Let’s review several examples of brain abnormalities that are difficult—if not impossible—to detect on computed tomography (CT).
Whenever a patient has had multiple seizures, or if there is a strong clinical suspicion that a seizure is secondary to an underlying brain lesion (and the CT scan is normal), magnetic resonance imaging (MRI) is usually warranted.
Case 1: A small metastatic lesion is best viewed on contrast-enhanced MRI
Small metastatic lesions can lead to seizures, but are difficult (or even impossible) to see on non-contrast MRI. Our first case features two MRI scans with contrast that demonstrates small metastatic lesions. One of these lesions is within the cortex and could represent the source of the seizures.
Case 2: A cerebellar metastasis is more evident on enhanced MRI
In the next case, a patient’s cerebellar metastasis is much more evident on an enhanced MRI scan than on the non-contrast fluid-attenuated inversion recovery (FLAIR) scan.
Case 3: A herpes encephalitis abnormality is better viewed using FLAIR MRI
Our third case involves a patient with seizures secondary to herpes encephalitis. The abnormality in the patient’s right temporal lobe is much more apparent on the FLAIR MRI compared to CT (Fig. 3).
Case 4: A small cavernoma is best viewed with gradient echo MRI
In our fourth case, the patient had a cavernoma involving the left insular cortex. In this circumstance, the CT scan might show some high attenuation within the cavernoma, but the CT appeared normal since the lesion was small and barely visible on T2-weighted MRI. Small cavernomas are best seen on gradient echo or susceptibility-weighted MRI images.
Case 5: A cavernoma is much more evident on MRI
Keep in mind the concept of conspicuity when considering the role of CT and MRI in patients with seizures. In our fifth case, even though the CT demonstrates a small area of high attenuation in the white matter, the cavernoma is much more conspicuous on MRI.
Case 6: Left-sided mesial temporal sclerosis on MRI in a patient with epilepsy
In our sixth case, the MRI from a patient with epilepsy demonstrated left-sided mesial temporal sclerosis. The right hippocampus is normal and appears larger and lower in signal intensity than the left hippocampus. This is a typical finding with this disease and would be almost impossible to detect on CT. In appropriate circumstances, the patient may be a candidate for surgery and would have a high likelihood of improved seizure control afterward.
So, we’ve covered imaging approaches for patients with seizures. In patients with one or more new seizures, even when the CT scan is normal, consider MRI with contrast when you have a reasonably high degree of suspicion that the patient has an underlying brain lesion. But, in patients with epilepsy, there is almost no reason to start with CT because an MRI is much more likely to demonstrate subtle changes in the brain that may be the source of the seizures.
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