MRI generally allows for a rapid and usually accurate diagnosis. Because of the widespread availability of CT in emergency departments, some patients will come to diagnosis by way of CT, although this technique is generally inferior to MRI in terms of sensitivity and specificity. The CT appearance of the diffuse pontine glioma typically is a low-density lesion in the brainstem with associated compression and obliteration of the surrounding cisterns. The fourth ventricle is often distorted and displaced posteriorly, and obstructive hydrocephalus is common. There is little, if any, enhancement, even with HGG. Less frequently, the lower-grade gliomas affecting the brainstem are isodense or hyperdense and contain cystic areas. Exophytic portions of the tumor are only moderately well delineated on CT. If the diagnosis of a brainstem glioma is suspected by CT, the diagnosis should be confirmed with an MRI (3-5).
The presence and extent of infiltrating brain stem gliomas is best seen with MRI. The higher-grade infiltrating gliomas are usually seen as masses with decreased signal intensities on T1-weighted images and increased signal intensity lesions on T2-weighted images. The tumors typically involve and expand the entire pons, and will often extend into the higher brainstem regions and beyond, as well as caudally into the medulla and upper cervical spinal cord. These tumors tend not to enhance with contrast agents, or do so only minimally. In some cases the tumor seems surprisingly demarcated and localized to one brain stem site, which is often associated with a less aggressive tumor (3-5). Because the higher grade gliomas can disseminate early in the course of the disease, many centers are performing complete spinal cord MRIs with and without contrast at the time of diagnosis to investigate for leptomeningeal spread (8,10-12,30-32).
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