Magnetic Resonance Imaging

Since 1983. MRI has come to play a major role in the pre- and postoperative evaluation of patients with pineal region masses. Superior anatomic localization is the first and foremost reason. MRI can show direct sagittal images of the pineal region, demonstrating the gland's relationship to the tectum of the midbrain, corpus callosum and posterior third ventricle. Coronal and axial images complement the sagittal views.

The combination of T1-weighted, proton density, and T2­weighted images allows detection of abnormal alterations in signal intensity within the pineal gland and adjacent structures, such as the thalamus or brain stem. Tumors or disease processes characterized by an abnormal increase in interstitial water content within the lesion appear as regions of increased signal intensity on proton density images. CSF and fluid­filled cystic spaces appear high in signal intensity on T2-weighted images. Thus, the frequently occurring pineal cysts (a normal anatomic finding) are shown by a combination of T1 - and T2­weighted images. On T1-weighted images, the cyst is hypointense and on proton density and T2-weighted images, bright.

The normal pineal gland tissue enhances with gadolinium on MRI. This is because the pineal gland lacks a blood-brain barrier. Thus, contrast enhancement within the pineal gland, in and of itself, does not denote abnormality. Mamourian and Towfighi used MRI to obtain images from six patients with pineal cysts measuring between 7 and 15 mm in size and found that with immediate imaging, enhancement initially showed a rim-like margin, but with delayed imaging (60 to 90 minutes) the cysts also enhanced because of diffusion of the contrast material into the cyst. Tamaki et al. evaluated 32 cases of pineal cysts and found that they did not enlarge on follow-up studies over the next 3 months to 4 years. None of the patients in either series was symptomatic secondary to the pineal cyst. It is thought that pineal cysts may arise from incomplete fusion of the third ventricular diverticulum that gives rise to the pineal gland. However, pineal cysts with both glial and ependymal linings have been found.

Calcification is best seen by CT and poorly seen by MRI. Calcification may be seen as a focal hypointensity, when the calcification occupies a sufficient portion of the volume of the slice. It is possible to have a pineal neoplasm that is not larger than the normal-size pineal gland but is identifiable on CT because of the presence of calcification too early in life. Such is the case in trilateral retinoblastoma. Under these circumstances it is possible to have the tumor not seen on MRI because the calcification cannot be visualized and the pineal gland is not increased in size and the enhancement of the pineal gland is considered a normal phenomenon.

MRI shows flowing blood as a hypointense flow void within the lumen of the vessel. This is true for both arteries and veins. Thus, on the routine T1-weighted, proton density and T2­weighted images, the internal cerebral veins, vein of Galen and straight sinus can be identified clearly, along with their relationship to any pineal mass. Should the mass be a vascular malformation, then the hypointense flow voids that make up the mass can be identified as such and characterized as a vein of Galen malformation.

Despite the superior anatomic demonstration of a pineal mass by MRI it is not always possible to determine the site of origin. Thus, sometimes it is not possible to determine whether a mass in this region has arisen from the tectal plate, pineal gland or adjacent thalamus.

In the demonstration of dissemination of tumor into the subarachnoid pathways, gadolinium-enhanced MRI of the brain and spinal canal has proved superior to contrast­enhanced CT and/or myelography with a water-soluble agent. Thus, the method of choice for determining the presence or absence of disseminated tumor is gadolinium-enhanced MRI of the brain and spine. This should be done before surgery in order to avoid confusion with postoperative blood products in the form of methemoglobin, which can be bright on T1-weighted images. MRI is also superior to CT in demonstration of blood products, whether acute, subacute or chronic. This is important in the case of occult vascular malformations, such as those arising in the midbrain, thalamus, splenium or corpus callosum, masses that may mimic pineal region tumors. In these instances. the pattern of signal intensity changes on T1-weighted, proton density and T1-weighted images may help characterize the presence of blood products, giving a pattern that suggests the presence of a vascular anomaly.

With teratoid tumors, the presence of fat produces increased signal intensity on T1-weighted images. That this is fat and not methemoglobin can be verified by the use of a fat­suppression pulse sequence, which will turn the high signal of fat to a low one but will leave the high signal of methemoglobin unaffected.

Germinomas on MRI appear as masses hypo- to isointense to gray matter on T1-weighted images. On proton density images they are often of slightly increased signal intensity, whereas on T1-weighted images they are most often iso- to hypo­intense. The reason for this decrease in signal intensity on long time to repetition (TR) images seems to be related to their dense cellularity. This signal intensity change is not unique to germinomas but occurs in lymphomas and primitive neuroectodermal tumors as well. The pineoblastoma is a primitive neuroectodermal tumor and has a similar signal intensity change on T1-weighted images. Germinomas enhance intensely. They are radiation-responsive and often disappear on imaging following the initial 3000 rad. Gadolinium-enhanced T1­weighted images of the brain and spinal canal are used pre- and postoperatively to evaluate for disseminated disease. Choriocarcinomas and embryonal cell carcinomas arising in the pineal region have a more variable signal intensity on MRI. This is due to the frequent presence of haemorrhage in the tumor. Haemorrhage can have a variety of signal intensity appearances, including areas of hypo-, iso- or hyperintensity on T1-weighted, proton density, and T2-weighted images, depending upon the chemical state of the blood (oxy- or de oxyhemoglobin, intra- or extracellular methemoglobin, or hemosiderin). Teratomas of the pineal region often contain fat, which can be seen as a zone of increased signal intensity on T1-weighted images. Fat decreases in signal intensity on long TR images as the time to echo (TE) increases, and it disappears on fat­suppressed sequences. Pineoblastomas are a form of primitive neuroectodermal tumor. Calcification, if present in these tumors, is seen poorly or not at all on MRI. On T1­weighted images they are hypo- to isointense masses; on proton density images the masses are of slightly increased signal intensity; and on T2-weighted images they are hypointense. They enhance strongly with contrast medium. The pineocytoma has a signal intensity pattern somewhat different from that of the pineoblastoma. On T2-weighted images pineocytomas are more often somewhat increased in signal intensity. Again, contrast enhancement is usually present. Astrocytomas arise from adjacent structures, such as the tectum of the mid­brain, thalamus, and splenium of the corpus callosum, and intrinsically from within the pineal gland. These tumors are usually of low signal intensity on T1-weighted images and of increased signal intensity on proton density and T2-weighted images. Enhancement is variable and may or may not be present.

Arteriovenous malformations (AVMs) and fistulae within the thalamus and midbrain drain into the adjacent venous structures, such as the vein of Galen and straight sinus (the vein of Galen malformation), giving rise to their enlargement. These high-flow vascular structures, both arterial and venous, become enlarged hypointense flow voids on T1-weighted, proton density, and T2­weighted images. Gadolinium can produce some enhancement within portions of the nidus of the AVM. Magnetic resonance angiography serves a role in anatomically delineating the feeding arteries, the nidus, and the draining veins.

Magnetic Resonance Angiography and Magnetic Resonance Spectroscopy

Within the last 14 years, magnetic resonance angiography has come into its own as a diagnostic technique. Two methods, time-of-flight and phase-contrast, have been used to produce images of flowing blood within vessels. Resolution remains a problem but is adequate to show major feeding arteries, the nidus of an AVM and draining veins. This is useful in the region of the pineal gland when there is a vein of Galen malformation. It can also be useful in giving a clear anatomical picture of the configuration of the internal cerebral veins, vein of Galen and straight sinus when a surgical approach is being contemplated for a solid tumor that is displacing or encasing these structures.

Proton magnetic resonance spectroscopy in the last 13 years has begun to come into its own as a diagnostic technique. Single­voxel spectroscopy, using a 2 x 2 x 2 cm voxel size, can show the levels of choline, phosphocreatine, creatine, N-acetylaspartate (NAA), and lactate within the region studied. Preliminary work in paediatric brain tumors has shown that elevation of choline, a cell membrane metabolite, is increased to a larger extent in malignant tumors than in benign tumors. By calculating ratios of choline to NAA, an index of the tumour's relative malignancy can be determined.