Since functional Magnetic Resonance Imaging(ƒMRI) is a new way for us to "see" which areas of the brain are more active and which areas are less active, it can be used as a substitute for intra- and extra-operative brain mapping or for the intracarotid amobarbital test, known as the Wada test.
When we speak to someone or listen to someone speak to us, certain areas of our brains become more active. These areas are critical for speech, and it is important to be able to locate the precise areas for each patient, prior to any resection of these areas because everyone's brain is quite different. Brain tissue does not "re-grow" nor re-generate. Once gone, these functions are forever lost. It is for these reasons that individual differences are important, and ƒMRI allows us to respect and preserve these vital functions.
To determine the exact location of speech or memory or motor functions, a patient has a normal MRI scan of their brain, but at the end of these scans, specialized protocols are added. These special protocols search for tiny amounts of increased blood flow, and they record its location. Each scan takes about 6 minutes, and it is sometimes necessary to scan different areas, so functional scans can take about an hour. Nothing is injected into the patient, so the scan is non-invasive and safe. The patient has to be able to follow very simple instructions while he or she is being scanned, such as listen to a sentence and squeeze a rubber ball.
This is in contrast to intra- or extraoperative brain mapping, a procedure that requires placing small metal electrodes directly on the surface of the brain and stimulating tiny areas. When these areas are stimulated, the area directly underneath the electrode is made temporarily dysfunctional, and the patient is tested during the times of stimulation. This procedure requires at least one, and possibly two craniotomies, is highly invasive and therefore represents considerable risk to the patient.
ƒMRI also can substitute for the Wada test, which is part of the presurgical evaluation in a patient who may have epilepsy. The Wada test requires a catheter to be inserted into an artery at the groin and threaded into other arteries leading to the brain. The anesthestic agent, sodium amobarbital is injected into one of the carotid arteries thus putting that side of the brain "to sleep". The patient's ability to speak, understand speech, and remember things is evaluated. After the drug effect has worn off, the process is repeated with the other hemisphere. The Wada test helps determine which cerebral hemisphere is "dominant" for speech and if memory is functional on one or both sides of the brain. The test requires brief hospitalization.
ƒMRI provides significant advantages over either procedure:
• Outpatient vs inpatient
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