Roughly 25-50% of all intracranial malignancies are estimated to be metastases in hospitalised patients. There is a great variation in imaging appearances, often presenting a diagnostic challenge.

Epidemiology

Five primary tumors account for 80% of brain metastases:

1. lung cancer

2. renal cell carcinoma

3. breast cancer

4. melanoma

5. gastrointestinal tract adenocarcinomas (the majority colorectal carcinoma)

MRI features

T1: Typically shows iso- to hypointense signal. Hemorrhagic lesions may have an intrinsic high signal. Non-hemorrhagic melanoma metastases may have a high signal do to the paramagnetic properties of melanin. 

T1C+: The enhancement pattern can be uniform, punctate, or ring-enhancing, but it is usually intense. Delayed sequences may show additional lesions. 

T2: T2 imaging is usually hyperintense, though hemorrhage may alter this. 

FLAIR: FLAIR sequencing typically shows a hyperintense signal and hyperintense peri-tumoral oedema. 

DWI/ADC: The oedema is out of proportion with tumour size and appears dark on DWI. 

ADC demonstrates facilitated diffusion in oedema.