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Hi Suchada, I mean the FA and MD should only contain a single volume. FA is a scalar measure of the total diffusion within a voxel. Your raw DTI images (tenors) are derived from your DWIs. Depending on which software package (dtifit in FSL, 3dDWItoDT in AFNI, etc) you used for your raw DTI data calculation, the raw DTI volumes varies. (about DWI and DTI: http://en.wikipedia.org/wiki/Diffusion_MRI) But even the raw DTI data usually will not contain more than 42 volumes. 3 by 3 matrix, We use eigenvalue. Thus, 3 volumes for basic lamda1, 2 and 3. Hope I made more sense this time. Sorry for the confusion. Thanks. Yingying >>> Suchada Tantisatirapong <[log in to unmask]> 10/20/2011 10:44 AM >>> Hi Yingying, Isn't scalar image, FA, MD and etc derived from DTI images which is obtained from different epi directions? Raw DTI images contain only single volume? Cheers Suchada ________________________________ From: FSL - FMRIB's Software Library [[log in to unmask]] On Behalf Of Yingying Wang [[log in to unmask]] Sent: 20 October 2011 15:34 To: [log in to unmask] Subject: Re: [FSL] Confusion between DTI and DWI data Hi Suchada, From your description, data1 should be your DWIs. data2 should be your DTI. You can probably see the difference between the images using fslview. DWI - diffusion weighted images: different volumes with different contrasts for white matter due to the anisotropic diffusion of water molecule in WM, while the diffusion of gray matter and CSF is roughly isotropic. Then, we can model the water diffusion using a simple ellipsoid model and get the DTI data which can be FA, MD, RD, AD. Those are the scalar images with one single volume not like DWI with different volumes representing different directions. Hope this is helpful to you. Cheers, Yingying >>> Suchada Tantisatirapong <[log in to unmask]> 10/20/2011 8:52 AM >>> Hi, I got the dicom images which are claimed to be DTI images. Those data have different file sizes, e.g. data1 contains 42 files and each file is about 1500 kB; data2 contains 55 files and each file is about 130 kB. After convert those file from dicom to nifty, I saw that the data1 has 42 volumes (bval and bvec is generated normally) while data2 has only 1 volume (no bvec and bval is generated). I guess the data2 is dwi images. Could you please tell me what is going on here?? Thank you for your time Regards, Suchada