EDUCATION EXHIBIT The Diaphragmatic Crura and Retrocrural ...

1289EDUCATION EXHIBIT

Carlos S. Restrepo, MD • Andres Eraso, MD • Daniel Ocazionez, MD Julio Lemos, MD • Santiago Martinez, MD • Diego F. Lemos, MD

The retrocrural space (RCS) is a small triangular region within the most inferior posterior mediastinum bordered by the two diaphrag-matic crura. Multiplanar imaging modalities such as computed to-mography and magnetic resonance imaging allow evaluation of the RCS as part of routine examinations of the chest, abdomen, and spine. Normal structures within the retrocrural region include the aor-ta, nerves, the azygos and hemiazygos veins, the cisterna chyli with the thoracic duct, fat, and lymph nodes. There is a wide range of normal variants of the diaphragmatic crura and of structures within the RCS. Diverse pathologic processes can occur within this region, including benign tumors (lipoma, neurofibroma, lymphangioma), malignant tumors (sarcoma, neuroblastoma, metastases), vascular abnormalities (aortic aneurysm, hematoma, azygos and hemiazygos continuation of the inferior vena cava), and abscesses. An understanding of the anat-omy, normal variants, and pathologic conditions of the diaphragmatic crura and retrocrural structures facilitates diagnosis of disease pro-cesses within this often overlooked anatomic compartment. ©RSNA, 2008 • radiographics.rsnajnls.org

The Diaphragmatic Crura and Retrocrural Space: Normal Imaging Appearance, Variants, and Pathologic Conditions1

ONlINE-ONly CME

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lEARNING OBJECTIVESAfter reading this article and taking the test, the reader

will be able to:

Describe the nor- ■

mal imaging appear-ance of the retro-crural space and its normal variants.

Identify the ■

anatomic structures that are normally encountered in the retrocrural space.

List the imaging ■

findings associated with the spectrum of pathologic condi-tions that may affect the retrocural space.

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TEACHING POINTS

Abbreviations: AIDS = acquired immunodeficiency syndrome, IVC = inferior vena cava, RCS = retrocrural space

RadioGraphics 2008; 28:1289–1305 • Published online 10.1148/rg.285075187 • Content Codes: 1From the Department of Radiology, University of Texas Health Science Center, 7703 Floyd Curl Dr, Mail Code 7800, San Antonio, TX 78229-3900 (C.S.R., D.O.); the Department of Radiology, Veterans Administration Medical Center, Washington, DC (A.E.); the Department of Radiol-ogy, University of Vermont College of Medicine, Burlington, Vt (J.L., D.F.L.); and the Department of Radiology, Duke University Medical Cen-ter, Durham, NC (S.M.). Recipient of a Certificate of Merit award and an Excellence in Design award for an education exhibit at the 2004 RSNA Annual Meeting. Received September 12, 2007; revision requested October 10; final revision received February 7, 2008; accepted February 19. C.S.R. is a research consultant for Bayer; all other authors have no financial relationships to disclose. Address correspondence to C.S.R. (e-mail: [email protected]).

©RSNA, 2008

1290 September-October 2008 RG ■ Volume 28 • Number 5

IntroductionWith the advent of computed tomography (CT) as an imaging modality for evaluating the tho-rax and abdomen, the retrocrural space (RCS) gained importance as an anatomic landmark among radiologists. Multiplanar imaging modali-ties such as CT and magnetic resonance (MR) imaging have allowed evaluation of the RCS as part of routine examinations of the chest, abdomen, and spine. There is a wide range of normal variants of the diaphragmatic crura and of structures within the RCS. Similarly, diverse pathologic processes can occur within this region including congenital, inflammatory, neoplastic, vascular, and infectious conditions.

In this article, we review the relevant embryol-ogy and anatomy of the RCS as well as the nor-mal variants and pathologic conditions occurring within this small but important imaging landmark.

EmbryologyThe diaphragm forms from the 4th to 12th weeks of embryonic life. By the 4th week of embryonic life, the coelom or body cavity appears as a horse-shoe-shaped cavity in the cardiogenic and lateral mesoderm; this cavity will later give origin to the thoracic and peritoneal cavities. By the 6th week, the pleuropericardial membranes extend medially and their free edges fuse with the mesentery of the esophagus and with the septum transversum, sepa-rating the pleural cavities from the peritoneal cav-ity. Further growth of myoblasts will ensure pleu-roperitoneal openings, forming the posterolateral elements of the diaphragm. The dorsal mesentery of the esophagus constitutes the median portion of the diaphragm. The diaphragmatic crura develop from myoblasts that grow into the dorsal mesen-tery of the esophagus (1) (Fig 1).

Anatomic ConsiderationsIn cross-sectional imaging, the RCS can be defined as a triangular region that represents the most inferior portion of the posterior me-diastinum, with no true boundaries from the posteromedial aspect of the thoracic cavity. The space is delimited anteriorly and laterally by the diaphragmatic crura as they pass from the antero-lateral aspects of the vertebral bodies and ascend to decussate in front of the aorta. Posteriorly, boundaries include the ventral aspect of the distal thoracic and proximal lumbar vertebral bodies. The RCS communicates with the posterior medi-astinum and the retroperitoneum, being a poten-tial pathway for extension of diverse pathologic conditions between these spaces. The contents of the RCS include fat, vascular structures (ie, aorta

and arterial branches, azygos and hemiazygos veins), neural structures (ie, sympathetic trunks and splanchnic nerves), lymph structures (ie, lymph nodes, thoracic duct, and cisterna chyli), and other less distinct structures (eg, ascending lumbar venous plexus) (2) (Fig 2).

Figure 1. Embryologic origin of the diaphragm. Dia-gram shows the four embryologic structures that give rise to the diaphragm: the horizontal septum transver-sum (ST), bilateral pleuroperitoneal membranes (ppm), dorsal mesentery of the esophagus (dme), and body wall muscular fibers (Bw). Ao = aorta, Es = esophagus, IVC = inferior vena cava.

Figure 2. Diagram of an axial section through the distal thoracic spine shows the normal components of the RCS and their relations to the IVC (large blue structure), adrenal gland, stomach, and vertebral body. Normal structures within the RCS include the aorta (red), thoracic duct (green), splanchnic nerves (yellow), lymph nodes, and azygos and hemiazygos veins (small blue structures).

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RG ■ Volume 28 • Number 5 Restrepo et al 1291

Diaphragm and Diaphragmatic CruraThe peripheral diaphragm is formed by muscu-lar fibers attached to three different anatomic regions: the sternum, inferior ribs, and lumbar region. The lumbar portion attaches to the me-dial and lateral lumbocostal arches (arcuate liga-ments) and to the anterolateral surfaces of the lumbar vertebrae as bilateral musculotendinous pillars, known as the diaphragmatic crura (Fig 3).

There are two pairs of lumbocostal arches or arcuate ligaments: the medial and lateral arcuate ligaments. The medial ligament arches over the psoas major muscle and is continuous medially with the corresponding crus, attaching itself to the lateral surface of the L1 or L2 vertebral body.

Laterally, the medial arcuate ligament attaches to the anterior aspect of the transverse process of the first lumbar vertebra. The lateral arcuate liga-ment arches over the quadratus lumborum and attaches laterally to the 12th rib and medially to the transverse process of L1 (3).

The term crura, the plural of crus, is derived from the Latin word cruralis, meaning “leg.” The tendinous aspect of the right crus attaches to the ventral surface of the lumbar vertebral bodies and to the intervertebral fibrocartilage of the first three lumbar vertebrae. The right crus is longer and broader than the left. The shorter left crus at-taches to corresponding structures of the first two lumbar vertebral bodies only. As the two crura ascend, their medial fibers are conjoined, form-ing an arch ventral to the aorta and just above the celiac trunk. This sometimes indistinct arch is known as the median arcuate ligament. Other medial fibers of the right crus continue to sur-round the distal esophagus, forming the esopha-geal hiatus, with some superficial fibers extending around the left margin of the distal esophagus and the deeper fibers running along the right margin. A fasciculus of the medial aspect of the left crus crosses the aorta ventrally and runs along the lateral deeper fibers of the right crus toward the vena cava hiatus (Figs 4, 5).

The aorta, the thoracic duct, some lym-phatic trunks, and sometimes the azygos and

Figure 3. Diagram (inferior view) shows the diaphragm and the insertions of the diaphragmatic crura on the upper lumbar vertebral bodies. The right crus is longer and broader than the left. Ao = aorta, CTD = central tendon of the diaphragm, Es = esophagus, LAL = lateral arcuate ligament, LC = left crus, MAL = medial arcu-ate ligament, Ps = psoas, QL = quadratus lumborum, RC = right crus.

Figure 4. Axial T2-weighted MR image shows the normal appearance of the diaphragmatic crura at the level of the aortic hiatus. Normal retrocrural fat typi-cally has high signal intensity on T1- and T2-weighted MR images.

Figure 5. Coronal reformatted image from multisec-tion contrast-enhanced CT shows the anatomic rela-tionship between the diaphragmatic crura (arrowhead) and the psoas muscles (arrows).


aorta, but left-sided or retroaortic locations have also been described (2,6) (Fig 6).

The cisterna chyli can be seen in up to 52% of patients at lymphangiography, 1.7% at CT, and 15%–96% at MR imaging (2,7,8). It has a vari-able morphology including tubular (most com-mon; 30%–42.5% of cases), rounded, oval, plexi-form, and fusiform (2). At CT it has low attenu-ation, near that of water. Delayed enhancement has been described at CT after oral ingestion of ethiodized oil or intravenous injection of contrast material (9–11) (Fig 7).

AortaAt the level of the aortic hiatus, the aorta is slightly to the left of midline. Important arterial branches that arise from the descending aorta within this region are the lower posterior inter-costal and subcostal arteries.

hemiazygos veins pass through the aortic hiatus, which is the most posterior of the diaphragmatic apertures (4).

Lymph Nodes, Cisterna Chyli, and Thoracic DuctThe lymph nodes of the RCS are lymphatic draining stations for the posterior diaphragm, posterior mediastinum, and lumbar spine and can measure up to 6 mm along the short axis (5).

The cisterna chyli is a bulbous dilatation formed by the convergence of lymphatic channels at the level of the upper lumbar vertebral bod-ies. It receives two afferent lumbar and intestinal lymphatic trunks and finally ascends as the tho-racic duct. It is usually located to the right of the

Figure 6. Normal contents of the RCS. Axial (a) and coronal reformatted (b) contrast-enhanced CT images show a mildly enlarged cisterna chyli (arrow) slightly to the right of the aorta. The cisterna chyli appears as an oval structure with near water attenuation.

Figure 7. Axial (a) and coronal (b) T2-weighted MR images show the thoracic duct (ar-rows). The thoracic duct is normally seen at MR imaging as a fluid-containing tubular structure, with low signal intensity on T1-weighted images and high signal intensity on T2-weighted and half-Fourier acquisition single-shot turbo spin-echo or fast field echo images.


of competent valves. The absence of valves is also responsible for the potential retrograde venous flow, which may contribute as a pathway for dis-tant spread of neoplasms and infections (Fig 8).

Sympathetic Trunk and Splanchnic NervesThe thoracic sympathetic trunk passes either dorsal to the medial arcuate ligament or runs through the diaphragmatic crura to become the lumbar sympathetic trunk. The medial branches of the lower sympathetic ganglia supply the aorta and unite to form the greater, lesser, and lowest splanchnic nerves, all of which descend obliquely and lateral to the distal thoracic vertebral bodies. After piercing the diaphragmatic crura, sympa-thetic fibers from the greater and lesser splanch-nic nerves end in the celiac ganglia. Understand-ing the anatomy of the sympathetic trunk and splanchnic nerves within the retrocrural region has gained importance with the advent of imag-ing-guided techniques for percutaneous blockade of the celiac plexus.

Anatomic Vari- ants and Anomalies

Anomalies and Variations of the Diaphragmatic CruraIn comparison with other more common con-genital diaphragmatic anomalies like agenesis or Bochdalek, Morgagni, and hiatal hernias, anom-alies affecting the crura are often less symptom-atic and are usually detected incidentally during imaging (12,13).

Partial duplication of the diaphragm may involve the crura. It is thought to result from improper timing in the interaction of the lung buds and septum transversum. The condition is associated with cardiovascular malformations and ipsilateral pulmonary maldevelopment. The du-plicated accessory diaphragm extends obliquely upward and backward to attach to the third to seventh ribs posteriorly. This anomaly is more frequently right sided; at cross-sectional imag-ing, the lower lobe of the lung may be partially or completely contained within the accessory diaphragm and the true hemidiaphragm (13,14) (Fig 9).

Discontinuity of the diaphragm between the crura and the lateral arcuate ligaments is a normal variation encountered in 11% of nor-mal patients and should not be confused with diaphragmatic rupture (15). An association with aging suggests that atrophy can be related to an

Azygos and Hemiazygos VeinsThe azygos vein is right sided and most fre-quently arises from the junction of the lumbar azygos with the right ascending lumbar and sub-costal veins. It enters the chest through the right crus or through the RCS via the aortic hiatus and ascends along the right anterolateral surface of the thoracic spine. In a similar fashion, but on the left side, the hemiazygos vein originates at the junction of the left ascending lumbar and left subcostal veins.

Paravertebral Venous PlexusA network of valveless venous systems freely communicates along the entire extent of the vertebral column. The relative increased venous pressure of this venous network enables blood to flow from the vertebrae to the caval venous system. There is free communication between the veins of the neck, thorax, abdomen, and pelvis and the vertebral venous plexus owing to the lack

Figure 8. Paravertebral venous plexus. Diagram shows the anterior external vertebral venous plexus (AEVV), with veins that can be seen in the RCS. AIVV = anterior internal vertebral (epidural) venous plexus.

Figure 9. Normal crural variation in a 50-year-old man. CT image shows a duplicated or accessory right hemidiaphragm (arrow). This is an uncommon anom-aly that can be seen as an incidental finding or in asso-ciation with recurrent pulmonary infections related to pulmonary sequestration. Note the second linear area of increased attenuation within the right RCS.


of venous blood from the lower abdomen, pelvis, and lower extremities into the azygos or hemi-azygos venous systems (17). An enlarged azygos vein receives venous blood from the renal portion of the IVC and passes within the RCS, accessing the thorax to drain into the superior vena cava. At axial CT or MR imaging, the enlarged azygos vein can be recognized as a well-defined tubular structure that parallels the aorta behind the dia-phragmatic crura and extends cephalad to com-municate with the azygos arch. There is intense enhancement after intravenous contrast material administration (Fig 12).

Other IVC congenital anomalies that manifest as enlarged retrocrural azygos and hemiazygos veins include absence of the infrarenal IVC and duplication of the IVC with azygos or hemiazygos continuation (18,19). Knowledge and recognition of these vascular anomalies avoids misinterpreta-

acquired loss of diaphragmatic muscular integ-rity. However, underlying congenital weakness of the diaphragm may coexist and may become ap-parent only with aging (16) (Fig 10).

Congenital Anomalies of the IVCSeveral congenital anomalies of the IVC are asso-ciated with variations of the azygos and hemiazy-gos anatomy within the RCS.

Absence of the hepatic segment of the IVC with azygos continuation is probably the most well studied of these anomalies with a prevalence of 0.6%. Although previously associated with congenital cardiovascular anomalies, asplenia, and polysplenia syndromes, this congenital anom-aly has been increasingly identified in asymptomatic patients (Fig 11). Failure to develop a communication between the right subcardinal and hepatic venous system during embryogenesis has been postulated to cause interruption of the hepatic segment of the IVC, resulting in loss of continuity with the prerenal IVC. The atrophy of the right subcardinal vein results in shunting

Figure 10. Normal crural variation in a 63-year-old man. (a) CT image shows an incomplete right hemidi-aphragm (straight arrow) with a smaller but similar dehiscence on the left (curved arrow). (b) CT image shows herniation of fat (arrow) into the RCS and pos-terior mediastinum.

Figure 11. Azygos continuation of the IVC (het-erotaxia syndrome) in a 58-year-old man. CT image shows an abnormally dilated azygos vein (arrow) within the RCS in association with polysplenia (*).

Figure 12. Absence of the infra- and intrahepatic portions of the IVC with azygos continuation in a 34-year-old woman. Contrast-enhanced CT image shows enlarged azygos and hemiazygos veins (straight arrows) with multiple venous collaterals (curved arrow).


Other nonneoplastic conditions that can involve the diaphragmatic crura include infec-tious processes such as intracrural abscess and traumatic rupture. Thickening of the crura has been described as an indicator for diaphrag-matic injury in the setting of trauma (20,21). However, normal thickness variations related to respiration or the patient’s age limit the use of this parameter as an indicator of traumatic diaphragmatic injury.

Abnormalities of the RCS

Lymphadenopathy.—Lymph nodes dorsal to the diaphragmatic crura drain the posterior me-diastinum and diaphragm as well as the lumbar regions (3). Detection of enlarged retrocrural lymph nodes exceeding 6 mm in diameter should be considered suspiciously abnormal (Fig 15).

Enlarged retrocrural lymph nodes are associ-ated with inflammatory, infectious, and most frequently neoplastic conditions. Cephalic spread of metastatic genitourinary or gastrointestinal malignancies or caudal extension of thoracic malignances such as esophageal and lung carci-nomas may manifest as enlarged lymph nodes within the retrocrural region (22,23). In addition, lymphoma—either Hodgkin or non-Hodgkin—is a common malignant cause of enlarged lymph nodes or nodal masses within the RCS. As with other anatomic compartments, there are limita-tions to use of size criteria to determine malig-nant involvement because metastases can occur in normal-sized nodes.

Inflammatory conditions that manifest with generalized lymph node enlargement such as

tion of enlarged azygos and hemiazygos veins as retrocrural masses or adenopathy.

Pathologic Processes

Abnormalities of the CruraPrimary neoplasms of the diaphragmatic crura are rare. Lipomas, desmoids, and muscular tumors such as leiomyosarcomas and rhabdomyosarcomas have been described (Fig 13). Malignancies of the diaphragmatic crura occur via local or hemato-genous spread of tumor. Tumor extension from malignant intrathoracic processes such as pleural mesothelioma and metastatic lung or esophageal malignancies can involve the diaphragmatic crura due to their anatomic proximity (4,12) (Fig 14). Similarly, malignant abdominal neoplasms, es-pecially within the retroperitoneum, can extend through the paravertebral space and RCS with subsequent diaphragmatic crural invasion.

Figure 13. Crural lipoma in a 40-year-old man. Axial contrast-enhanced CT image shows a fat attenu-ation lesion (arrow) within the left crus.

Figure 15. Retrocrural metastasis in a 67-year-old woman with renal cell carcinoma. Contrast-enhanced CT image shows metastatic renal cell carcinoma as a markedly enhancing and enlarged retrocrural mass (arrows).

Figure 14. Metastases from lung carcinoma in a 62-year-old man. Contrast-enhanced CT image shows abnormal thickening of the left hemidiaphragm (ar-row) and left crus (arrowheads).

TeachingPoint


pathetic chain along the paravertebral region and range from malignant masses (neuroblastoma) to benign tumors (ganglioneuroma). The child’s age at presentation, the presence of skeletal metasta-ses, calcification within the tumor, and the pattern of enhancement at both CT and MR imaging can help differentiate between the three (29,30).

Although CT has been the most commonly used imaging modality for assessment of ganglion cell tumors and is especially useful in detect-ing calcification within the masses, MR imaging is superior in evaluating the extent of the more malignant neurogenic tumors owing to its supe-rior multiplanar imaging capability (29). At MR imaging, neuroblastoma—the most common of the three—has prolonged T1 and T2 relaxation times, appearing with low signal intensity on T1-

sarcoidosis, lymphangioleiomyomatosis, and amyloidosis may occasionally manifest as abnor-mally enlarged retrocrural lymph nodes (24,25). Enlarged lymph nodes associated with myco-bacterial infection or lymphangioleiomyomatosis usually have low attenuation values at nonen-hanced CT (23). Disseminated tuberculosis, Mycobacterium avium-intracellulare complex, and lymphadenopathy in acquired immunodeficiency syndrome (AIDS) patients are infectious causes for enlarged lymph nodes within this region; however, the presence of bulky nodes larger than 1.5 cm is unusual (23,26,27) (Table).

Retrocrural Neoplasms.—Primary neoplasms that arise in or extend to involve the RCS re-semble the imaging presentation of other primary neoplasms of the posterior mediastinum. These can be categorized according to their cell of ori-gin into neurogenic, mesenchymal, germ cell, and lymphoid tumors.

Retrocrural neurogenic tumors can be divided into three categories: ganglion cell tumors, nerve and nerve sheath tumors, and tumors of the paraganglia (28). Tumors derived from ganglion cells such as neuroblastoma, ganglioneuroblas-toma, and ganglioneuroma are the most common posterior mediastinal masses encountered during early childhood. These tumors are derived from sympathetic ganglion cells arising from the sym-

Causes of Retrocrural Lymphadenopathy

Inflammatory and miscellaneous conditionsSarcoidosisLymphangioleiomyomatosisAmyloidosis

InfectionsAIDSTuberculosisM avium-intracellulare complex

LymphomaMetastatic disease

NeuroblastomaSarcomaMalignant pleural mesotheliomaTesticular germ cell tumorEsophageal carcinomaLung carcinomaGastric carcinomaOvarian carcinomaCervical carcinomaColorectal carcinomaRenal cell carcinomaProstate carcinomaBladder carcinoma

Figure 16. Large posterior mediastinal neuroblas-toma extending into the RCS in a 3-year-old boy. (a) Axial T1-weighted MR image shows a large mass (arrow) displacing the left hemidiaphragm anteriorly (arrowheads). (b) Coronal T2-weighted MR image shows the left diaphragm pushed inferiorly (black ar-row) by the RCS mass (arrowheads). The right crus is normal (white arrow). MR imaging characteristics of low signal intensity on T1-weighted images and high signal intensity on T2-weighted images are typical of neurogenic tumors.

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dentally detected in young to middle-aged women. The central cystic degeneration of neurilemomas can be readily recognized at both CT and MR im-aging; however, the peripheral calcification is bet-ter depicted with nonenhanced CT. Ten percent of nerve sheath tumors demonstrate intraspinal extension with neural foramina enlargement and have a “dumbbell” appearance (33). Overall, these benign nerve sheath tumors manifest as smooth oval masses with well-defined margins and are of-ten indistinguishable with imaging alone.

Paraganglioma is an even less common ag-gressive neurogenic tumor that can occur within the RCS as well. These hypervascular tumors arise from paraganglionic cells near the aorti-cosympathetic chain (34) and more frequently affect patients during the second or third decade of life. Patients with functional paragangliomas have hypertension and biochemical evidence of excess catecholamine production. Paragan-gliomas range from 1 to 6 cm in diameter and demonstrate brisk enhancement after intravenous administration of contrast material on both CT and T1-weighted MR images. The typical uni-form high signal intensity on T2-weighted MR images has been reported in 80% of studies (28). m-iodobenzylguanidine scanning can be diagnos-tic in functioning tumors (35). Owing to their hypervascularity, it is important to consider these paraganglionic tumors as a potential cause of spontaneous acute hemorrhage within the RCS and retroperitoneum.

As elsewhere within the mediastinum, mesen-chymal neoplasms of the RCS are infrequently encountered. Among tumors of mesenchymal origin, lipoma and liposarcoma are probably the least uncommon and can appear de novo or gain access into the RCS from the posterior mediasti-num or retroperitoneum. These fat-containing tumors are slow growing and usually large at pre-sentation (36). Other less frequently encountered neoplasms of mesenchymal origin include other types of sarcomatous tumors (rhabomyosarcoma, leiomyosarcoma, and fibrosarcoma), malignant fibrous histiocytoma, leiomyoma, lymphangioma, and tumors of blood vessel origin like hemangio-pericytoma and hemangioma (30,37,38). Simi-larly, these may be retrocrural in origin or extend into the RCS from the posterior mediastinum or retroperitoneum (Fig 17).

Among tumors of germ cell origin, teratoma is a neoplasm incidentally found in asymptomatic patients. Mediastinal teratomas, although unusual, have been more frequently reported in the anterior mediastinum; however, a few have been reported in the posterior mediastinum along the paravertebral

weighted images and high signal intensity on T2-weighted images (31) (Fig 16).

Nerve sheath tumors such as neurofibroma and neurilemoma are usually benign neurogenic tumors that may equally enlarge to occupy the RCS. Nerve sheath tumors that occupy the RCS often arise from lower intercostal or distal tho-racic sympathetic nerves, often in patients with neurofibromatosis. Neurilemomas are usually large encapsulated tumors that often occur with cystic degeneration and peripheral calcification. On the other hand, neurofibromas are unencap-sulated solid tumors without the cystic compo-nents typical of neurilemomas (28).

Neurofibromas are typically rounded hypoat-tenuating tumors at nonenhanced CT and dem-onstrate homogeneous enhancement after intra-venous contrast material administration at both CT and T1-weighted MR imaging (32). Neuri-lemomas are usually asymptomatic tumors, inci-

Figure 17. Retroperitoneal rhabdomyosarcoma with RCS involvement in a 6-year-old boy. (a) Axial con-trast-enhanced CT image shows a paravertebral mass (arrow) causing anterior displacement of the aorta. (b) Coronal T1-weighted MR image shows the isoin-tense lobulated paraspinal mass (arrows) extending into the posterior mediastinum through the RCS.


well-defined soft-tissue masses at multiple levels along the distal thoracic paraspinal region. Faint enhancement after intravenous contrast material administration is demonstrated at both CT and MR imaging. These lobulated soft-tissue masses may have areas of fat attenuation at CT (Fig 19).

MR imaging features of extramedullary he-matopoiesis depend on the level of activity of the hematopoietic lesion. In active hematopoietic lesions, there is intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images in relation to muscle, with slight enhancement after administration of gado-linium contrast material. In older inactive lesions, low signal intensity on both T1- and T2-weighted

regions (39). When teratomas manifest with pathognomonic features such as the presence of fat, fluid, soft tissue, and especially calcification, they can be readily recognized and usually differ-entiated from other more aggressive fat-containing tumors such as liposarcomas (40,41).

Seminomatous and nonseminomatous ma-lignant germ cell tumors involve the RCS by metastatic spread of tumor from a gonadal pri-mary or by direct invasion from posterior medi-astinal or retroperitoneal primaries. Malignant germ cell tumors arise from primitive germ cells that failed to migrate into the scrotum during development. The majority of malignant germ cell tumors occur in men between the ages of 20 and 35 years. Seminomatous tumors usually manifest as well-marginated, large, bulky masses that demonstrate homogeneous attenuation at CT, reflecting their uniform cellular nature (42). Nonseminomatous germ cell tumors are also large aggressive tumors, with irregular margins and a heterogeneous appearance due to areas of hemorrhage and necrosis (40).

Lymphoma encompasses a diverse group of lymphoid neoplastic processes that are well known to cause enlarged retrocrural lymph nodes or nodal masses. Involvement of retrocrural and para-aortic lymph nodes is generally accompa-nied by a similar presentation involving other lymphatic nodal groups in the mediastinum or retroperitoneum. Both Hodgkin disease and non-Hodgkin lymphoma can involve retrocru-ral lymph nodes. Fifty percent of patients with non-Hodgkin lymphoma have positive high para-aortic lymph nodes at presentation compared to 25% with Hodgkin disease (43). Patients with non-Hodgkin lymphoma present with larger nodal masses compared to those with Hodgkin disease. A bulky nodal mass larger than 1.5 cm that surrounds and displaces the aorta away from the spine is the typical presentation of non-Hodg-kin lymphoma within the RCS (Fig 18).

Extramedullary hematopoiesis is an unusual benign neoplastic process that can manifest as soft-tissue masses within the retrocrural region. This unusual condition arises as a compensa- tory phenomenon in patients with long-standing anemia. There is reactive expansion of the eryth-ropoietic tissue that has extruded from the vertebral bodies and posterior ribs, resulting in bilateral paraspinal masses (44). Although more frequently described in patients with thalassemia, it may also be seen in those with sickle cell dis-ease, hereditary spherocytosis, myelosclerosis, and other causes of anemia and myeloprolifera-tive disorders (45). CT usually demonstrates

Figure 18. Non-Hodgkin lymphoma in a 39-year-old woman. Contrast-enhanced CT image shows bulky right retrocrural lymph nodes (black arrow) and an-terior aortic displacement (white arrow). In addition, pleural fluid is identified in the posterior costophrenic angles bilaterally.

Figure 19. Extramedullary hematopoiesis in a 52-year-old woman with thalassemia. Nonenhanced CT image shows abnormal bilateral paraspinal soft-tissue masses in the lower thoracic region (arrows).

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such as infection and malignant transformation (48). Foregut malformations arise from abnormal budding of the primitive foregut between the 3rd and 7th weeks of embryonic development (49). Foregut duplication cysts can be divided into enterogenous and bronchogenic cysts. When de-tected at CT, these lesions demonstrate variable attenuation values without enhancement after intravenous contrast material administration. At MR imaging, the cysts have variable signal inten-sity on T1-weighted images and high signal inten-sity on T2-weighted images (50,51).

Vascular Disorders.—The aorta is the largest anatomic structure confined within the RCS. Disease processes of the aorta, including rup-tured aneurysm and pseudoaneurysm, traumatic rupture, and inflammatory conditions such as aortitis, can all manifest as abnormalities radio-logically evident within the retrocrural region.

Infiltrative appearing soft-tissue attenuation that replaces the normal retrocrural fat surround-ing the aorta is the characteristic CT appearance of acute periaortic hematoma. Periaortic hema-toma within the RCS is present in patients with traumatic aortic injury or aneurysm rupture as well as in periaortic bleeding caused by translum-bar access for aortography (52). The clinical set-ting and ancillary imaging findings such as aortic size, contour abnormalities, extravasation of con-trast material, and displacement from mass effect of adjacent anatomic structures aid in identifying the cause and nature of the retrocrural periaor-tic hematoma (Fig 20). In the setting of trauma, periaortic hematoma within the retrocrural re-gion can also be present secondary to posterior diaphragmatic rupture or vertebral body and rib fractures or related to dissection of a hematoma from a more proximal thoracic aortic laceration (53,54) (Fig 21).

Chronic periaortitis encompasses a series of inflammatory conditions with a suspected autoimmune pathogenesis. Among these enti-ties, which can manifest as circumaortic fibrotic changes, the most representative are giant cell ar-teritis, Takayasu arteritis, periaortic fibrosis, and inflammatory aortic aneurysm; all are known to involve the distal thoracic and proximal abdomi-nal aorta. Periaortic inflammatory changes within the RCS can be well depicted with cross-sectional imaging (CT or MR imaging) or angiography and recently with positron emission tomography (Fig 22). Delayed enhancement of the thickened aortic wall after intravenous contrast material ad-ministration suggests aortitis at both CT and MR

images is seen secondary to deposition of iron, whereas high signal intensity on both image types is visualized when there is significant fatty infil-tration (46).

Foregut Malformations.—Although usually found in the middle mediastinum within the paratracheal or subcarinal region, foregut mal-formations within the posterior mediastinum and retroperitoneum extending to occupy the RCS have been described in isolated case reports. Simultaneous retrocrural presentation of pulmo-nary sequestration with gastric and esophageal duplication cysts has been described (47). Rec-ognizing these malformations usually prompts surgical resection in order to reach a definitive diagnosis and to prevent potential complications

Figure 20. Ruptured aortic aneurysm in a 73-year-old man. Contrast-enhanced CT image shows a rup-tured aneurysm of the aorta (*). There is anterior dis-placement of the crura, effacement of the retrocrural fat, and a hemorrhagic fluid collection within the left pleural space (arrow).

Figure 21. Traumatic aortic injury in a 25-year-old man after blunt thoracic trauma. Contrast-enhanced CT image shows a periaortic hematoma (arrowheads) extending to the RCS. An aortic pseudoaneurysm was identified at the aortic isthmus.


imaging. Infectious aortitis caused by bacterial and mycobacterial infections may have a similar imaging appearance.

Other vascular abnormalities evident within the retrocrural region can involve smaller vessels such as the intercostal arteries and veins as well as the perivertebral venous plexus. Portal hyper-tension, caval occlusion, and other central venous occlusions can alter the normal venous drainage, altering the flow within the azygos-hemiazygos venous system. Obstructive central venous pro-cesses and portal hypertension can manifest as varices of the intercostal veins, ascending lumbar veins, and perivertebral venous plexus (5,55,56). Hypertrophied intercostal arteries traversing the RCS can be seen in patients with chronic pulmo-nary inflammation, long-standing active tuber-culosis, pulmonary thromboembolism, chronic obstructive pulmonary disease, and severe bron-chiectasis (57) (Fig 23).

Compression of the renal artery by the crura has been recognized as an unusual cause for re-nal artery stenosis and renovascular hypertension (58). An atypical high and posterior origin of the left renal artery appearing entrapped within the RCS by the diaphragmatic crus can be adequately recognized with contrast-enhanced helical CT and especially with CT angiography (59) (Fig 24).

Inflammatory Conditions.—Retroperitoneal fibrosis is an uncommon chronic inflamma-tory condition of unknown pathogenesis that is characterized by replacement of the retroperi-toneal tissues, especially fat, with fibrous tissue.

Figure 22. Giant cell aortitis in a 72-year-old woman. Contrast-enhanced CT image shows circumaortic in-flammation (arrow) affecting the proximal descending abdominal aorta at the level of the RCS.

Figure 23. Dilated ascending paravertebral veins and intercostal arteries in a 40-year-old man with chronic pleuropulmonary tuberculosis. Chronic in-flammation results in hyperemia and hypertrophied vessels. (a) Contrast-enhanced CT image of the thorax shows dilated ascending and intercostal veins (arrows). (b) CT image shows abnormal prominence of the intercostal arteries (arrows) bilaterally.

Figure 24. Crural entrapment of the renal artery in a 35-year-old man with atypical high origin of the right renal artery. Axial contrast-enhanced CT image shows entrapment of the right renal artery (arrow) by the right crus. Crural entrapment of renal arteries is an unusual but recognized cause of renal artery stenosis.


uation values of the paraspinal fat have been well demonstrated with CT and have allowed specific diagnosis of this unusual inflammatory condition (64). Other iatrogenic causes for lipomatosis such as that caused by antiretroviral agents in patients with human immunodeficiency virus infection can also trigger increased accumulation of fat within the retrocrural region (65,66) (Fig 25).

Spondylosis deformans is a very common de-generative disease of the spine that manifests as os-teophyte formation along the anterolateral aspect of the vertebral bodies. When large enough, the bone excrescences arising from the distal thoracic ver-tebral bodies often project into the RCS. Thoracic spine osteophytes are more frequently encountered along the right anterolateral aspect, presumably be-cause the pulsations of the descending aorta on the left side inhibit bone production (5) (Fig 26).

Pancreatic pseudocysts are encapsulated collec-tions containing necrotic debris, pancreatic secre-tions, and blood degradation products that origi-nate from the pancreas after pancreatitis. Retro-peritoneal and transhiatal extension of pancreatic inflammation explains the unusual presentation of pancreatic pseudocysts within the retrocrural region and posterior mediastinum. A pancreatic pseudocyst within the RCS can be suspected when there is rapid development of a fluid collection in this region in a patient with the clinical diagnosis of pancreatitis (Fig 27). A history of alcohol abuse, recurrent pancreatitis, and the appropriate labora-tory test results demonstrating increased amylase and lipase levels support the diagnosis (67). CT and MR imaging both clearly demonstrate the cystic nature of the lesion and its communication with an intraabdominal pancreatic pseudocyst.

Although typically affecting the retroperitoneum between the kidneys and the pelvic brim, retro-peritoneal fibrosis can manifest de novo or extend beyond the diaphragmatic crura into the poste-rior mediastinum as fibrosing mediastinitis (60).

As elsewhere, fibrosis within the retrocrural re-gion appears as a mantle or plaque of soft tissue encasing the aorta. At both CT and T1-weighted MR imaging, the plaque enhances variably after intravenous contrast material administration, depending on the activity of the inflammatory process (61). MR imaging demonstrates low sig-nal intensity on T1-weighted images and low to moderate signal intensity (lower than that of fat but higher than that of muscle) on T2-weighted images (62). It has a similar behavior as when present below the kidneys, where there is encase-ment without displacement of adjacent structures like the aorta. Retroperitoneal fibrosis within the RCS can be mistaken for lymphadenopathy.

Steroid-induced retrocrural lipomatosis has been reported (63). The characteristic low atten-

Figure 25. Lipomatosis secondary to antiretroviral therapy in a 47-year-old man with AIDS. CT image shows increased retrocrural fat (arrows).

Figure 27. Intracrural and subphrenic infected pan-creatic pseudocyst in a 38-year-old woman with acute pancreatitis. Contrast-enhanced CT image shows a hy-poattenuating fluid collection in the pancreas (*) that extends into the subphrenic space and fluid within the right crus (arrow).

Figure 26. Spondylosis in a 65-year-old patient. CT image shows a hypertrophic and bridging spinal osteo-phyte (arrow) projecting into the RCS. The bone spur deforms and displaces the crus.


(69,70). However, MR imaging is superior to CT in detecting associated vertebral, intervertebral disk, and epidural involvement. Use of intrave-

nous contrast material significantly increases the sensitivity for detecting spondylodiscitis and paraspinal abscesses. Abscess walls are hyper-vascular and typically enhance after intravenous contrast material administration.

Retrocrural abscesses manifest as single or multiple fluid collections just dorsal to the crura with associated obliteration of the normal retro-crural fat planes (Fig 29). Anterolateral crural displacement and abscess wall enhancement after intravenous contrast material administration can also be identified. The presence of calcification suggests chronicity, such as in more indolent infectious processes like tuberculous infections. Tuberculous spondylitis more frequently involves

Infections.—Infections involving the distal tho-racic and proximal lumbar spine can extend to involve the RCS by extraspinal extension into the prevertebral and paravertebral soft tissues. Paraspinal abscesses are associated with pyogenic spondylitis—particularly due to Staphylococcus—and tuberculous spondylitis (68) (Fig 28). In-fectious spondylitis occurs more frequently in men, with the highest frequency during the sixth decade of life. Diabetes, immunosuppression, intravenous drug abuse, sickle cell anemia, and urinary tract infections are known predisposing factors for infectious spondylitis. The prevalence of tuberculous spondylitis is higher in developing countries and is increasing because of the resur-gence of tuberculosis, especially in patients with AIDS.

Infectious spondylitis may manifest as de-struction of the vertebral body endplates and of the intervertebral disk. Associated compression deformities of the vertebral bodies can also be identified. Hematogenous spread of microorgan-isms into the vertebral bone marrow through perivertebral arterial branches or venous plexuses is the most common source of infection. How-ever, direct inoculation secondary to iatrogenic causes is another possible route for development of infectious spondylitis and paraspinal abscesses. Once the vertebral body is involved, subligamen-tous spread of infection can occur. Extension into the prevertebral soft tissues occurs by discharge through sinus tracts.

CT and especially MR are the imaging mo-dalities of choice for detection of paraspinal abscesses in patients with infectious spondylitis

Figure 28. Paraspinal abscess in a 34-year-old man with AIDS and methicillin-resistant Staphylococcus au-reus infection. Unenhanced CT image shows a paraspi-nal abscess (arrowhead) in the RCS. Arrow = fluid within the left crus.

Figure 29. Retrocrural paraspinal abscess due to M avium-intracellulare infection in a 37-year-old man with AIDS. Sagittal (a) and axial (b) T2-weighted MR images of the proximal lumbar spine show a large para-spinal abscess with high signal intensity (arrow) in the RCS. The abnormal signal intensity of the vertebral bodies and intervertebral disks is consistent with multi-level spondylodiscitis.

TeachingPoint


tients with distal esophageal perforation; however, it is not specific to this condition. CT more read-ily demonstrates the presence of gas within the RCS than does radiography (Fig 30).

Pleural effusion is perhaps the most frequently detected abnormality occupying the RCS. Free pleural fluid first accumulates within the poste-rior pleural recess in patients being imaged in the supine position. As little as 15 mL of pleural fluid is needed to demonstrate pleural fluid occupy-ing the posterior recess at CT or MR imaging. Axial CT or MR imaging demonstrates pleural fluid within the posterior pleural recess posterior and medial to the diaphragmatic crus, displac-ing the crus anteriorly and laterally. This pattern contrasts with that of upper abdominal intraperi-toneal fluid collections, which lie anterolateral to the diaphragmatic crura (74).

Acute traumatic fractures of the distal thoracic and proximal lumbar vertebrae can be associated with paravertebral hematomas that can occupy the RCS. These fractures are usually related to high-energy blunt trauma. Other associated conditions such as anticoagulation or underlying pathologic conditions like ankylosing spondylitis can predispose to vertebral body fractures and development of a paraspinal hematoma. The he-matoma results from tearing of the anterior lon-gitudinal ligament at the fractured vertebra and from hemorrhage produced by disruption of the small anterior vertebral branching arteries and venous plexus.

Retrocrural hematomas can be readily de-tected with CT, since it is commonly used as the imaging modality in the setting of trauma. Be-sides the imaging findings related to the fracture of the adjacent distal thoracic or proximal lumbar vertebra, CT of retrocrural hematomas demon-strates infiltration of the normal retrocrural fat planes with soft-tissue attenuation present behind the crura (5). Associated aortic displacement with bulging and thickening of the diaphragmatic crura can also be present (Fig 31).

ConclusionsDespite the small area that encompasses the RCS, there is a wide variety of pathologic condi-tions within this region, which is located at the posterior confines of the inferior thorax and up-per abdomen. An understanding of the anatomy, the normal variants, and the multiple pathologic conditions affecting the diaphragmatic crura and retrocrural structures facilitates the diagnosis of disease processes within this often overlooked anatomic compartment.

the distal thoracic and upper lumbar vertebrae; therefore, the presence of a retrocrural abscess with associated spondylitis favors the diagnosis of tuberculosis (71).

Other Abnormalities.—Pneumomediastinum can be recognized by the presence of gas within the RCS outlining the aorta. The combination of paraspinal and extrapleural diaphragmatic gas at chest radiography was described by Naclerio (72). The sharply angulated radiolucent shadow that resembles a V and is known as the V sign of pneumomediastinum corresponds to gas that ex-tends between the parietal pleura and the medial portion of the left hemidiaphragm, outlining the descending aorta (72,73). This presentation of pneumomediastinum was initially reported in pa-

Figure 30. Posttraumatic pneumomediastinum in a 45-year-old man. Contrast-enhanced CT image shows air within the RCS (arrowheads) and surrounding the aorta. Subcutaneous emphysema is present within the right chest wall.

Figure 31. Retrocrural hematoma secondary to vertebral and rib fractures in a young male patient. CT image shows a comminuted vertebral body (ar-rowhead) with a paraspinal hematoma (*) in the RCS, along with effacement and displacement of the left crus.


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This article meets the criteria for 1.0 AMA PRA Category 1 CreditTM. To obtain credit, see www.rsna.org/education /rg_cme.html.

RG Volume 28 • Volume 5 • September-October 2008 Restrepo et al

The Diaphragmatic Crura and Retrocrural Space: Normal Imaging Appearance, Variants, and Pathologic Conditions Carlos S. Restrepo, MD, et al

Page 1290 In cross-sectional imaging, the RCS can be defined as a triangular region that represents the most inferior portion of the posterior mediastinum, with no true boundaries from the posteromedial aspect of the thoracic cavity. The space is delimited anteriorly and laterally by the diaphragmatic crura as they pass from the anterolateral aspects of the vertebral bodies and ascend to decussate in front of the aorta. Posteriorly, boundaries include the ventral aspect of the distal thoracic and proximal lumbar vertebral bodies. Page 1295 Lymph nodes dorsal to the diaphragmatic crura drain the posterior mediastinum and diaphragm as well as the lumbar regions (3). Detection of enlarged retrocrural lymph nodes exceeding 6 mm in diameter should be considered suspiciously abnormal (Fig 15). Page 1296 Primary neoplasms that arise in or extend to involve the RCS resemble the imaging presentation of other primary neoplasms of the posterior mediastinum. These can be categorized according to their cell of origin into neurogenic, mesenchymal, germ cell, and lymphoid tumors. Page 1298 Both Hodgkin disease and non-Hodgkin lymphoma can involve retrocrural lymph nodes. Fifty percent of patients with non-Hodgkin lymphoma have positive high para-aortic lymph nodes at presentation compared to 25% with Hodgkin disease (43). Page 1302 CT and especially MR are the imaging modalities of choice for detection of paraspinal abscesses in patients with infectious spondylitis (69,70). However, MR imaging is superior to CT in detecting associated vertebral, intervertebral disk, and epidural involvement.

RadioGraphics 2008; 28:1289–1305 • Published online 10.1148/rg.285075187 • Content Codes:

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