Sonographic Characteristics of the Thymus in Healthy Canine Neonates

The thymus is important for neonatal immune development; however, information on its ultrasonographic appearance in newborn puppies is limited. Because the thymus occupies a substantial portion of the cranial mediastinum in neonates, it may be confused with pathological mediastinal masses during thoracic imaging. Defining its normal ultrasonographic appearance is therefore crucial for accurate clinical interpretation. This study aimed to describe the topographic anatomy and ultrasonographic features of the thymus in neonatal dogs. Ten puppy cadavers were examined. Under ultrasound guidance, the thymus was injected with methylene blue, and its location confirmed by dissection. Subsequently, transcutaneous ultrasonographic examinations were performed in 40 clinically normal puppies aged 1-5 days. In all puppies, the thymus was consistently visualized from the left side of the thorax within the precardiac mediastinum. In the transverse imaging plane, the thoracic portion of the thymus typically appeared triangular, with variations related to respiratory movements and cardiac and vascular pulsations. The thymic parenchyma was predominantly homogeneous and hypoechoic with scattered hyperechoic areas and fine lines. In some cases, the echotexture was more heterogenous, showing fusiform hypo- and hyperechoic linear structures. The thymus was easily differentiated from adjacent thoracic structures due to its distinctive echotexture. The thymus in newborn puppies demonstrates a consistent and recognizable ultrasonographic pattern, allowing reliable identification using transcutaneous ultrasonography. These findings provide a useful reference for clinical and diagnostic evaluation of the neonatal canine thymus.