Myasthenia Gravis

In some cases, MRI is superior to CT scanning for visualizing the thymus and for differentiating it from the surrounding soft tissue. In healthy children younger than 5 years, MRI shows the thymus to have a quadrilateral shape and biconvex lateral contours.  [ 9 ]  In older children and adolescents, the thymus is triangular with straight, lateral margins. On T1-weighted images, the thymus appears homogeneous with a signal intensity slightly greater than that of muscle; on T2-weighted images, the signal intensity is close to that of fat. Mass lesions in the mediastinum have sufficiently different imaging characteristics to allow their distinction on MRI from normal structures and fat, and MRI produces excellent cross-sectional images in the mediastinum without contrast enhancement; with CT, contrast material is often needed to properly identify a mass and to avoid mistaking blood vessels for a mass lesion. Encasement or invasion of the vasculature, esophagus, and trachea and

CT scanning is highly sensitive for thymomas. If the thymus appears grossly asymmetrical or if it has a lobular configuration, the diagnosis of a thymoma should be strongly considered. However, thymomas cannot be distinguished from other thymic masses on CT scans unless fat is visible. Thymomas are usually homogeneous and show mild enhancement with the use of contrast. They tend to grow to one side of the mediastinum or the other. (Images of thymomas appear below.)  [ 2 ]  Thymoma. CT scan shows a homogeneous thymic mass.  View Media Gallery Malignant thymoma. Chest CT scan in a 61-year-old man with myasthenia gravis demonstrates a large, lobulated mass with punctate calcification in the anterior mediastinum; these findings are consistent with a thymoma.  View Media Gallery Invasive thymoma. Contrast-enhanced CT scan shows a thymic mass with variable attenuations invading the adjacent mediastinum. Bilateral pleural effusions are also present, indicating pleural invasion

Thymic cyst On CT scans, a thymic cyst appears homogeneous with water attenuation. The attenuation may vary, depending on the contents of the cyst. High attenuation may be present if the cyst contains proteinaceous fluid or blood from hemorrhage. A neoplasm with cystic degeneration may closely mimic a thymic cyst; associated soft tissue attenuation may help in their differentiation. Thymic rebound After initiation of chemotherapy, CT scans may reveal a decrease in the size of the thymus. Rebound in the form of overgrowth occurs a few months after completion of chemotherapy. Criteria for rebound include an increase in size of greater than 50%, as compared with the baseline volume. Degree of confidence Chest CT scanning performed with and without contrast enhancement is clearly superior to routine radiography. A small thymic tumor can easily be missed on a chest radiograph, whereas CT scans distinctly delineate the tumor. In addition, enhancement often helps in clearly diff

CT scanning reveals a fatty mass interspersed with varying amounts of thymic soft tissue. The whole mass often consists of adipose tissue except for a thin rim of thymic tissue; this finding is consistent with soft tissue attenuation and mimics a pure lipoma of the mediastinum. (See the image below.) Thymolipoma. CT scan of the chest shows a large anterior mediastinal mass displacing the mediastinal vascular structures to the right and projecting into the left thorax. The tumor is composed of soft tissue and fat, and a few punctate calcifications are present.  View Media Gallery

Thymic carcinoma belongs to a group of uncommon epithelial neoplasms. They are characterized by cytologic atypia and anaplasia. On CT scans, they often show central necrosis in a large tumoral mass, with invasion and infiltration of adjacent structures in the mediastinum. Because of their aggressive nature, they are likely to produce hematogenous and lymphatic spread, locally and distally.

On CT scans, the thymus is enlarged, symmetrically or asymmetrically; this enlargement often makes it difficult to differentiate the thymus from the enlarged surrounding lymph nodes. At times, differentiating the thymus from a thymoma is difficult, although the clinical picture and the presence of other sites of lymphadenopathy are often helpful in the diagnosis. (See the image below.) Thymolymphoma. Contrast-enhanced CT scan demonstrates a large, lobulated, soft tissue mass in the anterior mediastinum that posteriorly displaces the aorta and pulmonary artery.  View Media Gallery

Seminoma On CT scans, seminomas are usually large and homogeneous, with soft tissue attenuation. Areas of low attenuation often are present secondary to necrosis and hemorrhage. Nonseminomatous tumor Nonseminomatous tumors include embryonal carcinoma, endodermal sinus tumor, choriocarcinoma, and mixed germ cell tumor. On CT scans, these lesions are often large and heterogeneous, with large (>50%) areas of low attenuation. They may contain areas of calcification.