The OASIS datasets require a registration process, so are not ideal for replication. However, to see the issue look at some of the PET images on OpenNeuro, like this one. Notice that the first baseline volume only has noise, while subsequent volumes show clear signal from tissue with radiotracer uptake. The timeline at the bottom of MRIcroGL allows you to select a volume from the timeseries, and also shows the intensity of the selected voxel across time points. Note that the signal from this voxel was 0 for the first volume, but increases dramatically for subsequent volumes.
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