It is an ampulla-shaped dilation at the beginning of the thoracic duct; it may be up to one and a half centimeter in diameter, it is normally anterior to the initial lumbar vertebrae, posterior to the aorta, and medial to the right medial pillar of the diaphragm. In some cases, where the origin of the thoracic duct is particularly high – e.g. at the level of the last thoracic vertebrae