Hemorrhage or not? Determining the cause of high attenuation on brain computed tomography (CT)

Not all high attenuation in the brain is due to hemorrhaging. When you see high attenuation on a computed tomography (CT) scan, several other diseases should be considered—especially if they are more consistent with the patient’s clinical history!

Magnetic resonance imaging (MRI) and computed tomography angiography (CTA) are valuable when evaluating other possible causes of high attenuation on brain CT, which may include the following:

• Vascular malformations of the brain
• Sarcoidosis
• Meningiomas and lymphomas

Case 1: Diagnosing a cavernous angioma

A 27-year-old patient presented to the emergency room with new, severe headaches. Her CT demonstrated an area of high attenuation in the inferior left frontal lobe. The patient had a complete work-up for nontraumatic hemorrhaging that included a CTA (which was normal), followed by MRI.

Corresponding to the high attenuation seen on CT, the patient’s magnetic resonance T2-weighted scan revealed a lesion with a dark rim and mixed high and low signal in the center. This appearance on both CT and MRI is typical for a cavernous angioma.

Cavernous angiomas are vascular malformations of the brain that can occasionally hemorrhage, but their natural history in any individual patient may be difficult to predict.

Case 2: Diagnosing an arteriovenous malformation

In this next case, a 26-year-old woman presented with headaches to the emergency room. Her CT showed two areas of high attenuation. These were initially thought to be due to a hemorrhage, but proved to be dilated blood vessels from an underlying arteriovenous malformation (AVM).

You should be aware of the CT findings for an unruptured AVM so that they are not mistaken for an acute hemorrhage, and appropriate additional imaging is ordered. Arteriovenous malformations can be distinguished from cavernomas on an MRI, but digital subtraction angiography (DSA) is the definitive test.

Look for rounded or linear high attenuation caused by dilated blood vessels within or on the brain CT scan, with or without calcifications.

Normal flowing intravascular blood has a higher attenuation than the normal brain, which explains why dilated blood vessels can be mistaken for parenchymal or subarachnoid hemorrhages. Often, you can determine if this high attenuation is due to blood vessels by how deeply it extends into the brain. But in some cases, an MRI will be necessary to establish the diagnosis.

Case 3: Confirming an arteriovenous malformation with catheter angiography

In this case, the patient has a large, clearly evident AVM (Fig. 4). However, it still could be mistaken for a hemorrhage at first glance!

The characteristic feature of an AVM on catheter angiography is the early appearance of veins. This is a sign of the underlying pathology since blood flow is bypassing the capillaries from direct connections between the small arteries and veins. The nidus (or core) of the AVM can be seen on an angiogram (Fig. 5).

When an arteriovenous malformation hemorrhages, it can be the result of venous hypertension. But, you should also consider the possibility of a ruptured intranidal or supplying artery aneurysm. Aneurysms of vessels supplying the AVM are presumed to be the result of the chronically elevated blood flow to the AVM. The patient in this case had an unruptured aneurysm of the anterior communicating artery (ACoA).

Case 4: High attenuation surrounded by low attenuation is a sign of an arteriovenous malformation

A 35-year-old patient presented with headaches and no history of trauma or anticoagulation. The CT revealed a very small area of high attenuation with surrounding low attenuation. This was the only indication of the patient’s underlying AVM. The low attenuation indicates abnormal brain tissue near the AVM, due to vascular shunting or the result of a previous hemorrhage.

Case 5: Calcification near an arteriovenous malformation

A CT from a patient demonstrated a larger AVM that was near the cerebellum (Fig. 7). Note the multiple dilated blood vessels and a small calcification in this image.

Case 6: Confirming an arteriovenous malformation using a digital subtraction angiography

In this case, a patient presented with a new seizure, and on the CT scan you can see some high attenuation in the patient’s left temporal lobe (Fig. 8). While this could be mistaken for hemorrhaging, you now know that an AVM and cavernomas can also appear as high attenuation in the brain on CT.

If there is any question regarding the nature of high attenuation on CT, consider a follow-up CT, a CTA, or a magnetic resonance angiography (MRA). For this patient, an MRA showed some subtle flow-related enhancement in the temporal lobe that was suspicious for an AVM but a diagnosis could not be confirmed. So, a digital subtraction angiogram was ordered.

The carotid injection on the patient's DSA confirmed the diagnosis of an AVM based on the findings of a nidus and the appearance of an early vein. The term early vein indicates that it appears during the early phases of the angiography when no veins are visible elsewhere.

Case 7: Digital subtraction angiography confirms an arteriovenous malformation

Unfortunately, arteriovenous malformations often go unrecognized and many will present with hemorrhaging as the first clinical symptom. Patients with hemorrhaging from an AVM tend to be younger on average than those with ruptured aneurysms.

When a large parenchymal hemorrhage was evident on a non-contrast CT from a 30-year-old patient, a vascular cause was suspected. However, a CTA did not demonstrate any abnormal blood vessels in the region of the hemorrhage. Since a vascular cause was suspected (and CTAs may not show a small AVM), a digital subtraction angiography was ordered.

The DSA from this patient demonstrated a small, peripheral AVM with an abnormal cluster of blood vessels, a large supplying blood vessel from the anterior cerebral artery, and early filling of the superior sagittal sinus.

Computed tomography errors can be divided into errors of detection and errors of interpretation. We have seen several examples where high attenuation in the brain from vascular malformations may be mistaken for a hemorrhage. Rarely, you may also encounter difficulty with the interpretation of high attenuation outside the brain.

In one patient, the CT findings were initially interpreted as consistent with a subdural hemorrhage (Fig. 12). However, the absence of a typical history in the patient led to further imaging.

An MRI scan demonstrated homogenous enhancement in the extra-axial space and additional sites of meningeal involvement—without the characteristic features of blood products. A biopsy of the dura revealed that the extra-axial high attenuation was from sarcoidosis rather than the result of a hemorrhage.

Meningiomas and lymphomas also frequently appear as masses with high attenuation on CT. This can be due to calcification or dense cell packing, and can be mistaken for a hemorrhage in the brain, ventricles, or subdural space.

So, you can now recognize that not all abnormal high attenuation on CT is due to a hemorrhage! These other diseases should be considered whenever you see an area of high attenuation on head CT that is not consistent with the clinical history. MRI with MRA, CTA, and in some cases DSA exams will be of value in the investigation of other causes of high attenuation on brain CT.

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