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Subject:

Errors associated with ImCalc images in t-test

From:

Aaron Soufer <[log in to unmask]>

Reply-To:

Aaron Soufer <[log in to unmask]>

Date:

Fri, 4 Apr 2008 10:21:14 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (53 lines)

Dear SPM experts,

 I am using SPM2 to  design a t-test between two groups of PET images.
 I encounter errors every time I try to do so with images derived from
a subtraction function in ImCalc.  I will outline the procedures and
problems briefly :

1)      I used ImCalc to subtract  baseline condition from active
condition.  "i1-i2"
2)      I prepared a t-test between these two groups under the PET
model.  "compare populations: one scan/patient (two sample t-test)",
with proportional scaling global mean=50, threshold masking relative
(.8), Global calculation=mean voxel value, with no non-sphericity
correction.
3)      After estimating the model, I chose the results option and
defined the contrast.  However, when I tried to name the comparison,
the program encountered an error.  The MATLAB window read:

??? Attempted to access x(1); index out of bounds because numel(x)=0.

Error in ==> spm_Tcdf at 92

F(ml) = 0.5 + atan(x(mlx))/pi;

Error in ==> spm_getSPM at 445

      Ps = (1 - spm_Tcdf(Z,df(2))).^n;

Error in ==> spm_results_ui at 260

[SPM,xSPM] = spm_getSPM;

??? Error while evaluating uicontrol Callback

A few helpful pieces of the puzzle:
-This analysis works when I used small group sizes (4 per group), but
exhibited these errors with larger groups.
-My friend advised me to set up larger group comparisons in this
model: no global normalization, no grand mean scaling, Threshold
masking (absolute) set to –Inf, omitting global calculation, and no
non-sphericity correction.  This allowed me to retrieve results,
however they were scattered pixels outside of the brain and did not
appear to be valid.

Is there a way to solve this error so that I may compare two groups of
subjects' "active-baseline" scans.  If there is no way to solve the
problems associated with the ImCalc images, then is there another
statistical model that I can use to compare groups in such a way?

Thank you!

Aaron

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