MayBE this would help:

https://theolb.readthedocs.io/en/latest/imaging/measuring-cell-fluorescence-using-imagej.html

There is already a similar topic here with several answers

https://www.researchgate.net/post/How-can-I-measure-fluorescence-intensity-and-distribution-in-ImageJ

May I ask why ?

to determine the correct time of injection ? liver-CD ratio ?

Some companies have inbuilt intensity measurements (Stryker, Novadaq, Medtronic etc.)