Objective To investigate the diagnostic value of the T2WI gray scale ratio for Hashimoto's thyroiditis(HT).Methods The T2WI-iterative decomposition of water and fat with echo asymmetry and least square estimation(IDEAL)quantitation sequence water images of 22 HT cases were analyzed retrospectively.The gray scale ratio of the thyroid,sternocleidomastoid muscle,trachea cavity,and subcutaneous fat at the same layer were measured on the picture archiving and communication systems(PACS).The gray scale ratios of thyroid/sternocleidomastoid muscle(T/M),thyroid/trachea cavity(T/Tr),and thyroid/lipid(T/L)were calculated.The intraclass correlation coefficient(ICC)was used to evaluate the consistency among the measurements,and the optimal threshold for distinguishing HT from non-HT was determined via the receiver operating characteristic(ROC)curve.The Spearman correlation analysis was used to analyze the correlation between T/M,T/Tr,T/L ratios,and titers of thyroid peroxidase antibody(TPO-Ab)and thyroglobulin antibody(Tg-Ab),respectively.Results On the T2WI-IDEAL quantitation sequence water images,the(x)±s of T/M,T/Tr,T/L ratios for HT and non-HT were 2.17±0.47 and 1.62±0.21(t=14.90,P<0.001),9.40±3.24 and 4.87±2.93(t=11.42,P<0.001),1.66±0.32 and 1.21±0.31(t=7.51,P<0.001),respectively.The area under the curve(AUC)of T/M,T/Tr,and T/L ratios for diagnosing HT were 0.89,0.86,and 0.85,respectively;the optimal thresholds were 1.90,3.50,and 1.36,and the sensitivity and specificity were 72.7%and 100%,100%and 40.5%,95.5%and 29.7%,respectively.The T/M ratio had a moderate correlation with TPO-Ab(r=0.513,P<0.05),and T/Tr,T/L ratios had a mild correlation with TPO-Ab,respectively.Conclusion The T/M ratio in the T2WI gray scale ratio can quantitatively and objectively distinguish HT from non-HT to some extent and is correlated with TPO-Ab.It has extremely high specificity and holds promise as a non-invasive imaging method for the diagnosis of incidental HT.