Identifying brain abscesses and encephalitis on brain computed tomography (CT)

Let’s examine several patient cases with brain abscesses and encephalitis on computed tomography (CT). Brain abscesses can resemble brain tumors on CT, but they can usually be distinguished from neoplasms using magnetic resonance imaging (MRI). When MRI is not sufficient, surgery with biopsy may be necessary for the diagnosis.

In our first case, the patient’s CT demonstrated a right frontal mass effect with an obvious shift of the midline structures. While a brain neoplasm (primary or metastatic) should be in the differential for this finding, you should at least consider infection as well.

The MRI demonstrated an abnormal signal on the diffusion-weighted image in the center of this lesion indicating restricted diffusion in this region. When accompanied by rim enhancement, this finding is typical for a brain abscess (with pus causing the diffusion restriction).

The CT in our second case shows low attenuation with mass effect in the right thalamus. The contrast-enhanced CT scan in the patient demonstrated rim enhancement of the mass. This finding can be seen with primary tumors, metastatic tumors, and brain abscesses.

The diffusion-weighted magnetic resonance imaging (MRI) from the same patient shows restricted diffusion in the center of an enhancing rim. A high signal on the trace image of the diffusion-weighted MRI that corresponds with a low signal on the apparent diffusion coefficient image establishes that there is restricted diffusion of water in the lesion. This is usually from pus or necrosis. This abscess was drained during surgery.

Our next case presented to the emergency room with headaches that started after a previous visit to Mexico. Their CT shows a large left-sided mass with ventricular enlargement. The MRI demonstrated rim enhancement, but the diffusion-weighted imaging was normal.

These findings were unusual for neoplasm, and MR spectroscopy was consistent with infection. This proved to be the result of brain infection from cysticercosis, which regressed spontaneously.

These next two patient cases had mass effect from swelling and hemorrhaging in the medial temporal lobes secondary to herpes encephalitis. While not all cases of herpes encephalitis will lead to a hemorrhage, nearly all cases of herpes infection of the brain involve the medial temporal lobe.

There are many other brain infections to consider, such as Creutzfeldt-Jakob disease (e.g., mad cow disease). Our next case demonstrates typical findings of this prion disease on diffusion-weighted MRI including symmetric restricted diffusion in the basal ganglia and thalamus. These findings are frequently overlooked because the symmetry suggests a normal finding even though it is not!

Another type of brain infection to consider is progressive multifocal leukoencephalopathy. This infection is demonstrated in our next case featuring an immunocompromised patient. Typically we see asymmetric T2 prolongation in the white matter of the brain without substantial mass effect. Any enhancement is variable, but when present, will usually be peripheral.

Keep in mind that CT scans for Creutzfeldt-Jakob disease and multifocal leukoencephalopathy may be normal!

Errors of interpretation and incorrect patient management may occur when an infection is thought to be a tumor on CT. This is precisely why you should be guarded with your diagnosis in patients with a newly discovered intracranial mass. In many cases, MRI along with the patient’s medical history will help establish the correct diagnosis.