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
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
> 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:
> (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
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