Introduction:
Intracranial aneurysms are abnormal focal dilatations of cerebral arteries, most commonly saccular (berry) aneurysms arising at arterial bifurcations. They account for approximately 80–90% of non-traumatic subarachnoid hemorrhages (SAH). Imaging plays a pivotal role in detecting, characterizing, and planning the management of aneurysms.
Imaging Modalities:
1. Non-Contrast CT (NCCT Brain):
- First-line imaging in suspected rupture (acute SAH).
- Sensitivity is:
- ~95% within first 24 hours,
- drops to <60% after 5 days.
- Hyperdensity in basal cisterns, sylvian fissures, or interhemispheric fissure suggests SAH.
- Cannot directly visualize aneurysms unless thrombosed or calcified.
2. CT Angiography (CTA):
- Primary tool for non-invasive aneurysm detection.
- Sensitivity ~95%, specificity ~90% compared to DSA.
- Provides information on:
- Size, shape, location,
- Neck width, dome-to-neck ratio, and aspect ratio.
⚖️ Important Ratios in CTA:
- Dome-to-neck ratio = Dome diameter / Neck width
- >1.5 is favorable for endovascular coiling.
- <1.5 suggests wide-necked aneurysm—may need adjuncts (stent/balloon).
- Aspect ratio = Dome height / Neck width
- >1.6 correlates with higher rupture risk.
3. MR Angiography (MRA):
- Performed with:
- Time-of-Flight (TOF) technique (non-contrast),
- Contrast-enhanced MRA (better for large/complex aneurysms).
- Preferred in:
- Asymptomatic screening (e.g., familial cases),
- Follow-up post intervention.
- Limitation: less sensitive for aneurysms <3 mm.
4. Digital Subtraction Angiography (DSA):
- Gold standard with highest spatial resolution.
- Detects aneurysms <2 mm.
- Allows dynamic evaluation of:
- Aneurysm neck, parent vessels, collateral circulation.
- Useful for pre-intervention planning.
5. MRI Brain (Conventional Sequences):
- May show:
- Flow void in large aneurysms,
- Thrombosed aneurysm as complex T1/T2 heterogeneous mass,
- Wall enhancement on post-contrast MRI (suggests inflammation).
- Black-blood MRI and high-resolution vessel wall imaging useful in assessing:
- Aneurysm instability,
- Dissection,
- Risk of rupture.
Common Locations:
| Location | % of Cases |
|---|---|
| Anterior communicating artery | ~30–40% |
| Posterior communicating artery | ~25% |
| Middle cerebral artery bifurcation | ~20% |
| Internal carotid artery | ~10% |
| Basilar tip / vertebrobasilar | ~5–10% |
Morphological Classification:
- Saccular (berry) – Most common.
- Fusiform – Spindle-shaped dilatation, often atherosclerotic.
- Dissecting – Associated with intimal tear.
- Mycotic – Infective etiology, often distal.
- Blister aneurysm – Fragile, broad-based, seen on dorsal ICA.
Post-Treatment Imaging:
- CTA / MRA for follow-up.
- DSA for:
- Residual aneurysm neck,
- Recanalization,
- New aneurysm detection.
- MRI for surrounding gliosis, hemosiderin, infarcts.
Key Points:
- CTA is the initial investigation of choice.
- Dome-to-neck ratio and aspect ratio aid in treatment planning and rupture risk.
- MRA is good for screening and follow-up.
- DSA remains gold standard for pre-surgical/intervention evaluation.
