Type:
Educational Exhibit
Keywords:
Normal variants, Ultrasound, Fluoroscopy, Paediatric, Anatomy, Tissue characterisation
Authors:
S. A. M. Ibrahim1, T. Salem Alyafei1, M. Sabawi1, N. M. Saloum2, M. M. H. Ahmed1; 1Doha/QA, 2Doha, Doha/QA
DOI:
10.26044/ecr2019/C-0975
Background
The thymus is a lymphatic organ located in the antero-superior mediastinum.
When sizeable,
it consist of two lateral lobes touching in the midline,
situated partly in the thorax,
partly in the neck. The two lobes are usually asymmetric in size.
They are occasionally united,
so as to form a single mass.
Sometimes they are separated by an intermediate lobe.
The gland extends from as high as the lower border of the thyroid gland to the fourth costal cartilage downwards.
It originates from the third and fourth branchial pouches,
and contains elements derived from all three germinal layers.
Themain function is the generation,
differentiation and maturation of T lymphocytes,
being one of the central lymphoid organs and playing an important role in cellular immunity.The normal thymus is a dynamic organ that changes during life,
making differentiation of normal thymus from thymic disorders difficult for radiologists.
It grows until reaching its maximum absolute weight in adolescence (35g),
starting thereafter a process of involution in which the number of epithelial cells decrease and the amount of adipose tissue increases (15g at 60 years of age).
Fatty replacement starts at puberty and occurs more rapidly in males than females.
There can be a wide variation in size between patients.
Normal Variants of the Thymus
The two most common anatomic variants of the thymus are either superior or posterior extension of the normal thymus.
These variants usually are seen before 2 years of age.
The thymus may extent superiorly to the level of the lower neck or posteriorly to the middle or posterior mediastinal compartment.
Imaging An ectopic thymus rarely presents as a mass and is most commonly discovered incidentally as thymic tissue extension on cross sectional imaging with similar signal characteristics and density as the orthotopic thymus on magnetic resonance imaging (MRI) and CT,
respectively.