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NEUROMEG  March 2008

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Subject:

MaxFilter HPI Sphere Centres

From:

Rik Henson <[log in to unmask]>

Reply-To:

Rik Henson <[log in to unmask]>

Date:

Fri, 14 Mar 2008 11:04:25 +0000

Content-Type:

text/plain

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Parts/Attachments

text/plain (68 lines)

Dear Neuromeg list.

We have two questions about the centre of the spherical expansions in 
Maxfilter:


1. Occasionally MaxFilter bails out with the error:

    "ERROR: sphere fitted to isotrak extends outside of the sensors! (r0 
= (14 3 36) mm, rad = 11 cm)"

However, when we run the Neuromag utiltity:

    /neuro/bin/util/fit_sphere_to_points

(or our own sphere-fitting code), we seem to get sensible fits, eg 
Centre = [1.347 7.374 42.871] (Radius=98.608).

The MaxFilter manual implies that the MaxFilter sphere-fitting is more 
sophisticated/robust than simple methods, so should we not trust the 
"fit_sphere_to_points" results above? Or is the error actually with 
MaxFilter? If the latter, should we just "bypass" this MaxFilter bug by 
passing "-frame head -origin 1.347 7.374 42.871" (in this example)?


2. Related to question 1 above, we have noticed that the sphere fit by 
MaxFilter and the sphere fit by fit_sphere_to_points tend to differ, 
sometimes vastly so, with the origins differing by 10mm in one or 
another dimension. (When we independently fit a sphere using SPM 
functions in matlab, the origin is consistently very close (within 1 mm) 
to the fit_sphere_to_points result). Here are some examples:

Subject 1:
   -0.7770    5.3440   35.9980          fit_sphere_to_points origin
   -0.9496    5.1591   36.2188          SPM origin
    2.2          6.2        38.6                 MaxFilter Inside Origin

Subject 2:
   -9.5500    0.8510   39.8750
   -9.5891    1.4581   39.3087
   -10.1        5.5        36.9

Subject 3:
   -0.6850   11.9050   51.6610
   -0.5834   11.6485   51.4186
  -10.4        12.5        46.6

So should we ALWAYS use independent sphere fitting, rather than relying 
on MaxFilter? (we expect not, but just wanted to get your attention! ;-) 
Or are we misunderstanding what MaxFilter actually does?

Many thanks
Rik Henson, Jason Taylor, Danny Mitchell

-- 

----------------------------------------
Dr Richard Henson
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge
CB2 7EF, UK

Tel: +44 (0)1223 355 294 x522
Fax: +44 (0)1223 359 062

http://www.mrc-cbu.cam.ac.uk/~rik.henson
----------------------------------------

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