(Radiographics. 2002;22:S61-S78.)
© RSNA, 2002
MEDIASTINUM, CHEST WALL, AND DIAPHRAGM |
Fat-containing Lesions of the Chest1
Scott C. Gaerte, MD,
Cristopher A. Meyer, MD,
Helen T. Winer-Muram, MD,
Robert D. Tarver, MD and
Dewey J. Conces, Jr, MD
1 From the Department of Radiology, Indiana University School of Medicine, Indiana University Hospital, 550 N University Blvd, Indianapolis, IN 46202-5253. Presented as an education exhibit at the 2001 RSNA scientific assembly. Received February 1, 2002; revision requested March 26 and received May 22; accepted June 4. Address correspondence to S.C.G. (e-mail: scgaerte@iupui.edu).
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Abstract
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Although most lesions that occur in the chest have a nonspecific soft-tissue appearance, fat-containing lesions are occasionally encountered at cross-sectional computed tomography (CT) or magnetic resonance imaging. The various fat-containing lesions of the chest include parenchymal and endobronchial lesions such as hamartoma, lipoid pneumonia, and lipoma. Endobronchial hamartoma usually appears at CT as a lesion with a smooth edge, focal collections of fat, or fat collections that alternate with foci of calcification. Mediastinal fat-containing lesions include germ cell neoplasms, thymolipomas, lipomas, and liposarcomas. The most frequent CT manifestation of the germ cell neoplasm teratoma is a heterogeneous mass with soft-tissue, fluid, fat, and calcium attenuation. Cardiac lesions with fat content include lipomatous hypertrophy of the interatrial septum and arrhythmogenic right ventricular dysplasia. Diagnosis of the former is made with CT when a smooth, nonenhancing, well-marginated fat-containing lesion is identified in the interatrial septum. Finally, fat may herniate into the chest at several characteristic locations. When such a lesion is identified, the time required for differential diagnosis is significantly reduced, often allowing a definitive radiologic diagnosis. Sagittal and coronal reformatted images can add valuable information by showing diaphragmatic defects and hernia contents.
Index Terms: Arrhythmogenic right ventricular dysplasia, 51.1959 • Fat, CT, **.121122 • Fat, MR, **.1214 • Hamartoma, 60.314 • Heart, abnormalities • Heart, neoplasms, 51.312, 51.32 • Hernia, 71.152, 795.153, 795.154 • Lipoma and lipomatosis, 51.312, 51.59, 60.319 • Lung, infection, 60.21 • Lung neoplasms, 60.314 • Mediastinum, neoplasms, 67.315 • Pleura, neoplasms, 66.317
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LEARNING OBJECTIVES FOR TEST 3
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After reading this article and taking the test, the reader will be able to:
- Describe the CT and MR imaging features of benign and malignant intrathoracic tumors.
- Narrow the differential diagnosis of various fat-containing lesions solely on the basis of their location within the chest.
- Select CT and MR imaging techniques for optimal evaluation of multiple intrathoracic lesions.
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Introduction
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A majority of lesions in the chest have a nonspecific soft-tissue appearance at imaging, but occasionally, lesions have fat attenuation at computed tomography (CT) or the signal intensity characteristics of fat at magnetic resonance (MR) imaging. By correlating lesion location with fat characteristics, one can significantly reduce the time required for differential diagnosis. In this article, we discuss and illustrate the various imaging characteristics of fat-containing lesions that manifest in endobronchial, parenchymal, mediastinal, cardiac, and pleural locations. We also describe intrathoracic herniations that contain fat.
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Endobronchial Lesions
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Lipoma
Endobronchial lipomas are rare, constituting only 0.1% of all pulmonary tumors and roughly 3.2%–9.5% of all benign endobronchial tumors (1,2). They are well-circumscribed mesenchymal tumors that originate from submucosal or interstitial adipose tissue. These tumors are pliable and pedunculated with a predilection for mainstem bronchi (3).
Common symptoms include a persistent cough (81% of cases), chest pain, dyspnea, recurrent fever and pneumonia, and wheezing (4,5). Hemoptysis is uncommon, owing to the avascular nature of lipomas, but can occur as a result of postobstructive infection (4,6).
Conventional radiographs typically reveal normal findings or postobstructive changes. At CT, the tumor is often a pedunculated homogeneous lesion with attenuation around -100 HU. Bronchoscopic biopsies are frequently nondiagnostic owing to the tumor’s thick fibrous capsule or are misleading if atypical cells are found secondary to chronic inflammation. In such instances, helical CT is extremely valuable in the diagnosis of endobronchial lipomas (3).
Hamartoma
Hamartomas constitute roughly 8% of all lung neoplasms (7), and an endobronchial location is described in 3%–20% of all pulmonary hamartomas (7). These tumors may contain cartilage, fat, fibrous tissue, and epithelial components. Endobronchial hamartomas typically contain more fat tissue than parenchymal ones. Symptoms result from obstruction and include chronic cough, hemoptysis, or fever (8).
Endobronchial hamartomas are often inflamed, which results in nonspecific bronchoscopic findings similar to those of bronchogenic carcinoma (8). Conventional radiography and CT may show postobstructive changes such as atelectasis or pneumonia. Characteristic CT findings include a lesion with a smooth edge, focal collections of fat, or collections of fat alternating with foci of calcification (Fig 1) (9).
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Figure 1a. Endobronchial hamartoma. (a) CT scan (mediastinal window) shows an endobronchial lesion containing fat in the right middle lobe bronchus (arrow). (b) CT scan obtained at the same level (lung window) shows postobstructive atelectasis in the middle lobe. (Case courtesy of H. Page McAdams, MD, Duke University Medical Center, Durham, NC.)
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Figure 1b. Endobronchial hamartoma. (a) CT scan (mediastinal window) shows an endobronchial lesion containing fat in the right middle lobe bronchus (arrow). (b) CT scan obtained at the same level (lung window) shows postobstructive atelectasis in the middle lobe. (Case courtesy of H. Page McAdams, MD, Duke University Medical Center, Durham, NC.)
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Parenchymal Lesions
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Lipoma
Although lipomas are the most common benign neoplasms in humans, lung parenchymal involvement is rare. They are well-circumscribed mesenchymal tumors that originate from adipose tissue. The most common parenchymal location is in the periphery surrounded by normal lung tissue (4). Lipomas have an insidious onset that typically goes unnoticed until a well-delineated abnormality is identified at routine chest radiography (10).
Parenchymal lipomas appear as soft-tissue-opacity lesions on chest radiographs and may be mistaken for pneumonia in patients with a history of fever. At CT, lipomas are well-differentiated lesions with homogeneous fat attenuation (10).
Lipoid Pneumonia
Lipoid pneumonia is an uncommon condition that results from the chronic aspiration of mineral, animal, or vegetable oils into the lungs (11). Once inhaled, the oil is phagocytosed by macrophages that fill the alveoli and distend the alveolar walls, causing acute and chronic pneumonitis (11,12). Over time, the macrophages transport the oil from the alveoli to the interlobular septa, resulting in localized granulomas and pulmonary fibrosis (12,13).
Predisposing factors include neuromuscular disorders and esophageal abnormalities (11). Lipoid pneumonia is most often seen in debilitated adults or in children with anatomic defects such as a cleft palate. Symptoms include progressive dyspnea (14).
Conventional radiography can show airspace consolidation, an irregular masslike lesion, or a reticulonodular pattern (Fig 2) (11). The most common locations for lipoid pneumonia are the dependent portions of the lungs (13). The characteristic CT finding is lung consolidation with fat attenuation. A "crazy-paving" pattern of septal thickening and centrilobular interstitial thickening superimposed on ground-glass attenuation has been described (11). Thin-collimation CT minimizes partial volume effect and improves spatial resolution (11). Traction bronchiectasis and cystic changes consistent with fibrosis can be seen.
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Figure 2a. Lipoid pneumonia. (a) Posteroanterior radiograph shows basilar airspace disease in a reticulonodular pattern. (b) CT scan (lung window) shows bilateral basilar ground-glass attenuation. The crazy-paving pattern of superimposed septal and interstitial centrilobular thickening is not clearly seen on this image. (c) CT scan (mediastinal window) allows confirmation of a low-attenuation consolidation (arrows).
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Figure 2b. Lipoid pneumonia. (a) Posteroanterior radiograph shows basilar airspace disease in a reticulonodular pattern. (b) CT scan (lung window) shows bilateral basilar ground-glass attenuation. The crazy-paving pattern of superimposed septal and interstitial centrilobular thickening is not clearly seen on this image. (c) CT scan (mediastinal window) allows confirmation of a low-attenuation consolidation (arrows).
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Figure 2c. Lipoid pneumonia. (a) Posteroanterior radiograph shows basilar airspace disease in a reticulonodular pattern. (b) CT scan (lung window) shows bilateral basilar ground-glass attenuation. The crazy-paving pattern of superimposed septal and interstitial centrilobular thickening is not clearly seen on this image. (c) CT scan (mediastinal window) allows confirmation of a low-attenuation consolidation (arrows).
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The diagnosis of lipoid pneumonia is often determined from a group of radiographic and clinical features, long-term stability, and the absence of other causes of pulmonary disease (11). Treatment involves discontinuation of the offending agent and radiographic follow-up to ensure stability.
An endogenous form of lipoid pneumonia results from the accumulation of lipid-filled macrophages beyond a bronchial obstruction such as a lung tumor (15). Chest radiographs are nonspecific and show an area of consolidation often distal to a tumor. CT may show an area of increased attenuation spreading distal to the tumor shadow corresponding to the accumulation of foamy macrophages and eosinophilic proteinaceous material (15).
Hamartoma
Hamartomas are the most common benign pulmonary neoplasm. The prevalence of hamartomas at autopsy has been reported at nearly 0.25% (7). Pulmonary hamartomas often originate in the fibrous connective tissue beneath the mucous membranes of the bronchial wall (7,9). Hamartomas may be chondromatous or leiomyomatous, the former being more common (16). They are unencapsulated, lobulated tumors with connective tissue septa (7).
Patients present between the 4th and 7th decades of life, with a male predilection of 2:1 to 3:1 (7). Typically, hamartomas manifest incidentally as solitary nodules in the periphery of the lung. However, they can arise in all parts of the lung, with the hila being the least common locations.
Conventional radiographs show a sharply demarcated pulmonary nodule that may contain popcorn calcifications characteristic of the chondroid calcifications in hamartomas. This pattern is a reliable indicator of a benign lesion but is present in only a minority of cases. At CT, the reported prevalence of calcification in hamartomas varies from 5% to 50% (17). Fat is identified in up to 50% of hamartomas at CT and may be localized or generalized within the nodule (Fig 3) (9,17). Intranodular fat at -40 HU to -120 HU is considered a reliable indicator of a hamartoma, and its presence may help obviate needle aspiration biopsy (17).
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Figure 3a. Parenchymal hamartoma. (a) CT scan shows a solitary pulmonary nodule containing a focus of low attenuation. (b) At further evaluation of the CT scan, the area of low attenuation is identified as fat (-41 HU), a finding that is consistent with a hamartoma.
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Figure 3b. Parenchymal hamartoma. (a) CT scan shows a solitary pulmonary nodule containing a focus of low attenuation. (b) At further evaluation of the CT scan, the area of low attenuation is identified as fat (-41 HU), a finding that is consistent with a hamartoma.
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Mediastinal Lesions
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Lipoma and Lipomatosis
Lipomas are well-circumscribed mesenchymal tumors that originate from adipose tissue (18). They occur predominantly in the anterior mediastinum and are reported to represent 1.6%–2.3% of all primary mediastinal tumors (4). They have an insidious onset that typically goes unnoticed until a well-delineated abnormal shadow on a routine chest radiograph is identified (10). Because of the slow growth of the lesions, the presenting symptoms are often due to mass effect (ie, compression of the primary bronchi, esophagus, phrenic nerve, or vagus nerve). Symptoms can include dysphagia, dyspnea, dry cough, jugular distention, and cardiac arrhythmias (18).
Mediastinal lipomas resemble other soft-tissue masses at chest radiography and are commonly categorized by location as localized mediastinal (usually at the cardiodiaphragmatic angle), cervicomediastinal (extending into the neck), or transmural (penetrating the chest wall, usually in the upper anterior mediastinum) (Fig 4a) (10). At CT, lipomas have homogeneous fat attenuation of approximately -100 HU (Fig 4b). MR imaging can also show the extent and fatty nature of the lesion (Fig 4c, 4d). Simple excision of this well-demarcated tumor can be performed if it is symptomatic.
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Figure 4a. Mediastinal lipoma. (a) Posteroanterior chest radiograph shows a lobulated mass in the right paratracheal region. (b) CT scan shows a well-demarcated fatty mass surrounding the right brachiocephalic artery. (c, d) Coronal T1-weighted MR images (repetition time msec/echo time msec = 451/12) further show the fatty nature of the lesion. (Case courtesy of Charles S. White, MD, University of Maryland, Baltimore.)
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Figure 4b. Mediastinal lipoma. (a) Posteroanterior chest radiograph shows a lobulated mass in the right paratracheal region. (b) CT scan shows a well-demarcated fatty mass surrounding the right brachiocephalic artery. (c, d) Coronal T1-weighted MR images (repetition time msec/echo time msec = 451/12) further show the fatty nature of the lesion. (Case courtesy of Charles S. White, MD, University of Maryland, Baltimore.)
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Figure 4c. Mediastinal lipoma. (a) Posteroanterior chest radiograph shows a lobulated mass in the right paratracheal region. (b) CT scan shows a well-demarcated fatty mass surrounding the right brachiocephalic artery. (c, d) Coronal T1-weighted MR images (repetition time msec/echo time msec = 451/12) further show the fatty nature of the lesion. (Case courtesy of Charles S. White, MD, University of Maryland, Baltimore.)
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Figure 4d. Mediastinal lipoma. (a) Posteroanterior chest radiograph shows a lobulated mass in the right paratracheal region. (b) CT scan shows a well-demarcated fatty mass surrounding the right brachiocephalic artery. (c, d) Coronal T1-weighted MR images (repetition time msec/echo time msec = 451/12) further show the fatty nature of the lesion. (Case courtesy of Charles S. White, MD, University of Maryland, Baltimore.)
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Excessive unencapsulated infiltrative fat deposition is known as mediastinal lipomatosis (Fig 5). Lipomatosis is commonly associated with obesity and exogenous steroid administration.
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Figure 5a. Mediastinal lipomatosis. Initial chest radiographs (not shown) demonstrated upper mediastinal widening with soft-tissue opacity at the level of the anteroposterior window. (a) CT scan (mediastinal window) shows a fatty lesion with mass effect on the superior vena cava and on the azygos vein (arrow). (b) CT scan (lung window) shows pulmonary fibrosis, which prompted the patient’s treatment with steroids.
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Figure 5b. Mediastinal lipomatosis. Initial chest radiographs (not shown) demonstrated upper mediastinal widening with soft-tissue opacity at the level of the anteroposterior window. (a) CT scan (mediastinal window) shows a fatty lesion with mass effect on the superior vena cava and on the azygos vein (arrow). (b) CT scan (lung window) shows pulmonary fibrosis, which prompted the patient’s treatment with steroids.
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Thymolipoma
Thymolipomas are rare, benign, slow-growing tumors of the anterior part of the superior mediastinum that contain an admixture of thymic parenchyma and mature adipose tissue, with the former accounting for 10%–33% (19,20). These tumors represent 2%–9% of all thymic neoplasms and are often large (21). Most patients are asymptomatic, with the lesion detected incidentally. Symptoms such as cough, dyspnea, arrhythmias, and chest pain may occur secondary to the displacement of mediastinal structures. In rare cases, thymolipomas are associated with myasthenia gravis, cystic lymphangioma, aplastic anemia, thyrotoxicosis, or Graves disease, with a mean patient age at presentation of 22 years (22).
These soft, pliable tumors conform to the heart border and may be mistaken for cardiomegaly or diaphragmatic elevation on chest radiographs. A mediastinal mass or mediastinal widening is usually identified (23). A thymolipoma can be misinterpreted as a pleural or pericardial tumor, a pericardial effusion, atelectasis, or sequestration. When detected early, they are small, lobulated soft-tissue lesions anterior to the heart base (22).
At CT, thymolipomas appear as primarily fatty tissue mixed with soft-tissue attenuation that represents thymic tissue. This intermixed thymic tissue often has the appearance of linear whorls. The fat content usually constitutes 50%–85% of the lesion but has been reported to account for as much as 95% of the tumor (23). The sharp borders of the lesion delineate a well-defined capsule, with no invasion of surrounding structures (22). T1-weighted MR imaging shows fat signal intensity mixed with intermediate signal intensity similar to that of skeletal muscle. When the anatomy is clearly delineated, a connection to the superior mediastinum can often be identified (23).
Teratoma and Teratocarcinoma
Teratomas are germ cell neoplasms that contain one or more well-differentiated tissues from the ectoderm, mesoderm, or endoderm. Only 3% of germ cell tumors are mediastinal, with teratomas accounting for 75% of them (24,25). These tumors usually occur in the anterior mediastinum near the thymus but may rarely occur in the posterior mediastinum (24).
Teratomas are spheric, lobulated, encapsulated multicystic neoplasms that typically occur in young patients. Benign lesions have no gender predilection, whereas malignant lesions have a strong male predilection. Patients may be asymptomatic at presentation or may complain of chest pain, dyspnea, cough, upper respiratory complaints, and fever (25).
Conventional radiography often depicts teratomas as soft-tissue-opacity lesions in the anterior mediastinum. Roughly 21% of teratomas demonstrate calcification, whereas 6% have a radiolucency suggestive of fat and only 2% have a visible fat-fluid level (25). CT is the imaging technique of choice. At CT, 89% of teratomas have a well-defined margin that is described as lobulated more often than smooth (25). The most frequent CT manifestation is a heterogeneous mass with soft-tissue, fluid, fat, and calcium attenuation (Fig 6). This combination is identified in 39% of teratomas. Seventy-six percent of all teratomatous lesions contain some fat (25). Teratomas can be distinguished from other mediastinal tumors by their cystic appearance as well as by the frequent inclusion of fluid, fat, calcifications, or teeth (25). Fat-suppressed MR imaging sequences can be used to confirm fat content.
Teratocarcinomas are malignant teratomas that often demonstrate unique characteristics. They are typically more nodular or poorly defined than benign teratomas and can mold to and compress surrounding structures (Fig 7). These malignant forms demonstrate fat less often (40% of cases) than do benign teratomas (90%) and are more likely to appear solid (26). Malignant teratomas can show a thick enhancing capsule after administration of contrast medium (26,27).
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Figure 7. Teratocarcinoma. CT scan shows a lobulated mass with soft-tissue (curved arrow) and fat (straight arrow) attenuation. No calcifications were identified. The irregular margins of the mass with respect to lung parenchyma suggest local invasion, consistent with a malignant germ cell neoplasm.
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Lipoblastoma
Lipoblastomas are rare soft-tissue mesenchymal tumors of embryonic white fat that occur during infancy and early childhood (28). More than 90% of cases are diagnosed in children less than 3 years of age, with nearly 75% occurring before the age of 12 months (29). Lipoblastomas typically occur in the extremities (70% of cases), with the remainder of cases in mediastinal, retroperitoneal, or paravertebral areas (29).
These tumors are divided into two categories. The more common superficial well-defined mass is known simply as a lipoblastoma. The second form is a deep, unencapsulated infiltrative lesion known as lipoblastomatosis (28). As with the other benign mediastinal tumors, symptoms result from local mass effect.
Lipoblastomas are seen as a soft-tissue-opacity mass on conventional radiographs. These fat-containing lesions are characterized by predominant intratumoral stranding that can often be identified on CT scans (Fig 8a). This tumor can often be differentiated from other mediastinal fat-containing tumors on the basis of the patient’s age and clinical history (28). Myxoid liposarcomas may have a similar CT appearance but are relatively uncommon in children (29). If further evaluation is necessary, MR imaging can be performed. The most notable characteristics at MR imaging include intratumoral streaks and whorls that correspond to the tumor’s fibrovascular network (Fig 8b, 8c). As with teratomas, cystic changes are seen. However, lipoblastomas lack the fluid or calcific components often seen in teratomas (28).
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Figure 8a. Lipoblastoma in a 7-year-old patient. (a) CT scan shows a heterogeneous fat-attenuation lesion at the thoracic inlet, a finding that is virtually diagnostic for a lipoblastoma. (b, c) Axial fast spin-echo T2-weighted (5,500/108) MR image (b) and coronal T1-weighted (500/14) MR image (c) show characteristic intratumoral streaks and whorls due to the tumor’s fibrovascular network. (Case courtesy of Mark J. Kransdorf, MD, Mayo Clinic, Jacksonville, Fla.)
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Figure 8b. Lipoblastoma in a 7-year-old patient. (a) CT scan shows a heterogeneous fat-attenuation lesion at the thoracic inlet, a finding that is virtually diagnostic for a lipoblastoma. (b, c) Axial fast spin-echo T2-weighted (5,500/108) MR image (b) and coronal T1-weighted (500/14) MR image (c) show characteristic intratumoral streaks and whorls due to the tumor’s fibrovascular network. (Case courtesy of Mark J. Kransdorf, MD, Mayo Clinic, Jacksonville, Fla.)
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Figure 8c. Lipoblastoma in a 7-year-old patient. (a) CT scan shows a heterogeneous fat-attenuation lesion at the thoracic inlet, a finding that is virtually diagnostic for a lipoblastoma. (b, c) Axial fast spin-echo T2-weighted (5,500/108) MR image (b) and coronal T1-weighted (500/14) MR image (c) show characteristic intratumoral streaks and whorls due to the tumor’s fibrovascular network. (Case courtesy of Mark J. Kransdorf, MD, Mayo Clinic, Jacksonville, Fla.)
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Cardiac Lesions
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Lipoma
Lipomas constitute roughly 8%–12% of all primary tumors of the heart and pericardium (4,30). A majority of cardiac lipomas are extramyocardial and are either subendocardial or subpericardial. Subendocardial tumors are often small and sessile, whereas subpericardial lipomas may be larger, resulting in anginal pain from external compression of the heart (Fig 9) (4). As the tumor develops, various symptoms may arise, depending on the location, mobility, and size of the mass. Symptoms such as congestive heart failure, supraventricular and ventricular arrhythmia, valve obstruction, syncope, and sudden death have all been reported with cardiac lipomas (30).
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Figure 9. Subpericardial lipoma in a 28-year-old man with ventricular tachycardia. CT scan shows a large, fat-attenuation lesion that surrounds and elevates the left anterior descending artery, a finding that is consistent with a subpericardial lipoma.
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Imaging is often diagnostic. At transesophageal echocardiography, a lipoma manifests as an echogenic, well-demarcated mass. CT more accurately shows an oval, nonpedunculated mass with fat attenuation (Fig 10). True lipomas are encapsulated masses of adipose tissue that usually occur in the parietal or visceral pericardium. Cardiac MR imaging can help determine the intramyocardial extent of the tumor and its relation to coronary arteries (30).
Liposarcoma
Primary cardiac liposarcomas are extremely rare, but when they occur, they usually originate from the right side of the heart. These tumors may invade locally, infiltrate the heart, or metastasize to the lungs. They are classified histologically as well-differentiated (lipomalike), myxoid, round cell, or pleomorphic. Local extension is typical for well-differentiated and myxoid liposarcomas. Round cell and pleomorphic tumors show infiltrative growth, early metastasis to the lungs, and frequent local recurrence after surgical resection. Affected patients are often asymptomatic until cardiac constriction develops. Patients then present with chest pain, dyspnea, dysrrhythmia, and signs of congestive heart failure (31).
Conventional radiographs may demonstrate an enlarged cardiac silhouette and left lower lobe atelectasis. CT shows a hypoattenuating lipomatous tumor in the anterior mediastinum that often encircles the heart over time. MR imaging furtherdelineates the extent and fatty nature of the lesion (Fig 11).
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Figure 11a. Well-differentiated cardiac liposarcoma. Coronal T1-weighted (1,068/25) MR image (a) and axial T2-weighted (4,000/140) MR image (b) show a lobulated high-signal-intensity mass along the right border of the heart. (Case courtesy of Mark J. Kransdorf, MD, Mayo Clinic, Jacksonville, Fla.)
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Figure 11b. Well-differentiated cardiac liposarcoma. Coronal T1-weighted (1,068/25) MR image (a) and axial T2-weighted (4,000/140) MR image (b) show a lobulated high-signal-intensity mass along the right border of the heart. (Case courtesy of Mark J. Kransdorf, MD, Mayo Clinic, Jacksonville, Fla.)
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Lipomatous Hypertrophy of the Interatrial Septum
Lipomatous hypertrophy of the interatrial septum (LHIS) is a benign condition characterized by fat accumulation in the interatrial septum (32). At histologic analysis, LHIS manifests as an unencapsulated mass that can contain adipocytes, myocytes, and mild interstitial fibrosis (33). LHIS is a result of adipose cell hyperplasia and has been linked to increasing patient age and obesity. In contrast to lipomas, the adipose tissue in LHIS is unencapsulated and confined to the interatrial septum without an intercavitary or extracardiac component. It may surround adjacent vascular structures but does not demonstrate invasion (32). The lesion is often asymptomatic but is associated with atrial arrhythmias, altered P-wave configurations, recurrent pericardial effusions, and sudden cardiac death (34).
Diagnosis is made with CT when a smooth, nonenhancing, well-marginated fat-containing lesion in the interatrial septum is identified (Fig 12a). The lesion characteristically takes on a dumbbell shape, with relative sparing of the oval fossa (Fig 12b) (35). When fat attenuation is identified at CT, the diagnoses of myxoma, rhabdomyoma, fibroma, fibroelastoma, and mesothelioma can be excluded (32). MR imaging can be used to confirm the composition of the lesion.
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Figure 12a. Lipomatous hypertrophy of the interatrial septum. (a) CT scan shows a smooth, well-marginated fat-containing lesion in the interatrial septum. (b) Axial T1-weighted MR image obtained in a different patient shows the characteristic dumbbell shape of the lesion with sparing of the oval fossa (arrow).
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Figure 12b. Lipomatous hypertrophy of the interatrial septum. (a) CT scan shows a smooth, well-marginated fat-containing lesion in the interatrial septum. (b) Axial T1-weighted MR image obtained in a different patient shows the characteristic dumbbell shape of the lesion with sparing of the oval fossa (arrow).
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Arrhythmogenic Right Ventricular Dysplasia
Arrhythmogenic right ventricular dysplasia (ARVD), or arrhythmogenic right ventricular cardiomyopathy, is a cardiac abnormality characterized by patchy replacement of the right ventricular myocardium by fatty or fibrous tissue (Fig 13) (36). At histologic analysis, the fatty and fibrous replacement tissue is seen to border on and embed strands or sheets of cardiac myocytes (37). The most commonly affected locations include the right ventricular apex, pulmonary infundibulum, and subtricuspid region (38).
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Figure 13a. Arrhythmogenic right ventricular dysplasia in a 24-year-old woman with recurrent ventricular tachycardia. (a) Axial fast spin-echo inversion recovery T2-weighted MR image shows that the apical portion of the right ventricular myocardium is replaced by fat (arrows). The more proximal right ventricular myocardium is normal (arrowheads). (b, c) End-systolic (b) and end-diastolic (c) cine MR images obtained at the same level as a show that the apical right ventricular myocardium is akinetic and fails to thicken normally. (Case courtesy of David A. Lynch, MD, University of Colorado Health Sciences Center, Denver.)
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Figure 13b. Arrhythmogenic right ventricular dysplasia in a 24-year-old woman with recurrent ventricular tachycardia. (a) Axial fast spin-echo inversion recovery T2-weighted MR image shows that the apical portion of the right ventricular myocardium is replaced by fat (arrows). The more proximal right ventricular myocardium is normal (arrowheads). (b, c) End-systolic (b) and end-diastolic (c) cine MR images obtained at the same level as a show that the apical right ventricular myocardium is akinetic and fails to thicken normally. (Case courtesy of David A. Lynch, MD, University of Colorado Health Sciences Center, Denver.)
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Figure 13c. Arrhythmogenic right ventricular dysplasia in a 24-year-old woman with recurrent ventricular tachycardia. (a) Axial fast spin-echo inversion recovery T2-weighted MR image shows that the apical portion of the right ventricular myocardium is replaced by fat (arrows). The more proximal right ventricular myocardium is normal (arrowheads). (b, c) End-systolic (b) and end-diastolic (c) cine MR images obtained at the same level as a show that the apical right ventricular myocardium is akinetic and fails to thicken normally. (Case courtesy of David A. Lynch, MD, University of Colorado Health Sciences Center, Denver.)
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The most common clinical presenting symptom is ventricular arrhythmia (37). This condition is often first detected in adolescents and young adults, and the mean patient age at presentation is 30 years. Males are affected more often by a ratio of 3:1 (39). The presenting symptom may be sudden cardiac death. The left ventricle may also be affected in a minority of cases (biventricular dysplasia) (37).
Echocardiography and angiocardiography are capable of showing focal or global right ventricular enlargement with associated wall motion abnormalities such as hypokinesis, akinesis, and dyskinesis. Echocardiography is a noninvasive examination, but visualization of the right ventricle is often unsatisfactory. Conversely, angiography of the right ventricle is invasive but clearly demonstrates wall motion abnormalities and dyskinetic wall bulges. Unfortunately, similar right ventricular abnormalities can be seen in patients who have had myocardial infarctions or have previously undergone cardiac surgery (38).
Cine MR imaging is a noninvasive and repeatable examination that can show fatty or fibrofatty replacement of the right ventricular myocardium as well as global and focal dilatations and wall motion abnormalities (38). A spin-echo T1-weighted MR image may show increased signal intensity due to fatty infiltration of the right ventricular myocardium. Unfortunately, the areas of fibrotic degeneration are not always distinguishable from normal myocardium. Use of additional MR imaging sequences such as the chemical shift–selective breath-hold cine technique has been shown to improve evaluation of areas of normal-signal-intensity myocardium. The areas of fatty infiltration and fibrosis often will not show systolic thickening and contraction, whereas dysplasia, wall thinning, and systolic and diastolic dysfunction may be visible (36,39).
Recently, major and minor criteria have been described in an attempt to diagnose ARVD more definitively (40). Many facets of this condition are still being explored, and there is ongoing debate as to the best modality for the evaluation of ARVD. Currently, no standard exists to properly evaluate the sensitivity and specificity of MR imaging.
A more benign form of ARVD, termed fatty replacement of the right ventricle, has also been described. Unlike with ARVD, the right ventricular wall is normal in size or thickened and does not show myocyte atrophy or inflammation. Arrhythmias are far less common in this condition (41). The distinction between these two conditions is not always clear, but the currently used histologic distinction is that fatty replacement of the right ventricle is not associated with any fibrous tissue replacement, which is present in patients with ARVD (41). A recent article described the use of cardiac MR imaging to follow up patients with monomorphic right ventricular extrasystoles (mean follow-up, 15 years) (42). In this study, localized adipose replacement of the right ventricle without worsening arrhythmias was seen in eight of 11 patients, and these eight cases were believed to represent focal fatty replacement of the right ventricle.
Adipomatosis cordis, or cor adiposum, is another similar entity often discovered at routine autopsy. In this condition, a continuous layer of fatty tissue covers the ventricles without involvement of the cardiac fibers (37).
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Pleural and Extrapleural Lesions
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Lipoma
Pleural lipomas are benign soft-tissue neoplasms that originate from the submesothelial layers of parietal pleura and extend into the subpleural, pleural, or extrapleural space. They are soft, encapsulated fatty tumors that demonstrate slow growth (4). In addition, lipomas can occasionally arise from the diaphragm. Lipomas of diaphragmatic origin are predominantly found in a posterolateral location and have a 2:1 predilection for the left side.
Most patients with these types of lipomas remain asymptomatic until incidental detection of the lesions at radiography (Fig 14). Symptoms may include nonproductive cough, back pain, exertional dyspnea, or a sensation of heaviness in the chest (4). Pleural and diaphragmatic lipomas appear as soft-tissue lesions on chest radiographs and may become extremely large (Fig 15). At CT, they are homogeneous and demonstrate fat attenuation (approximately -100 HU) (10). If the lesion is near the diaphragm, the differential diagnosis may include hernias and localized eventrations.
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Figure 14a. Pleural lipoma. (a) Posteroanterior radiograph shows a smoothly marginated mass in the right apex. (b, c) CT scans obtained with lung (b) and mediastinal (c) windowing show the mass with fat attenuation, a finding that is consistent with a pleural lipoma.
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Figure 14b. Pleural lipoma. (a) Posteroanterior radiograph shows a smoothly marginated mass in the right apex. (b, c) CT scans obtained with lung (b) and mediastinal (c) windowing show the mass with fat attenuation, a finding that is consistent with a pleural lipoma.
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Figure 14c. Pleural lipoma. (a) Posteroanterior radiograph shows a smoothly marginated mass in the right apex. (b, c) CT scans obtained with lung (b) and mediastinal (c) windowing show the mass with fat attenuation, a finding that is consistent with a pleural lipoma.
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Figure 15a. Pleural lipoma. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show soft-tissue increased opacity in the inferior right pleural space. (c) The low attenuation of the mass on this CT scan helps confirm the presence of fat. On more superior scans (not shown), there was compressive atelectasis of the right lower lobe. (Case courtesy of James Pike, MD, Methodist Hospital, Indianapolis, Ind.)
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Figure 15b. Pleural lipoma. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show soft-tissue increased opacity in the inferior right pleural space. (c) The low attenuation of the mass on this CT scan helps confirm the presence of fat. On more superior scans (not shown), there was compressive atelectasis of the right lower lobe. (Case courtesy of James Pike, MD, Methodist Hospital, Indianapolis, Ind.)
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Figure 15c. Pleural lipoma. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show soft-tissue increased opacity in the inferior right pleural space. (c) The low attenuation of the mass on this CT scan helps confirm the presence of fat. On more superior scans (not shown), there was compressive atelectasis of the right lower lobe. (Case courtesy of James Pike, MD, Methodist Hospital, Indianapolis, Ind.)
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Extrapleural fat is fat outside the parietal pleura in the chest wall. It is a component of the loose connective tissue of the endothoracic fascia and is most abundant along the posterolateral aspects of the fourth through eighth ribs bilaterally (43,44). The resulting soft-tissue shadow that is produced can be confused with pleural thickening at conventional radiography. Pleural plaques may calcify, whereas extrapleural fat does not. When this finding is not present, secondary features are used to differentiate between these two entities. Extrapleural fat is typically bilateral, symmetric, and located along the midlateral chest wall. Pleural plaque is asymmetric, is randomly distributed, and spares the apices and costophrenic angles. Definitive diagnosis is made at CT when the lesion in question shows fat attenuation of approximately -100 HU.
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Diaphragmatic Hernias
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Morgagni Hernia
Hernias through the foramen of Morgagni represent 2%–3% of all diaphragmatic hernias (Fig 16) (45). They are not usually diagnosed until adulthood. The underlying developmental defect allows herniation of abdominal contents betweenthe fibrotendinous elements of the sternal and costal portions of the diaphragm (46). The defect is anterior and retrosternal in location and is usually a right-sided process (90% of cases) (47).
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Figure 16. Cross-sectional drawing depicts the common locations of juxtacaval fat and of Morgagni, Bochdalek, and hiatal hernias. Arrow = juxtacaval fat, green = lumbar trigone (foramen of Bochdalek), pink = esophageal hiatus, yellow = foramen of Morgagni.
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The contained abdominal contents may include, in order of decreasing frequency, the omentum, colon, stomach, liver, and small intestine (45). Morgagni hernia can be associated with trauma, severe effort, and obesity. At routine chest radiography, it usually appears as a rounded mass in the right cardiophrenic angle, adjacent to the anterior portion of the chest wall (Fig 17). The differential diagnosis may include a pericardial fat pad, pericardial cyst, or solid tumor (46). Further evaluation and diagnosis can be performed with CT or MR imaging. Sagittal and coronal reformatted images are often helpful in demonstrating the diaphragmatic defect and identifying the contents of the hernia.
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Figure 17a. Morgagni hernia. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a large round anterior right cardiophrenic mass. (c) CT scan shows a retrosternal hernia that includes the omentum and colon.
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Figure 17b. Morgagni hernia. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a large round anterior right cardiophrenic mass. (c) CT scan shows a retrosternal hernia that includes the omentum and colon.
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Figure 17c. Morgagni hernia. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a large round anterior right cardiophrenic mass. (c) CT scan shows a retrosternal hernia that includes the omentum and colon.
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Bochdalek Hernia
Hernias through the foramen of Bochdalek are developmental defects in the posterior part of the diaphragm. These hernias are usually diagnosed in infants who present with clinical symptoms of pulmonary insufficiency. The herniated contents contain fat and omental tissue in a majority of cases, but other retroperitoneal and intraperitoneal structures can infrequently be involved.
Bochdalek hernias have a reported prevalence of 6% in the adult population (48). Many cases go undiagnosed because affected adults are commonly asymptomatic. Late-manifesting hernias may be due to congenital herniation, trauma, physical exertion, pregnancy, sneezing, or coughing. Left-sided hernias are more common (70%–90% of cases), presumably owing to the protective effects of the liver (48).
On conventional radiographs, the hernia may appear as a lung-base soft-tissue-opacity lesion seen posteriorly on lateral images. CT usually demonstrates fat above the diaphragm and is extremely beneficial in revealing organ entrapment. Coronal and sagittal reformatted images show the defect to best advantage (Fig 18).
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Abstract
LEARNING OBJECTIVES FOR TEST...
