[fluidcruft]

I probably wasn't clear in context because different meanings of sensitivity seem to have crossed paths. I was referring to the physics of signal detection in MRI vs PET, not clinical disease detection. MRI only detects signal from an extremely small fraction of the protons in the sample. You have no chance of detecting individual protons. MRI makes up for that with brute force of proton numbers in tissue.

Modalities like PET can detect events from a far greater fraction of the isotopes present. But they are limited in spatial resolution by the physics. This is why MRI is said to be a very insensitive modality.

See for example the discussion about MRI vs other modalities here:

https://wikipedia.org/wiki/Molecular_imaging

which notes that MRI has a sensitivity around 10^−3 to 10^−5 mol/L whereas PET is many orders of magnitude more sensitive at 10^-11 to 10^-12 mol/L.

So what I meant was if there's an analogy that you're looking at a skyscraper, MRI can "see" which which rooms have floodlights turned on. PET could detect whether there's a single candle burning somewhere in the building, but it can't tell you which room it's in.