An investigation of the relationship between sequential change in heart to mediastinum ratio of Tc-99m MIBI and pharmacokinetic parameters with a mathematical compartment model analysis

Objectives The heart to mediastinum ratio (H/M) of Tc-99m sestamibi (MIBI) has been reported to be useful for evaluation of myocardial damage in various heart diseases. The uptake of Tc-99m MIBI is relatively unstable immediately after the injection. We hypothesized that it may take some time for stabilization of H/M. The purpose of this study was to investigate the sequential change of H/M in several variations of time interval between the radiotracer injection and scanning (TI). Moreover, the relationships between the H/M and pharmacokinetic parameters were examined.

Methods Fifty-four patients with various heart diseases were studied. After an injection of Tc-99m MIBI (720MBq), the dynamic planar imaging was performed on a gamma camera system for 24 minutes. ROIs were drawn over the whole heart and mediastinum to obtain the H/M. Ten of 3-minute TI data (from 3-6 to 21-24) were extracted from the continuously collected data. Furthermore, influx (k1), outflux (k2) and specific uptake (k3) rate constants for the myocardium were obtained through a mathematical compartment model analysis. Finally, 540 of H/M and time-activity curves were analyzed. Tukey's multiple comparison test was performed to estimate whether there was a significant difference of H/M in each TI or not, and Pearson’s correlation coefficients were calculated to evaluate the relationships between H/M and each rate constant.

Results The H/M gradually increased with TI. The H/M in small TI (3-6) was significantly smaller than it in the others (p < 0.05). There were significant relationships between H/M and k2 in 5 minutes or more after the injection (p < 0.05).

Conclusions According to the H/M itself and the relationship between H/M and k2, scanning time might have to be 5 minutes or more after the radiotracer injection to estimate H/M