Hi, I was interested in downloading the FDG-PET images for analysis, so I searched for the coregistered, averaged FDG images in the IDA. When I went through the data, I noticed some of the data had voxel values of less than 1, while others are in the 1000s-10000s range. What do these mean? Are some of these data intensity normalized or are they different types of scans?
Thank you!
They are not intensity normalized. The images you downloaded are not fully pre-processed. Coregistered & averaged is Step 2; see: http://adni.loni.usc.edu/methods/pet-analysis/pre-processing/
A rough intensity normalization processed has been applied to the fully pre-processed PET images (see Step 4 of the description here): Co-reg, Avg, Std Img and Vox Siz, Uniform Resolution. Those Step 4 (fully pre-processed) images do not have the difference in scale that you mentioned. However this initial scaling is simply to put the voxel values on a uniform scale and is not intended to replace any subsequent intensity normalization that users choose to apply.
