Approach to Imaging of Mediastinal Conditions in the Adult
Generating a relevant different diagnosis for mediastinal processes rests on the principles of localization and characterization.
Once a process or mass can be localized to the mediastinum, it should be localized within the mediastinum. Many of us use an approach first championed by Ben Felson. Using a lateral radiograph or sagittal CT or MR, a line is drawn from the anterior tracheal wall to the posterior inferior vena cava. This line separates the anterior mediastinum from the middle mediastinum. A second line is drawn 1 cm posterior to the anterior margin of the vertebral body. This line separates the middle from the posterior mediastinum. This approach can be useful in creating concise, meaningful differential diagnoses.
After localization, CT or MR should be performed for lesion characterization. Knowing whether a lesion has a significant vascular, fluid or fat component can be very helpful in suggesting a more specific diagnosis.
KeywordsCT MR Mediastinum ITMIG (International Thymic Malignancy Interest Group) Anterior mediastinum Middle mediastinum Posterior mediastinum
To understand an approach to the adult mediastinum based on localization of lesions within the mediastinum and attenuation/intensity features on cross-sectional imaging.
To highlight conditions that disregard the compartmental model.
To use some cases to show the additional value of MR in evaluating the adult mediastinum.
The mediastinum is an anatomic space defined by the thoracic inlet superiorly and the diaphragm inferiorly. It extends from the sternum to the vertebral bodies. Yet, despite its landmarks, there are no structures that completely separate the mediastinum from the neck above or the retroperitoneum below. Imaging of the mediastinum and generating a relevant different diagnosis rest on the principles of localization and characterization.
Once a process or mass can be localized to the mediastinum, it should be localized within the mediastinum. Many of us use an approach first championed by Ben Felson. Using a lateral radiograph or sagittal CT or MR, a line is drawn from the anterior tracheal wall to the posterior inferior vena cava. This line separates the anterior mediastinum from the middle mediastinum. A second line is drawn 1 cm posterior to the anterior margin of the vertebral body. This line separates the middle from the posterior mediastinum. No anatomic structures actually divide the mediastinal compartments, but this approach can be useful in creating concise, meaningful differential diagnoses. Keep in mind certain processes may involve more than one compartment and that a large mass may be hard to localize.
After localization, cross-sectional imaging (either CT or MR) should be performed for lesion characterization. Knowing whether a lesion has a significant vascular, fluid or fat component can be very helpful in suggesting a more specific diagnosis. PET/CT is used mainly to evaluate lymph node metastases in lung cancer. It is used as well in the evaluation of solid mediastinal masses.
This approach of localization and characterization will provide the interpreting radiologist a solid foundation in imaging the mediastinum.
3.2 Anterior Mediastinum/Prevascular Compartment
The visualization of fat intensity or attenuation can also be very helpful. In the anterior or prevascular mediastinum, most fatty masses are benign. As described above, an anterior mediastinal mass with fat and fluid suggests a teratoma (benign germ cell tumor) (Fig. 3.5). If the mass is purely fat, it may be the very rare thymolipoma but more likely will be a fat pad or anterior hernia (Morgagni hernia). Coronal or sagittal images are quite helpful in depicting vessels originating below the diaphragm. Visualization of these are key in separating hernias from fat pads or fatty tumors. In our practices, infarction of a pericardial fat pad or fat within the Morgagni hernia may present with chest pain. In the era of the frequent use of CT in the evaluation of chest pain, these areas of fat necrosis can simulate a neoplasm. Awareness of this potential pitfall will allow the patients to be treated appropriately.
Vascular or hyperenhancing lesions may present within the anterior or prevascular mediastinum. Often, these are related to the heart, ascending aorta. Rarely, ectopic parathyroid adenomas may present as vigorously enhancing anterior prevascular masses.
Most anterior mediastinal masses are of thymic origin. Patient age and attenuation characteristics are key in narrowing the differential diagnosis. MR can be helpful in characterizing cystic lesions and in diagnosing thymic hyperplasia.
3.3 Middle Mediastinum/Visceral Compartment
Most middle mediastinal or visceral compartment masses represent lymphadenopathy, foregut duplication cysts, vascular lesion, or esophageal processes. They usually present with right paratracheal widening on a frontal chest radiograph or occasionally the doughnut sign on a lateral examination. As with the anterior mediastinal conditions, assessment of attenuation or intensity can be helpful. In the ITMIG model, the heart is included in this compartment. In our experience, left ventricular and left atrial conditions may occasionally simulate middle mediastinal, visceral masses.
Unlike the anterior mediastinum or visceral compartment, fat cannot be considered benign. Although esophageal or tracheal lipomas and esophageal fibrovascular polyps contain fat, so may mediastinal liposarcomas. Although rare, these lesions may insinuate through the mediastinum and often have a predilection for the middle mediastinum.
Most mediastinal lymphadenopathy will present in the middle mediastinum or visceral compartment. Occasionally, these nodes will be calcified. Most often these calcified nodes are indicative of an old granulomatous process such as healed tuberculosis or histoplasmosis or sarcoidosis, but care must be taken to remember that certain tumors also tend to present with calcified mediastinal lymph nodes, including ovarian serous adenocarcinomas, mucinous colon neoplasms, and osteosarcomas.
Another potential for a perceived middle mediastinal mass on radiography will be a dilated esophagus. Although a distal mass may also result in esophageal dilatation, it is usually only achalasia that results in esophageal widening that can be seen on a chest radiograph.
Most middle mediastinal masses will consist of lymphadenopathy, duplication cysts, vascular lesions, or esophageal masses. Attenuation characteristics of the lymphadenopathy compared to skeletal muscle can be helpful at honing in on the correct diagnosis.
3.4 Posterior Mediastinum/Paravertebral Compartment
As with the other compartments, attenuation or intensity can be helpful. On CT, a potential pitfall is that myelin-rich neurogenic lesions may look cystic. For this reason, we often rely on MR with posterior mediastinal/paravertebral lesions. True posterior mediastinal cystic lesions are rare. Although neuroenteric cysts exist, they are often associated with vertebral anomalies and rarely encountered de novo in adults. Instead, a cystic lesion in the posterior mediastinum is much more likely to represent a lateral meningocoele or post-traumatic nerve root avulsion.
Fatty lesions are unusual in the posterior mediastinum but when encountered may invoke extramedullary hematopoiesis. Although rare, in patients with anemia, extramedullary hematopoiesis may develop in this space. The etiology of this condition remains unknown. Some authors have postulated that it develops from extruded marrow, while others have suggested that it develops from totipotent cells in the paravertebral space. When the patient is anemic, extramedullary hematopoiesis will present with bilateral masses that enhance similar to the spleen without a connecting bridge. As the patient returns to normal hematocrit, the yellow marrow will take over. The net effect is bilateral posterior mediastinal fatty masses. In the elderly, Bochladek hernias should be included in the differential diagnosis. Often the diaphragmatic defect can be seen.
Hypervascular lesions in in the posterior mediastinum or paravertebral space are less helpful than with the other compartments. Most often these are related to an aneurysmal aorta or enlarged collateral vessels as with aortic coarctation. As described above, extramedullary hematopoiesis may be seen with bilateral hypervascular paravertebral masses.
Most posterior mediastinal masses are neurogenic in origin. Osseous lesions, however, should not be forgotten. Mutiplanar reconstructions and MR are particularly helpful in this compartment.
3.5 Conditions that Disregard the Compartment Model
Certain conditions tend to disregard the compartment model of the mediastinum. Even with these lesions understanding the attenuation or intensity can be helpful. These include infection and hematoma, which will result in fat stranding and soft tissue attenuation throughout the mediastinum, often in more than one compartment.
Lymphangiomas and hemangiomas also tend to disregard the compartment model. The former tend to be fluid in their attenuation and insinuate throughout, while the latter will be higher in attenuation.
Of course, lung cancer may present with metastases to any compartment and unfortunately and tends to metastasize to more than one region.
Infections, lymphangiomas, and hematomas tend to disregard the compartmental model.
Location and characteristic approach to mediastinal masses in the adult
Germ cell tumor
Aorta or collaterals
More than one
A solid approach to the adult mediastinum uses the patient’s age, lesion cross-sectional attenuation/intensity, and lesion location to generate the differential diagnosis.
This compartment model is based on certain anatomic landmarks (Fig. 3.1).
Although most lesions will be characterized by CT, MR may have additional value in characterizing cystic lesions, diagnosing thymic hyperplasia and separating neurogenic from osseous lesions in the posterior mediastinum/paravertebral compartment.
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