PET/CT results

I finally got some verbal results -there's high activity in pelvic region with a 2cm "something". I'm supposed to be having an MRI to get a clearer picture. The 2 cm thing may be a lesion -I was told if it is, it wouldn't show up on the MRI. IF it does show up -I need to get it biopsied. I have OVC stg 2B grade 2. Could this be a recurrence? My last CA in NOvember was normal. I thought PET/CT scans were pretty accurate -what's the difference between that and an MRI?

[MickeyVicki]

An MRI is very precise and can image fluids as well as tissues in the body.

[PFunk]

This is what I don't really understand.  Why do doctors always do the CT scan BEFORE doing an MRI.  CT scans are notoriously terrible at imaging pelvic organs.