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Question Posted 09/16/13:
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!
Response posted 04/10/14 by Susan Landau:
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.
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