Recognizing a subarachnoid hemorrhage on CT

In this video, from our Brain CT Essentials course, you'll learn how to recognize and correctly localize subdural, epidural, and subarachnoid hemorrhages on CT.

Alexander Mamourian, MD
Alexander Mamourian, MD
29th Sep 2020 • 4m read

Detection of a subarachnoid hemorrhage on CT is critical to the care of your patient, since it can be the first and only sign of a ruptured aneurysm. In this video, from our Brain CT Essentials course, you'll learn how to recognize and correctly localize subdural, epidural, and subarachnoid hemorrhages on CT.

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Video transcript

Detection of subarachnoid hemorrhage on CT is critical to the care of the patient, since it can be the first and only sign of a ruptured aneurysm. But you need to be aware that the findings of subarachnoid hemorrhage can be very subtle on CT, so that you will examine the scan appropriately.

When subarachnoid hemorrhage is suspected as the cause of headaches, a good place to begin your search on CT is the interpeduncular cistern. This space is at the level of the midbrain and as its name indicates, it lies between the two cerebral peduncles. This space is normally filled with low attenuation cerebrospinal fluid. So when the triangular cistern is visible because it's white, that means it's filled with blood.

You should also carefully exam the ambient cisterns lateral to the midbrain, where subarachnoid hemorrhage is visible in these two cases. When blood involves the posterior fossa but extends no higher than the ambient cisterns. It suggests what is called perimesencephalic hemorrhage. In those patients the bleeding is frequently not due to a ruptured aneurysm and maybe venous and origin.

The clinical course may also be more benign, but it is still necessary to exclude a ruptured aneurysm, since this is a diagnosis of exclusion. Carefully look at all the cortical sulci as well, since frequently this will be the only sign of non-traumatic subarachnoid hemorrhage. This patient CT scan demonstrated a small subarachnoid hemorrhage in a single sulcus, that was confirmed on a FLAIR MR scan.

The CT scan on your right is from another patient with a small subarachnoid hemorrhage. Notice that the asymmetric high attenuation in this case is less sharply defined, compared with the case we just saw, and shown here on your left again. The conspicuity with the hemorrhage is dependent on the quality of the CT, the technique used to create the CT images, and the slice thickness.

Hemorrhage is usually more apparent on reconstructions of one to two millimeter. For example, notice how much easier it is to see the small subarachnoid hemorrhage in this case, when you look at the one millimeter reconstructions of the same data set. The image on your left is the usual five millimeter reconstructions, and while that is standard at many sites, it can make the high attenuation of a small subarachnoid hemorrhage less apparent than the thinner sections.

The middle and right images are both one millimeter reconstructions of the same data, and both more clearly demonstrate the subarachnoid hemorrhage. The reason that high attenuation blood is more difficult to recognize on thick sections is that high attenuation is offset by low attenuation of cerebrospinal fluid when they are both included in a large voxel.

The blending of high and low attenuation can make the voxel resemble the surrounding brain and therefore much less apparent. We always view CT on axial images but in patients with suspected trauma or subarachnoid hemorrhage reconstructions in coronal and sagittal views can be very helpful for the detection of the hemorrhage, as well as assigning it to the correct compartment.

In this case, the high attenuation evident on CT in the right cerebellar hemisphere could be parenchymal. But, its location in the subarachnoid space is much easier to establish when the scan is displayed as a coronal reconstruction. Even though subarachnoid hemorrhage is usually the only abnormality evident after aneurysm rupture.

In a small percentage of cases aneurysm rupture can also result in intraventricular hemorrhage, subdural hemorrhage, or parenchymal hemorrhage, with or without subarachnoid hemorrhage. In this case, blood was evident in four compartments - that is subarachnoid, subdural, parenchymal, and intraventricular after rupture of a middle cerebral artery aneurysm.

But in patients who present with parenchymal hemorrhage without subarachnoid hemorrhage, you should first consider the possibility of an underlying arteriovenous malformation, venous occlusion, cavernoma, or dural fistulas. The enlarged blood vessels that supply an arteriovenous malformation or duralal fistula, because they are usually on the surface of the brain, can be mistaken for subarachnoid blood on noncontrast CT.

While the findings in this case resemble subarachnoid hemorrhage we've just seen, notice that the high attenuation appears to be within the brain itself rather than the cortical sulci. That finding suggests that these are abnormally enlarged blood vessels and associated with a vascular malformation rather than intracranial hemorrhage.

Keep in mind that normal intravascular flowing blood will look whiter than normal brain due to its high attenuation in those patients who don't have anemia. The catheter angiogram in this patient demonstrated a dural fistula with large straining veins. That finally accounts for the abnormal vessels that we saw on the noncontrast CT scan.

So carefully review the CT scan for subarachnoid hemorrhage using multiplanar reconstructions and thin sections, since its presence can be a critical finding in the workup of patients with headaches. And keep in mind that not all linear high attenuation is hemorrhage on CT.