"Straddling across boundaries"-thoracoabdominal lesions: spectrum and pattern approach.

The thoracoabdominal region consists of the inferior thorax and superior abdomen and is separated by the diaphragm. Although the diaphragm appears to act as a barrier in this region, various lesions can straddle across the diaphragm and lie contiguously in both the thorax and the abdomen. Thoracoabdominal lesions can extend across the diaphragm either through its various natural openings or through abnormal defects. The natural openings lie in the midline and include the hiatuses for the inferior vena cava, the esophagus, and the retrocrural space, which includes the aortic hiatus and the prevertebral and paravertebral spaces. Abnormal defects include congenital defects in fusion, that is, foramina of Morgagni and Bochdalek and acquired diaphragmatic rupture. Very large lesions can also displace the diaphragm, either inferiorly or superiorly, and thus appear to pseudoextend across this region. Using a pattern approach based on the location and route of extension, thoracoabdominal lesions can be classified as central and lateral lesions. Central lesions form a large group, and based on their location, they can be further classified as central anterior, central tendon, inferior vena cava, esophageal, and retrocrural pathologies. Both central and lateral thoracoabdominal lesions form a diverse spectrum and can be congenital, neoplastic, inflammatory, iatrogenic, or traumatic in etiology. Morphologically, these can consist of solid masses, cystic lesions, and ill-defined collections extending across the diaphragm. This article depicts the imaging appearance of the wide spectrum of lesions straddling across the diaphragm. Familiarity with these pathologies can help in better understanding the continuum formed by the thoracoabdominal region and the various routes of transdiaphragmatic extension.