How to accurately detect subarachnoid hemorrhages on brain computed tomography (CT)

Click here to find out about the five tips for accurate detection of a nontraumatic hemorrhage on brain CT!
Last update12th Dec 2020

Detection of subarachnoid hemorrhages on computed tomography (CT) is critical to your patient’s care because it can be the first and only imaging sign of a ruptured aneurysm.

Since subarachnoid hemorrhage findings can be very subtle on a brain CT scan, the following five tips will help you accurately detect subarachnoid hemorrhages:

  1. Look at the interpeduncular cistern.
  2. Look carefully at basilar cisterns and all cortical sulci (including the Sylvian fissures) and if there is any doubt, you may be able to confirm using magnetic resonance imaging (MRI).
  3. Use thin sections for reviewing the CT imaging.
  4. Use coronal and sagittal view reconstructions.
  5. Keep in mind that high attenuation in the cortical sulci is not always due to hemorrhaging.

Tip 1: Look for subarachnoid hemorrhages at the interpeduncular cistern

A good place to begin detecting subarachnoid hemorrhage—especially when subarachnoid blood is suspected as the cause of headaches—is the interpeduncular cistern.

This space is at the level of the midbrain and lies between the two cerebral peduncles. The interpeduncular cistern is normally filled with low attenuation cerebrospinal fluid, so when the triangular interpeduncular cistern appears white, that means it’s filled with blood.

Figure 1. Computed tomography (CT) scan of the interpeduncular cistern appearing white, indicating that it’s filled with blood.

Tip 2: Look carefully for subarachnoid hemorrhages at the basilar cisterns and all cortical sulci (including the Sylvian fissures)

Ambient cisterns

The next tip for detecting subarachnoid hemorrhages is to carefully examine the ambient cisterns lateral to the midbrain. If blood involves the posterior fossa but extends no higher, the CT scan findings suggest the diagnosis of a perimesencephalic hemorrhage.

Oftentimes, the bleeding from a perimesencephalic hemorrhage is not due to a ruptured aneurysm and can be venous in origin. The clinical course of these patients is usually more benign. However, it is still important to exclude any underlying aneurysms since a perimesencephalic hemorrhage is a diagnosis by exclusion.

Figure 2. Computed tomography (CT) scans show the posterior fossa of the ambient cisterns appearing white, suggesting that it is filled with blood. If the blood does not extend higher than the posterior fossa, it may be from a perimesencephalic hemorrhage or aneurysmal rupture.

Cortical sulci (including the Sylvian fissures)

After assessing the ambient cisterns, carefully look at all the cortical sulci. This is frequently the site of nontraumatic subarachnoid hemorrhages. Subarachnoid hemorrhages in a single sulcus can be further confirmed on fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI).

Figure 3. Computed tomography (CT) and fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) scans highlighting a small single-sulcus subarachnoid hemorrhage.

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Tip 3: Use thin-section imaging

Detecting a small subarachnoid hemorrhage can be difficult because its high attenuation is not always sharply defined on a brain CT. The clarity of the hemorrhage is dependent on the quality of the CT, the technique used, and the slice thickness of the image reconstructions. Hemorrhages are usually more apparent on 1–2 mm reconstructions.

Figure 4. Differences in the clarity of the hemorrhage are due to differences in CT quality, technique, and slice thickness.

For example, it is often easier to detect a small subarachnoid hemorrhage when you look at a 1 mm reconstruction. While a 5 mm reconstruction is standard at many sites, it can make the attenuation of a small subarachnoid hemorrhage less apparent than thinner sections.

Figure 5. A small subarachnoid hemorrhage on brain computed tomography (CT) images. The high attenuation is less apparent on a standard 5 mm reconstruction than on a 1 mm reconstruction.

High attenuation (e.g., blood) is more difficult to recognize on thick sections because it may be offset by low attenuation of the cerebrospinal fluid when they are both included within a large voxel. The blending of high and low attenuation can make the voxel resemble the surrounding brain and therefore much less apparent on CT.

Tip 4: Use coronal and sagittal view reconstructions

Computed tomography is always viewed as axial images. But, when trying to detect subarachnoid hemorrhages in patients with suspected trauma or other potential causes of hemorrhaging, reconstructions in coronal and sagittal views are valuable. These reconstructions can help detect the subarachnoid hemorrhage as well as assigning it to the correct compartment.

Figure 6. An area of high attenuation in the right cerebellar hemisphere that could be parenchymal is evident on this axial computed tomography (CT) image. Its location in the subarachnoid space is easier to establish when the scan is displayed as a coronal reconstruction.

Tip 5: Keep in mind that high attenuation in the cortical sulci is not always due to hemorrhage

A subarachnoid hemorrhage is usually the only abnormality detected after aneurysmal rupture. But in a small percentage of cases, aneurysmal rupture can result in an intraventricular hemorrhage, a subdural hematoma, or a parenchymal hemorrhage with or without subarachnoid hemorrhage.

Rarely, blood may even be evident in four compartments—the subarachnoid, subdural, parenchymal, and intraventricular compartments—as seen after this rupture of a middle cerebral artery aneurysm (Fig. 7).

Figure 7. Brain computed tomography (CT) images showing the rare occurrence of an intraventricular hemorrhage, a subdural hematoma, and a parenchymal hemorrhage with or without subarachnoid hemorrhages after a middle cerebral artery (MCA) aneurysmal rupture.

In patients who present with a parenchymal hemorrhage without a subarachnoid hemorrhage, you should consider the possibility of an underlying arteriovenous malformation, venous occlusion, cavernoma, or dural fistula.

The enlarged blood vessels that supply an arteriovenous malformation or dural fistula can be easily mistaken for subarachnoid blood on non-contrast CT scans because they usually are evident on the surface of the brain.

Sometimes, the area of high attenuation appears linear but extends too deeply into the brain to be within a normal cortical sulcus. This finding suggests that there are abnormally enlarged blood vessels associated with a vascular malformation, rather than an intracranial hemorrhage.

In patients without anemia, keep in mind that normal intravascular blood will have higher attenuation than normal brain tissue.

Figure 8. Computed tomography (CT) and catheter angiogram of an arteriovenous malformation (AVM). The CT scan highlights the high linear attenuation extending deeply into the brain. This suggests that there are abnormally enlarged blood vessels associated with a vascular malformation, rather than an intracranial hemorrhage. A catheter angiogram confirms a dural fistula with large draining veins.

The presence of a subarachnoid hemorrhage can be a critical finding for the care of patients with headaches. These five tips will help improve your detection accuracy for subarachnoid hemorrhages and subsequent patient management!

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Recommended reading

  • Mamourian, A. 2015. Learn to Read CT Angiography of the BrainPart 1: Aneurysms. Apple Books.

About the author

Alexander Mamourian, MD
Alex is a Professor Emeritus of Radiology at the University of Pennsylvania.
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