Objective To explore the values of contrast-enhanced ultrasound (CEUS) characteristics and quantitative parameters in diagnosing ambiguous thyroid nodules confirmed to category 4 of China Thyroid Imaging Reporting and Data System (C-TIRADS) by fine needle aspiration biopsy (FNAB).

Methods The CEUS characteristics and parameter ratios of 72 patients with ambiguous thyroid nodules confirmed to category 4 of C-TIRADS by FNAB were retrospectively analyzed. Quantitative analysis software was used to obtain CEUS time-intensity curve (TIC) of thyroid nodules; the time to peak (TTP) of contrast agents, ascent slope (AS) of radiography, peak intensity (PI), mean transit time (MTT) and area under the curve (AUC) in nodules were compared between two groups; the ratios of nodules to various imaging parameters of surrounding normal thyroid tissues were compared between two groups, including the ratio of time to peak of contrast agents of nodules to surrounding normal thyroid tissues (R-TTP), the ratio of peak intensity of contrast agents of nodules to surrounding normal thyroid tissues (R-PI), the ratio of ascent slope of nodules to surrounding normal thyroid tissues (R-AS), and the ratio of the area under the TIC curve of nodules to surrounding normal thyroid tissues by CEUS imaging (R-AUC); the value of CEUS in differential diagnosis of ambiguous thyroid nodules confirmed to category 4 of C-TIRADS by FNAB was analyzed.

Results The age of patients in thyroid malignant nodule (TMN) group was significantly smaller than that in thyroid benign nodule (TBN) group (P < 0.05). There were significant differences in the way of contrast agents perfusion, the degree of contrast agents perfusion, whether the nodular perfusion was even, the speed of contrast agents perfusion compared with the surrounding normal thyroid tissues, and whether the nodular boundary was clear after perfusion between the two groups (P < 0.05). In the TMN group, the TTP was significantly higher than that in the TBN group, while the PI, AS and AUC were significantly lower than those in the TBN group (P < 0.05). In the TMN group, the R-TTP was significantly higher than that in the TBN group, while the R-PI, R-AS and R-AUC were significantly lower than those in the TBN group (P < 0.05). For the diagnosis of benign and malignant thyroid nodules by CEUS perfusion characteristics, the sensitivity, specificity, missed diagnosis rate, misdiagnosis rate, ratio product, Yuden index, positive predictive value, negative predictive value, positive likelihood ratio, negative likelihood ratio and total coincidence rate were 83.02%, 73.68%, 16.98%, 26.32%, 13.69, 0.567, 89.79%, 60.87%, 3.15, 0.23 and 80.56% respectively.

Conclusion CEUS characteristics and quantitative parameters can improve the diagnostic accuracy of ambiguous thyroid nodules confirmed to category 4 of C-TIRADS by FNAB, which can provide a reliable basis for clinical decision-making.