〔Abstract〕 To assess and to compare the diagnostic performance of cervical ultrasound and 18 F-fluorode- oxyglucose ( 18 F-FDG PET/CT) in identifying cervical lymph node metastases in patients with differentiated thyroid cancer (DTC) following total thyroidectomy , focusing on both early and delayed imaging phases . Methods A ret- rospective review was performed on 83 DTC patients with DTC who had undergone total thyroidectomy . A total of 143 cervical lymph nodes , comprising both metastatic and non-metastatic nodes , were evaluated using 18 F-FDG PET/CT and ultrasound . Sensitivity , specificity , and overall diagnostic accuracy were calculated for each imaging modality. Differences in lymph node detection rates across cervical regions using ultrasound were analyzed , along with nodal size parameters (long and short axis diameters) , to investigate potential limitations of ultrasound in this context. Additionally , PET/CT parameters including axial ratio , early and delayed maximum standardized uptake values (SUV max ) , and changes in SUV max between imaging timepoints ( ΔSUV max ) were compared between meta- static and non-metastatic lymph nodes to evaluate the diagnostic value and underlying mechanisms of PET/CT in this clinical setting. Results 18 F-FDG PET/CT demonstrated a sensitivity of 91 . 58% (98/107) , a specificity of 72. 22% (26/36) , and an overall diagnostic accuracy of 86. 71% (124/143) in identifying cervical lymph node metastases post-total thyroidectomy in DTC patients . In comparison , ultrasound showed a lower sensitivity of 50. 47% (54/107) , but a higher specificity of 97. 22% (35/36) , with an overall accuracy of 62. 24% (89/ 143) . Notably , ultrasound exhibited a significantly higher false-negative rate in the first cervical lymph node group compared to the second group [56. 6% (30/53 ) vs 43 . 4% (23/53 ) ; χ2 = 20. 396 , P < 0. 001] . In delayed PET/CT imaging , metastatic lymph nodes showed a significantly increased SUV max compared to early-phase imaging (Z = - 8. 147 , P < 0. 001) . Using an SUV max threshold of 2. 5 , delayed imaging identified a greater number of positive lymph nodes than early imaging ( χ2 = 18. 127 , P < 0. 001) . Furthermore , metastatic lymph nodes exhibi- ted a significantly lower axial ratio than non-metastatic nodes (Z = - 4. 529 , P < 0. 001) . Both early and delayed SUV max values were significantly higher in malignant nodes compared to benign ones (Z = - 3 . 689 , P < 0. 001 and Z = - 4. 550 , P < 0. 001 , respectively) . Additionally , ΔSUV max was markedly elevated in metastatic lymph nodes (Z = - 4. 189 , P < 0. 001) . Conclusion Dual-phase 18 F-FDG PET/CT proves to be superior to ultrasound in di- agnosing cervical lymph node metastasis in patients with differentiated thyroid cancer (DTC) following total thyroid- ectomy . Delayed-phase imaging offers higher tumor-to-background ratio , which enhances the visibility of metastatic lymph nodes . In addition , dual-phase 18 F-FDG PET/CT plays greater advantages in the differential diagnosis be- tween metastatic lymph nodes and non-metastatic lymph nodes .