How to model observed quantity tha is an average over compartments

Dear Nmusers, we have rich concentration data from tissues measured with MRI techniques. >From other results, we know that the following model is a good approximation of biology | C <> T1 <> T2 | However, we cannot observe T1 and T2 directly, since these are mixed on cellular level and our resolution in mm-range. We only see the tissue-average of T1 and T2. I assume that I have to tweak $ERROR to model the averaging. Could someone point me to a similar example? I could not find an example on the list that has a CMT in $ERROR. Dieter Menne