a 43-year - old woman underwent neck sonography as part of a routine medical checkup . six years earlier she had received a renal transplant , and the kidney involved had been functioning well up to the time of the checkup . neck ultrasonography revealed a 2.91.4-cm hypoechoic mass at the inferoposterior aspect of the right lobe of the thyroid ( figs . the lesion , located outside the thyroid capsule , had an ill - demarcated margin and contained multiple hyperechoic foci which appeared to be small calcifications . the thyroid gland was normal in echogenicity and size , and no focal lesion was observed . subsequently , on the basis of the sonographic findings , a tumor of the right parathyroid gland was suspected , and needle aspiration biopsy ( nab ) was planned . further sonography suggested that the hyperechogenic foci were mobile . to rule out zenker 's diverticulum , neck ct scans were obtained , and an unusual air pocket was found on the right side of the trachea . 1c ) , and multiplanar reformatted imaging showed that a slit - like structure linked it with the trachea ( fig . after reviewing all the above findings , a paratracheal air cyst was diagnosed . a 65-year - old man presented with anterior neck pain . sonography indicated that the thyroid gland was normal , but at the inferoposterior aspect of its right lobe , an ill - defined hypoechoic mass was discovered ( figs . esophagography revealed no evidence of esophageal diverticulum , but neck ct demonstrated an ill - defined air pocket , which communicated with the trachea ( figs . 2c , chest radiographs revealed mild emphysematous change but did not depict the air pocket in the right lower neck region . a 43-year - old woman underwent neck sonography as part of a routine medical checkup . six years earlier she had received a renal transplant , and the kidney involved had been functioning well up to the time of the checkup . neck ultrasonography revealed a 2.91.4-cm hypoechoic mass at the inferoposterior aspect of the right lobe of the thyroid ( figs . the lesion , located outside the thyroid capsule , had an ill - demarcated margin and contained multiple hyperechoic foci which appeared to be small calcifications . the thyroid gland was normal in echogenicity and size , and no focal lesion was observed . subsequently , on the basis of the sonographic findings , a tumor of the right parathyroid gland was suspected , and needle aspiration biopsy ( nab ) was planned . further sonography suggested that the hyperechogenic foci were mobile . to rule out zenker 's diverticulum , neck ct scans were obtained , and an unusual air pocket was found on the right side of the trachea . 1c ) , and multiplanar reformatted imaging showed that a slit - like structure linked it with the trachea ( fig . sonography indicated that the thyroid gland was normal , but at the inferoposterior aspect of its right lobe , an ill - defined hypoechoic mass was discovered ( figs . esophagography revealed no evidence of esophageal diverticulum , but neck ct demonstrated an ill - defined air pocket , which communicated with the trachea ( figs . 2c , chest radiographs revealed mild emphysematous change but did not depict the air pocket in the right lower neck region . pathologic diagnoses of these cysts vary ( tracheal diverticulum , or lymphoepithelial or bronchogenic cyst ) , but all are lined by ciliated columnar epithelium and communicate with the trachea ( 1 - 4 ) . the differential diagnosis of a paratracheal air collection includes pharyngocele , esophageal diverticulum , laryngocele , apical hernia of the lung , and apical paraseptal blebs or bullae ( 1 , 2 ) . because of the reverberation artifact at ultrasonography , zenker 's diverticulum may be seen as a mass with hyperechoic foci , mimicking a thyroid nodule , but the shape of the mass changes as the patient swallows , and the hyperechoic foci are mobile ( 5 , 6 ) . a barium swallow study is used to confirm zenker 's diverticulum and pharyngocele . laryngocele can be identified at ct as abnormal dilatation of the saccule of the laryngeal ventricle ( 7 ) , and ct also shows that apical hernia of the lung is contiguous with the remainder of the lung . at chest radiography , the hernia tends to enlarge on forced expiration and shrink on inspiration ( 8) . apical paraseptal blebs or bullae , which are air cysts within the lung , can be recognized on ct scans . in most previously reported cases , paratracheal air cysts were identified at routine chest radiography and ct , and were located at the right posterolateral aspect of the trachea at the level of the t1-t2 vertebral body ( 1 - 4 ) . ( 4 ) reported that if a right paratracheal air cyst was observed at ct , this could be a sign of obstructive lung disease , clinically , and of the presence of emphysema , radiologically . at initial ultrasonography surprisingly , the cyst appeared as a hypoechoic mass - like lesion at ultrasonography , though ct revealed no fluid density within it . we constructed a paratracheal air cyst model by trapping a balloon between slices of pork ( figs . 3a , b ) , and ultrasonography of the model demonstrated a hypoechoic mass with multiple echogenic foci ( fig . this outcome was probably due to the strong reflection of the ultrasound beam from the anterior wall of the balloon , and the multiple echogenic foci detected within the mass may be due to reverberation artifact . this phantom model explains why the paratracheal air cyst was seen as a hypoechoic mass containing hyperechoic foci . moreover , compression by the sonographic transducer distorts the wall , making it more irregular , and this invokes artifacts from many different directions . as a result , when sonography depicts a mass - like lesion containing hyperechoic foci at the right inferoposterior aspect of the thyroid gland and to the right of the trachea , the possibility of a paratracheal air cyst should be considered . ct may help disclose the nature of the lesion before performing an invasive procedure such as nab .