The application of extraocular muscle maximum standardized uptake value of 99mTc-diethylenetriamine pentaacetic acid orbital single photon emission computed tomography/computed tomography in the assessment of Graves’ orbitopathy
Authors: Lu Chao , Yu Yang , Wang Shen , Yin Xue , Zheng Hongyuan , Li Xiangxiang , Jia Qiang , Zheng Wei 
Published: November 2024

Purpose
To study the feasibility and value of assessing patients with Graves’ orbitopathy (GO) in 99mTc-diethylenetriamine pentaacetic acid (DTPA) orbital single photon emission computed tomography/computed tomography (SPECT/CT) with extraocular muscle maximum standardized uptake value (SUVmax).

Methods
A total of 235 patients who underwent 99mTc-DTPA orbital SPECT/CT were included in this study, including 176 patients with GO and 59 patients with Graves’ disease (GD) during the same period as a control group. The SUVmax of four extraocular muscles, including the right medial rectus muscle (RMR), right lateral rectus muscle (RLR), left medial rectus muscle (LMR) and left lateral rectus muscle (LLR), was compared between patients with active and inactive GO and patients with GD, and Spearman’s correlation analyses were performed between extraocular muscle SUVmax and patients’ clinical activity scores (CAS) as well as between extraocular muscle SUVmax and serologic indices. The diagnostic efficacy of SUVmax for GO activity was evaluated by plotting the receiver operating characteristic (ROC) curve for each subject. The consistency of extraocular muscle activity assessed by SPECT/CT was compared with MRI.

Results
Pairwise comparisons showed that the SUVmax of the extraocular muscles in the active group was significantly higher than that in the inactive group and the control group (P < 0.05); and the SUVmax of the medial rectus muscle (MR) in both eyes of patients with GO (both active and inactive patients) were higher than those of the lateral rectus muscle (LR) (P < 0.05), and the LMR was the most prominent (P < 0.05); while the difference between SUVmax in the inactive group and SUVmax in the GD group was not statistically significant (P > 0.05). SUVmax was positively correlated with CAS, thyroid-stimulating hormone, total cholesterol, and low-density lipoprotein and negatively correlated with free triiodothyronine and free thyroxine, and above all P < 0.05. SUVmax did not significantly correlate with the duration of GO (P > 0.05). The area under the ROC curve for SUVmax of the four extraocular muscles was 0.758, 0.751, 0.799, and 0.742 (P < 0.05). The optimal cutoff values for distinguishing between active and inactive SUVmax were 1.185, 0.755, 0.885, and 0.780. When the SUVmax of the extraocular muscles is higher than the cutoff value, it can be determined as the GO active phase. The sensitivity of RLR was higher compared with other extraocular muscles, 87.5%, and the specificity of RMR was higher compared with other extraocular muscles, 97.4%. There was moderate agreement between the assessment of extraocular muscle activity by 99mTc-DTPA SPECT/CT and MRI in the 87 patients with GO (Kappa = 0.439, P = 0.000), with a compliance rate of 82.7% (72/87).

Conclusion

99mTc-DTPA orbital SPECT/CT of the extraocular muscles SUVmax not only assesses the inflammatory response activity of the extraocular muscles in GO patients, but also SUVmax combined with CAS provides a more accurate and comprehensive overall assessment, treatment planning, and efficacy prediction for GO clinics.