Hyperplasia And Cyst


Thymic hyperplasia

Thymic hyperplasia and normal thymus share the same characteristics on MRI. In the case of thymolipoma, T1-weighted MRIs reveal high signal intensity, which represents fat; strands of intermediate intensity represent thymic tissue.

Thymic cyst

T1-weighted MRIs of thymic cysts reveal low signal intensity; T2-weighted images show high signal intensity consistent with the fluid component of the lesion. T1-weighted images naturally show high signal intensity if the cyst contains blood from hemorrhage or if it is rich in proteinaceous fluid.
Qualitative evaluation of gross thymic morphology (ie, size, shape, margins, and signal intensity) is usually sufficient for distinguishing normal thymus from abnormal thymus. The abnormal thymus generally is enlarged, multilobular, or inhomogeneous because of the presence of cystic degeneration, hemorrhage, septations, fibrosis, or calcification, as seen on pathologic sections. In patients with lymphoma, associated lymphadenopathy is helpful in distinguishing normal thymus from abnormal thymus.
On T1-weighted spin-echo images, thymic lipomas have areas of high signal intensity, because of their fat content; the signal intensity is similar to that of subcutaneous fat, with areas of intermediate signal intensity reflecting the presence of soft tissue. Although thymic lipomas can attain a large size, they invariably do not invade surrounding structures. However, they can cause mass effect on the surrounding structures because of their size.
MRI findings in thymic carcinoid tumors are nonspecific and are identical to those of thymoma.
On T1-weighted images, thymic carcinoma has higher signal intensity than muscle; on T2-weighted images, there is an increase in signal. Heterogeneous signals often reflect the presence of necrosis, cystic degeneration, or hemorrhage.
Teratomas have various appearances on MRI, depending on the composition of the tumor. They commonly contain fat, which is of high signal intensity on T1-weighted images. Cystic changes may also be present; such changes have low signal intensity on T1-weighted images, but they have increased signal intensity on T2-weighting.
MRI often shows the inhomogeneous nature of seminomas.

Lymphoma and residual tumor

The MRI signal characteristics of untreated lymphomas are different from those of treated lymphomas. Untreated lymphomatous tissue has high signal intensity, whereas a homogeneous, hypointense pattern is characteristic of inactive residual fibrotic masses in patients receiving successful therapy for lymphoma. A heterogeneous pattern with mixed hypointensity and hyperintensity is often seen in untreated nodular sclerosing Hodgkin disease. A heterogeneous pattern with mixed areas of low and high signal intensity on T1- and T2-weighted images is seen in lesions containing mixed fat (high signal intensity) and fibrous tissue (low signal intensity). This pattern is seen after treatment of patients with sterilized tumors. (Positron emission tomography [PET] CT may play a role in assessing for residual lymphoma.) [3, 4, 10, 5, 6, 11] 

Degree of confidence

With MRI of the thorax, motion artifacts may occur. Breathing motion and pulsation of the heart and great vessels can markedly degrade image quality. Hence, in general, fast imaging sequences and artifact reduction techniques must be used for MRI of the mediastinum and chest.
MRI provides information similar to that provided by CT scanning in the evaluation of thymomas. MRI is particularly useful when an intravenous contrast agent cannot be administered for use with CT because the patient is allergic to the agent.

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