[fluidcruft]

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.

[lostlogin]

Maybe the state of play has changed, as we scan for this indication in MR, not PET.

Have you a link to something as my understanding is that small lesions are better found with MR?

Or is this a rule that applies to high end research work and hasn’t hit clinical practice yet? Maybe a limitation of the isotopes used clinically?

[haldujai]

I am a radiologist.

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.

Not sure where this is coming from.

[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.

[lostlogin]

Thanks, it’s reassuring that we aren’t way off piste.