This study focuses on implementing Yellow 560 for the direct intraoperative visualization of Fluorescein Sodium stained intracranial lesions to facilitate extend of surgery, develop better treatment protocols, and improve the prognosis of a wide array of neurosurgical diseases. More specifically, for the patients who are undergoing surgical intervention for the treatment of their brain aneurysm, tumor, arteriovenous malformation or fistula, the investigators will inject the dye intraoperatively to assess for residual aneurysm, tumor or in general residual lesion which must be corrected.

The research design is a prospective clinical study. All patients enrolled in the study will prepare for surgery as per standard neurosurgical indications, procedures and institution protocols. At the time of the anesthesia induction, with the patient under general anesthesia, Fluorescein Sodium 10% (100mg/1mL) at a dose of 500mg (100mg/ml) will be administered intravenously (the optimal dosage will be determined within the study as the most minimal dose for adequate visualization will be used). Surgery will continue with the aid of the Yellow 560 microscope with a minimum delay of 10 minutes after injection to reduce the amount of leakage of Fluorescein Sodium into the surrounding tissue.

Conventional microsurgical and biopsy procedures and techniques including histopathology and intraoperative imaging will be performed through the completion of the surgery. The surgeon will resect and remove what he believes is the tumor based on previously known criteria (standards of care) regardless of what the Fluorescein angiography demonstrates. For vascular lesions, fluorescein sodium 10% (100mg/1mL) will be injected and used to assess its application after the conventional methods have confirmed the exclusion of the aneurysm. No patient's care will be affected by the results of the Fluorescein angiography.

No other additional neurosurgical instruments will be used. Patients will be treated post-operatively with conventional adjunctive therapies and imaging as deemed necessary by the attending neurosurgeon for the evaluation of extent of surgery. Follow-up data (including post-operative images assessing outcomes and complications) will be collected on all patients on standard post-operative visits. Intra-operative films or recordings and all patient data may be saved and used according to HIPAA allowances.

Inclusion Criteria:

• Adult patients (age ≥ 18)
• Diagnosed by preoperative imaging modalities to have a brain tumor (including metastatic brain tumors) or vascular lesions (aneurysm, arteriovenous malformation or arteriovenous fistula) requiring surgical intervention.
• The patient is determined by a board certified neurosurgeon to have a tumor or vascular lesion that would take up fluorescein
• Patient or legally authorized representative provides written informed consent to enroll in this study