Hi Matt / Scott / Dianne-
 
on our TIM Trio I have gotten down to 1.5*1.5*2 mm voxels with DWI-EPI at fair SNR with a single average using a 32 channel head coil. The previous stripe artefact has been eliminated on the most recent numaris version, the vibration artefact still bothers us. For peripheral nerve studies using TSE- or SSFP-DWI can be advantageous.
Cheers-
Andreas

________________________________

Von: FSL - FMRIB's Software Library im Auftrag von Matt Glasser
Gesendet: Mo 15.09.2008 06:02
An: [log in to unmask]
Betreff: Re: [FSL] voxel size vs structure size



Scott,

I guess I wasn't really clear there.  I have seen reducing the FOV to reduce
SNR (even when voxel size was kept constant) for a physics reason that I did
not really understand.  What I meant to say was to only acquire a few slices
in the region you are interested in so that you can acquire many averages in
a reasonable amount of time.  Regarding SNR, I thought that 4-6 averages at
60 directions and 1.3mm isotropic would produce good results (expect to scan
for a long time though if you are going for whole brain coverage, however).
SNR is definitely not too low if you acquire the averages (this on a TIM
Trio, I don't off hand recall what coil we used). 

Peace,

Matt.

-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Scott Kolbe
Sent: Sunday, September 14, 2008 10:52 PM
To: [log in to unmask]
Subject: Re: [FSL] voxel size vs structure size

Hi Matt
you mentioned reducing the FOV,
have you tried this on your Trio? what was the SNR like at 1.3mm? we
tried this a couple of years ago and got some really ugly zebra stripes
in our images.
cheers
Scott


--
========================
Scott Kolbe
MS Imaging Group
Howard Florey Institute &
Centre for Neuroscience
University of Melbourne
VIC, Australia, 3010.

ph:       +61 3 8344 1887
email:    [log in to unmask]
website:  http://www.neuroimaging.org.au/index.php?id=383




Kochunov, Peter wrote:
> For a project like this one might think about using a TSE-DTI sequence.
You won't have many of the limitations of the EPI-DTI sequence, although it
will be only practical if you can sufficient SNR through multiple averages.
> cheers
> pk
> 
>
> ________________________________
>
> From: FSL - FMRIB's Software Library on behalf of Dianne Patterson
> Sent: Sun 9/14/2008 4:28 PM
> To: [log in to unmask]
> Subject: Re: [FSL] voxel size vs structure size
>
>
> Thanks so much...that gives me a place to start!
>
> -Dianne
>
>
> On Sun, Sep 14, 2008 at 2:01 PM, Matt Glasser <[log in to unmask]> wrote:
>
>
>       Dianne,
>
>       
>
>       I think you ought to be able to get much better resolution than
that.  I have scanned a human at 1.3mm isotropic on a Siemens 3T scanner.
If you are specifically interested in small nerves, it would probably be
best to get as high resolution as possible and scan a limited field of view
with many averages.  To measure the FA of a small structure accurately, you
would need at least one voxel without partial voluming (or devise a method
to correct for partial voluming).  I have not personally done this, but
theoretically you could use higher resolution anatomical imaging to estimate
the fraction of the structure that was present in the DTI voxel, and if you
knew the mean FA of whatever else was in the voxel, you could estimate the
FA in the structure of interest.  There may be issues with such an approach
that others may point out, and I could see it as difficult to get past
reviewers, however, and it would rely on perfect alignment between the
anatomical image and DTI (i.e. any EPI susceptibility distortion would have
to be corrected).  Increasing the number of directions will not compensate
for poor spatial resolution.  Increasing the number directions will help in
reconstructing multiple fiber directions for tractography, but it sounds
like you are more interested in quantitative DTI values.
>
>       
>
>       Peace,
>
>       
>
>       Matt.
>
>       
>
>      
> ________________________________
>
>
>       From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On
Behalf Of Dianne Patterson
>       Sent: Friday, September 12, 2008 10:00 PM
>       To: [log in to unmask]
>       Subject: [FSL] voxel size vs structure size
>
>       
>
>       Dear Group,
>      
>       We have a 3 tesla GE scanner and we get 2.6 mm isotropic voxels in
the brain.
>       I recently talked to a colleague who would love to image nerves in
the neck that are about 3 mm thick.
>      
>       Can anyone tell me:
>       1) Whether 2.6 mm voxels would be appropriate for identifying and
measuring FA along such a small diameter structure.
>       2) What the relationship needs to be between voxel size and
structure size
>       3) If increasing angular resolution can compensate for the lack of
spatial resolution
>      
>       I appreciate your kindness and support..thankyou,
>      
>       Dianne
>      
>       --
>       Dianne Patterson, Ph.D.
>       [log in to unmask]
>       University of Arizona
>       SHLS 328
>       621-5105
>
>
>
>
>