“Straddling Across Boundaries”— Thoracoabdominal Lesions: Spectrum and Pattern Approach Ananya Panda, MD, Ashu Seith Bhalla, MD, Raju Sharma, MD, Arundeep Arora, MD, and Arun Kumar Gupta, MD The thoracoabdominal region consists of the inferior thorax and superior abdomen and is separated by the dia- phragm. 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 open- ings 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 con- genital 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 etiol- ogy. Morphologically, these can consist of solid masses, cystic lesions, and ill-defined collections extending across the diaphragm. This article depicts the imaging appear- ance of the wide spectrum of lesions straddling across the diaphragm. Familiarity with these pathologies can help in better understanding the continuum formed by the thor- acoabdominal region and the various routes of trans- diaphragmatic extension. Introduction The thoracoabdominal region consists of the inferior thorax and superior abdomen and is separated by the diaphragm. Although the diaphragm serves as a barrier between the thorax and the abdomen, various structures are contiguous across it and hence lesions can straddle across the diaphragm and lie in thora- coabdominal region. 1 Etiologically as well as morpho- logically, thoracoabdominal lesions form a diverse spectrum. These lesions can extend through either natural or pathologic defects in the diaphragm. Hence it is important to have a pattern approach to their diagnoses. This article describes the embryology, the relevant anatomy, a pattern approach, and the spec- trum of thoracoabdominal lesions. Embryology and Anatomy The diaphragm is a fibromuscular structure that develops from the fusion of 4 separate structures between 4th and 12th weeks of gestation. The septum transversum in the embryo forms the central tendon of the diaphragm. The pleuroperitoneal membranes, initially separating the chest from the peritoneal cavity in the embryo, grow medially and anteriorly to fuse with the septum transversum and form the postero- lateral parts of the diaphragm. Subsequently, myo- blasts from the body wall penetrate into the pleuroperitoneal membranes to form the peripheral muscular fibers, and the dorsal mesentery of the esophagus invaginates into diaphragm from behind Curr Probl Diagn Radiol 2015;44:122–143. & 2015 Mosby, Inc. All rights reserved. 0363-0188/$36.00 + 0 http://dx.doi.org/10.1067/j.cpradiol.2014.11.005 From the Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi, India. n Reprint requests: Ashu Seith Bhalla, MD, Room No. 66, Department of Radiodiagnosis, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029, India. E-mail: ashubhalla1@yahoo.com, ashubhalla2@gmail.com. Curr Probl Diagn Radiol, March/April 2015 122