Distinguishing between intra- and extra-axial tumors on brain computed tomography (CT)
When examining computed tomography (CT) scans, you will eventually see a scan with an unexpected intracranial mass causing displacement or compression of the brain. As you consider the nature of the mass, keep in mind that mass effect alone does not mean that the patient has a brain tumor; infections, inflammatory diseases, and demyelinating diseases can also cause displacement of normal structures.
Before you can make any predictions about the composition of the mass, you should first determine if it resides outside the brain (extra-axial) or within the brain (intra-axial). This is because the range of possible diagnoses differs in these two spaces. For example, an extra-axial mass is not due to acute demyelination or a glial tumor.
Deciding whether a mass is intra-axial or extra-axial
When you press your finger deeply into an inflated balloon, the finger may appear to be inside, but it is still in fact outside the balloon. In the same way, an extra-axial mass can press on the surface of the brain and may look to be inside, but it is still outside the brain.
So, be thoughtful about this first step of assigning location! An error in predicting the location of a mass will very likely result in errors when predicting the tumor’s histology and expected behavior. And in some cases, it will be difficult or impossible to be sure about where the tumor resides.
We will also be looking at several magnetic resonance imaging (MRI) scans in this article, since tumors are better seen on MRI. Once you see common findings on an MRI, it becomes easier to recognize subtle abnormalities when viewing CT scans.
Review CT images in two perpendicular planes to distinguish between intra- and extra-axial masses
It’s crucial to begin your search by carefully reviewing the CT images reconstructed in two perpendicular planes. This is particularly helpful when the mass is along a fissure or near the surface of the brain.
This approach is especially helpful whenever you see a mass surrounded by brain tissue. In some cases, it may seem reasonable to conclude that it is arising from the brain (e.g., intra-axial). Before you leap to that conclusion, keep in mind that you should look at the tumor in at least two planes before making a decision. Remember, collect the evidence first!
You will notice on this MRI that the mass is in the interhemispheric fissure (Fig. 2). In the axial view, the tumor appears to be surrounded by brain tissue. But when you look at the tumor on the coronal view, you can now see that the tumor is arising from the dura along the skull base and pressing up into the brain. This tumor proved to be an extra-axial meningioma.
In the next example, based on the axial view you might predict that the tumor is intra-axial because this axial section is not close to the surface of the brain (Fig. 3). As well, the tumor is not near a fissure and nearly reaches the center of the brain.
However, when you view this tumor in the coronal plane (perpendicular to the axial slice), you can see the broad dural attachment of the tumor and some subtle bone reaction. This proved to be another meningioma—but even larger than the first example!
Next, let’s look at another case of an extra-axial mass that appeared to be within the brain on axial MRI imaging (Fig. 4). When you look at the mass on the sagittal plane you can see that the mass arises from the skull base and projects upwards into the brain substance—like a finger pressing into a balloon.
While MRI scans are used here to illustrate this concept, most current CT scanners can provide high quality, multiplanar reconstructions that provide comparable information.
When unclear after CT, enhanced MRI may help to distinguish between intra- and extra-axial masses
On a 45-year-old patient’s CT scan (Fig. 5), we can see that the fourth ventricle is displaced by a mass on the patient’s left side. This displacement is more evident when you compare it with a CT section of a different patient that shows a normal fourth ventricle.
Notice that the mass itself is difficult to see on CT, and its presence is inferred by the displacement of the fourth ventricle. This should be a reminder of why the fourth ventricle is one of the brain structures you should look for on all head CT scans.
Determining the location of the mass is critical! If the mass is extra-axial, it would most likely be a meningioma or vestibular schwannoma in this location. But, if the findings suggest that it is intra-axial, metastatic disease, subacute infarction, and tumefactive multiple sclerosis are all possible diagnoses in adults.
The MRI scan in this case clearly demonstrated that the mass was extra-axial in origin based on its broad attachment to the petrous apex and enhancement that extends into the internal auditory canal. Note again the mass effect on the fourth ventricle, due to the displacement of the brain by the tumor.
When determining if a mass is intra-axial or extra-axial, it can be helpful to look for six findings, which we will demonstrate with case studies:
- Skull changes
- Displacement of the brain cortex
- Displacement of large blood vessels
- Multi-compartment involvement
- Location in the midline or along Sylvian fissures
- Blood supply to the mass
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Skull changes can help establish that a mass is extra-axial
Bone changes can help establish that a mass is extra-axial. A contrast-enhanced CT scan demonstrates another extra-axial mass (Fig. 7). Note the widening of the left internal auditory canal when compared to the other side.
Skull changes can also support the diagnosis of a vestibular nerve sheath tumor (as in this case). These tumors arise from the eighth cranial nerve and while they grow very slowly, they eventually can cause hearing loss and sometimes vertigo.
When unsure after MRI, a CT may help distinguish between an intra- or extra-axial mass
Even when you have an MRI to review, you should consider obtaining a CT when you are not sure if a mass is extra-axial. For example, on the MRI from a patient with a history of breast cancer, a rounded enhancing mass was evident and appeared to be extra-axial in origin.
A CT was also ordered since the imaging was consistent with a meningioma (which is usually a benign, slowly growing tumor) and a dural metastasis (which would be of more immediate concern). The bone-filtered CT image demonstrated bone formation along the inner table of the skull that corresponded to the mass seen on MRI.
This finding, called hyperostosis, is characteristic of a meningioma rather than metastatic disease. This information proved to be a great comfort to the patient!
You may have noticed that the tumor appears to be in two different locations when you look at the MRI and CT images (Fig. 8). This is a common pitfall and is the result of a discrepancy in the standard plane of reconstruction used for MRI and CT. For a variety of reasons, the convention with CT is that the images are reconstructed at an angle while magnetic resonance images are simply displayed in the plane perpendicular to the long axis of the patient.
Displacement of the brain cortex indicates an extra-axial mass
In our next example, a large enhancing mass was seen on a contrast-enhanced CT scan. In considering the location of the mass, you should notice that there is evidence of displacement of the frontal lobe cortex by the mass (Fig. 9). This displacement is visible because the cortex has a slightly higher attenuation than the white matter. The finding was confirmed on the MRI scan.
Recall the analogy of a finger pressed into a balloon. Displacement of the brain cortex argues strongly that the tumor’s origin was extra-axial since it displaced the outer layer of the brain as it enlarged.
As well, the tumor was associated with skull changes. This finding indicated that the tumor arose from the skull itself, and proved to be a cavernous hemangioma of the skull based on surgical pathology.
Displacement of large blood vessels
Another finding to look for when trying to determine if a mass is intra- or extra-axial is the displacement of large blood vessels. In our next case, the vertebral arteries (which lie on the surface of the brain) are displaced away from the skull base. This sign indicates that the tumor is extra-axial since a tumor arising from the brainstem would displace the arteries towards the skull base.
Multi-compartment involvement is common with extra-axial tumors
The next case illustrates multi-compartment involvement typically seen with meningiomas. On MRI, the enhancing tumor occupies the sphenoid sinus, the left cavernous sinus and middle fossa, and the internal auditory canal in the posterior fossa (Fig. 11).
Be careful when the mass is in the midline or along the Sylvian fissures
In our next case, the mass appears to be surrounded by brain tissue with considerable edema extending into the left frontal lobe. While both features suggest that this could be an intra-axial mass, it proved to be an extra-axial mass arising from the interhemispheric fissure.
Remember that edema can be seen with both intra- and extra-axial tumors. For that reason, edema is an unreliable sign for the location of a tumor.
Tumor blood supply from arteries that supply brain tissue suggests an intra-axial tumor
Deciding whether a mass is intra- or extra-axial in some cases can be quite difficult. Every now and then you will encounter a tumor where the location remains uncertain even with multiplanar reconstructions and the use of contrast CT and MRI.
In our next case, a patient’s non-contrast CT demonstrated a displaced and compressed fourth ventricle. The mass appears just posterior and to the left of the fourth ventricle and seems intra-axial since it appears to be surrounded by brain tissue. However, we have seen that this sign is unreliable when viewed on only one plane of imaging.
The patient had no history of cancer to support the diagnosis of metastatic disease (the most common intra-axial tumor in the cerebellum in adults), so more testing was ordered.
The contrast-enhanced CT image from the same patient shows enhancement of multiple enlarged blood vessels. In this image, the tumor again appears intra-axial (Fig. 14). However, an MRI scan in the coronal plane shows the tumor has a broad attachment along its inferior border suggestive of extra-axial origins.
In this case, because of the tumor’s hypervascularity, a preoperative digital subtraction angiography (DSA) was performed. The exam demonstrated that the blood supply to the tumor arose primarily from an artery that supplies the brain—the posterior inferior cerebellar artery—which favors an intra-axial tumor. The mass proved to be a primary brain tumor called a hemangioblastoma during surgery.
When reviewing imaging scans of a patient with a newly diagnosed brain mass, before you make any decision regarding its histology, consider whether the tumor arose from inside the brain or outside the brain.
Multiplanar reconstructions, skull changes, displacement of the cortex, and in some cases blood supply are helpful findings. Remember to be particularly careful with your decision when the tumor is in the midline, along the Sylvian fissure, or near the periphery of the brain.
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