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A couple of comments: (1) VBM is not the best measure of density; (2) Unmodulated images will give you a percentage of grey matter per voxel -- not necessarily density (or tissue density) (3) Since you are looking a how much tissue you have per voxel and not modulating, you are looking at a property of the tissue. Since tissue properties (like FA, MD, T2 time, etc.) are independent of brain volume, there is no need to include TBV, or TICV. (4) If you switch to modulated images, then TBV or TICV might need to be included in the model depending on how the modulation was done. See http://dbm.neuro.uni-jena.de/vbm/segmentation/modulation/ for more details. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Fri, Feb 4, 2011 at 3:08 PM, Ben Becker <[log in to unmask]> wrote: > Dear SPM-experts, > > I’ve computed a VBM analysis using the DARTEL-toolbox to investigate GM density differences in SPM8. The aim of the study was to investigate differences in GM density in two groups of patients. I set up the analysis according to the SPM8 manual (chapter 26 Dartel tools) using unmodulated GM images of the patients. > > My questions: > Do I have to incorporate the individual total intracranial volume in the second-level between-group comparison as a covariate or does the analysis already account for this nuisance variable? So is it possible to analyze the date using an unpaired t-test or do I have to use the total intracranial volume as a covariate in the second-level analysis? > > Thanks and kind regards > > Ben >