PET scans are limited in resolution when you get down to the sub-5 mm or so range due to scanner technology and fundamental limits of the physics of positron/electron annihilation & photon emission.
A typical MRI (i.e. alas, not what this article is describing) can usually resolve something at that size and identify characteristics like diffusion restriction or contrast enhancement which can confirm metastasis.
Also, in the brain, PET scans (at least the most common, FDG, which is based on glucose) are extremely limited in utility because of the baseline high glucose metabolism of the brain, which makes it hard to distinguish from the metabolic activity of a tumor.
The flip-side of that is that PET is vastly multiple orders of magnitude far more sensitive than MRI. So while PET may not be able to localize as well as MRI, it can detect smaller things if the targeting of the radioisotope is good.
FDG PET/CT is not used to stage intracranial metastases due to background brain activity significantly reducing sensitivity. You’re likely only detecting lesions 1cm or greater in the brain, or ones which demonstrate low metabolic activity and appear dark.
MRI is much more sensitive and specific and is the standard of care for staging in my practice and as per the NCCN clinical guidelines. I haven’t been to any institution or heard of one where PET is used for staging of brain metastases.
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:
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.
I am unsure of the exact state of play but believe that small mets (eg a few mm in size) are better seen with MRI. MR is probably easier to get than PET too.
There are usually a few radiologists lurking here and they would have better knowledge than me (I’m an MR tech).
