Hi Venkateswaran,
just to add to what Steve said earlier (because I'm on holiday but I still check my emails from time to time!)...
Having several size of kernel is useful in general to make people aware of the influence of such smoothing on the results.
In very simple terms, the smaller the kernel is, the more detailed your results might be (e.g. something in the substantia nigra might be seen at 3mm sigma - so roughly 7mm FWHM - but "disappear" at 4mm or above), while the larger it is, the more confident you might be in it.
The general rule is that the FWHM of the kernel should be anywhere between at least twice the resolution of your images and the maximum size of the atrophy expected (so more than 4mm FWHM and not more than 10-12mm FWHM, unless you're in a severe case of neurodegeneration, in which case you might not trust the non-linear registration anyway).
Here, we tend to use 3 or 3.5mm sigma for the Gaussian kernel.
Hoe this helps,
Gwenaëlle
PS: FWHM = sigma * sqrt(8*log(2)) ~ sigma * 2.3
--- En date de : Ven 28.8.09, Rajagopalan, Venkateswaran <[log in to unmask]> a écrit :
> De: Rajagopalan, Venkateswaran <[log in to unmask]>
> Objet: [FSL] Smoothing FSL-VBM
> À: [log in to unmask]
> Date: Vendredi 28 Août 2009, 18h15
>
>
>
>
> Dear FSL
> users,
>
> I am new to VBM kind of
> analysis. I was going through FSL-VBM user documentation
> where it was mentioned that t-statistic map will be created
> for a range of FWHM =4.6 to 9 mm and it was suggested
> that based on t-stats maps one can decide the
> size of smoothing kernel for the data in hand. Can anyone
> please tell how i can decide i.e. what to look for in
> t-stats map which will help me to decide the appropriate
> smoothing kernel.
>
> In general also i
> don't know how to choose an appropriate size for
> smoothing kernel, i read a little bit about Matched
> filter theorm to decide on the selecting the kernel size but
> couldn't understand much.
>
> Any help in this regard is
> greatly appreciated.
>
> Thanks in
> advance.
>
> Regards
> venkateswaran
>
> ===================================
> P
> Please
> consider the environment before printing this
> e-mail
>
> Cleveland Clinic is ranked one of the top hospitals
> in America by U.S. News & World Report (2008).
> Visit us online at http://www.clevelandclinic.org for
> a complete listing of our services, staff and
> locations.
>
>
> Confidentiality Note: This message is intended for use
> only by the individual or entity to which it is addressed
> and may contain information that is privileged,
> confidential, and exempt from disclosure under applicable
> law. If the reader of this message is not the intended
> recipient or the employee or agent responsible for
> delivering the message to the intended recipient, you are
> hereby notified that any dissemination, distribution or
> copying of this communication is strictly prohibited. If
> you have received this communication in error, please
> contact the sender immediately and destroy the material in
> its entirety, whether electronic or hard copy. Thank you.
>
|