Methodological aspects of tear flow determination by means of a radioactive tracer

Theoretical considerations on a simplified biological system representing the tear pathways lead to the assumption of an exponential pattern of elimination of tracer substances from the conjunctival sac. The tracer used in the present study was technetium, Tc99m, as sodium pertechnetate. The detection system consisted of a gamma camera coupled to a digital system, a minicomputer and a magnetic tape unit. After instillation of 10 microliter normal saline solution containing Tc99m the decay of activity was followed by means of an activity-time function curve to which exponential curves were approximated by the computer. As a parameter for tear flow the fractional turnover rate was calculated from 7 to 15 min after instillation. An initial faster elimination was found in the first 7 min following instillation. Corrections for background radiation, evaporation of water and transconjunctival transport of Tc99m were estimated.