Sorry, my initial reply went to just to Laura, and not the list.
-MH
-------- Forwarded Message -------- From: Laura Verga ildestinodiunombrello@gmail.com To: Michael Harms mharms@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 mharms@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, pikase@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 > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > The information in this e-mail is intended only for the person to whom it is > addressed. If you believe this e-mail was sent to you in error and the e-mail > contains patient information, please contact the Partners Compliance HelpLine at > http://www.partners.org/complianceline . If the e-mail was sent to you in error > but does not contain patient information, please contact the sender and properly > dispose of the e-mail.