Sorry, my initial reply went to just to Laura, and not the list.
-MH
-------- Forwarded Message --------
From: Laura Verga <ildestinodiunombre...@gmail.com>
To: Michael Harms <mha...@conte.wustl.edu>
Subject: Re: [Freesurfer] Different slices number for control group
Date: Tue, 8 Jun 2010 17:13:14 +0200
Hi MIchael,
thanks so much for your answer. Firstly: you're absolutely right about
the number of slices. My mistake. I meant 150.
I will surely contact my physicist.
Thanks again and sorry for being imprecise.
2010/6/8 Michael Harms <mha...@conte.wustl.edu>
Hi Laura,
What do you mean in saying that the images of the control group
are
taken from a "different acquisition session"? Different
subjects are
always scanned in a different "session"...
Assuming that all subjects were scanned on the same scanner, the
differing number of slices in the acquisitions for the two
groups could
still be a major confound for a group comparison, depending on
the
sequence used. For example, with an MPRAGE acquisition, many k-
space
lines are acquired during each "TR", so changing the number of
slices
will in principle change the contrast and spatial filtering
properties
of the acquisition, even if all other parameters are identical.
In your
case, the change in the number of slices is quite dramatic. You
probably need to talk to your local MR physicist regarding the
sequence
that you used, the details of how it is implemented on the
Philips
scanner, and whether a change in number of slices will affect
the
contrast and spatial filtering due to the manner in which the
Philips
sequence loops through k-space.
BTW: 128 sagittal slices of 1 mm thickness is only 128 mm, which
is not
enough to cover the L/R dimension of a human head. Were those
really
the parameters?
cheers,
-MH
On Tue, 2010-06-08 at 10:50 +0200, pik...@tin.it wrote:
> Hi freesurfers!
>
> We want to estimate the differences of cortical thickness
between two
> groups. The images of the control group are taken from a
different
> acquisition session. the only differences would be the number
of
> slices (200 vs 128).
> It is still possible to compare the two groups?
>
> Images were acquired on the sagittal plane on a 3T Phillips
scanner,
> SENSE coil, voxels 1x1x1, matrix 256x256, T1 MPR.
>
> Thanks so much for your time
>
> Laura
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