Summary: The intratissual diffusion of a tracer and the geometry of the detection system are considered and a generalization of Kety’s equation [S. S. Kety, Measurement of regional circulation by the local clearance of radioactive sodium. Am. Heart J. 38, 321-328 (1949)] for the measurement of regional microcirculation is derived. Non-monoexponential shapes of time-activity curves are examined and explained.