Your description is more than a bit vague...
...but I'll guess anyways - that you have a tube where you assume the volume emission rate has a cylindrical variation only depending on the radius that you want to estimate from some kind of image. For that problem you should be able to get the solution from the inverse abel transform. A recent file exchange submission for this (I haven't checket its quality yet) is: Abel-inverse-Abel-algorithms
HTH
