Midbrain gliomas are focal lesions. Upward extension to the thalamus and downward extension to the pons are present in some cases. The margins are well defined in focal midbrain gliomas, and most of these tumors are more or less round or spherical. Hydrocephalus is commonly present in association with midbrain tumors. Before the availability of CT scan and MRI for diagnosis of intracranial lesions, tectal gliomas could be found only in autopsy evaluations. Kernohan and Sayre described tectal tumors as the smallest lesions in the body that they had found as a cause of death of a patient (104).
- Typically not seen on CT: CT scan may be unsuccessful in demonstrating a midbrain tumor, especially when the tumor is isodense and nonenhancing. Midbrain lesions may be missed on CT scans even after contrast injection.
- Midbrain gliomas best seen on MRI: MRI is the modality of choice for the diagnosis of midbrain tumors. MRI enables identification of important imaging features and localization of tumors with multi-planar views. It allows precise evaluation of the growth pattern and accurate preoperative diagnosis in most cases.
- Focal lesions on MRI: Gliomas in the midbrain are focal lesions. Upward extension to the thalamus and downward extension to the pons are present in most cases. The margins are well defined in focal midbrain gliomas, and most of these tumors are more or less round or spherical.
- Enhancement varies with location in midbrain: Gadolinium enhancement, calcifications, cysts, and exophytic nature are observed in both low- and high-grade gliomas. Most tectal plate gliomas have a solid consistency without intense enhancement after intravenous contrast. Central calcification may be noted rarely (33, 36). Tegmental tumors are cystic in 50% of cases with intense rim enhancement after intravenous contrast.
- DTI may be helpful for surgical planning: Diffusion tensor imaging coupled with tractography enables visualization of the relationship between lesions and functional tracts. These studies may be useful when planning surgery in appropriate cases (30).
- Differentiating high-grade tumors by metabolic activity: FDG-PET may be helpful in differentiating between anaplastic astrocytoma and glioblastoma among high-grade tumors. FDG-PET is a procedure that permits measurement of regional cerebral glucose metabolism. Hypermetabolism compared with surrounding structures in FDG-PET is typically noted in glioblastoma (34). Indolent tumors may display a PET appearance similar to that of normal brain (35).