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NEUROMEG  March 2008

NEUROMEG March 2008

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Subject:

Re: MaxFilter HPI Sphere Centres

From:

Jukka Nenonen <[log in to unmask]>

Reply-To:

Jukka Nenonen <[log in to unmask]>

Date:

Fri, 14 Mar 2008 13:49:04 +0200

Content-Type:

text/plain

Parts/Attachments:

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text/plain (86 lines)

Hi Rik,

1) message "sphere fitted to isotrak extends outside of the sensors!"
usually indicates that the initial head position determined during the
recording may have a problem, and the resulting transformation may
put the head in a wrong position. Thus, sphere fir to isotrak may be OK,
but the radius extending outside of sensors indicates that also the
head 'pierces' through the dewar bottom.

2) maxfilter origin fit does some iterations, on each roundthe point 
furthest
from the sphere is dropped. Thus, points on the nose and gace are typically
omitted, and therefore the result may deviate from 'fit_sphere_to_points'.

Best regards, Jukka

Rik Henson wrote:
> 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. Jukka Nenonen
Manager, Method development
Elekta Neuromag Oy
Street address: Siltasaarenkatu 18-20A, Helsinki, Finland
Mailing address: P.O. Box 68, FIN-00511 HELSINKI, Finland
Tel: +358 9 756 240 85 (office), +358 400 249 557 (mobile),
      +358 9 756 240 11 (fax),    +358 9 756 2400 (operator)
E-mail: [log in to unmask]
http://www.neuromag.com

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