# Sonographic Characteristics of the Thymus in Healthy Canine Neonates

## TL;DR

This study describes the ultrasound appearance of the thymus in healthy newborn puppies to help veterinarians distinguish it from abnormal masses.

## Contribution

The study provides the first detailed description of the ultrasonographic features of the thymus in neonatal dogs.

## Key findings

- The thymus in puppies is consistently visualized on the left side of the thorax within the precardiac mediastinum.
- The thymus has a distinct hypoechoic texture with scattered hyperechoic areas, making it distinguishable from other thoracic structures.
- Its ultrasonographic appearance is reliable and recognizable, aiding in clinical diagnosis.

## Abstract

The thymus is an important organ for the immune system in newborn puppies, but its appearance on ultrasound is not well described. In this study, we examined the thymus in healthy neonatal dogs using transcutaneous thoracic ultrasonography. The anatomical position of the thymus was first confirmed in cadavers and then evaluated in live puppies during the first five days of life. In all animals, the thymus could be clearly identified and showed a consistent location and characteristic ultrasound appearance. These results provide a practical reference for veterinarians and may help distinguish normal thymic findings from pathological changes in neonatal dogs.

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.