Normal Thymus

Etiology:
Imaging CXR:
— Gone by ~ 5 years old
— Findings that help diagnose a normal thymus include 1) thymic wave sign = undulating thymus contour caused by ribs making impression on soft thymus that appear like waves and 2) thymic sail sign = triangle-shaped inferior margin of a normal thymus seen more commonly on the right upper lobe than the left upper lobe
Imaging US:
— Use as a problem solving tool
— Thymus has starry sky or dot-dash appearance
Imaging CT: Gone by ~ 17 years old
Imaging Fluoroscopy: Moves with inspiration and expiration – increases in size in expiration and decreases in size in inspiration
Note: Thymus shrinks when stressed by infection or malignancy and rebounds when disease resolves or treatment is over so make sure enlarging thymus not due to recurrent tumor in oncology patients
DDX: Upper lobe consolidation due to pneumonia or atelectasis
— Pulmonary vascularity can be seen through a normal thymus
— Pulmonary vascularity cannot be seen through pneumonia or atelectasis
DDX: Anterior mediastinal tumor
— Thymus is soft and is displaced by surrounding structures
— Anterior mediastinal tumor is hard and displaces surrounding structures
DDX: Lymphadenopathy
— Lymph nodes are hard and cause airway obstruction and airway shift and lymph nodes are more mass like and resemble lobes of thymus less
— Thymus is soft and compresses and displaces nothing, moves with inspiration and expiration – increases in size in expiration and decreases in size in inspiration
Complications:
Treatment:
Clinical:

Radiology Cases of Normal Thymus

CXR of normal thymus — CXR PA (left) and lateral (right) show a small soft tissue prominence to the left of the spine in the superior mediastinum. The heart is normal in size, the pulmonary vascularity is not congested, and the lungs are well expanded and clear.

CXR and CT of normal thymus — CXR AP (above) shows a soft-tissue lesion filling the entire right upper hemithorax. The pulmonary vascularity can be seen through the lesion. Coronal (below left) and axial (below right) CT with contrast of the chest shows the homogenous low density soft tissue lesion to arise in the anterior mediastinum and extend to the right, to not compress or displace any adjacent structures, and to have the minor fissure as its inferior border.

CT of pediatric normal thymus — Axial (above), coronal (below left) and sagittal (below right) CT with contrast of the chest show a low density triangular soft tissue mass centered in the anterior mediastinum that is not compressing or displacing any adjacent structures.

Radiology Case of False Positive Thymus Enlargement Due to Rotation

CXR of normal thymus on rotated CXR — CXR AP rotated to the right shows complete opacification of the right lung apex. Repeat CXR without rotation was unremarkable.

Radiology Cases of Ectopic Cervical Thymus

US and CT of cervical thymus — Transverse+sagittal US of the neck (above) shows the mass to have a starry sky appearance. Axial+coronal CT with contrast of the neck shows the mass to be inferior to the thyroid and contiguous with the thymus.

Radiology Cases of Normal Thymus Sail Sign

CXR of normal thymus with sail sign — CXR AP and lateral shows a prominent anterior mediastinal mass whose right inferior margin appears triangular and sail-like in appearance.

Radiology Cases of Normal Thymus Wave Sign

CXR of normal thymus with wave sign — CXR AP shows an anterior mediastinal mass with a wave-like appearance to the left mediastinal contour.

CXR of normal thymus with thymus wave size — CXR AP (left) shows an oval in shape right-sided superior mediastinal mass whose lateral border has a wave-like appearance (thymus wave sign). CXR lateral (right) shows the mass to be laying on top of the right minor fissure. Heart size and pulmonary vascularity are normal and the lungs are clear.