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Thymus

The thymus is a primary lymphoid organ located in the anterior superior mediastinum, posterior to the sternum and anterior to the great vessels of the heart. It consists of two asymmetrical lobes joined in the midline and is composed of cortex and medulla supported by a connective tissue capsule. The thymus is large and active during childhood, playing a vital role in T-lymphocyte maturation and immune system development, but undergoes physiological involution after puberty, becoming progressively replaced by fat in adults. Despite involution, thymic remnants can persist throughout life and may give rise to pathologies such as thymic hyperplasia, thymoma, or thymic carcinoma.

Synonyms

  • Thymus gland

  • Mediastinal thymus

  • Primary lymphoid organ of the anterior mediastinum

Function

  • Maturation and differentiation of T-lymphocytes (T-cells)

  • Establishes central immune tolerance

  • Provides an essential role in immune system development during childhood

  • Source of hormones such as thymosin and thymopoietin, which regulate T-cell activity

MRI Appearance

T1-weighted images:

  • Thymus shows intermediate signal intensity, slightly lower than surrounding muscle

  • Fatty replacement in adults increases signal intensity on T1

  • Helps differentiate normal thymus, hyperplasia, and tumors

T2-weighted images:

  • Thymus demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Thymic hyperplasia may appear heterogeneous, while cystic changes or tumors appear more hyperintense

 

T1 Post-Contrast (Gadolinium-enhanced):

  • Normal thymus shows mild, homogeneous enhancement

  • Thymic hyperplasia enhances diffusely but maintains soft margins

  • Thymomas and thymic carcinomas may show heterogeneous, nodular, or irregular enhancement

CT Appearance

Non-contrast CT:

  • In children: soft tissue density triangular structure in the anterior mediastinum

  • In adults: fatty replacement with soft tissue remnants, making it appear heterogeneous

  • Calcifications may be seen in tumors

Contrast-enhanced CT (CECT):

  • Normal thymus enhances homogeneously

  • Hyperplasia appears diffusely enlarged with preserved shape and smooth borders

  • Thymomas or malignancies may present as lobulated, heterogeneous enhancing anterior mediastinal masses, sometimes invading adjacent structures

MRI images

Thymus anatomy MRI coronal image -img-00000-00000

CT image

Thymus anatomy ct axial image -img-00000-00000