Response of Brain Metastases After Gamma-Knife Radiosurgery Using Dynamic Susceptibility-weighted Contrast-enhanced Perfusion Magnetic Resonance Imaging

Inclusion Criteria:

* Patient must have newly diagnosed brain metastases visible on brain MRI. A biopsy of the lesion is not required as long as the patient has a biopsy-proven malignancy elsewhere and a clinician deems the lesion to be metastatic.
* At least one brain metastasis must be ≥ 1cm to allow adequate quantitative imaging measurement for DSC-PMR.
* Patient must be clinically eligible for and elect to be treated with GK alone without WBRT.
* Patient must be ≥ 18 years of age.
* Patient must have Karnofsky Performance Status (KPS) of at least 60
* Patient may be part of other clinical trials (as long as no other local treatments beyond GK such as WBRT or other local therapy are indicated to the brain) or imaging studies.
* Patient or legally authorized representative must be able to understand and willing to sign a written informed consent document.

Exclusion Criteria:

* Patient must not have any medical contraindications to MRI (e.g., unsafe foreign metallic implants, incompatible pacemaker, inability to lie still for long periods, severe to end-stage kidney disease or on hemodialysis).
* Patient must not be pregnant or breastfeeding.
* Patient must not have an estimated glomerular filtration rate (eGFR) \< 60 mL/min/1.73m2.
* Patient must not have melanoma.
* Patient must not have hemorrhagic lesions.