To compare the safety and efficacy of treating small aneurysms with coils specially designed for small aneurysms to historically reported rates of safety and efficacy for the treatment of larger aneurysms.

This is a prospective cohort study whereby patients who undergo endovascular treatment with coils for intracranial aneurysms will be studied for initial procedural and 12-18 month post-treatment outcome. The procedural failure rate (defined above) of treating small aneurysms with specially designed coils will be compared to pre-specified historical occurrences. This evaluation will occur within the framework of "non-inferiority" (i.e. comparable success to coiling of large aneurysms).

Inclusion Criteria:

• Patient presenting with a ruptured or un-ruptured cerebral aneurysm appropriate for endovascular treatment as determined by the neurovascular team (neurosurgeon/neurointerventionalist)
• Patients between (and including) 18 and 90 years of age.
• Patient HUNT AND HESS Grade 0-3.
• Where required, patient has given fully informed consent to endovascular coiling procedure. If patient cannot consent for themselves, appropriate written consent has been sought from their surrogate, or from appropriate power of attorney.
• Aneurysm < 4mm in maximum diameter.
• Patient is willing and able to return for clinical evaluation and follow-up imaging evaluation (angiography or MRA) at both 6-months and 12-18 months after endovascular treatment.
• The aneurysm has not previously been treated (by coiling or clipping).
• A three-dimensional angiogram has been performed.
• Placement of at least one "Nano" coil, at, or at least close to the neck of the aneurysm.

Exclusion Criteria:

• Patient has more than one aneurysm requiring treatment in the current treatment session. If a patient has multiple aneurysms, but only one will be treated at the time of enrollment, they are eligible for the trial. (Additional aneurysms may be treated at a later date, and may be treated with any coil type that the operator chooses).
• Patient has an H&H score of 4 or 5 after subarachnoid hemorrhage (SAH).
• Medical or surgical co-morbidity such that the patient's life expectancy is less than 1 year.
• Less than 80% by length of Stryker endovascular coils are implanted.

Patients with cerebral aneurysms less than 4mm