I had the same issue - my diagnostic MRI didn't show up lymph node involvement, but a later PET did. My consultant at the time did say that, having seen exactly which node was affected on the PET, they could then tell it on the MRI with hindsight. I was anxious about relying on an MRI post-treatment, particularly because of lymph involvement and in the end my consultant said he'd book a PET rather than MRI to draw a line in the sand under it. Unfortunately it hasn't done that, as a very small area of unusual activity showed up on my cervix. My consultant and gynae nurse say this isn't uncommon, as the PET shows activity in cells, as opposed to the MRI which shows actual mass/changed shape/size, etc. Activity could be scar tissue forming, or continued radiation effects, and the PET can't tell them that. Obviosuly, because it could be residual cancer cells I now need further tests - which mean further discomfort and anxiety for me and further cost to the NHS. I guess this is why MRIs are usually used for the first 3 month scan, particularly in cases where it is highly unlikely that there is residual disease. I'm now due to have an MRI in the next few weeks - my nurse pointed out that it will be compared to other scans (from diagnosis and during treatment) so it isn't just a stand alone thing - I hadn't taken that into account when I asked for a PET. Hopefully that will confirm no residual disease, if not I'll have a biopsy. I guess I'm glad to be under close observation and I don't regret pushing for the PET, but if you trust your consultant and they are confident that the MRI is enough then I'm sure it will be - I trust mine, but he ummed and ahhd a bit when I queried MRI v PET. If they see anything suspicious they will follow up with a PET anyway.
Boundie - I think I gave such an impression of querying everything and being prone to thinking the worst, my consultant thought a thorough full body PET would finally reassure me - might that be why they've booked you in for another one?