Unusually large cisterna chyli: US and MRI findings
We report a rare appearance of cisterna chyli appearing as a giant cystic structure on routine abdominal ultrasonography (US). Diagnosis was established with color Doppler US and after magnetic resonance imaging. This report describes an unusual appearance of the cisterna chyli that radiologists should be familiar with, especially on the routine conventional gray-scale US to avoid mistaking it for a pathologic condition.
KeywordsLarge cisterna chyli Imaging findings
The cisterna chyli is a dilated lymphatic sac in the retroperitoneal space, usually to the immediate right of the abdominal aorta, and represents the origin of the thoracic duct. It is seen in approximately 50% of lymphangiographic studies  and 20% of autopsies . The cisterna chyli is joined by two lumbar and intestinal lymphatic trunks that originate at the level of the L1-L2 vertebral body, after which it continues in the cephalic direction as the thoracic duct . Despite this classic description, the cisterna chyli has a highly variable appearance. The appearance of the normal cisterna chyli and variations in normal anatomy were described by Pinto et al.  who examined 200 patients with magnetic resonance imaging (MRI) using highly fluid-sensitive sequences.
The appearance of the cisterna chyli on MRI has seldom been described [3, 4, 5, 6]. To our knowledge, an unusually large cisterna chyli with an anteroposterior or transverse diameter larger than 2 cm is rare and limited to the study reported by Lee and Cassar-Pullicino . The giant cisterna chyli described in their study consisted of contrast-enhanced MRI with US and computed tomographic correlation in three patients . The craniocaudal dimension of the cisterna chyli was reported as 5 cm in two patients and 7 cm in another, with a transverse diameter of 2 cm in these patients. In a more recent study, Erden et al.  evaluated the detectability, configuration, location, and dimensions of the cisterna chyli on MRI in 125 patients and reported that the greatest dimensions in the craniocaudal, anteroposterior, and transverse diameter of the cisterna chyli were 10, 1, and 1 cm, respectively. The cisterna chyli with a transverse diameter of 3.2 cm and an anteroposterior diameter of 2.5 cm described in our patient appears to be very rare and not reported previously in the literature to our knowledge.
Pinto et al.  identified the cisterna chyli in 30 of 200 patients (15%) who were evaluated with routine abdominal MRI protocols that include HASTE sequence. They described the cisterna chyli as a variably shaped, fluid-filled structure in the retrocrural space and reported that the maximum caliber was 2.3 cm. In our case, the cisterna chyli appeared as a saccular cystic structure with a greatest dimension of approximately 4.2 cm in the caudocephalic direction.
An unusually large cisterna chyli may mimic other pathologic conditions of the retroperitoneum at conventional gray-scale US, and it may be mistaken for a retrocrural lymph node on computed tomographic images. However, depiction of fluid content of the cisterna chyli continuous with the afferent lymphatic trunks permits its accurate identification on MRI.
Radiologists should be familiar with anatomic variations, including an unusually large size, of the cisterna chyli to avoid mistaking it for a pathologic entity.