Specific Imaging Finding

Metastatic neoplasms to the brain can occur anywhere; however, most are found at the supratentorial peripheral gray–white matter junction, followed by the cerebellum and basal ganglia. They are discrete, multiple or solitary masses, with variable degree of vasogenic edema in the surrounding white matter. The edema and mass effect are often very prominent and out of proportion to the lesion size except with cortical and very small metastases, where edema may be minimal. A majority are hypodense on CT and of low T1 signal. Tumors with high nuclear/cytoplasm ratios or mucinous contents, such as adenocarcinomas, are CT hyperdense. Necrotic and cystic tumors are T2 hyperintense. Highly cellular tumors show iso- to hypointense signal, while mucinous contents and calcifications lead to very low T2 signal, typically seen with adenocarcinomas. Non-hemorrhagic metastases always enhance with contrast, either in a nodular or ring-like pattern, typically with irregular but sharp margins. Volumetric T1WI improves detection of punctate deposits without edema. Metastatic tumors show variable MRI diffusivity characteristics. However, the center of ring-enhancing lesions is characteristically dark on DWI and bright on ADC maps, consistent with high diffusivity, with very rare exceptions. Perfusion studies reveal increased rCBV of the enhancing tumor, with delayed signal recovery on the signal intensity–time curve, in contrast to normal brain and gliomas. MRS shows nonspecific increased choline and decreased NAA levels. Both the MRS spectra and rCBV are essentially normal in the adjacent non-enhancing edema.