TO THE EDITOR:
I read with interest the article of Ohno et al. (1), and I was rather surprised to see the methodology used for the calculation of mean transit time (MTT). The area-over-height principle for the calculation of MTT is valid for a retention function. The mirror of the wash-in curve of 99mTc-galactosyl-human serum albumin (GSA) is not a retention function; the parameter (Δ over the peak value of the count) calculated by the authors does not, therefore, correspond to MTT. The authors used as a reference an article on dynamic 133Xe ventilation scintigraphy (2). The pulmonary washout curve of 133Xe can be assimilated to the retention function of a single-compartment model. The pulmonary wash-in curve during continuous and constant administration of 133Xe can also be applied for the calculation of MTT using the same principle. It is, however, obvious that after an intravenous administration, the kinetics of 99mTc-GSA in the liver are different from those of 133Xe in the lung during constant continuous administration of the tracer or during washout. The area over peak activity calculated by the authors systematically underestimates MTT. The underestimation is also highly variable depending on the shape of the plasma curve, the extraction rate of the liver, and the shape of the liver retention function.