Vols. Videos

Imaging Evaluation of SAH and Aneurysm

Last Updated: October 1, 2018


  • Subarachnoid Hemorrhage (SAH)
    • Ruptured intracranial aneurysm (80% of non-traumatic cases).
    • Vascular malformations (up to 5% of cases).
    • Imaging workup
      • Non-enhanced CT (NECT) of the head (sensitivity 98%)
      • CTA
        • Performed if NECT positive
        • Specificity 100%, sensitivity 96-99.7% for >4mm aneurysms
      • MRI/MRA

        • Uncommonly used for acute SAH
    • For more information, please see the corresponding chapter in Radiopaedia
  • Intracranial Aneurysms

    • Incidence of intracranial aneurysms is 2-3% in the general population
    • Saccular aneurysms
      • Typically arise at arterial branch points
      • Far more common than intracranial arterial dissections and dissecting pseudoaneurysms
      • May be associated with high velocity flow phenomena
        • Feeding vessels to dural AVFs
        • Feeding and intra-nidal AVM vessels
        • Vessels associated with infection, trauma, and neoplasm
          • Cause fewer than 5% of all aneurysms
      • Acute onset of severe headache is the typical presentation indicating aneurysm rupture
      • Progressive sequela

        • Acute hydrocephalus
        • Vasospasm
        • Cerebral edema
        • Ischemic infarct
        • Herniation
        • Coma
        • Death
    • For more information, please see the corresponding chapter in Radiopaedia

  • Imaging workup

    • CTA superior to MR for detection
    • MRI/MRA for surveillance of unruptured/treated aneurysms
    • DSA
      • Gold standard for evaluation of aneurysms detected by cross sectional angiography and for pre- and postoperative characterization
      • Allows endovascular access for embolization
      • Rotational angiography using C-arm greatly enhances spatial and temporal resolution compared to CT


Non-enhanced CT

  • Technique
    • Axial 5mm collimation is typical
    • If acquired helically, reformatted images in coronal/sagittal planes can be made available in difficult cases, though are generally not needed
  • Findings

    • Hyperdense (50-75 HU) foci or collections in sulcal and/or cisternal subarachnoid spaces representing subarachnoid hemorrhage (SAH)
      • High sensitivity of detection (>75%)
      • Variable appearance can suggest aneurysm location, with SAH often thickest near site of rupture
        • Interhemispheric cisterns (AComA aneurysms)
        • Suprasellar cistern (IC-PComA, AComA aneurysms)
        • Sylvian fissure (MCA bifurcation aneurysms)
        • Prepontine, cerebellopontine cisterns, 4th ventricle (PICA, BA, VA aneurysms)
      • Perimesencephalic subarachnoid hemorrhage (see DDx)

        • Distinct pattern of SAH
        • Centered on the basal cisterns around the midbrain, interpeduncular cistern
        • For more information, please see the corresponding chapter in Radiopaedia

        • See Figures 1 and 2

    • Fisher CT grading in SAH

      • 1: No SAH visible
      • 2: Diffuse, thin layer (< 1 mm)
      • 3: Localized clot or thick layer (> 1 mm)
      • 4: Intraventricular blood
      • See Figure 3
    • Detectable sequela of SAH

      • Intraventricular hemorrhage
      • Hydrocephalus (can be seen at initial presentation, up to 90%)
      • Cerebral edema
      • Ischemic infarct
      • Mass effect
      • Herniation
      • Terson’s syndrome
        • Vitreous hemorrhage seen in up 13% of SAH cases
        • Associated with severe SAH

Figure 1: Perimesencephalic SAH - Axial NECT image of the head demonstrates hyperdense blood products layering within the perimesencephalic cisterns consistent with SAH (Fisher grade 2).

Figure 2: Perimesencephalic SAH - Axial NECT image of the head demonstrates hyperdense blood products layering within the perimesencephalic cisterns eccentric to the left consistent with SAH (Fisher grade 2).

Figure 3: Non-perimesencephalic SAH - Axial NECT image of the head demonstrates hyperdense blood products layering within the suprasellar cisterns anterior the midbrain and expanding into the sylvian cisterns consistent with SAH (Fisher grade 3). Note the enlargement of the temporal horns indicating developing hydrocephalus.

  • Limitations
    • Insensitive for detection of the aneurysm itself given lack of contrast and collimation, though large and giant aneurysms and resultant mass effect can be detected
      • Thrombosed aneurysms may be identifiable, appearing hyperdense to parenchyma
      • Aneurysm may demonstrate mural calcifications
    • Beam hardening artifact obscuring SAH

      • Bone (Skull base and posterior fossa)
      • External or foreign body metallic artifact
      • High attenuation of previously treated aneurysms by endovascular coil mass or aneurysm clips
  • DDx

    • AVM or AVF, intracranial and cervical spine
    • Nonaneurysmal perimesencephalic SAH
      • Potentially venous origin
      • Diagnosis of exclusion
    • Intracranial arterial dissection
    • Traumatic SAH

      • For more information, please see the corresponding chapter in Radiopaedia

    • Cavernous malformations
    • Vasculitis or other vasculopathy
    • Leptomeningeal metastasis, hemorrhagic metastasis
    • Leptomeningeal infection (meningitis), infectious (mycotic) aneurysms
    • Diffuse cerebral edema (Pseudo-subarachnoid hemorrhage sign)

      • Diffuse hypoattenuation of the brain parenchyma makes normal arteries look hyperdense, can mimic SAH
      • For more information, please see the corresponding chapter in Radiopaedia

      • See Figure 4

    • Reversible cerebral vasoconstriction syndrome (RCVS)
    • Recent intrathecal contrast administration
    • Subarachnoid leakage of recent intra-arterial contrast injection, often in area of recent infarct
    • See Figure 5

Figure 4: Pseudo-subarachnoid hemorrhage sign – Axial NECT images of the head for two different patients demonstrate diffuse cerebral edema indicated by diffuse hypoattenuation of the brain parenchyma. Note the relative hyperdensity of the normal cerebral arteries within the subarachnoid space, mimicking SAH.

Figure 5: Contrast extravasation - Axial NECT image of the head demonstrates leakage of contrast into the subarachnoid space following intra-arterial contrast injection for angiography and thrombectomy, due to increased vascular permeability from recent completed infarct.

CT Angiography

  • Technique
    • Thin collimation (1-2mm) is typical
    • Reconstructions
      • Reformatted imaging in coronal/sagittal planes
      • Maximum intensity projection (10mm slabs)
      • Volumetric reconstructions
    • Iodinated contrast injection rates of 4–5 mL/sec of highly concentrated medium (iodine, 350–370 mmol/mL) are preferable
    • Can be done concomitantly with NECT
  • Findings

    • Patent aneurysms typically demonstrate uniform postcontrast enhancement
    • High sensitivity for aneurysm detection
      • 90-95% sensitive for aneurysms ≥ 2 mm
      • 96-99% sensitive for aneurysms ≥ 4 mm
    • High specificity (~100%)
    • CTA is superior to MRA for detection and overall characterization of saccular aneurysm geometry
    • May detect dissecting aneurysm
    • Demonstration of active extravasation

      • Focal enhancement within area of hemorrhage or hematoma (“spot sign” - rare)
  • Limitations

    • Beam hardening artifact
      • Bone (Skull base and posterior fossa)
        • Can obscure PComA and AComA aneurysms as well as posterior circulation aneurysms
        • Dual energy CT has promise in improving detection of aneurysms in these locations as threshold HU values can be selectively removed
      • Metallic artifact

        • High attenuation of endovascular coil mass or aneurysm clips can obscure
    • 3D reconstruction and surface shading software may be prone to artifacts
    • Limited or judicious use in patients with iodine-based allergy and/or renal dysfunction
    • may require contrast dose adjustment considerations for subsequent catheter angiography
    • Typically, proceed to conventional angiography if NECT is consistent with SAH and CTA is negative
  • Pitfalls

    • Vessel loops and infundibula can mimic or be mistaken for aneurysms on CTA as well as MRA
    • Vessel Infundibulum
      • < 3 mm, conical protrusion with vessel arising directly from apex (may not be definitive in some cases)

Figure 6: On this lateral digital subtraction angiography image, the small infundibulum is clearly visible at the origin of the posterior communicating artery from the distal internal carotid artery (arrow), but even without the visible presence of the posterior communicating artery, small focal outpouching in this area implies an infundibulum rather than aneurysm.


  • Technique
    • Multiple pulse sequences available to enhance various tissue properties as well as dynamic properties (flow, velocity)
    • Ability to detect vascular flow
      • High-velocity flow in arteries, veins, and aneurysms appears as a signal loss on MRI (“flow voids”)
      • Occurs when protons in flowing blood exit the selected slice too quickly to acquire both the 90- and 180-degree pulses used to produce a spin echo
    • Generally does not require IV contrast
    • Spoiled Gradient-recalled echo (SPGR) contrast-enhanced T1 images (3-D acquisition) may show best view of aneurysm
  • Findings

    • Very high sensitivity for detection of SAH
      • FLAIR (87%) and SWI (88%)
      • Combined FLAIR and SWI (~100%)
    • Blood products (intra-axial or extra-axial) demonstrate five unreliable prototypical stages of evolution that are apparent on MRI:
    • Hyperacute

      • Intracellular oxyhemoglobin
      • T1 isointense
      • T2 isointense to hyperintense
    • Acute (1 to 2 days)

      • Intracellular deoxyhemoglobin
      • T2 signal intensity drops
      • T1 remains intermediate-to-dark
    • Early subacute (2 to 7 days)

      • Intracellular methemoglobin
      • T1 signal gradually increases becoming hyperintense
    • Late subacute (7 to 14-28 days)

      • Extracellular methemoglobin:over the next few weeks, as cells break down, extracellular methemoglobin leads to an increase in T2 signal
    • Chronic (>14-28 days)

      • Hemosiderin - can persist indefinitely, depending on the integrity of the blood brain barrier
      • Low T1 and T2 signal
      • Superficial siderosis (hemosiderin coating the cortex)
        • Uncommon
        • Often due to chronic, repeated SAH
        • Can cause atrophy and dysfunction of associated cortex
    • Acute SAH will demonstrate patterns anatomically analogous (cisterns/sulci) to other axial imaging with variable signal depending on pulse sequence and predominate blood product composition

      • T1-weighted imaging
        • Isointense to mildly hyperintense (compared to CSF)
      • T2-weighted imaging

        • Hyperintense
        • Less sensitive given similar appearance to CSF
      • FLAIR (Fluid Attenuated Inversion Recovery)

        • Hyperintense
        • Highly sensitive within the first 5 days after SAH as CSF signal is nulled
      • Gradient recalled echo - T2*WI (GRE) and susceptibility-weighted imaging (SWI)

        • Sequences show hemorrhage as black signal dropout as iron is prone to susceptibility artifacts
        • T2*-weighted MRI is very useful in diagnosing prior SAH and may also reveal the location of a ruptured aneurysm
        • Tends to exaggerate volume of blood with progressive stages (“blooming artifact”)
      • Diffusion Weighted Imaging (DWI)

        • Generally negative, though blood products will behave variably with progressive stages
        • May see parenchymal restricted diffusion (ischemic infarct) in territories of vessels affected by vasospasm
    • Aneurysm Detection

      • MRI is also highly sensitive for aneurysm detection
        • Direct visualization of aneurysmal flow void
        • Direct visualization of intra-aneurysmal contrast enhancement, particularly on SPGR post-contrast T1
        • Pulsation artifact
          • Supportive finding seen in patent aneurysms, accentuated by contrast administration because of the increased intravascular signal
        • Direct visualization of large and giant intracranial aneurysms and giant aneurysms

          • Large and giant aneurysms are often partially thrombosed and can be confused with hematoma
          • MRI is superior to CT and conventional angiography in characterizing large complex aneurysms
          • Sequela related to mass effect (ie. parenchymal edema on FLAIR)
  • Limitations

    • Prone to multitude of artifacts that can be both helpful and difficult to interpret
    • Generally, less cost-effective, less available than CT
    • CT is more easily performed in uncooperative, combative patients due to speed of acquisition
    • MRI is contraindicated in the evaluation of postsurgical patients with a ferromagnetic aneurysm clip. Modern aneurysm clips are generally non-ferromagnetic for this reason.
  • Pitfalls

    • Flow-related enhancement and echo rephasing can cause increased signal intensity within normal vascular structures simulating aneurysms
    • False diagnosis of basilar artery aneurysms is possible secondary to CSF pulsation artifact in the prepontine cistern
    • Depending on the imaging parameters, turbulent and/or slow-flowing blood within a vessel or an aneurysm can have high or mixed signal intensity rather than a flow void
    • Pneumatized bone (most notoriously, an aerated anterior clinoid) may be misinterpreted as an aneurysm flow void

MR Angiography

  • Technique
    • 3D time-of-flight (TOF)
      • Spatial resolution of modern 3D TOF MRA on the order of 1mm3
      • Axial source images
      • Maximum intensity projection (MIP) images derived from source data
      • 85-95% sensitive for aneurysms ≥ 2-3 mm
    • Contrast-Enhanced MRA

      • Useful in surveillance of coil-treated aneurysms
      • Metal suppression techniques can be employed, still being studied
    • MRA should generally be considered complementary to conventional MRI
    • MRA screening in patients with suspected familiar forms of cerebral aneurysms

      • Dynamic contrast-enhanced MRA and DSA may prove useful as complex and slow flow patterns may generate artifacts limiting evaluation of vessel lumen on conventional MRI
    • Vessel wall imaging

      • Useful in evaluation or confirmation of dissection
      • Time-of-flight (TOF) MR angiography can also demonstrate subacute intramural hematoma because of incomplete suppression completely of (stationary) tissues with intrinsic T1 shortening (phase-contrast MR angiography and contrast-enhanced MR angiography demonstrate only the vessel lumen)
      • High resolution T1-weighted images with fat saturation can demonstrate crescent-shaped hyperintense false lumen around eccentric (narrowed) flow voids
  • Pitfalls

    • Intrinsic T1 shortening (T1 hyperintensity), such as in subacute hemorrhage or thrombosis, may simulate flow on TOF MRA

Digital Subtraction Angiography

  • Pre-procedural evaluation
    • Pre-procedural laboratory data
      • Platelet count, prothrombin time, international normalized ratio, partial thromboplastin times to evaluate for a bleeding diathesis
      • Blood urea nitrogen and creatinine to look for renal dysfunction
    • Renal insufficiency predisposes to contrast-induced nephropathy

      • Prehydration with sodium bicarbonate (130 mEq/L IV solution at 3.5 mL/kg bolus over 1 hour, then 1.2 mL/kg/hr during the procedure and for 6 hours after the procedure)
      • Administration of N-acetylcysteine (600 mg orally at 24 and 12 hours before and after the procedure) may prevent contrast-induced nephropathy
      • Noninvasive imaging and previous angiography should be reviewed for preoperative clinical assessment and treatment planning
    • Physical examination

      • Vital signs and cardiopulmonary status
      • Complete neurological examination and documentation of any preprocedural deficits
      • Extremity pulse evaluation, including bilateral dorsalis pedis and posterior tibial pulses for transfemoral approaches, and Allen’s test for patients and procedures requiring radial artery access
    • Contrast Allergy

      • True contrast allergy is rare
      • If suspected, premedication with prednisone 50 mg orally at 24, 12, and 1 hour as well as diphenhydramine 50 mg orally or intravenously 1-hour preoperatively is a standard protocol to prevent an allergic reaction.
    • Informed consent prior to an angiogram should include an estimate of complication risk

      • Neurological complications
        • Cerebral ischemic events that occur as a result of thromboembolism, air emboli (more common)
        • Disruption of atherosclerotic plaques and vessel wall injury such as perforation, dissection (less common)
      • Overall rate of neurological complications is in the range of ~1%

        • Transient/reversible complications occurring about 2x the rate of permanent complications
      • Relative risk of complications increases with the following:

        • Prolonged angiography procedure time
        • Worse underlying primary disease process
        • Underlying comorbidities (atherosclerotic carotid disease, recent cerebral ischemic event, advanced age, hypertension, diabetes, renal insufficiency)
  • Technique

    • Conventional 4-vessel angiogram considered gold standard for aneurysm detection and characterization
    • Biplane angiography
    • Sterile prep
    • Trans-femoral approach is typical
    • Modified Seldinger technique, short introducer sheath commonly used
    • Longer sheath useful when high arterial tortuosity or high atherosclerotic burden might impair catheter navigation
    • Catheter selection varies, typically 4F or 5F, advanced over hydrophilic wires
    • Heparinized saline, double flushes
    • Contrast - hand injected or power injected
      • Bilateral internal and external carotid arteries
        • Thorough evaluation of the anterior communicating artery complex
        • May be necessary to perform an internal carotid artery cross-compression study
      • Bilateral vertebral arteries, including bilateral posterior inferior cerebellar arteries

        • Can be performed via a dominant vertebral artery
    • Multiple fluoroscopic projections
    • Stereoscopy

      • Views obtained in slightly different projections, viewed with prism glasses or by crossing eyes
      • Can be valuable in complex cases, particularly in understanding venous anatomy and in AVM/AVF cases
    • Three-dimensional (3D) rotational angiography reconstruction modeling
    • Leads to more sensitive/specific aneurysm detection
    • Provides invaluable anatomic information about aneurysm neck and geometry
    • Catheter angiography effectively demonstrates anatomic detail such as aneurysm size, geometry, and neck-to-dome ratio
    • Very high spatial resolution compared to current cross-sectional imaging paradigms

      • Spatial resolution of flat-panel volume CT (rotational angiography using a C-Arm) ranges between 200 and 300 μm in high-resolution mode
      • Spatial resolution of multislice CT scanners is up to 600μm
    • Time-resolved, dynamic study

      • Demonstrates collateral circulation and flow dynamics within the corresponding vascular territories
      • Catheters can be exchanged over wires for triaxial micro catheter systems in endovascular therapies
  • Findings

    • Saccular aneurysm
      • Saccular outpouching at arterial branch point
      • Size and Morphology
        • Small (< 0.3 cm) to giant (> 2.5 cm)
        • Vary in complexity from round to ovoid with daughter lobe(s)
        • Narrow to wide-necked
        • Branch vessel may be incorporated into aneurysm neck or dome
          • Can lead to technical difficulties or preclude coil embolization
          • Flow-diverting embolization devices becoming widely available for aneurysms with wide necks, unfavorable for coiling alone
        • Anterior carotid circulation (85%-90%)

          • Anterior communicating artery (30%)
            • See Figure 6
          • Posterior communicating artery (25%)

            • See Figure 7
          • Middle cerebral artery bifurcation/trifurcation (20%)
        • Posterior vertebrobasilar circulation (10%-15%)

          • Basilar artery trunk and bifurcation (basilar tip) (10%)
          • Vertebral-posterior inferior cerebellar artery (3%)
        • Rare locations (<<1%) include

          • Pontine perforators, persistent trigeminal artery, fenestration aneurysms
        • Multiple aneurysms (20%), can be familial cases
      • Evaluate for segmental luminal narrowing of vasospasm in adjacent territory
      • Active extravasation can be detected (rare)
      • For more information, please see the corresponding chapter in Radiopaedia

    • Blister Aneurysm

      • Smooth bleb-like outpouching from arterial side wall
      • Typically small (< 6 mm length), wide neck
      • Not necessarily associated with major arterial branch point
      • Along the supraclinoid ICA near the carotid terminus
        • MCA, ACA, ACoA, basilar artery are much more rare
      • May be angiographically occult

        • CTA may demonstrate subtle, asymmetric bulging of supraclinoid ICA
        • Best evaluation with high-resolution DSA, multiple projections, and 3D angiography
      • For more information, please see the corresponding chapter in Radiopaedia

    • Fusiform aneurysms resulting from advanced atherosclerotic disease

      • Exaggerated dolichoectasia (“elongated and dilated”) with focal fusiform aneurysmal dilatations (basilar artery diameter of > 4.5mm)
      • Tend to involve the vertebrobasilar arteries more often than carotid circulation
      • Tend toward large or giant aneurysms (> 2.5 cm), indolent growth
      • Propensity for cranial nerve compression syndromes
        • Vertebrobasilar ectasia on trigeminal nerve
        • Cavernous-supraclinoid carotid ectasia on optic nerve, chiasm
      • Increased incidence of vertebrobasilar occlusions, brain stem ischemia/infarct
      • Older patients
    • Fusiform aneurysms resulting from vasculopathy

      • Segmental ectasia due to inherited collagen-vascular diseases, viral, neurocutaneous syndrome, post-radiation
      • Younger patients
    • Dissecting Aneurysm and Pseudoaneurysms

      • Luminal irregularity, abrupt narrowing/dilatation
      • Pseudoaneurysmal saccular outpouching may be seen
      • Locations
        • Petrous/cavernous ICA, vertebral artery (more common)
      • Relatively rare
      • Potential etiologies of pseudoaneuryms

        • Trauma (penetrating or blunt)
        • Spontaneous dissection, underlying vasculopathy
        • Infection, inflammation (mycotic aneurysm) usually occurring in unusual peripheral locations
          • See Figure 8
        • Associated with neoplasm
    • Aneurysms associated with Vascular Malformations (discussed in other sections)

Figure 7: Anterior Communicating Artery Aneurysm – The large aneurysm arising from the anterior communicating artery demonstrates an ovoid appearance and homogeneous contrast filling on CT during the arterial phase (top row left). After injection of the left internal carotid artery during digital subtraction angiography (top row right), the aneurysm also begins to fill during the arterial phase. T2-weighted MR image of the aneurysm (bottom row) shows the typical smooth black flow-void that is characteristic of non-thrombosed aneurysms. A small amount of adjacent hyperintense edema may be reactive to the mass effect.​

Figure 8: Posterior Communicating Artery Aneurysm – This P-Comm Aneurysm appears pedunculated but is clearly shown to incorporate the proximal (carotid) aspect of the posterior communicating artery on this 3-D reformat from DSA (top row left). A small bulbous outpouching from the aneurysm apex portends a higher risk for hemorrhage. CT Angiography images can be reformatted (top row right) to create images that may be helpful for anatomic localization for the neurosurgeon planning to clip an aneurysm. Axial FLAIR MR image (bottom row) demonstrates heterogeneity of this aneurysm due to flow-related signal changes.

Figure 9: Mycotic Aneurysm – The 3-D reformatted CT Angiography (top row left) shows an aneurysm in an unusual location arising from the distal posterior cerebral artery (arrow). The contrast-enhanced T1-weighted MRI (top row right) also demonstrates the location of this enhancing aneurysm in the lateral left occipital lobe with surrounding hypointense edema. More inferiorly, contrast-enhanced MRI (bottom row left) also shows several small enhancing abscesses in and around the basal ganglia, often arising contemporaneously with mycotic aneurysms due to their shared etiology. These lesions are often associated with hemorrhage, as demonstrated by the low signal intensity areas on SWI MR image (bottom row right).

  • Pitfalls
    • DSA negative in 15% of aSAH, consider repeating DSA in 5-7 days
    • Non-aneurysmal perimesencephalic SAH
      • Up to 95% of cases will have normal cerebral angiogram, source of bleeding not identified
      • Thought to be venous-origin bleed
    • Evaluation and detection of saccular aneurysms is dependent on optimal projections
    • Spontaneous partial or complete aneurysm thrombosis and presence of vasospasm
    • Failure to evaluate the ECA circulation may result in failure to demonstrate a dural AVF as cause of SAH

Summary and Recommendations

Imaging recommendations include initial evaluation of suspected SAH with nonenhanced CT (NECT). If NECT is positive for SAH, the study should be followed by multiplanar CTA which gives important complementary anatomic detail to DSA. Proceed directly to DSA if NECT is consistent with SAH, but CTA fails to demonstrate the culprit aneurysm. Consider MRI with or without MRA in cases of large complex aneurysms with suspected mass effect and resultant cranial nerve dysfunction as well as in cases where DSA and CTA fail to demonstrate an aneurysm. Lastly, rely on MRI with MRA in the surveillance of unruptured and treated aneurysms.

For more information, please see the corresponding chapter in Radiopaedia, and the Giant Cerebral Aneurysms chapter within the Brain Tumor Mimics sub-volume of the Neurosurgical Atlas.

Contributor: Daniel Murph, MD

DOI: https://doi.org/10.18791/nsatlas.v2.03.03.01


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