MRI initially provided two types of images, designated T1 and T2. For brain tumors, the former generally showed a well-demarcated area of low density, and the latter showed bright whiteness that encompassed a more extensive region owing to the signal of the surrounding brain edema. With the availability for general usage in 1988 of gadolinium contrast for MRI, a new set of criteria of usage and differential diagnostic considerations in brain imaging have quickly evolved. T1 gadolinium imaging is the most precise way to image a brain tumor, and patients can often be followed up during and after treatment with that type of study alone. Such an approach is easier for patients because it reduces the length of time otherwise spent on T2 scanning. Now and then, T2 images are useful. For example, T2 images, besides showing the extent of edema, also delineate the demyelinating effects of radiation on white matter. FLAIR images, a variant of T1, are even better for this.
GADOLINIUM MRI CHARACTERISTICS OF BRAIN TUMORS
|Metastases||These are remarkably variable.
Some enhance brightly and solidly with gadolonium. Others are in ring
configuration. Many are invisible with contrast CT.
|Acoustic neuromas||These are invariably intensely
contrasted by gadolinium, even more reliably than by CT.
|Meningiomas||Same as for acoustic neuromas.|
|Pituitary adenomas||These always enhance less than
the normal pituitary gland. MRI is superior in every way to CT,
especially when thin slices and magnified views are ordered.
|Glioblastoma||These are almost always in ring
|Anaplastic astrocytomas||These are sometimes solidly
bright; they are often patchy, may be noncontrasting, and may look like
|Low-grade astrocytomas||These do not enhance. They are
often invisible by CT or are imaged only as vague low density.
|Oligodendrogliomas||These generally do not enhance
unless anaplastic and are often invisible on CT unless they are
|Primary brain lymphomas||These usually exhibit
homogeneous enhancement and are smoothly rounded. Periventricular
location is common. They are multiple in about a fourth of cases. This
lesion does not often look like glioblastoma but is easily mistaken for
metastases if multiple.
Cerebral angiography seldom is used in the diagnosis of brain tumors. In a few circumstances, neurosurgeons, in preparation for surgery, require a more precise knowledge of the pattern and position of blood vessels, which can be obtained only by angiography. The procedure is also used to embolize highly vascular meningiomas or to study cerebral dominance by injection of barbiturate into the carotid artery (the Wada test) in left-handed individuals who are to have surgery near language areas. Preoperative determination of cerebral localization helps surgeons to plan the extent of surgery and to avoid creation of postoperative language deficits in the patient.
Examination of the spinal fluid has limited indication in the diagnosis of brain tumors. One is to rule out an inflammatory disorder mimicking a brain tumor. Another is to establish the diagnosis of benign intracranial hypertension in patients with uninformative MRIs. In addition, spinal fluid cytology may be useful for determining instances of malignant meningitis secondary to metastatic neoplasms in association with spinal spread of medulloblastoma in some children and in identifying primary lymphomas of the brain in cases in which MRI changes are ambiguous.
The routine electroencephalogram (EEG) has no role in the diagnosis of brain tumors and does not assist in the choice of anticonvulsant drugs for patients with brain tumor. However, specialized intraoperative neurophysiologic techniques, such as depth electrode studies and intraoperative monitoring, may be useful in identifying and removing epileptogenic areas adjacent to brain tumors or to avoid resection of critical brain regions adjacent to tumors.
Positron emission tomography (PET) is able to quantify biochemical functions, such as oxygen and glucose utilization, within tumors as well as in normal brain tissue. PET scanning is a powerful research tool of limited availability for routine clinical purposes. Its spatial resolution is inferior to that of both CT and MR. In patients with brain tumors who develop recurrent symptoms after radiation therapy, PET can differentiate, with about 70% accuracy, radiation-induced injury from tumor recurrences. These disorders often appear identical on MRI.