Central nervous system disorders

VI./3.2.: Computed tomography – CT

CT images the metastases as hypo-, iso- and hyperdense structures compared to the adjacent cerebral parenchyma. Hypodensity is characteristic of metastases of lung, breast and renal origins. Usually it is surrounded by broad edema – thus metastasis cannot be differentiated on unenhanced images.

Hyperdense metastases are well visible on the unenhanced CT scan: this type of metastasis is frequent in cancers of GI origin and small cell lung cancer. Hemorrhagic metastases are also hyperdense. Metastases of melanoma and choriocarcinoma tend to be so (bleeding is usually accompanied by acute neurologic symptoms). The rarely parenchymal lymphoma and leukemia (chloroma) deposits (lymphomas tend to be leptomeningeal) are mildly hyperdense. Obviously, it cannot be caused neither by calcification nor bleeding, but hypercellularity, large nuclei. Mixed density is characteristic of metastases of renal and pulmonary origin as well besides melanomas. If the metastasis is isodense and there is no surrounding edema, detection by unenhanced CT is impossible. Therefore, contrast enhanced examination must be performed definitely.

As long as the metastasis is small, usually enhances weakly, enhancement can be annular, especially in case of large metastases (central necrosis, but there are primarily cystic metastases as well, e.g. RCC – fluid-fluid level can be observed in these). Annular enhancement of metastasis is thick, in contrast to the thin peripheral enhancement of the abscess, which might be helpful in the differentiation from the abscess. Enhancement of vanishing hematomas can be also annular. Attention! Certain lesions enhance delayed only! If there is no other choice available for searching metastasis (MRI), another CT scan is worthy to be performed with a larger dose of contrast agent (infusion) or even following an hour long awaiting time.

CT can demonstrate bony destruction, spreading to bone and bone metastases well.