martedì 10 novembre 2009
Aberrant cervical thymus
Ultrasound demonstrates unique linear echogenic septa and multiple discrete echogenic foci characteristic for thymic tissue.
T1-weighted MRI demonstrates low signal intensity slightly greater than muscle.
T2-weighted MRI demonstrates high signal intensity slightly lower than that of fat but greater than muscle.
Coronal T2-weighted image identifying normal thymus in its expected location in the anterior mediastinum, as well as, the aberrant thymus in the right neck. Note that the aberrant and native thymus demonstrate the same signal characteristics on MRI.
Diagnosis: Aberrant cervical thymus
Aberrant or ectopic thymus is an uncommon entity that may be encountered in early childhood. Recognition of this entity can obviate unnecessary surgery. Aberrant and most cases of ectopic thymus do not require therapy or surgery. Ultrasound is the initial imaging modality of choice for diagnosis.
The thymus is a lymphoepithelial gland that functions in T-cell lymphopoiesis and is important for immune system development. Development usually begins in the 6th gestational week from the third and fourth pharyngeal pouches. The tissue descends along a path from the angle of the mandible to the thoracic inlet, and reaches the anterior mediastinum by the 12th week.
The thymus is typically located in the anterior mediastinum immediately posterior to the sternum and anterior to the great vessels. Each of the two lobes of the thymus is divided by primary connective tissue septa carrying blood vessels to parenchymal lobules, which are composed of a cortex and medulla, both of which are developed by the 12th week. The lobules are further divided by secondary connective tissue septa carrying blood vessels from the surface of the cortex to the corticomedullary boundary. The cortex contains only capillaries, whereas, the medulla has both small arteries and arterioles. The thymus increases in size until puberty, where it can range from 30 to 40 grams. After puberty, there is gradual fatty involution that replaces the lymphoid components.
Abnormally located thymic tissue can be categorized as being aberrant or ectopic. Patients typically present clinically with a painless, nontender mass. Aberrant thymic tissue is found along the expected normal path of descent to the anterior mediastinum from the angle of the mandible to the thoracic inlet. Aberrant thymus is commonly located in the lateral neck or in the suprasternal region, and is usually an asymptomatic, incidental finding. Ectopic thymic tissue can be found in any other location, such as the pharynx, trachea, posterior neck or mediastinum, and esophagus. Ectopic thymus can occasionally be life-threatening, such as with airway obstruction.
Ultrasound is the initial imaging modality of choice. It does not require sedation such as with MR and CT; does not involve ionizing radiation such as with CT; and does not utilize contrast media such as with MR and CT. Thymus has been found to have a characteristic and unique appearance demonstrating linear echogenic septa and multiple discrete echogenic foci throughout the gland, believed to represent connective tissue septa and their associated vasculature. The cortex of the lobules is typically hypoechoic versus the more echogenic medulla. The unique ultrasound pattern allows for easy differentiation of thymus from liver, spleen, and thyroid. In cases of thymic tissue near the mandible, one can also differentiate thymus from salivary glands (including parotid and submandibular) that are typically more homogeneous with fine internal echoes.
Occasionally MRI may be obtained for further evaluation. Normal thymic tissue in children, regardless of location, is typically homogeneously low-signal intensity on T1-weighted sequences (slightly greater than muscle) and high-signal intensity on T2-weighted sequences (slightly less or equal to fat) owing to the higher water content of the thymus. The appearance of the thymus may vary depending on age, and may also be completely or partially cystic. MRI can be useful to compare the signal characteristics of normal and abnormally positioned thymic tissue, as well as, in confirming the presence of normal thymus.
Recognition of aberrant or ectopic thymus is important because these entities usually do not require therapy or surgery, except in circumstances where there is evidence of airway obstruction. The thymus is important for immune development. Ultrasound can be used to confidently identify tissue as being of thymic origin. It is important to identify the normal thymus as well because the aberrant or ectopic thymus may be the only functioning thymic tissue within the patient. MR and/or CT may be used to supplement ultrasound findings.