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Subject:

Re: multiple volume simulation

From:

Ivana Drobnjak <[log in to unmask]>

Reply-To:

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

Date:

Thu, 17 Apr 2008 14:32:03 +0100

Content-Type:

text/plain

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text/plain (37 lines)

Hello Bjorn,

No it is not a structural image.  The first volume will have the  
strongest signal strength due to saturation effects i.e.  spins will  
not be fully recovered (the Mz component will not reached its  
maximum) at the time of the RF pulse of the following volumes. In  
addition, the contrast is different because different tissue types  
require different amount of time to recover, so some take short time  
(T1 for white tissue type is around 500ms for 1.5T scanner) and some   
take long time (T1 for CSF is around 2500ms for 1.5T scanner) which  
means that you get less contrast between them in the volumes after  
the first. If you play with your T1 values and your TR you can change  
your contrasts...  Also note that if your TR is large then the spin  
density becomes also quite prominent in the contrast. All these  
parameters are in the file called MRpar.

Hope this helps,
Ivana


On 17 Apr 2008, at 13:48, Bjorn Roelstraete wrote:

> Dear all,
>
> i noticed that, when i simulate several volumes in POSSUM, the  
> first volume is always much more detailed than the next ones. Is  
> the first volume some kind of structural image, perhaps?
>
> Thank you,
> -- 
> Bjorn Roelstraete
> Ghent University
> Department of Data-analysis
> H. Dunantlaan 1, B-9000 Gent, Belgium
> Tel: 32-9-2646434
> E-mail: [log in to unmask]
>

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