On a 32 bit computer, the most memory that SPM or any other program can
use at any time is 4Gbytes (or sometimes only 2Gbytes).  This is because
the largest number that can be represented with 32 bits is
4,294,967,295.  A 150Mbyte file probably has 2 bytes per voxel, so is
probably about 512x512x300 in size.  Then add a bunch of deformations
and their inverses, a few tissue class images etc, and the amount of
memory needed soon adds up.

64-bit computers can handle images of these dimensions, but it seems
that 32 bit computers simply do not have enough memory to run this part
of SPM.

You are essentially searching for the model that most accurately
characterises the anatomical patterns of difference among the
populations.  I have my own preferred model for doing this, but most
investigators want to use a method that localises volumetric differences
of grey matter to specific regions.  To make interpretations in terms of
localised volume differences, then the pre-processed data should
accurately reflect the tissue volumes you are interested in.  The more
accurate the models used for pre-processing, then the more
straight-forward is the interpretation.  I wouldn't have the first clue
about trying to assign any physiological interpretation to patterns of
difference that did not account for the stretching and shrinking that
occurs through spatially normalising the data.

Best regards,
-John

On Mon, 2010-02-15 at 11:43 +0000, Dennis Chung Ming-Tak wrote:
> On Fri, 12 Feb 2010 17:15:36 +0000, John Ashburner wrote:
> >Realign, Coreg, Normalise and the old Segment are pretty much the same
> >as in SPM5.  However, SPM8 has a new version of the segmentation (see my
> >email from a few minutes ago) that I think will achieve more accurate
> >spatial normalisation than the older versions.
>
> Given the improvements you listed in the other post, does this dramatically
> change the structure of the output files?  I only ask because my system (1GB
> physical + 3GB virtual memory in 32 bit XP) reports "out of memory" when I try
> to import certain new segmented images - specifically 150ish megabyte
> MPRAGE files - into DARTEL.  The new segmented images (i.e. c1 to c5) were
> produced and can be viewed but this "out of memory" seemed to pop up when
> the program tries to produce the DARTEL files (i.e. rc1 etc.).
>
> I should note that the old segmented files of these MPRAGE images were
> processed through DARTEL without trouble and the 50ish megabyte SPGR files,
> which were segmented using the new method, could also be successfully
> imported into DARTEL.
>
> >Dartel is a still more accurate approach to spatial normalisation,
> >although it is a bit more complicated to use.
>
> I've been following this modulated vs. unmodulated debate here on the mailing
> list.  I understand that Dr. Ashburner's position is that an unmodulated
> comparison merely reflects registration error but others such as Mechelli 2005
> and Gaser etc. suggest that unmodulated analyses reveal brain matter
> density/concentration.  Now, with the improvement of registration through the
> use of new segmentation and DARTEL, does this change the debate in any
> way?  If not, I'm sorry for beating a dead horse.  Of course, I am assuming we
> are not studying subjects with significant deformation or volume reduction
> which would interfere with registration.
>
> Thank you for your attention.
>
> Sincerely,
> Dennis
>
>


--
John Ashburner <[log in to unmask]>