(Radiographics. 1999;19:1161-1176.)
© RSNA, 1999
Navigating the Thoracic Inlet1
Caroline Chiles, MD,
Kirkland W. Davis, MD and
Daniel W. Williams, III, MD
1 From the Department of Radiology, Division of Radiologic Sciences, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1088. Recipient of a Magna Cum Laude award for a scientific exhibit at the 1997 RSNA scientific assembly. Received December 28, 1998; revision requested January 7, 1999 and received January 28; accepted January 28. Address reprint requests to C.C.
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Abstract
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The thoracic inlet is often seen on the "edge of the film" at computed tomography (CT); consequently, lesions affecting this structure are easily overlooked. A vascular abnormality that may be overlooked is venous thrombosis. The CT appearance of jugular vein thrombosis varies with the age of the lesion: In the acute phase, there is often loss of soft-tissue planes surrounding an enlarged, peripherally enhancing thrombus. In the chronic phase, the jugular vein appears as a tubular, nonenhancing "mass" without loss of surrounding fat planes. Intrathoracic goiters typically manifest as well-defined, markedly enhanced inhomogeneous lesions that are continuous with the cervical thyroid gland. Thyroid adenomas are typically round or oval low-attenuating lesions that enhance after contrast material administration. Thyroid carcinomas may manifest as single or multiple, irregularly shaped low-attenuating areas with or without calcification. Primary tracheal malignancies may appear as smooth or irregular, sessile or pedunculated intraluminal filling defects. Tracheomalacia manifests as destruction of the tracheal walls with soft-tissue narrowing of the tracheal lumen, whereas esophageal abnormalities manifest as thickening of the esophageal wall, dilatation of the esophageal lumen, or both. Schwannomas manifest as well-circumscribed lesions with soft-tissue attenuation that enhance after contrast material administration. Neurofibromas tend to have lower attenuation than schwannomas. Lymphangiomas typically have a cystic appearance with near water attenuation. Familiarity with the normal anatomy of the thoracic inlet as well as the CT features of related abnormalities is critical for correct diagnosis and prompt treatment.
Index Terms: Arteriovenous malformations, 94.14 • Esophagus, neoplasms, 71.321 • Head and neck neoplasms, 20.36 Neck, infection, 27.21, 27.24 • Lung neoplasms, 61.3222, 64.3222 • Nerves, neoplasms, 27.364 • Thorax, neoplasms, 32.30, 474.30 • Thyroid, neoplasms, 273.368, 273.373 • Trachea, diseases, 275.1496, 275.373 • Veins, jugular, 907.751
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INTRODUCTION
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Boaters recognize inlets as potentially hazardous areas because of a combination of wind, current, shoals, and other geophysical considerations. The thoracic inlet can also be hazardous and therefore demands the radiologist's close attention. Because the thoracic inlet is often seen on the last image of a computed tomographic (CT) examination of the neck or the first image of a CT examination of the chest, it is at the "edge of the film" at CT. As a result, lesions of the thoracic inlet, like lesions seen at the edge of a chest radiograph, are easily overlooked.
In this article, we review the normal anatomy of the thoracic inlet and discuss and illustrate the CT appearance of a variety of abnormalities involving the structures of the inlet. These abnormalities include vascular anomalies, diseases of the thyroid gland and lymph nodes, neoplasms of the trachea and esophagus, superior sulcus tumors, thoracic outlet syndrome, nerve sheath tumors, neoplasms of the thoracic spine and chest wall, infections of the lower neck and thoracic inlet, fibromatosis colli, lymphangiomas, fatty tumors, and abnormal thoracic configuration.
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NORMAL ANATOMY
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As with many regions of the body, the symmetry of the normal thoracic inlet makes abnormalities easier to recognize. Identification of the muscles of the neck and upper chest as well as the veins and arteries entering and exiting the thorax helps in the detection of enlarged lymph nodes or other masses (Fig 1).
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Figure 1. Normal anatomy of the thoracic inlet. Axial contrast material-enhanced CT scan obtained at the level of the thyroid gland shows the anterior jugular vein (aj), anterior scalene muscle (as), biceps muscle (b), common carotid artery (cc), internal jugular vein (ij), long muscle of the neck (lc), levator muscle of the scapula (ls), middle scalene muscle (ms), greater pectoral muscle (p), sternocleidomastoid muscle (sc), trachea (t), and trapezius muscle (tp).
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In this era of cross-sectional imaging, it is useful to think of the neck in terms of adjacent anatomic spaces separated by fascial layers (1,2). These layers tend to prevent the spread of infection or neoplastic processes from one compartment to the next. The cervical fascia is divided into superficial and deep components. The superficial fascia is composed of the subcutaneous tissues (mostly fat) that encircle the neck. Blood vessels, superficial lymph nodes, and cutaneous nerves all reside within the superficial cervical fascia. Deep to superficial fascia lies the deep cervical fascia, which can itself be divided into three major components: the superficial or investing layer, the middle or visceral layer, and the deep or prevertebral (perivertebral) layer. These three layers are not usually visible at CT or magnetic resonance (MR) imaging, although their locations can be inferred by knowing their relationships to various anatomic structures that are visible at cross-sectional imaging. The superficial layer of the deep cervical fascia extends from the skull base to the thoracic inlet and encircles the neck deep to the superficial fascia. The middle layer of the deep cervical fascia is also attached to the skull base and descends into the mediastinum posteriorly, where it forms the anterior wall of the retropharyngeal space. Anteriorly, this fascia extends from the hyoid bone to the thoracic inlet. The muscular component of this layer encloses the strap muscles, whereas the visceral component surrounds the neck viscera. The deep layer of the deep cervical fascia encircles the prevertebral and paraspinal muscles and also extends from the skull base into the mediastinum. The alar component of this layer forms the posterior and lateral walls of the retropharyngeal space and, along with the prevertebral layer proper, delimits a potential space called the danger space. The prevertebral component encloses the vertebra and surrounding musculature, brachial plexus trunks, phrenic nerve, cervical plexus, and vertebral artery and vein. All three layers of the deep cervical fascia contribute to the formation of the carotid sheath.
These layers of the cervical fascia define a group of contiguous spaces within the neck. At the level of the thoracic inlet, these include the visceral space, anterior and posterior cervical spaces, carotid space, retropharyngeal and danger spaces, and perivertebral space. Important structures within the visceral space include the trachea, esophagus, thyroid and parathyroid glands, recurrent laryngeal nerve, juxtavisceral lymph nodes, and infrahyoid strap muscles. The anterior and posterior cervical spaces are not actually enclosed by their own fascia but are delimited by other fascial compartments. The posterior cervical space lies posterolateral to the carotid space between the sternocleidomastoid and paraspinal muscles. This space contains mostly fat, although the spinal accessory nerve and spinal accessory chain of deep cervical nodes also reside here. The anterior cervical space is a small, fat-filled space located in the anterolateral neck, lateral to the visceral space and anterior to the carotid space. The carotid space extends from the aortic arch to the skull base and is enclosed by the carotid sheath, the tough fascial covering that receives contributions from all three components of the deep cervical fascia. Major structures within the infrahyoid carotid space include the carotid artery, internal jugular vein, vagus nerve, internal jugular lymph nodes, and portions of the sympathetic plexus. The retropharyngeal space extends from the skull base into the upper mediastinum and is located posterior to the visceral compartment, medial to the carotid space, and anterior to the danger and perivertebral spaces. This space contains only fat within the infrahyoid neck. The retropharyngeal space is important primarily in that it can serve as a pathway for the spread of infections or tumors of the neck into the mediastinum. The danger space is located just posterior to the retropharyngeal space and is actually a potential space located between the two components of the deep layer of the deep cervical fascia (ie, the alar portion anteriorly and the prevertebral portion proper posteriorly). The danger space extends from the skull base into the mediastinum and terminates at the level of the diaphragm. This space contains fat and provides another means by which neck infections or tumors can spread into the chest. The danger space cannot be distinguished from the retropharyngeal space at cross-sectional imaging. Finally, the perivertebral space is enclosed by the deep layer of the deep cervical fascia and has a midline location posterior to the retropharyngeal space. The perivertebral space extends from the skull base to approximately the T4 level in the posterior mediastinum, although some anatomists consider this space to extend to the level of the coccyx. The perivertebral space is divided into the anteriorly located prevertebral portion and the posteriorly located paraspinal portion by the lateral fascial attachments to the vertebral transverse processes. Within the prevertebral portion of the perivertebral space lie the prevertebral muscles, scalene muscles, brachial plexus, phrenic nerve, vertebral body, and vertebral artery and vein. Within the paraspinal portion of the perivertebral space reside the paraspinal muscles and the posterior elements of the vertebra.
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VASCULAR STRUCTURES
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Anomalous Vessels
An otherwise normal blood vessel in an anomalous position can mimic a mass or a lymph node on unenhanced images. Familiarity with the most common vascular anomalies helps avoid misinterpretation. An aberrant right subclavian artery occurs in approximately one of every 200 patients. This anomalous vessel arises from the aortic arch or proximal descending aorta distal to the left subclavian artery and passes behind the esophagus as it travels to the right axilla (Fig 2) (3). Dilatation of the vessel due to an aneurysm may produce the appearance of a mass (4,5).
Most venous anomalies can be seen only on images obtained below the level of the thoracic inlet. However, there can be marked asymmetry in the size of the jugular veins (Fig 3). The right jugular vein is usually larger than the left. Reflux into the jugular vein during intravenous administration of contrast material into the brachial veins can also cause dilatation and asymmetry of the jugular veins.
Jugular Vein Thrombosis
Patients with jugular vein thrombosis typically have a history of central line placement, drug abuse, or cancer. Clinical findings vary according to the age of the thrombosis. In the acute phase, the neck is tender and swollen, and abscess is often suspected. In the chronic phase, a nontender, palpable mass is present in the region of the carotid sheath. Imaging findings also vary with the age of the lesion (6,7). In the acute setting, there is often loss of soft-tissue planes surrounding an enlarged, peripherally enhancing thrombus within the internal jugular vein (Fig 4). This peripheral enhancement may represent the vasa vasorum in the wall of the vein. MR imaging demonstrates loss of normal flow void within the vessel and development of high signal in the subacute phase because of the paramagnetic effect of methemoglobin (8). In the chronic phase, the jugular vein appears as a tubular, nonenhancing "mass" without loss of surrounding fat planes (Fig 5). Another common finding in acute internal jugular venous thrombosis is edema in the retropharyngeal space (9).
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Figures 4, 5. Venous thrombosis. (4) Axial contrast-enhanced CT scan shows a filling defect (arrow), a finding that is consistent with thrombosis of the left internal jugular vein. (5) T1-weighted spin-echo MR image shows increased signal intensity in a thrombosed left subclavian vein (arrow).
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Figures 4, 5. Venous thrombosis. (4) Axial contrast-enhanced CT scan shows a filling defect (arrow), a finding that is consistent with thrombosis of the left internal jugular vein. (5) T1-weighted spin-echo MR image shows increased signal intensity in a thrombosed left subclavian vein (arrow).
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THYROID GLAND
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The thyroid gland is located at the level of the cricothyroid ligament with its isthmus anterior to the trachea. Abnormalities of the thyroid gland are common. Thyroid nodules are reported as incidental findings in 4%–8% of adults (8). Although most of these are benign adenomas, follicular nodules, or cysts, malignancy is always a consideration (10,11). The risk of malignancy is higher in patients who are under 20 or over 60 years of age, have a history of radiation therapy to the neck or upper chest, or have a family history of thyroid cancer or multiple endocrine neoplasia syndrome. The nodules are also more likely to be malignant if they arise within a long-standing goiter (8,9). Approximately 12,000 new cases of thyroid cancer are detected each year in the United States. Metastases to the thyroid gland are less common but may originate from primary neoplasms in the lung, breast, or kidney or from melanoma (10,12,13).
At unenhanced CT, the normal thyroid gland has attenuation values of 70–120 HU (14). The appearance of an intrathoracic goiter is usually characterized by continuity with the cervical thyroid gland, marked enhancement after intravenous administration of contrast material, well-defined margins, inhomogeneity, and, often, focal calcifications (15).
Multinodular goiter causes enlargement of the thyroid gland, and multiple low-attenuating foci may be visible within the gland (Fig 6). A multinodular goiter may compress adjacent structures including the trachea (Fig 7). Varying degrees of focal or global enlargement of the gland, often with heterogeneous attenuation, are seen with autoimmune, inflammatory, and infectious diseases of the thyroid gland. Thyroid adenomas are typically round or oval and have lower attenuation than the rest of the gland at unenhanced CT. However, adenomas do enhance after intravenous administration of contrast material.
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Figures 6, 7. (6) Multinodular goiter. Axial contrast-enhanced CT scan demonstrates multiple low-attenuating nodules in an enlarged gland. (7) Goiter. Axial contrast-enhanced CT scan demonstrates tracheal stenosis due to external compression.
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Figures 6, 7. (6) Multinodular goiter. Axial contrast-enhanced CT scan demonstrates multiple low-attenuating nodules in an enlarged gland. (7) Goiter. Axial contrast-enhanced CT scan demonstrates tracheal stenosis due to external compression.
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Carcinomas of the thyroid gland include (in order of worsening prognosis) papillary, follicular, medullary, and anaplastic cell types (10). Thyroid carcinomas may manifest at CT as single or multiple irregularly shaped areas with or without calcification and with lower attenuation than the remainder of the gland (16,17) (Fig 8). These characteristic CT findings are not reliable for distinguishing between benign and malignant lesions. However, ancillary findings that support the diagnosis of an aggressive lesion may be present. These include lymphadenopathy, destruction of adjacent structures, loss of fat planes, and distant metastases (18,19).
Commonly encountered developmental anomalies of the thyroid gland include hemiagenesis and formation of a pyramidal lobe (Figs 9, 10).
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Figures 9, 10. (9) Hemiagenesis of the thyroid gland. Axial contrast-enhanced CT scan demonstrates absence of the left lobe, which is a typical finding in hemiagenesis. (10) Pyramidal lobe. Axial contrast-enhanced CT scan shows persistence of the distal portion of the thyroglossal duct. This condition is present in 50% of the population. P = pyramidal lobe.
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Figures 9, 10. (9) Hemiagenesis of the thyroid gland. Axial contrast-enhanced CT scan demonstrates absence of the left lobe, which is a typical finding in hemiagenesis. (10) Pyramidal lobe. Axial contrast-enhanced CT scan shows persistence of the distal portion of the thyroglossal duct. This condition is present in 50% of the population. P = pyramidal lobe.
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LYMPH NODES
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Lymph nodes that are visible on images through the thoracic inlet include deep cervical nodes along the course of the internal jugular vein, scalene nodes, supraclavicular nodes, and the highest mediastinal nodes within the superior mediastinum (Fig 11). Nodes greater than 5–7 mm in short-axis diameter are considered enlarged. The left supraclavicular lymph node, also known as the Virchow node, is named for the 19th century pathologist who associated enlargement of this node with gastric carcinoma. The highest mediastinal nodes lie above the plane of the left brachiocephalic vein and are classified as station 1 nodes according to the Regional Lymph Node Classification for Lung Cancer Staging adopted by the American Joint Committee on Cancer (20).
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TRACHEA
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The normal trachea has a coronal diameter of 13–25 mm in men and 10–21 mm in women and a sagittal diameter of 13–27 mm in men and 10–23 mm in women (21). Deviation from these normal dimensions can occur as a result of primary lesions of the trachea, extrinsic compression of the trachea by adjacent blood vessels, or masses of the neck and mediastinum. The diagnosis of "saber-sheath trachea," which by definition occurs when the coronal diameter measures less than 60% of the sagittal diameter, is associated with chronic obstructive pulmonary disease in 95% of cases (22).
The most common primary malignancy of the trachea is squamous cell carcinoma, which accounts for more than 50% of all primary tracheal malignancies (Fig 12); approximately 40% are adenoid cystic carcinomas and less than 10% are adenocarcinomas. These lesions may appear at CT as smooth or irregular, sessile or pedunculated intraluminal filling defects. Adenoid cystic carcinoma has also been described as resembling an iceberg in that the intraluminal component of the tumor is much smaller than the extraluminal component. The trachea may be directly invaded by local malignancies arising in the adjacent portions of the lung, thyroid gland, esophagus, and larynx. Metastases to the trachea may originate from primary malignancies in the colon, kidney, breast, and thyroid gland, as well as from melanoma and lymphoma (23–27).
Tracheomalacia may occur as either a congenital lesion (chondromalacia) or an abnormality acquired after prolonged intubation (28,29). As Figure 13 illustrates, tracheomalacia appears at CT as destruction of the tracheal walls with soft-tissue narrowing of the tracheal lumen. Stenosis of the trachea also occurs after prolonged intubation or other trauma.
Tracheal dilatation may be a sign of tracheobronchomegaly (Mounier-Kuhn syndrome) but also occurs in patients with severe pulmonary fibrosis (30,31).
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ESOPHAGUS
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At CT, the cervical esophagus can be seen extending between the hypopharynx and the thoracic inlet. It is usually flattened between the trachea and spine at this level and often indents the posterior trachea. At the thoracic inlet, the esophagus typically lies to the left of midline (32).
CT depicts abnormalities of the esophagus as thickening of the esophageal wall, dilatation of the esophageal lumen, or both (Fig 14) (33,34). Thickening of the esophageal wall may be due to either inflammatory or neoplastic processes. Inflammation and infection of the esophagus may produce esophagitis and esophageal wall thickening. Infections of the esophagus include Candida esophagitis, which produces plaques and small nodules throughout the esophagus in the immunosuppressed patient. Herpes, cytomegalovirus, and human immunodeficiency virus infections may produce ulcers or erosions in the esophagus (Fig 15) (35–37). Corrosive esophagitis causes ulcerations of the esophageal wall in early stages with stricture formations as a later sequela. A dilated upper esophagus should prompt a search for an obstructive distal lesion. Reflux esophagitis is usually confined to the lower and, occasionally, the middle esophagus (38).
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Figure 15. Esophagitis. Axial contrast-enhanced CT scan demonstrates thickening and irregularity of the esophageal wall due to Cytomegalovirus esophagitis. Flecks of contrast material are also seen.
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At least 90% of primary esophageal malignancies are squamous cell carcinomas, and the prognosis is poor for patients with these malignancies. A 5-year survival rate of 0% has been reported for patients with lesions in the cervical esophagus (39–41). The poor prognosis is attributable to the fact that esophageal carcinomas usually reach an advanced stage before symptoms develop (42). Predisposing factors include a history of smoking, heavy alcohol use, achalasia, radiation exposure, tylosis, and chronic inflammation. Adenocarcinoma is much less common than squamous cell carcinoma and is usually seen in the distal half of the esophagus in association with Barrett esophagus (43). Secondary tumor involvement of the esophagus commonly occurs as local extension from thyroid and laryngeal carcinoma (44). Hematogenous metastases to the esophagus are less frequent but may occur in patients with breast carcinoma, melanoma, or carcinoma of the gastrointestinal tract (45).
A primary esophageal carcinoma may be seen as asymmetric, irregular, circumferential, or focal thickening of the esophageal wall with luminal narrowing or less commonly as a focal mass (33,46). CT criteria for staging of esophageal carcinoma include the size of the tumor, the thickness of the esophageal wall, the degree of invasion of adjacent structures, and the presence of regional lymphadenopathy (47). CT may also demonstrate some distant metastases.
Other lesions that may be seen at the level of the thoracic inlet are duplication cysts and Zenker diverticula, which may be filled with gas, fluid, or solid material (44).
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SUPERIOR SULCUS
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A lung cancer originating in the apex of the lung may extend through the visceral and parietal pleura to involve the sympathetic nerve trunks including the stellate ganglion, thereby producing the symptom complex of a Pancoast tumor. Tumors that invade the chest wall are staged as T3 tumors unless they invade the vertebral body or extend into the neural foramen, in which case they are classified as T4 tumors.
Pancoast (48) described the characteristic signs and symptoms associated with these tumors, including pain in the shoulder or arm (along the distribution of the eighth cervical trunk and first and second thoracic nerve trunks), weakness and atrophy of the muscles of the hand, Horner syndrome (ipsilateral ptosis, miosis, and anhidrosis), and bone destruction. A lesion that does not invade the brachial plexus may be more accurately described as a superior sulcus tumor. However, some disagreement exists as to what constitutes the superior sulcus of the lung. It has been described as the "costovertebral gutter whose superior limit is the first rib arch" (49). Others are of the opinion that no such groove exists in the lung apex (50).
At posteroanterior and lateral chest radiography, a superior sulcus tumor may manifest as an apical cap or thickening, an apical mass, or bone destruction (Fig 16) (51). MR imaging is preferable to CT in the evaluation of suspected Pancoast tumor because it can provide coronal and sagittal images of the tumor and can better demonstrate the relationship of the tumor to the chest wall, brachial plexus, and cervical and thoracic vertebrae (Fig 17) (52,53).
An aggressive approach to the treatment of Pancoast tumors involves preoperative radiation therapy followed by en bloc resection of the tumor and the involved portion of the chest wall (Fig 18) (54,55). Resection of the tumor is generally achieved with either lobectomy or segmentectomy; removal of the entire tumor may require resection of the involved paravertebral sympathetic chain, stellate ganglion, lower trunks of the brachial plexus, and portions of the thoracic vertebrae (56).
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Figure 18a. Pancoast tumor. (a) Axial contrast-enhanced preoperative CT scan shows invasion of the left first (arrow) and second ribs by a superior sulcus tumor. (b) Axial contrast-enhanced CT scan obtained after surgery and external radiation therapy shows surgical clips at the site of en bloc resection of the chest wall (arrow).
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Figure 18b. Pancoast tumor. (a) Axial contrast-enhanced preoperative CT scan shows invasion of the left first (arrow) and second ribs by a superior sulcus tumor. (b) Axial contrast-enhanced CT scan obtained after surgery and external radiation therapy shows surgical clips at the site of en bloc resection of the chest wall (arrow).
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BRACHIAL PLEXUS
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The brachial plexus nerve roots pass through the scalene triangle formed by the anterior and middle scalene muscles and the first ribs. The nerve roots bind to the subclavian artery within the triangle to form a neurovascular bundle. This bundle travels beneath the clavicle, posterior to the pectoralis minor muscle, and into the axilla (57). Because the brachial plexus travels with the subclavian artery, the artery can be used as a landmark at MR imaging (Fig 19).
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Figure 19a. Tumor involvement of the brachial plexus. (a) Coronal T1-weighted spin-echo MR image of the brachial plexus shows slightly thickened upper, middle, and lower trunks (arrowhead) lying cephalad to the subclavian artery (A) and vein (V). (b) Coronal contrast-enhanced fat-suppressed MR image shows enhancement and thickening (arrows), findings that suggest extensive tumor involvement of the brachial plexus by a Pancoast tumor.
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Figure 19b. Tumor involvement of the brachial plexus. (a) Coronal T1-weighted spin-echo MR image of the brachial plexus shows slightly thickened upper, middle, and lower trunks (arrowhead) lying cephalad to the subclavian artery (A) and vein (V). (b) Coronal contrast-enhanced fat-suppressed MR image shows enhancement and thickening (arrows), findings that suggest extensive tumor involvement of the brachial plexus by a Pancoast tumor.
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Thoracic outlet syndrome refers to symptoms of compression of the brachial plexus and the subclavian artery and vein in the region of the thoracic inlet (or outlet). Thoracic outlet syndrome may often be managed conservatively with patient education and behavior modification. Surgical decompression is reserved for patients who do not respond satisfactorily to conservative treatment (58).
Motorcycle accidents are the most common cause of traction injuries of the brachial plexus. Surgical management and prognosis depend on accurate diagnosis of the location and extent of nerve rootlet avulsion (59). Criteria for the diagnosis of root avulsion at both CT myelography and MR imaging are the absence of one rootlet (partial avulsion) or both rootlets (complete avulsion) on the injured side, with or without an accompanying posttraumatic meningocele (Fig 20). The nerve rootlets should be visible on the intact side to ensure that the images are technically adequate.
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Figure 20a. Posttraumatic meningocele in a patient who had sustained a traction injury to the left arm and presented with weakness of the arm. (a) Posteroanterior chest radiograph shows a convexity at the left lung apex (arrow). (b) Axial CT scan obtained after intrathecal instillation of metrizamide demonstrates a posttraumatic meningocele extending through the left neural foramen (arrow).
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Figure 20b. Posttraumatic meningocele in a patient who had sustained a traction injury to the left arm and presented with weakness of the arm. (a) Posteroanterior chest radiograph shows a convexity at the left lung apex (arrow). (b) Axial CT scan obtained after intrathecal instillation of metrizamide demonstrates a posttraumatic meningocele extending through the left neural foramen (arrow).
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NERVE SHEATHS
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The most common nerve sheath tumors are schwannomas and neurofibromas. These two entities can be differentiated histologically but may have similar imaging appearances (60). Schwannomas and neurofibromas are frequently solitary, although they may also be associated with neurofibromatosis. In patients with neurofibromatosis, these lesions are often multiple. Schwannomas most commonly involve the sympathetic plexus but can also involve cervical spinal roots and lower cranial nerves, especially the vagus. Schwannomas are well circumscribed, have soft-tissue attenuation or signal intensity at CT or MR imaging, and enhance after intravenous administration of contrast material (61–63). The imaging characteristics of vascular schwannomas may be similar to those of paragangliomas.
Neurofibromas are also well-circumscribed tumors of peripheral nerves and may appear identical to schwannomas at MR imaging (62). At CT, neurofibromas tend to have lower attenuation than schwannomas, although there is considerable overlap (61,63). Plexiform neurofibromas, which are more infiltrative than the more typical well-circumscribed neurofibromas, are virtually diagnostic for neurofibromatosis (Fig 21).
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Figure 21. Neurofibromas in a 38-year-old woman with neurofibromatosis. Axial contrast-enhanced CT scan demonstrates multiple nonenhancing masses (arrows), predominantly within the perivertebral space.
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THORACIC SPINE AND CHEST WALL
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Secondary malignant involvement of the axial skeleton, including the ribs and thoracic spine, can occur by direct extension of a primary tumor, by hematogenous dissemination, or by subarachnoid spread. Metastases to bone typically have a mixed pattern of blastic and lytic lesions, usually from primary tumors of the breast and lung (Figs 22, 23). Blastic metastases are likely to originate from carcinoma of the prostate gland, whereas purely lytic lesions are more likely to come from tumors of the thyroid gland or kidney. After radiation therapy to the mediastinum or chest wall, the sternum may appear sclerotic. This appearance can be misinterpreted as possible metastases, particularly in patients with breast carcinoma.
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Figure 23. Metastatic disease to the axial skeleton. Sagittal T1-weighted spin-echo MR image reveals a soft-tissue mass (M) resulting from metastatic involvement of the manubrium and sternum. At CT, this mass was seen as a lytic lesion.
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In the spine, metastases may first be seen in the vertebral body with or without involvement of the pedicles. Rib lesions are more easily recognized when the rib is expanded. Recognition of a purely sclerotic lesion may require that CT scans be viewed at bone window settings.
Local invasion of the chest wall and thoracic spine may also occur in patients with bronchogenic carcinoma. If there is invasion of the chest wall, the tumor is classified as a T3 lesion. Surgical resection may be considered in the absence of distant metastases.
Soft-tissue tumors of the chest wall include lipomas and, less commonly, liposarcomas. Desmoid tumors and fibrosarcomas may also be encountered (Figs 24, 25). Although a lipoma can easily be diagnosed on the basis of its purely fatty attenuation, the diagnosis of other chest wall masses typically requires biopsy.
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Figures 24, 25. (24) Desmoid tumor. Axial contrast-enhanced CT scan depicts a soft-tissue mass in the right supraclavicular fossa (M). (25) Extraosseous sarcoma. Axial contrast-enhanced CT scan shows an inhomogeneous mass in the left trapezius muscle.
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Figures 24, 25. (24) Desmoid tumor. Axial contrast-enhanced CT scan depicts a soft-tissue mass in the right supraclavicular fossa (M). (25) Extraosseous sarcoma. Axial contrast-enhanced CT scan shows an inhomogeneous mass in the left trapezius muscle.
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MISCELLANEOUS ABNORMALITIES
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Infections of the Lower Neck and Thoracic Inlet
Infections of the lower neck and thoracic inlet may arise spontaneously or may be associated with recent surgery, trauma, or even cervical spine diskitis or osteomyelitis (64). Cellulitis, deep neck abscesses, and reactive or suppurative adenopathy are all manifestations of infections that involve this region of the body. The imaging features of cellulitis typically include reticulation within the subcutaneous fat, loss of deep fat planes, and thickening of the overlying skin or adjacent muscles. Deep neck abscesses are typically water-density masses that demonstrate peripheral contrast enhancement (64,65). Gas or air-fluid levels may occur within abscesses, especially if there is communication with the aerodigestive tract. Retropharyngeal space abscesses, often secondary to tonsillitis or pharyngitis, are particularly dangerous because they have a tendency to extend into the mediastinum and produce mediastinitis (Fig 26). Cervical adenopathy frequently accompanies upper respiratory tract infections but may also be seen in other conditions such as mononucleosis, tuberculosis (scrofula), cat scratch fever, and acquired immunodeficiency syndrome (66). Reactive nodes tend to be homogeneously enlarged, whereas suppurative nodes have low-attenuating centers with peripheral enhancement and can sometimes mimic an abscess (64).
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Figure 26. Retropharyngeal space abscess in a 42-year-old man with Down syndrome, sore throat, and dyspnea. Axial contrast-enhanced CT scan shows a large fluid collection in the retropharyngeal space (A) with peripheral enhancement and an air-fluid level.
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Deep neck infections tend to be confined to one of the fascial compartments within the neck (67,68). These infections frequently require surgical intervention as well as antibiotic therapy. Recurrent lower neck infections should prompt a search for a third or fourth branchial sinus tract. Complications of deep neck infections include carotid artery rupture and airway compromise.
Fibromatosis Colli
Fibromatosis colli is enlargement of the sternocleidomastoid muscle and is typically seen as a unilateral neck mass in a neonate, usually at about 2 weeks of age. The mass is right-sided in 75% of cases. Patient history often includes a difficult delivery, and the mass may enlarge further after its discovery (69). Ultrasonography (US) is the preferred imaging technique for diagnosis. CT may be helpful, especially when other diagnoses are a major consideration. At CT, fibromatosis colli manifests as isoattenuating enlargement of the sternocleidomastoid muscle with no definable mass (Fig 27). At US, an ill-defined isoechoic or slightly hypoechoic mass is seen enlarging the sternocleidomastoid muscle.
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Figure 27. Fibromatosis colli of infancy in a 2-month-old male infant with a right-sided neck mass. Axial contrast-enhanced CT scan demonstrates an enlarged right sternocleidomastoid muscle with normal attenuation (S).
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Lymphangiomas
Lymphangioma (cystic hygroma) is a rare, benign lesion of lymphatic origin that is typically discovered as a mass in the neck or axilla in children under 2 years of age. In adults, lymphangioma is more often found in the mediastinum in asymptomatic patients but may also occur as the sequela of a lymphangioma that was incompletely resected in childhood. The smoothly marginated mass may be located in the superior mediastinum and thoracic inlet or within the anterior, middle, or posterior mediastinum. At CT, lymphangiomas typically have a cystic appearance with uniform attenuation equal to or slightly higher than that of water (Fig 28) (70). Inhomogeneous attenuation or attenuation closer to that of muscle is sometimes seen.
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Figure 28. Lymphangioma in an adult patient. Axial contrast-enhanced CT scan demonstrates a smoothly marginated, low-attenuating neck mass representing a lymphangioma (cystic hygroma) (L). This lesion is more commonly detected in infancy.
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Fat And Fatty Tumors
Lipomas of the extracranial head and neck may be found within any of the fascially defined compartments, especially the posterior cervical space. Lipomas are composed of mature fat cells and are surrounded by a thin fibrous capsule. Lipomas are well-defined, nonenhancing masses that have the same MR imaging appearance as subcutaneous fat with all sequences (Fig 29). Nearly all lipomas are benign, although liposarcomas rarely occur.
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Abstract
INTRODUCTION
