Effects of different thresholds on SPECT/CT measurement of Graves' thyroid volume in hyperthyroidism
Authors: Gong Chengpeng , Zhang Yajing , Feng Fei , Hu Mengmeng , Li Kun , Pi Rundong , Shu Hua , Tang Rongmei , Wang Xiaoli , Tan Shilin , Hu Fan , Hu Jia 
Published: February 2023

Abstract
Purpose: To investigate the optimal threshold for measuring thyroid volume in patients with Grave's hyperthyroidism (GH) by SPECT/CT.
Materials and methods: A 53ml butterfly-shaped hollow container made of two 45-degree transparent elbows was put into a NEMA IEC phantom tank, and the butterfly-shaped container and the tank were filled with Na99mTcO4 with different radioactive concentrations respectively to form different tomographic target-to-background ratios (T/B) (200:1, 600:1, 1000:1) to simulate the thyroid of patients with GH. The phantom plane and SPECT/CT of different target ratios were acquired by the Discovery NM/CT 670 Pro SPECT/CT. With Thyroid software (Version 4.0) of GE Xeleris workstation, the thyroid area of the plane image was delineated and measured, the average long diameter of both lobes was substituted into the Allen formula to calculate the thyroid volume, and the calculation error was compared with the actual volume. Q-Metrix software was used to perform CT-based attenuation correction (CTAC), scatter correction (SC), resolution recovery (RR), ordered-subsets expectation maximization (OSEM) was used to reconstruct SPECT data, 20%, 25%, 30%, 40%, 50%, 60% thresholds were selected to automatically delineate the thyroid volume of interest and compared it with the real volume, and selected the most appropriate threshold. The optimal threshold determined by the model was used to measure the thyroid volume of 40 patients with clinically diagnosed GH, and the measured results were compared with the thyroid volume measured by radionuclide plane imaging method and B-ultrasound three-dimensional measurement method. ANOVA and least significant difference t-test were used to compare the differences in volume measurement of GH model with different target ratios and different thresholds, and SPECT/CT was evaluated using ANOVA, least significant difference t-test and Bland-Altman concordance test plot. The differences and consistency of thyroid volume in GH patients were measured by threshold automatic delineation method, ultrasound method and radionuclide plane imaging method.
Results: There was no significant difference in the measurement results between different T/B models (P > 0.05). The thyroid volume calculated by the plane formula method was higher than the real volume, with an average overestimation of 22.81%. With the increase of the threshold, the volume delineated by SPECT/CT threshold automatically decreased, and there were significant differences between groups with different thresholds (P < 0.001). With an average error of 3.73%, the thyroid volume of GH patients measured by 25% threshold automatic delineation method was similar to the results of ultrasound measurement, and the difference was not statistically significant (P > 0.05). Thyroid volume in patients with GH measured by radionuclide plane imaging method was significantly higher than that by ultrasound and SPECT/CT threshold automatic delineation method (P < 0.001). The Bland-Altman plot showed that the thyroid volume measured by the 25% threshold automatic delineation method was in good agreement with the ultrasound measurement.
Conclusions: The T/B has no significant effect on the measurement of thyroid volume in GH patients; radionuclide plane imaging method can significantly overestimate thyroid volume in GH patients, and 25% threshold automatic delineation method can obtain more accurate thyroid volume in GH patients.