Recognizing intra-axial tumors on brain computed tomography (CT) and magnetic resonance imaging (MRI)

Once you have determined that a mass is intra-axial, you should then consider whether it represents a primary or metastatic brain tumor or something entirely different! We are going to cover the key findings of intra-axial brain tumors on computed tomography (CT). We’ll also cover how to rule out other possibilities since not all intra-axial masses are brain tumors.

Before we get started, it’s important to note that both magnetic resonance imaging (MRI) and the patient’s history are very helpful in differentiating between all possible diagnoses. Many brain tumors cannot be seen on CT since they can resemble normal brain tissue, so obtaining an MRI is often helpful.

For example, a low-grade cerebellar tumor is only faintly visible on CT, but only if you know where to look based on the MRI results.

While there are a variety of primary brain tumors that can occur in the brain, the histology of the tumor frequently cannot be determined with certainty on CT or MRI alone. For that reason, most patients will be referred for a biopsy or resection.

Brain tumors can vary substantially in appearance from mostly cystic to mostly solid, non-enhancing to solidly enhancing, and sharply defined to diffuse. All of these imaging features weakly correlate with prognosis and tumor behavior.

It’s important to rule out the three possible diagnoses when considering whether an intra-axial mass is a brain tumor:

1. Subacute infarction
2. Brain infection
3. Tumefactive demyelination

You should know about your patient’s history so that you can avoid mistaking brain infection or demyelination for a neoplasm. The patient’s history may also help you avoid confusing a subacute infarction for a brain tumor—or the reverse.

In our first case, a 49-year-old patient presented to the emergency room with intermittent word-finding difficulty. While, in many ways, the CT findings resembled an infarction with both gray and white matter involvement, it had an unusually round configuration (Fig. 3). As well, on careful questioning, you learned the patient’s symptoms were waxing and waning over a week, which would be atypical for an infarction of this size.

An MRI was obtained based on the combination of the patient's history and CT scan. It showed that the patient had a primary brain tumor and not an infarction.

Ruling out infection or demyelination

Not all intra-axial masses that spare the cortex are neoplasms. Always consider the possibilities of infection and demyelination since both of them (in the acute phase) can resemble tumors.

In our next case, a patient’s CT scan demonstrates abnormally low attenuation, suggesting a brain tumor, with sparing of the overlying cortex (Fig. 4). Because of the mass effect, an MRI was obtained.

The MRI demonstrated a sharply defined zone of restricted diffusion in the same region as the brain mass. This finding can be seen with some highly cellular tumors like lymphoma, but it is typically a sign of pus within a bacterial abscess. The mass proved to be a brain abscess during surgery.

There are three imaging findings which should cause you to suspect that an intra-axial mass may be an underlying brain tumor:

1. Unexplained calcifications in the brain
2. Sparing of the cortex
3. Disproportionate mass effect to the symptoms

Finding 1: Unexplained calcifications

In our next case, a patient presented with intermittent word-finding difficulties. An intra-axial mass with calcification was noted in the left temporal lobe on CT (Fig. 6). This also proved to be a primary brain tumor called an oligodendroglioma.

In another case with unexplained calcification, the patient was involved in a car crash. On CT, the mix of high attenuation and adjacent low attenuation in the frontal lobe did not extend to the cortex (Fig. 7).

It is always tempting to attribute any imaging findings to the clinical situation, which was trauma in this case. This error is common and even has a name—confirmation bias. Always strive to be impartial and analytical if you want to contribute substantially to patient care as an imager.

As a result of confirmation bias, you might at first consider a hemorrhagic contusion for this finding in a trauma patient. However, the high attenuation looks more like the skull than blood products, and the overlying cortex of the brain is normal. Since the high attenuation might be calcium, the findings should lead you to at least consider a calcified primary brain tumor, which was the actual diagnosis in this case!

Finding 2: Sparing of the cortex

A 47-year-old patient presented with transient left arm weakness that had resolved by the time he had arrived at the emergency room. A small area of low attenuation was found on his CT, which was initially called an infarction (Fig. 8). This is an example of how you can go wrong by making a snap judgment, or what some call gunslinger radiology. But, it is always better to be accurate than fast in the case of clinical imaging.

On careful examination of the CT, the preservation of the cortex was distinctly unusual for an infarct, so an MRI scan was ordered.

The enhanced MRI scan demonstrated that the cause of the low attenuation on CT was a small subcortical enhancing tumor. This was resected and proven to be a primary brain tumor.

Finding 3: Disproportionate mass effect

The CT scan from our next case shows an area of low attenuation along the midline that conforms to the anterior cerebral artery territory (Fig. 9). There is also some mass effect with effacement of the sulci and a midline shift.

The mass could be attributed to an evolving, subacute infarction. However, the patient had no neurological deficits and presented only with headaches.

When you look at the coronal CT reconstruction of the scan, you can see there is in fact abnormally low attenuation on both sides of the brain. This proved to be a primary brain tumor. As was the case with this patient, primary brain tumors can be quite large without causing neurological deficits.

Not all brain tumors appear as masses with low attenuation; lymphoma typically has high attenuation on CT. Our next case features CT images from a patient with central nervous system (CNS) lymphoma (Fig. 10).

But beware! High-grade glial tumors and meningiomas may also appear to have high attenuation (due to their cellularity) on brain CT. So, high attenuation is not a specific sign of lymphoma. The high attenuation in our next case proved to be due to an anaplastic astrocytoma that extended across the corpus callosum, which is best seen on MRI (Fig. 11).

In our final case, notice that there is high central attenuation within an area of low attenuation that is associated with mass effect on the ventricle (Fig. 12). The high attenuation argues against the diagnosis of tumefactive demyelination, which should appear to have homogeneously low attenuation. Later, the attenuation proved to be another case of central nervous system lymphoma.

So, CT findings of unexplained calcifications, cortical sparing, and mass effect out of proportion to symptoms should make you consider an underlying intra-axial brain tumor.

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