This is an email from Rik Henson which references John. It addresses the fact that ImCalc defaults to writing out images in 2-byte signed integer format, which is can be problematic if you’re masking statistic images (which should written with floating point precision).
Subject: Masking with ImCalc
From: Rik Henson
Date: Fri, 14 May 2004 08:24:25 +0100 (03:24 EDT)
To: SPM@JISCMAIL.AC.UK
Several people have asked how to mask SPMs across different
analyses. This used to be possible using ImCalc in SPM99 (see Mailbase
archive). In SPM2 however, when ImCalc is called from the GUI, it
adopts various defaults, which include writing images as "uint16"
(rather than "float", which is the normal datatype for T/F imgs).
Thanks to John for pointing this out.
It is for this reason that the procedure of overwriting
spmT/F*imgs using ImCalc does not work from SPM2's GUI.
To override this default however, you can simply call
ImCalc from the command line instead, setting the
datatype to spm_type 'float':
spm_imcalc_ui([],[],'i1.*(i2>3.09)',{[],[],'float',0})
The empty 1st and 2nd arguments mean that SPM will prompt
you to select the input images and the output filename, as usual.
You will of course need to change the third argument to your
particular ImCalc equation for evaluation. The 4th argument
is a cell array of defaults, which includes the 'float' enforcement.
For more info, type "help spm_imcalc_ui".
Rik
NISOx 