Metastatic cancer to the central nervous system is primarily deposited by hematogenous spread in various anatomically distinct regions: calvarial, pachymeningeal, leptomeningeal, and brain parenchyma. A patient’s overall clinical status and the information needed to make treatment decisions are the primary considerations in initial imaging modality selection. Contrast-enhanced MR imaging is the preferred imaging modality. Morphologic MR imaging is limited to delineating anatomic deraignment of tissues. Dynamic susceptibility contrast-enhanced perfusion and diffusion-weighted physiology-based MR imaging sequences have been developed that complement morphologic MR imaging by providing additional diagnostic information.
Patients with neurologic symptoms often undergo noncontrast CT as an initial screening tool for the diagnosis of metastatic disease. The presence of a mass lesion by CT may be the first clue to the presence of a previously unidentified systemic neoplastic process. Conversely, asymptomatic patients with known systemic malignancy often undergo staging of the CNS prior to the initiation of medical therapy using MR imaging.
Patients with metastatic intracranial neoplasms often present with focal neurologic deficits, which are clinically indistinguishable from acute cerebral infarction. To further confuse the clinical diagnosis, metastatic neoplasms can resemble cerebral ischemia on initial imaging examinations. CT findings that favor the diagnosis of metastatic intracranial neoplasms include round hypodensity that preferentially involves the white matter with sparing of cortical gray matter that is not confined to a specific vascular territory. MR imaging is markedly useful in the evaluation of lesions initially detected by noncontrast CT imaging. Evolving subacute infarctions with normalizing ADC metrics should not demonstrate elevated CBV by DSC perfusion imaging.
The identification and classification of metastatic tumor can be a challenging task; however, with advanced CT and MR imaging of the CNS, multiple tools are available to correctly achieve this objective. DWI and DSC MR imaging, although currently investigational to some degree, add additional diagnostic information, allowing for improved tumor characterization.
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