The difference of the quantitative parameters using kinetic compartment model in Salivary gland scintigraphy in two kinds of people: hyperthyroidism and euthyroidism

Objectives: In the salivary gland dynamic imaging, the thyroid gland and salivary gland may competitive uptake ^99mTcO₄^- in the blood. Therefore, accurate quantitative parameters should be chosen when we evaluate the salivary gland function of the patient who is diagnosed with hyperthyroidism. Two kinds of quantitative parameters are commonly used in clinical: Uptake Percentage (UP) and Uptake Ratio (UR). In this paper, we evaluate the two kinds of quantitative parameters with kinetic compartment model.

Methods: We randomly select 19 patients with thyroid diseases during 2014.10 -2015.2, the object of the study is divided into hyperthyroidism group and control group. We use the dynamic scintigraphy of the salivary gland to get the data. On the dynamic image, we draw the region ROI of the ventricular and its background. Then we draw the ROI of the salivary glands, the thyroid gland and the corresponding backgrounds on the overlay image. Thus the TAC curve of ventricular, each gland and the corresponding backgrounds of the two groups of patients will be acquired. Compare the slope of the TAC curve of the two groups (that is K₁₂,the transfer rate from central compartment to peripheral compartment). We use the nonlinear fitting procedure to fit the ventricular data to a single compartment model, two compartment model and the three compartment model giving the best fit for the observed data. We use of Akaike information criterion (AIC) standard to select which compartment model is the best. We finally choose the two compartment model to simulate this process. We can further get the transfer rate of each gland and corresponding backgrounds. We use the rank test of the two independent samples to compare the transfer rate of the two group. P<0.05 is considered the differences have statistical significance. The two kinds of quantitative parameters are: Uptake Percentage (UP) and Uptake Ratio (UR). UP= the count of the gland /injected counts per second. This expression can be further simplified to: UP is the integral of Ct×K12×t from 0 to t timepoint/injected counts per second; UR is the count of the gland /the count of the background, this expression can be further simplified to: UR is the integral of the (Ct×K12×t/Ct×K12’×t) from 0 to t timepoint, (K12 represent the uptake rate of the gland; K12’ represent the uptake rate of the background). By comparing the two groups’ transfer rate, we can further select which quantitative parameter is better.

Results: By comparing the left and right side of the parotid gland and submandibular gland, the thyroid gland transfer rate and the corresponding backgrounds transfer rate of the left and right side of the parotid gland and submandibular gland, we use the rank test of the two independent samples to compare the transfer rate. We get the P values are 0.905,0.991,0.400,0.604,0.905,0.0006,0.661 respectively. Only the difference of the transfer rate of the thyroid was statistically significant. From the TAC curve of the two group, we can see that K12 of the hyperthyroidism group is faster than the control group. That is, the speed of radioactive count in the blood decrease faster in the hyperthyroidism group than the control group. Thus the UP can be affected by thyroid function, on the other hand, UR is only related to transfer rate. So whether the patients have hyperthyroidism, UR could accurately evaluate the salivary gland function. Conclusion: UR can reflect the function of the salivary gland accurately in patients with different physiological states relative to the UP.