Converting measured activity between isotopes

[silke]

Hello there, I have a question that I just can't seem to solve by myself (although compared to the other threads here it seems quite simple)
I did some FDG-PET scans for my master thesis: I inject a certain dose of F18-FDG and prepare these injections by measuring the right dose using a dosecalibrator.
Now I've accidentally measured the activity of a few injections while the calibrator was still set to 'Co57' instead of F18...
Can I convert the activity measured in Co57 to the actual activity in F18? I can't seem to find a way to do this but I feel like it should be possible if you know the half-life of both isotopes and the measured activity, right?
Like it should even be a simple mathmetical equation? But I really can't get my head wrapped around it somehow... If anyone knows whether this is (im)possible and has any formula for this conversion, that would really help me out!
Thanks in advance!

 

[mfb]

Co-57 mainly decays via electron capture, producing photons of variable energy afterwards. F-18 decays via beta+ decay, leading to two 511 keV photons. Without knowing much more about the detector, I don't see how you could relate them theoretically.
Why don't you test a sample of F-18 with both calibrator settings? That should give a conversion factor. I wouldn't expect nonlinearities to be an issue, but ideally your test sample(s) has/have an activity similar to what you had.