciated.
>>
>>
>> DG
>>
>>
>> --
>> David A Gutman, M.D. Ph.D.
>> Department of Psychiatry & Behavioral Sciences
>> Emory University School of Medicine
>>
>>
>>
>> --
>> David A Gutman, M.D. Ph.D.
>> Department of Psychiatry & Behavioral Sciences
>> Emory University School of Medicine
>>
>
>
>
> --
>
> Dr. med. Markus Gschwind, M.D.
> Laboratory for Neurology and Imaging of Cognition
> Dept of Neurosciences
> University Medical Center (CMU)
> 1 Michel-Servet - 1211 GENEVA - CH
>
> Tel 0041 (0) 22 379 5324
> Fax 0041 (0) 22 379 5402
> email: [log in to unmask]
> http://labnic.unige.ch
>
--
David A Gutman, M.D. Ph.D.
Department of Psychiatry & Behavioral Sciences
Emory University School of Medicine
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