Most common anterior mediastinal “mass” in neonates and infants

Has quadrilateral shape in infancy

Gradually becomes triangular-shaped in later childhood and teenage years

Thymus increases in weight until adolescence when it begins to involute

Most prominent in infancy

Visible on frontal radiograph until ˜ 5 years of age

Appearance may change with phase of respiration

Look for “spinnaker sail” sign, “notch” sign, and “wave” sign

May be asymmetric across midline

Can extend inferiorly to drape over heart, superiorly into neck, or posteriorly to involute between great vessels and trachea

Homogeneous appearance on CT and MR, enhancing homogeneously following contrast administration

Typical sonographic appearance of multiple linear echoes and discrete echogenic foci

Thymus can vary in size depending on intercurrent illness and stress

Maintains normal attenuation and signal on CT and MR respectively

Maintains normal configuration on cross sectional imaging

Also maintains normal gross architecture and histologic appearance

Most common anterior mediastinal mass in teenagers

Distorts shape of thymus, which assumes lobulated or biconvex contour

Mass usually crosses midline

May be homogeneous or heterogeneous soft tissue mass on CT and MR

Positron emission tomography (PET) imaging best identifies involved nodes and extent of involvement elsewhere

May become more heterogeneous while it is being treated or if it rapidly enlarges and outgrows blood supply

May be associated with involvement of lymph nodes in hila and in other mediastinal compartments

Pleural and pericardial involvement, especially effusions, not uncommon; lung involvement is unusual

May displace trachea and vessels

Superior vena caval invasion and occlusion may cause SVC syndrome

Pediatric patients (˜ 66%) usually present with symptoms

2nd most common extragonadal site after sacrococcygeal region

Occurs within or near thymus

Majority are mature teratomas (60%); seminoma is next most common

Tumors have lobulated or smooth contour on radiography; constituent elements may be identified, especially calcification

Intrinsic elements are better identified on CT

CT appearances may be homogeneously cystic or soft tissue in appearance, contain well-demarcated fat, fluid, soft tissue, or calcific elements, or may have heterogeneous soft tissue appearance

Calcifications are usually coarse

May be well demarcated or inseparable from vascular structures

Tumors are more commonly unilateral but may extend across midline

Formerly called cystic hygroma or lymphangioma

Usually detected on prenatal imaging or in neonatal period

Most often represents mediastinal extension from neck lesion but can be solely mediastinal

May be associated with more generalized lymphatic problem

Thin-walled fluid-filled structure, which may be focal but may also be multifocal or infiltrative and involve mediastinal compartments

May extend superiorly into neck between carotid artery and jugular vein

Usually thin walled and fluid filled

Rarely may have partial wall calcification

Can be congenital or postinflammatory

Thymic involvement occurs commonly in multisystem disease

Most commonly presents as diffuse enlargement but can have focal lesion, usually cystic area on CT

Contour may be smooth or lobulated

Involved thymus is heterogeneous in appearance on CT and MR

Irregular calcifications and cystic lesions may be present

Calcifications are usually subtle in comparison to those seen in germ cell tumors and are usually only visible on CT

Enlarged, involved gland can displace trachea and great vessels, unlike normal thymus

Appearance of thymus reverts to normal with therapy

Anteromedial parasternal defect of diaphragm, adjacent to xiphoid process

90% occur on right side; right cardiophrenic angle on radiography

More commonly asymptomatic than Bochdalek type of congenital diaphragmatic hernia

Contents of hernia are variable but most commonly contain omentum with liver and bowel less common; contents determine radiographic appearance

Nature of contents is more easily assessed on CT and MR

Occasionally diagnosed on barium study