Plain-Film and CT Evaluation of the Adult Mediastinum and Hilum: Pitfalls vs. Disease

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Introduction

Knowledge of the normal mediastinal anatomy and its variants is indispensable for proper interpretation of chest radiology. Mediastinal masses may be found inci- dentally in asymptomatic individuals, but they can also be symptomatic, depending on their size and location.

Cross-sectional imaging with computed tomography (CT) and magnetic resonance (MR) imaging plays an important role in the nonsurgical evaluation of mediasti- nal lesions.

In this article, we review the anatomy and normal vari- ants of mediastinal structures as well as the radiographic, CT, and MR appearances of some of the most common mediastinal diseases, with emphasis on those features that permit a focused differential diagnosis and help di- rect management.

Imaging Strategies in Mediastinal Diseases Conventional Chest Radiography

Many mediastinal masses are incidentally discovered on routine chest examinations obtained for other reasons.

Chest radiography is the first imaging modality used in those asymptomatic patients or when a mediastinal mass is suspected. The key to the chest radiograph is to deter- mine whether the lesion is actually within the medi- astinum or lung and to make an educated guess as to where the lesion is within the mediastinum. A lesion con- fined to the posterior mediastinum (paravertebral region) may be better evaluated by MR.

Computed Tomography

Computed tomography is currently the gold standard for the detection of mediastinal pathology. Initial nonen- hanced contiguous slices should be obtained to demon- strate calcifications and hemorrhage. The injection of in- travenous contrast will define the enhancement patterns of the lesions as well as their relationship with adjacent vascular and mediastinal structures.

Magnetic Resonance Imaging

Magnetic resonance has a complementary role in the evaluation of mediastinal masses. It is mainly used: (a) to solve unanswered questions after CT scan, (b) to assess mediastinal masses in patients allergic to iodinated con- trast material, and (c) to better assess the relationship of the mediastinal mass with adjacent structures, e.g., the pericardium, heart cavities, spinal canal, and vessels. For this last reason, MR is usually preferred to CT for the as- sessment of masses of suspected neurogenic origin.

Another indication for mediastinal MR is if the mediasti- nal mass is suspected to be a cyst (thymic, pericardial, foregut duplication, neuroenteric) and this suspicion can- not be confirmed by CT. With MR subtraction imaging (post contrast image minus pre-contrast image) only en- hanced structures are shown. Cysts should be invisible on these images.

Anatomic and Normal Variants Mimicking Mediastinal Pathology

Thymus

The thymus has a wide variation in appearances on cross- sectional imaging and its size and morphology are di- rectly related with age [1]. Whereas relatively large in the neonate and young infant, after puberty, there is a grad- ual reduction of the thymus size due to a progressive re- placement of the atrophied thymic follicles by adipose tissue. However, there is a broad variation in this involu- tion and significant residual thymic tissue may be present in individuals over the age of 30.

Superior Pericardial Recess

The superior pericardial recess is a semicircular space sur- rounding the ascending aorta. The most superior extent of this recess is at the level of the innominate artery. On rare occasions, pericardial effusion within the superior peri- cardial recess can simulate a cystic mediastinal mass [2].

IDKD 2007

Plain-Film and CT Evaluation of the Adult Mediastinum and Hilum:

Pitfalls vs. Disease

S. Bhalla

¹

, J. Cáceres

²

1

Mallinckrodt Institute of Radiology, Washington University School of Medicine, Barnes-Jewish Hospital, St. Louis, MO, USA

2

Department of Diagnostic Radiology, Hospital Universitari Vall d’Hebron, Universitat Autonoma de Barcelona, Barcelona, Spain

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Lymph Nodes

The maximal diameter of normal lymph nodes ranges from 6 to 14 mm. However, the short axis of mediastinal nodes has been advocated as the most predictable mea- sure of nodal enlargement; 1.0 cm is considered the up- per limit of normal [3]. When mediastinal fat is scanty, the collapsed esophagus may be confused with a lymph- node mass on CT and MR.

Left Pulmonary Artery

On rare occasion, the main and left pulmonary arteries are positioned at an unusually high level with respect to the aortic arch, thereby mimicking a mediastinal mass on CT or plain-chest radiographs [4].

Pericardial Fat Pad

An enlarged pericardial fat pad, usually associated with obesity, exogenous steroid therapy, and Cushing syndrome, may mimic a cardiophrenic angle mediastinal mass.

Mediastinal Lipomatosis

An excess of mediastinal fat, which is a benign condi- tion usually seen in obese patients or in patients under exogenous steroid use, results in significant mediastinal widening. Patients are asymptomatic and do not require therapy.

Classification of Mediastinal Masses

Classification of a mediastinal mass on CT and MR is based on two main features: location and attenuation/sig- nal. Although sometimes artificial, the mediastinum can be classified into three compartments: (a) anterior, com- prising the structures and tissue situated in front of a line drawn along the anterior border of the trachea and the posterior border of the heart; (b) middle, located be- tween this line and the anterior aspect of the vertebral bodies; and (c) paravertebral, when a mass is situated predominantly in the potential space adjacent to a verte- bral body [5].

The attenuations/signals of fat and fluid as well as high attenuation/signal (higher than muscle) may be of diagnostic help. A fat attenuation/signal may be seen in thymolipomas, germ-cell tumors, lipomatosis, fat pads, liposarcomas, extramedullary hematopoiesis, and esophageal fibrovascular polyps. A fluid attenuation/sig- nal may be seen with foregut duplication cysts, germ-cell tumors, thymomas, lymphomas, necrotic tumors, lym- phangiomas, pericardial cysts, and thymic cysts. High- attenuation/signal lesions include hemangiomas, Castleman’s disease, goiter, paraganglioma, vascular ab- normalities, and vascular metastases.

Anterior Mediastinum

The anterior mediastinum is the site of origin of the ma- jority of mediastinal masses, including thymoma and oth- er thymic disorders (cysts), lymphangioma, germ-cell neoplasms (mature teratomas and malignant germ-cell tu- mors), Hodgkin’s disease and non-Hodgkin’s lymphoma, and mesothelial cyst (pericardial) [5-8]. Residents and medical students sometimes like to use the mnemonic

‘3T’s’ to help with remembering thymoma, teratoma, and

‘terrible’ lymphoma. Though this can be helpful, it can al- so be misleading, as it allows one to forget the other thymic lesions that may be present in the anterior medi- astinum (cysts, carcinoid, carcinoma) and that not all an- terior germ-cell tumors are benign.

Thymoma

This is by far the most common primary tumor of the an- terior mediastinum. Men and women are equally affected and most of the lesions occur in patients >40 years.

Thymomas are rare in children and adolescents.

Myasthenia gravis occurs in approximately 30-50% of pa- tients with this tumor. Less commonly, patients may pre- sent with other parathymic syndromes, such as pure red cell aplasia or hypogammaglobulinemia. Radiologically, thymomas commonly manifest as well-defined, lobulated, homogeneous, or heterogeneous mediastinal masses usu- ally located anterior to the aortic root. Punctate or curvi- linear calcifications can be detected on conventional radi- ographs. In up to one third of cases, necrosis, hemorrhage, and/or cystic changes can be demonstrated on cross-sec- tional imaging studies (Fig. 1) [5]. Cross-sectional imag- ing does not reveal the malignant potential of a thymoma

Fig. 1. Thymoma. CT shows a mass in the anterior mediastinum

with uniform enhancement

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unless pleural seeding is present. Therefore, the term ‘in- vasive thymoma’ is reserved for the visualization of an an- terior mediastinal mass with pleural deposits.

Thymic Cysts

Thymic cysts represent approximately 3% of all anterior mediastinal masses and are most commonly found in children. They are generally considered developmental abnormalities caused by persistence of the thymopharyn- geal duct. Radiologically, thymic cysts manifest as well- marginated, rounded masses that are characteristically lo- cated in the anterior superior mediastinum. On CT scans, they appear as sharply delineated masses with near-water attenuation (Fig. 2). Thymic cysts are usually unilocular but some may be multiloculated, with occasional visual- ization of linear wall calcification. On MR, thymic cysts have a very high signal attenuation on T2-weighted im- ages but variable signal intensity on T1-weighted images, depending on the degree of proteinaceous material or he- morrhage present [5, 6]. They should not enhance with gadolinium. Occasionally, multiple thymic cysts may be present in HIV patients and in those with Langerhans cell histiocytosis.

Lymphangioma

Lymphangiomas are rare, benign lesions of lymphatic origin, representing 0.7-4.5% of all mediastinal tumors [5, 6, 8]. Most patients are asymptomatic, but occasion- ally they have symptoms of compression or pain. In adults, lymphangiomas are more often found in the me- diastinum, whereas in children, especially those <2 years of age, lymphangiomas are usually discovered as a mass

in the neck or axilla. CT scanning typically shows a uni- formly cystic lesion with an attenuation equal to or slight- ly higher than that of water and insinuating throughout multiple mediastinal compartments (Fig. 3). On MR imaging, the cyst contents have increased signal intensi- ty on T2-weighted images and can also have high signal intensity on T1-weighted images.

Germ-Cell Tumors

Most primary mediastinal germ-cell tumors are ter- atomas. These are a heterogeneous group of tumors that are derived from more than one germ-cell layer and thus contain different tissue types [9]. These tumors can be found in persons of any age, although they are most often detected in young adults. Teratomas account

Fig. 2. Teratoma. CT shows an anterior mediastinal mass with a combination of fat and soft-tissue attenuation

Fig. 3a, b. Pericardial cyst. a Posteroanterior chest radiograph shows a well-defined mass in the right cardiophrenic space. b CT shows the typical appearance of a pericardial cyst. Note the change in shape with the change in the patient’s position

a

b

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for 60-70% of mediastinal germ-cell tumors. Mature teratomas are by far the most frequent and occur more commonly in young women. Radiologically, such tu- mors manifest as a rounded or lobulated, well-defined, anterior mediastinal mass that usually protrudes to one side of the midline. The CT scan appearance of these lesions depends on the proportions of the various tis- sues of which they are composed. CT examination al- lows a confident diagnosis of a mature teratoma when a combination of fluid, soft-tissue, calcium, and fat at- tenuation is present [5, 9] (Fig. 2).

Germ-cell tumors are frequently cystic, and mature teratomas do not need to contain fat. When the soft-tis- sue component is proportionally greater than the cystic components, malignant germ-cell tumor may be suspect- ed. Seminoma is the most common primary mediastinal germ-cell tumor [5, 9].

Pericardial Cysts

These are believed to arise from the persistence of blind segments of the ventral parietal recesses of the pericardi- um. Seventy percent occur in the right cardiophrenic an- gle as a well-defined soft-tissue opacity on conventional chest radiograph [5, 7]. On CT, pericardial cysts typical- ly manifest as well-circumscribed, unilocular, nonen- hancing masses with low (near-water) attenuation con- tents. They are located adjacent to the cardiac border, typ- ically changing shape as the patient changes position (Fig. 3). On MR, pericardial cysts have low-signal inten- sity on T1-weighted images, homogeneous high-signal intensity on T2-weighted images, and do not enhance [7].

Pericardial cysts not infrequently communicate with the pericardial space and, therefore, may change in size be- tween two CT examinations.

Middle Mediastinum

The most common abnormalities of middle-posterior me- diastinum are developmental anomalies of the aortic arch and aortic aneurysms, esophageal lesions and develop- mental cysts (bronchogenic and enteric cyst) [5]. The pre- dominant mass in the middle superior mediastinum is the intrathoracic goiter.

Mediastinal Goiter

Mediastinal goiter is one of the most common mediasti- nal abnormalities seen in daily practice [6]. Patients are usually asymptomatic but rarely may present with pain or symptoms related to tracheal compression.

Radiographically, mediastinal goiter appears as a sharply defined, smooth or lobulated mass that usually causes displacement and narrowing of the trachea. Coarse or ring-like calcifications are commonly associated with the lesion being visible on plain films. CT can show the ex- tent of intrathoracic thyroid tissue and its connection with the thyroid gland (Fig. 4) [5, 6]. On CT, goiters should be heterogeneous in attenuation, with components that are of high attenuation. If the thyroid is enlarged and shows homogeneous low-attenuation, tumor infiltration should be suspected [7].

Developmental Cysts

Bronchogenic cyst is by far the most common form of foregut-duplication cysts, which result from abnormal ventral budding of the tracheobronchial tree.

Bronchogenic cysts represent 50-60% of all mediastinal cysts. Approximately 80% occur in the paratracheal or subcarinal region, but they may also have an intrapul- monary location [6, 8]. On CT scanning, bronchogenic cysts are typically unilocular nonenhancing masses of water attenuation, intimately related to the airway.

Calcification of the cyst wall can occur and sometimes the cyst’s attenuation on CT can be higher because they may contain milk or calcium. On MR imaging, the cystic contents can exhibit low- or high-signal intensity on T1- weighted images and typically have homogeneous bright signal intensity on T2-weighted sequences [8] (Fig. 5).

They should not enhance with gadolinium.

Enteric cysts are usually lined by alimentary epitheli- um. Esophageal duplication cysts comprise 10-15% of all alimentary-tract duplications and 60% are located adja- cent to the lower esophagus [6, 8]. They can be found within or attached to the esophageal wall and rarely com- municate with the esophageal lumen. On CT, esophageal duplication cysts appear as spherical nonenhancing intra- mural lesions with no esophageal communication, or sometimes showing a fibrous tract attached to the esophageal wall.

Often times, the foregut duplication cyst cannot be fur- ther classified as bronchogenic or enteric. No communi- Fig. 4. Intrathoracic goiter. Middle mediastinal mass, posterior and

lateral to the trachea. Note the typical coarse calcifications

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cation to the trachea, bronchi or esophagus is demon- strated. In these cases, one must wait for the final surgi- cal pathology, although it may not be more revealing. If the cyst has become infected or bled, the lining may be- come indistinct and further classification is not possible.

Paravertebral Region

This region is bounded in the front by the anterior surface of the vertebral column and in the back by the chest wall. The most common abnormalities of the paravertebral region are neoplasms of neural tissue arising from the peripheral nerves or sympathetic ganglia, infectious processes from the spine, bone and cartilage tumors, costovertebral traumatic lesions, and extramedullary hematopoiesis [6, 10].

Tumors of Neural Tissue

Neoplasms of neural tissue account for about 20% of all primary mediastinal neoplasms. Schwannoma, neu- rofibroma (plexiform and nonplexiform), and malig- nant schwannoma are tumors that arise from an inter- costal nerve [6]. Their radiographic appearance con- sists of sharply defined round, smooth, or lobulated paraspinal masses. In 50% of cases they are associated with bone abnormalities, including expansion of the neural foramina, erosion of the vertebral bodies, and erosion and deformity of ribs [6]. Ganglioneuroma, ganglioneuroblastoma, and neuroblastoma are neural tumors arising from the sympathetic ganglia. They usu- ally manifest radiographically as sharply defined ob- long masses located along the anterolateral surface of the vertebral spine (Fig. 6). At times, a schwannoma can emulate a foregut duplication cyst (low-attenuating from the myelin). One must look for enhancement or bone-remodeling to help in the identification, as nei- ther of these properties is routinely seen with duplica- tion cysts.

Extramedullary Hematopoiesis

Extramedullary hematopoiesis is a rare disease that is usually associated with chronic hematologic diseases in which there is inadequate production or excessive de- struction of blood cells [10]. Thalassemia and sickle-cell disease are the most common hematologic disorders as- sociated with foci of extramedullary hematopoiesis in or- gans such as liver, spleen, and lymph nodes. Unilateral or bilateral masses of hematopoietic tissue can be occasion- ally found in the paravertebral region. The radiographic appearance consists of smooth and lobulated paraverte- bral masses situated in the lower chest. On CT scans, they may have a large fatty component [10]. This uncommon disorder should be considered in the differential diagno- sis of a paravertebral mass in a patient with severe chron- ic anemia.

Fig. 6. Neurogenic tumor. T2-weighted tranverse MRI shows a pos- terior mediastinal mass. Ganglioneuroma confirmed at surgery Fig. 5a, b. Bronchogenic cyst. a T2-weighted transverse MRI shows

a high-signal round mass in the middle mediastinum. b T1-weight- ed coronal view confirms the location of the mass and its liquid content

a

b

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Vascular Abnormalities

Vascular anomalies of the mediastinum can occasionally be confused with mediastinal masses on plain film.

Cross-sectional imaging is helpful in diagnosing them but the clinician must be well-acquainted with these en- tities to avoid propagating the confusion.

Tortuous vessels are perhaps the most common medi- astinal vascular abnormality confused with mass on chest radiography. The right innominate artery (causing a right paratracheal opacity) and the left superior inter- costal vein are two of the more common enlarged, tortu- ous variants [11].

Anterior to the aortic arch in the para-aortic region, two venous anomalies are frequently encountered: the left-sided vena cava and partial anomalous return of the left upper lobe [12]. The former drains from the left bra- chiocephalic vein to the coronary sinus and may be pre- sent with a right-sided superior vena cava. The latter rep- resents the most common form of isolated anomalous drainage. In this entity the left upper lobe drains into the left brachiocephalic vein. The key to distinguishing par- tial anomalous return of the left upper lobe from a left su- perior vena cava is that the former does not connect to the coronary sinus, and no vessel is encountered anterior to the left mainstem bronchus. This is in distinction to the situation with a left superior vena cava, in which two ves- sels are encountered anterior to the left main-stem bronchus (the normal superior pulmonary vein and the left superior vena cava). Another venous anomaly is an azygos continuation of an interrupted inferior vena cava.

In this condition, the azygos vein is responsible for all in- frahepatic venous return. This entity should be suspected when the azygos vein is noticeably enlarged in a patient without superior vena cava thrombus or obstruction.

Aortic arch variations may also be encountered as an incidental mediastinal mass. They include double aortic

arch, right arch with aberrant left subclavian artery, and left arch with aberrant right subclavian artery. Detailed analysis of the branching vessels should allow for accu- rate diagnosis.

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