SUMMARY:AST is commonly associated with pyriform sinusϪthyroid fistula in children. Radiologic findings of AST are documented in a few reports. We report a new sign we term the "emerging echogenic tract sign," which reflects a patent air-containing pyriform sinusϪthyroid fistula on follow-up US. Recognition of this sign is an important finding suggesting an associated pyriform sinusϪthyroid fistula in a patient with AST and also suggesting the adequate timing of barium esophagography to confirm the fistula.ABBREVIATIONS: AST ϭ acute suppurative thyroiditis; US ϭ sonography A ST is a rare infectious disease affecting mainly children and young adults.1 That pyriform sinus fistula often causes suppurative thyroiditis has been well documented. [2][3][4] However, when a pyriform sinus fistula is not readily identified in a patient with AST, the correct diagnosis can be difficult.In the active phase of AST, demonstration of pyriform sinus fistula on US or esophagography is often difficult due to the inflammatory exudates within the fistula as well as a compression of the fistula by edema. With recovery of the thyroiditis, an echogenic tract suggesting the pyriform sinus fistula may emerge on US, and we termed this the "emerging echogenic tract sign." This finding is suggestive of an associated pyriform sinus fistula and also suggests that a subsequent barium esophagography will be helpful in demonstrating it.We report the emerging echogenic tract sign in the quiescence stage of AST with associated pyriform sinus fistula. Case Reports Case 1A 7-year-old girl was referred for evaluation of high fever and painful swelling in the left side of the neck for 5 days.US after admission ( Fig 1A) showed swelling of the left lobe of the thyroid gland with heterogeneous hypoechoic change of perithyroidal soft tissue. On a color Doppler study, increased signals of blood flow within the lesion were detected. Definite abscess formation was not seen. CT of the neck and US performed 6 days later (Fig 1B, -C) showed marked improvement of the inflammatory change of the left lobe of thyroid gland and perithyroidal soft tissue. Subsequent barium esophagogram (Fig 1D) was performed for possible presence of associated pyriform sinusϪthyroid fistula as the cause of AST, but it failed to demonstrate a fistula.Follow-up US performed 3 months after the initial US (Fig 1E, -F) showed a newly developed echogenic tract within the left lobe of the thyroid gland, suspicious for associated pyriform sinus-thyroid fistula. Repeat barium esophagography (Fig 1G) was performed and demonstrated this fistula. Case 2A 7-year-old girl presented with fever, poor oral intake, sore throat, and cough for 3 days and was referred for US. Initial US of the neck (Fig 2A) showed an ill-defined hypoechoic lesion with suspected microabscess in the left lobe of the thyroid gland and inflammatory change of overlying perithyroidal soft tissue. CT of the neck (Fig 2B) showed a heterogeneous low-attenuated lesion in the left lobe of the thyroid gland and inflammatory swellin...

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