The cisterna chyli is a dilated lymphatic sac in the retrocrural space usually to the immediate right of the abdominal aorta. It is located at the origin of the thoracic duct, usually described in anatomic studies as a saccular area of dilatation in the lymphatic channels that originates at the level of the L1–2 vertebral body and extends 5–7 cm in the caudocephalic direction. This region is also the location of the azygos and hemiazygos veins and the retrocrural lymph nodes. The cisterna chyli is joined by two lumbar and intestinal lymphatic trunks, after which it continues in the cephalic direction as the thoracic duct.

Despite this classic description, the cisterna chyli has a highly variable appearance. Complex anastomoses of lumbar lymphatics, which occur frequently, may result in a plexus rather than a single duct. Furthermore, multiple sacculations may be present in some of the convergent lymphatic channels. This wide variation in the origin of the thoracic duct has led some authors to prefer the descriptive phrase "abdominal confluence of the lymphatic trunks" to the term "cisterna chyli." The latter typically is reserved for cases in which a distinctly fusiform or saccular dilatation is present.

The main function of this ductal structure is the transport of ingested fat; approximately 100 mL of lymph per hour is transported through the channel. The caliber of the cisterna chyli may be altered by contraction waves, as has been demonstrated at fluoroscopy.

On MR Imaging, the cisterna chyli is considered present if a fluid collection with a diameter of 5 mm or more and signal intensity similar to that of bile or cerebrospinal fluid is visualized in the retrocrural space.

Shape is highly variable and includes the following morphologic patterns: a single straight thin tube, single straight thick tube, and single sausage-shaped tube; multiple parallel or converging tubes and tortuous tubes; a focal round or oval fluid collection; and a focal plexus.