Pixel-by-pixel mean transit time without deconvolution

Background Mean transit time (MTT) within a kidney is given by the integral of the renal activity on a well-corrected renogram between time zero and time t divided by the integral of the plasma activity between zero and t, providing that t is close to infinity. However, as the data acquisition of a renogram is finite, the MTT calculated using this approach might result in the underestimation of the true MTT. To evaluate the degree of this underestimation we conducted a simulation study. Methods One thousand renograms were created by convoluting various plasma curves obtained from patients with different renal clearance levels with simulated retentions curves having different shapes and mean transit times. Results For a 20 min renogram, the calculated MTT started to underestimate the MTT when the MTT was higher than 6 min. The longer the MTT, the greater was the underestimation. Up to a MTT value of 6 min, the error on the MTT estimation is negligible. Conclusion As normal cortical transit is less than 2 min, this approach is used for patients to calculate pixel-to-pixel cortical mean transit time and to create a MTT parametric image without deconvolution