Non-invasive techniques for the measurement of extraction fraction and permeability surface area product of 99Tcm DTPA in the human forearm

Only a very limited number of clinical studies have been reported on the measurement of endothelial permeability to hydrophilic solutes (molecular weight < 6000 Daltons) that normally gain free access to the extravascular space. Because of high extraction efficiencies into the extravascular space, the transfer rates of small solutes of molecular weight about 500 Daltons, like 99Tcm DTPA, are perfusion-dependent as well as diffusion-dependent. The authors describe non-invasive techniques for measurement of clearance and extraction fraction of 99Tcm DTPA into the extravascular space of the resting forearm using a scintillation probe, from which we then calculated permeability surface area (PS) product. Their values for extraction fraction of about 0.5 and for PS product of about 3 ml per minute per 100 ml tissue are comparable to values reported in the literature for resting skeletal muscle using more invasive techniques. (author)