Chest and Mediastinal Imaging

Anatomy

Multiple classification systems divide the mediastinum into distinct compartments for use by radiologists, surgeons, and anatomists. The Shields system is most commonly applied in clinical practice, whereas the Felson classification is more frequently used in radiologic interpretation. Under the Felson classification, the mediastinum is divided into anterior, middle, and posterior compartments. On a lateral chest radiograph, a line drawn from the anterior tracheal wall to the posterior inferior vena cava separates the anterior mediastinum from the middle mediastinum. The middle and posterior mediastinal compartments are separated by an additional line drawn 1 cm posterior to the anterior margin of the vertebral bodies.

More recent classification systems include the International Thymic Malignancy Interest Group classification, which defines mediastinal compartment boundaries based on anatomic structures identifiable on cross-sectional imaging.[1] The International Thymic Malignancy Interest Group classification is more inclusive and divides the mediastinum into prevascular (anterior), visceral (middle), and paravertebral (posterior) compartments.

Anterior Compartment

This mediastinal region extends from the posterior surface of the sternum to the anterior aspect of the great vessels and pericardium. The contents of the anterior mediastinum include lymph nodes, fat, thymus, and internal mammary arteries. Approximately 50% of all mediastinal masses arise in this compartment. The most common masses include thymic tumors, lymphoma, thyroid tumors, and germ cell tumors such as teratomas. Thyroid growths typically represent substernal extensions of masses originating from the cervical thyroid gland.

Middle Compartment

This compartment extends from the pericardium to the anterior aspect of the thoracic spine. The contents of this region include the heart, pericardium, aorta and its branches, the vena cava, trachea, and esophagus. The most frequent masses in the middle mediastinal compartment include lymphoma and metastatic lymphadenopathy. Other potential lesions include aortic aneurysms, pericardial and cardiogenic tumors, foregut inclusion cysts, and neoplasms of the esophagus and trachea.

Posterior Compartment

This anatomic area extends from the anterior aspect of the thoracic spine to encompass the spine and vertebral bodies. The contents of this compartment include the spinal cord, thoracic spine, neurovascular bundles, and sympathetic chain. Neurogenic tumors are the most common posterior mediastinal masses and include meningiomas, schwannomas, and meningoceles. Other lesions include bony metastases and paravertebral abscesses. The posterior mediastinum is also a site of extramedullary hematopoiesis, which can present as soft-tissue masses.

Plain Films

Chest x-rays are the initial imaging modality of choice for mediastinal evaluation. Outpatient studies typically include 2 views—posterior-anterior and lateral. This approach can identify emergent conditions, such as aortic dissection, pneumomediastinum, pneumoperitoneum, and pneumothorax, as well as infections or neoplasms. Single-view portable anteroposterior x-rays are obtained in patients in the intensive care unit, receiving mechanical ventilation, undergoing line or tube placement confirmation, or having difficulty positioning for posterior-anterior views.

Mediastinal widening occurs when the mediastinal width exceeds 8 cm on a posterior-anterior chest x-ray.[2] This finding constitutes a red flag, particularly in patients with traumatic injuries, with the most common causes being aortic dissection and mediastinal hematoma. Other etiologies include aortic aneurysms and mediastinal tumors. Additional radiographic signs assist in mediastinal evaluation. An age-related normal variant in older adults is aortic unfolding, resulting from elongation of the thoracic aorta.

The hilum overlay sign is useful for evaluating mediastinal masses on plain radiographs. Visualization of normal hilar structures through a mass indicates that the mass lies either anterior or posterior to the hila, allowing assessment of its location.[3] 

The right paratracheal stripe is a subtle line projecting through the superior vena cava. Thickening of this radiographic structure beyond 4 mm suggests lesions in this region, most commonly lymphadenopathy.

The aorticopulmonary window extends from the aortic knob to the left pulmonary artery. This area is typically concave or straight. A convex contour is considered abnormal and suggests mediastinal lymphadenopathy or aortic aneurysm.

Paraspinal lines result from the opposition between the pleural reflection and the vertebral bodies. These lines may be obscured by posterior mediastinal masses, including neurogenic tumors and metastases.

Chest fluoroscopy provides continuous, real-time imaging using x-rays. This imaging modality is used to detect diaphragmatic paralysis. Specialized studies performed under fluoroscopy include a barium swallow, which helps diagnose esophageal diseases such as ulcers, hiatal hernia, achalasia, and esophageal cancer.

Computed Tomography

CT is the modality of choice for evaluating mediastinal pathology. Multidetector CT of the chest is commonly employed in emergency and inpatient settings. Sagittal and coronal reformats provide critical perspectives and are extremely useful for assessing mediastinal masses. Three-dimensional reconstructions, including multiplanar reformation or reconstruction and maximum intensity projection sequences, further assist in evaluating mediastinal lesions, such as aneurysms. The primary limitation of this modality is the associated radiation dose.

Contrast-enhanced CT of the chest is typically performed when assessing mediastinal masses. Contrast administration permits clear delineation between vascular structures and adjacent anatomy, including lymph nodes and the esophagus. CT allows evaluation of masses based on their anatomic location. Measurement of the attenuation coefficient of a mass provides additional information to narrow the differential diagnosis.

The presence of air within the mediastinum indicates pneumomediastinum, which may be spontaneous, traumatic, or secondary to nontraumatic causes, such as asthma or chronic obstructive pulmonary disease. Iatrogenic causes, including endoscopic procedures and central vascular access, are additional frequent etiologies.

Mediastinal masses may demonstrate fluid, fat, or soft tissue attenuation. Fat-containing lesions in the anterior mediastinum strongly suggest teratoma. The differential diagnosis also includes lipoma and thymolipoma. Thymolipoma is a well-encapsulated thymic tumor that may be located in the cardiophrenic angle.[4]

Most teratomas exhibit variable tissue attenuation depending on ectodermal, mesodermal, and endodermal components. These lesions are typically solid-cystic, containing fat, soft tissue, and calcifications. Thymic cysts, also located in the anterior mediastinum, are typically unilocular.

Cystic lesions of the middle mediastinum commonly include bronchogenic cysts, foregut duplication cysts, and enteric cysts, representing frequent benign etiologies.[5] Pericardial cysts generally occupy the cardiophrenic angles and are unilocular. Posterior mediastinal cystic lesions are typically meningoceles. A meningocele consists of leptomeninges filled with cerebrospinal fluid protruding through defects in the vertebral body or along the intervertebral foramina.

Soft tissue masses in the mediastinum include substernal extensions of goiter in the anterior mediastinum, thymic hyperplasia, thymoma, thymic carcinoma, lymphoma, nonnecrotic metastatic lymph nodes, and masses arising from the trachea, esophagus, or neurogenic origins. Thymic hyperplasia, also referred to as thymic rebound, can occur after chemotherapy or systemic stress, such as that arising from recent surgery. This condition may be differentiated from thymic tumors by preservation of the triangular thymic contour. 

Thymoma accounts for 47% of anterior mediastinal tumors.[6] These neoplasms may invade adjacent thoracic structures, though metastasis is rare. Thymic carcinoma exhibits both local invasion and distant metastasis. Calcifications may be present in both thymoma and thymic carcinoma.

Posterior mediastinal soft tissue tumors predominantly include neurogenic tumors, such as schwannomas and neurofibromas. Schwannomas may extend through the intervertebral foramina, producing a characteristic dumbbell configuration.

The introduction of dual-energy CT has enabled the generation of virtual noncontrast images, permitting single-phase postcontrast acquisition and reducing radiation exposure.[7] In addition to the aforementioned pathologies, vascular malformations of lymphatic and venous origin in the anterior and middle mediastinum and neuroblastoma in the posterior mediastinum are common in pediatric patients and should be included in the differential diagnosis.[8]

Imaging for Acute Aortic Syndrome and Pulmonary Embolism 

An increase in aortic diameter greater than 4 cm in the ascending aorta is considered pathologic. An aortic aneurysm demonstrating growth of more than 10 mm per year is considered unstable, particularly when associated with intramural hematoma or an eccentric lumen.[9] Aortic aneurysms may arise from traumatic, degenerative, or congenital causes, including Marfan syndrome.

Aortic dissection is identified on CT angiography as a thin intraluminal flap within the aorta. The Stanford classification divides dissections into 2 types. Type A involves the ascending aorta and constitutes a surgical emergency. Type B involves only the descending aorta, sparing the ascending aorta. CT angiography protocols for aortic dissection include unenhanced, arterial, and venous phase sequences. The unenhanced phase is critical for evaluating intramural hematoma and hemopericardium. Intramural hematoma appears as a curvilinear hyperdense lesion within the aortic wall on unenhanced CT.

CT angiography studies are typically electrocardiographically gated to minimize motion artifacts. Three-dimensional volume-rendered reformats are particularly valuable for assessing the extent of dissection and involvement of branch vessels. Penetrating aortic ulcers are atherosclerotic plaques that disrupt the intimal layer.

Congenital vascular abnormalities include a right-sided aortic arch, double aortic arch, aberrant right subclavian artery, and left superior vena cava. These anomalies are uncommon but carry significant clinical implications.

Pulmonary embolism, a life-threatening condition, may be evaluated using CT pulmonary angiography. Imaging features allow the classification of pulmonary embolism as acute or chronic, based on the age of the embolus. The primary distinction between CT pulmonary angiography and CT angiography for aortic dissection lies in the region of interest for bolus-triggered scanning. The pulmonary artery is targeted for CT pulmonary angiography, whereas the ascending aorta is selected for aortic dissection imaging.[10][11]

CT pulmonary angiography offers the advantage of simultaneously evaluating the lungs for pulmonary infarction and assessing right heart strain, compared with ventilation-perfusion scanning. The primary limitation of CT is the higher radiation exposure.

Dual-energy CT is a more recent technique that enhances the detection of pulmonary embolism. This method generates iodine maps, allowing selective visualization of iodinated contrast within vessels and providing perfusion images that support identification of emboli.[12] Photon-counting CT is another advanced modality that reduces imaging artifacts and provides perfusion images to assist in the detection of pulmonary embolism.[13]

Triple rule-out studies assess the aorta, pulmonary arteries, and coronary arteries in a single examination and have been utilized in emergency settings to evaluate all 3 major causes of acute chest pain. The development of artificial intelligence–based pulmonary embolism detection systems has demonstrated significant promise by triaging positive cases in the background and serving as a second reader to improve detection rates.[14]

Magnetic Resonance

MRI is indicated for patients with contraindications to contrast-enhanced CT, such as severe contrast allergy or renal failure. MRI does not use ionizing radiation and may be safely performed in young and female patients. Disadvantages of MRI include higher cost, longer acquisition times compared to CT, and greater dependence on patient cooperation due to increased susceptibility to motion-related artifacts.

Standard MRI sequences for mediastinal evaluation include T2-weighted sequences in the axial and coronal planes, axial short τ inversion recovery sequences, in-phase and opposed-phase T1-weighted sequences in the axial plane, and diffusion-weighted sequences. Precontrast and postcontrast T1-weighted sequences are obtained during the early arterial phase, portal venous phase, and at a 5-minute delay.[15]

MRI offers superior soft-tissue resolution compared to CT, allowing differentiation of thymic and other malignant tumors from benign lesions and the detection of invasion into surrounding structures. The normal thymus and thymic hyperplasia contain small amounts of fat. Microscopic fat is difficult to evaluate on CT, but MRI can distinguish thymic hyperplasia from thymoma using T1-weighted in-phase and out-of-phase imaging. Decreased signal intensity on out-of-phase images relative to in-phase images indicates thymic hyperplasia, whereas this finding is absent in thymoma.

Diffusion-weighted imaging serves as a useful adjunct for differentiating benign from malignant masses. Malignant lesions demonstrate higher cellularity and exhibit increased signal intensity on diffusion-weighted imaging. MRI is particularly valuable for assessing intraspinal extension of posterior mediastinal masses, often through the neural foramina.[16]

MRI also facilitates the evaluation of congenital cardiac abnormalities and annual surveillance of aneurysms, as may be required in patients with connective tissue disorders such as Marfan syndrome. Gated cardiac MRI enables the assessment of cardiomyopathy and valvular heart disease.

Ultrasonography

US is widely used to evaluate cardiac function and anatomy through echocardiography. As a nonionizing imaging modality, US may be performed at the bedside to assess and quantify pleural effusions. Conventional US-guided biopsy and endobronchial US facilitate the detection and tissue sampling of mediastinal masses and lymphadenopathy.[17] Testicular US should be performed in male patients with a suspected or confirmed anterior mediastinal germ cell tumor to exclude a primary gonadal tumor.

Nuclear Medicine

Nuclear medicine techniques are used to assess mediastinal masses. These modalities include PET-CT, I-131 meta-iodobenzylguanidine, and technetium-99m sestamibi imaging.

PET-CT uses the radiopharmaceutical 18-F-fluorodeoxyglucose (FDG) for diagnosis, staging, and treatment monitoring of neoplasms. The study consists of separate PET and CT acquisitions, with CT used for attenuation correction and anatomic localization. CT is typically performed without intravenous contrast, although intravenous and oral contrast may be used, depending on the clinical indication. PET-CT is particularly useful for malignant mediastinal lesions, including lymphoma, esophageal carcinoma, and lung malignancies. Lymphomas demonstrate avid FDG uptake, allowing PET-CT to guide staging and restaging. Thymic tumors may also exhibit FDG uptake, which varies according to tumor grade.[18] PET-CT is a valuable tool for assessing therapeutic response in mediastinal neoplasms.

I-131 meta-iodobenzylguanidine imaging is used to confirm the presence of mediastinal pheochromocytomas.[19] Technetium-99m sestamibi imaging helps detect ectopic parathyroid tissue in the mediastinum.[20]

Angiography

Catheter-based angiography was historically used to assess vascular abnormalities before the widespread availability of CT. This technique exposes patients to high doses of ionizing radiation and requires intravenous contrast. An advantage of catheter-based angiography is its therapeutic capability. Bronchial artery embolization for hemoptysis is one of the most common clinical indications for this procedure.[21]

Patient Positioning

Patient positioning varies depending on the imaging modality used. Posterior-anterior radiographs are obtained with the patient facing the cassette and the x-ray beam directed from posterior to anterior. In an anteroposterior view, the cassette is positioned along the posterior aspect of the patient. For lateral films, the patient raises the arms above the shoulders, placing either side of the chest wall against the cassette. Portable radiographs are performed in the intensive care unit when the patient cannot stand erect.

Chest CT is performed with the patient in the supine position, typically with the arms raised above the head. MRI and PET scans are also acquired with the patient supine. US is generally performed with the patient supine. However, as a dynamic procedure, patient movement is permissible during imaging.

Clinical Significance

Accurate diagnosis of mediastinal lesions is essential given the complex anatomy and broad differential diagnosis, which includes vascular, neoplastic, infectious, congenital, and inflammatory conditions. A systematic approach that integrates clinical findings with imaging features is crucial to ensure timely and appropriate management, particularly in potentially life-threatening cases.

Diagnostic accuracy depends on selecting appropriate imaging and careful assessment. The most commonly employed modalities include plain radiographs and chest CT. MRI and PET-CT provide functional information useful for lesion characterization.