Hi Andy:

Thank you so much for your response! I am referring to the following README.txt that I downloaded along with the dataset. On line 340, it says value 84 refers to Corpus_Callosum, whereas there is no 84 in the IBSR_??_seg_ana.nii.gz files (I am using fslview to look at the images). However, the values for hippocampii and some other structures seem to be correct and coincide with the values in the image.

So I just wanted to make sure that I am looking at the right values within the image. A list of the unique values I found within the images IBSR_{01..18} are,

2 3 4 5 7 8 10 11 12 13 14 15 16 17 18 24 26 28 30 41 42 43 44 46 47 49 50 51 52 53 54 58 60 62

Does this mean that the images I downloaded simply do not have all ROIs (e.g. corpus callosum, left cerebral exterior, righ insula, left thalamus etc.), but only a subset of those in the list?

Please let me know if I am making a mistake somewhere or if I am looking in the wrong place! 
Thanks once again!
Jimit