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Pediatric Imaging

E2309. A Review of Normal and Abnormal Thymus in Pediatric Imaging

Borders H1,  Frost J1,  Betz B1,  Farrell C2,  Junewick J.1 1. Advanced Radiology Services, Grand Rapids, MI; 2. Grand Rapids Medical Education Partnes, Grand Rapids, Michigan

Address correspondence to J. Junewick (jjunewick@advancedrad.com)

Background Information: The ontogeny of the normal thymus in childhood and differentiating normal thymus from disease can confound pediatric and general radiologists; consequently, a review of the embryology, anatomy, and function of the thymus and its normal and pathologic appearance on various imaging modalities is beneficial.

Educational Goals/Teaching Points: In this exhibit we review the embryology, anatomy, and physiology of the thymus as it relates to normal, normal variants, and disease. Participants will recognize normal thymus on radiography, sonography, CT, MR, and scintigraphy. We illustrate benign diseases of the thymus and review immunologically mediated thymus disease, and illustrate malignant diseases.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques: The thymus develops from the ventral portion of the third and portions of the fourth paired pharyngeal pouches. The thymus has epithelial and lymphoid components; the lymphoid component contributes to T-cell immunity. The thymus gradually increases in size until adolescence but then rapidly involutes. It is a smoothly marginated bilobed pliable organ in the superior mediastinum. On chest radiography and fluoroscopy it occupies a relatively large portion of the superior mediastinum in infancy but becomes less conspicuous with age; the sail and thymus wave signs are hallmarks of normal thymus. On ultrasound, the thymus signature is uniformly hypoechogenic with punctate and linear echogenic foci. Age-related involution of the thymus results in fatty replacement, therefore density on CT decreases and T1 signal on MR increases with age; the thymus should enhance homogeneously on both CT and MR. The presence of various membrane receptors accounts for visualization of the thymus on FDG, radio-iodide, gallium, and octreotide imaging. Developmental, anatomic, and physiologic variants are illustrated (size, location, and rebound). Benign entities (thymus cysts, teratoma, lymphangioma), immunologic alterations (DiGeorge syndrome, Langerhans cell histiocytosis, chronic granulomatous disease, and immune deficiency), and malignant conditions (Hodgkin disease, T-cell leukemia/lymphoma, thymoma, germ cell tumors and others) of the thymus are illustrated and discussed.

Conclusion: Understanding and recognizing the wide range of normal and the various disease processes of the thymus will allow more confident and accurate diagnoses in children and young adults.