There are two types of neurologic tests for the brain: those that examine the structure of the brain and those that examine its function. The CAT scan and MRI look at the structure of the brain, whereas the electroencephalogram (EEG), SPECT scan, PET scan, and evoked studies examine the function of the brain.
MRI and CAT Scans
The MRI and CAT scan slice the brain radiographically into slabs. The MRI does this with magnetic fields; the CAT scan uses X-rays. The MRI provides more detail than the CAT scan. Hence, brain damage seen on an MRI – as small as 1-2mm in size – may escape detection by a CAT scan.
Being so very sensitive, the MRI commonly detects clinically silent (asymptomatic) "brain damage" in the normal population. For example, as we age it is common for myelin in the white matter to degenerate. An MRI can detect this myelin degeneration as white matter hyperintensities. The MRI is also sensitive to cerebral atrophy (brain shrinkage), another normal phenomenon as we age. Excessive numbers of white matter hyperintensities or excessive atrophy signal a possible neurologic illness, or injury.
MRA (Magnetic Resonance Angiography)
MRA, or magnetic resonance angiography, is a means of visualizing the carotid and vertebral arterial systems in the neck and brain without having to inject contrast into the bloodstream. The resolution is not as good as with conventional arteriography, but the patient is spared the risks of catheterization and allergic reactions to the dye. (In conventional arteriography, a catheter is threaded from the femoral artery in the groin backward up the aorta into a carotid or vertebral artery in the neck, and then dye is injected up the catheter. As the dye flows into the brain, X-rays are taken of the cerebral vasculature.)
Monitors the brain's electrical activity by means of wires attached to the patient's scalp. These wires act like an antenna to record the brain's electrical activity. The resting brain normally emits signals at a frequency of 8 to 13cps (cycles per second), called alpha activity; anything faster than this is called beta activity. Slower rhythms include theta activity (6-7cps) and delta activity (3-5cps).
Theta and delta activity occur in the normal brain as the patient descends into sleep. If the patient is awake, any slowing of electrical activity in a focal area of the brain may indicate a lesion there. Similarly, widespread slowing indicates a widespread disturbance of brain function, often due to a blood born insult like low blood sugar, drug intoxication, liver failure, etc. "Spiking" (sharp waves of electrical activity) discharges indicate an irritable area of cerebral cortex. If allowed to spread, the spikes can produce a seizure.
It is not uncommon for an EEG to be normal between seizures in patients with bona fide seizures. During a seizure, however, the EEG is almost invariably abnormal. Conversely, 15% of the population shows mild abnormalities on EEG, representing old head trauma, old strokes, migraine, viral infections, and most of the time for unknown reasons.
Quantitative EEG (QEEG, BEAM, Brain Mapping)
This test is performed in a way similar to EEG. Brain wave activity varies throughout the day depending on the state of alertness. Each area of the brain normally spends a characteristic amount of time in alpha, beta, theta and delta activity. Brain-mapping computers are now capable of creating a map of the brain's electrical activity depicting how long each area of the brain spends in each of the basic rhythms. By comparing the patient's map with that of a control population, it is possible to localize areas of focal slowing of electrical activity. Alone, a QEEG is insufficient to diagnose brain damage but in conjunction with other neurologic tests, QEEG can be confirmatory.
PET Scan (Positron Emission Tomography)
PET scanning (positron emission tomography) is based on the fact that the brain uses glucose for energy. By labeling a glucose molecule with a radioactive "tag" and then inhaling radioactive glucose and placing the patient's head under a large Geiger counter, one can identify abnormal areas of the brain that are underutilizing glucose. Because cyclotrons are needed to generate the radioactive gas, PET scanning is not widely available.
SPECT Scan (Single Photon Emission Computed Tomography)
SPECT scanning (single photon emission computed tomography) is similar to PET scanning in that a radioactive chemical is administered intravenously to the patient, but the radioactive chemical remains in the bloodstream and does not enter the brain. As a result, the SPECT scan maps the brain's vascular supply. Because damaged brain tissue normally shuts down its own blood supply, focal vascular defects on a SPECT scan are circumstantial evidence of brain damage. The advantage of a SPECT scan over a PET scan is its ready availability and relatively cheap cost. Recent studies have demonstrated abnormal SPECT scans after head trauma when the CAT and MRI were normal, suggesting that the SPECT scan is more sensitive to brain injury then either CT or MRI scans. Because the radioactive chemicals used in SPECT and PET scans are carried to all parts of the body by vascular tree, SPECT scans and PET scans are used judiciously in patients of reproductive age.
Evoked studies take advantage of the fact that each time a sensory system of the body – vision, hearing, touch – is stimulated, an electrical signal is generated in the brain. These electrical signals can be detected with electrical wires on the scalp.
A lumbar puncture (spinal tap) is used to analyze cerebrospinal fluid. An analysis of the fluid can help tell doctors, for example, if there is any bleeding in the brain or spinal cord areas.
Magnetic Resonance Spectroscopy (MRS)
This is an exciting new tool, used in conjunction with MRI, that detects the intra-cellular relationship of brain metabolites. Studies show that in an injured brain, the relationship between the amount of certain compounds in the brain changes in predictable ways, which can be picked up, non-invasively, by MRS. MRS holds great promise in the "objectivication" of brain injury. This data can and should be captured on MRI within six weeks of injury.
The CAT scan is superior to the MRI in detecting fresh blood in and around the brain, while the MRI is better at detecting the remnants of old hemorrhaged blood, called hemosiderin. CAT scans are often repeated to insure that a brain injury is not becoming more extensive, usually in the early stages of ER treatment.
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