Magnetic resonance imaging, the scientific term behind a brain MRI, is a relatively new technique which uses the quantum mechanical characteristics of protons in your body tissues to create an image. The specific details of the science behind an MRI is quite complex. In general, the patient is placed in a large magnet which aligns all their protons into the same spin. A radiofrequency pulse stimulates these protons, most abundant in water, and an electrical coil around the head detects the signals that result. In the end, the computer uses complex mathematics to convert these signals into an image which the physicians read and interpret. The appearance of an MRI scanner is similar to a CT Scan in that it is a large donut-shaped tube. The patient's lays on a table while their head is inside the "donut". Unlike a CT scan, no x-rays are used to produce a brain MRI so there is no exposure to radiation. The patient does not feel anything during the performance of the study although some patients complain of claustrophobia while in the scanner. Sometimes a brain MRI is performed with injection of a contract agent. This is basically a dye that is injected into the blood which shows up brightly on the MRI scan. It can help to make some diagnoses more clearly visible, such as many brain tumors. Not all medical conditions have specific findings on MRI. For example, a patient in coma due to uncontrolled diabetes would typically show nothing abnormal in their brain. Rather, brain MRI is good at showing structural changes in or around the brain. For example, MRI is excellent at detecting tumors of the nervous system. They can also show changes in the tissue of the brain consistent with stroke and hemorrhage, or bleeding into the brain. MRI is more expensive and slower to perform than CT scan, so they are not used for every patient. Pathologies which can be easily diagnosed quickly on a CAT scan, such as acute head trauma and bone pathology, will use that type of brain scan. MRI however is more versatile and shows better resolution of subtle details than a CT scan so it is used frequently for stroke, tumors, spinal disease and other pathology Normally, on x-ray the blood vessels are relatively invisible. However, when injected with a radio-opaque dye, the blood vessels show up clearly on an x-ray. This technique is used to visualize the brain blood vessels in a cerebral angiogram but is also used in other parts of the body, such as in a coronary angiogram of the heart. For a standard angiogram, a small catheter is placed into the blood vessels that supply the brain, the carotid arteries and the vertebral arteries. The catheter is placed through the femoral artery in the thigh and maneuvered up to the head with x-ray guidance. When the catheter is in place, dye is injected into the blood stream while x-rays are taken to allow clear visualization of the blood vessels. X-rays are taken at several time points and from several angles to visualize both the arteries and the venous system in the head. Some centers now have 3-dimensional angiography machines that rotate around the head while the dye is injected to create a 3D model of the blood vessels. Additionally, some CT scanners have the capability of producing a 3D image of the blood vessels called a CT Angiogram. This has the advantage that no catheter is placed into the artery as the dye can be injected into the general circulation through a standard intravenous catheter (an IV). In general a cerebral angiogram is safe, but it does have some risks. The catheter in the artery can occasionally cause injury to the artery leading either to bleeding or an arterial dissection and subsequent clotting. This can lead to stroke in rare cases. Additionally, the site of catheter insertion in the leg can be prone to hematoma formation and/or infection. These insertion site complications are generally well tolerated and easily treated in the great majority of patients. Angiograms are also commonly performed to evaluate blockages of the blood vessels to the brain as in acute stroke or vasospasm. In addition to being a diagnostic test for these various pathologies and others, some procedures are performed through the catheter while performing an angiogram. These endovascular procedures, usually performed by an interventional neuroradiologist or neurosurgeon, include coiling of aneurysms, embolization of arteriovenous malformations, angioplasty or stenting of obstructed arteries and retrieval of clots during acute stroke. In some types of brain tumors, such as meningiomas, the blood vessels supplying the tumor are embolized prior to surgical removal of the tumor.What Is It?
A Brain MRI (alternately called a brain scan, MRI scan, or magnetic resonance imaging) is a neurological test which produces a high-resolution image of the contents of the head. It is a commonly used study to evaluate patients with neurological complaints to investigate for diseases of the brain. MRI scans can be performed for other parts of the body as well, including the spine, extremities and chest.What Is It Used For?
As mentioned above, an MRI produces a high-resolution image of the internal parts of the body. There are several different types of MRI imaging which focus on different characteristics of the tissues. By interpreting these various images that are produced, a radiologist or other physicians can help to make diagnoses of medical conditions.Cerebral Angiogram
What Is It?
A cerebral angiogram is a neurological test which evaluates the blood vessels that supply the brain.What Is It Used For?
An angiogram is an excellent test to evaluate the blood vessels of the brain and any abnormalities or pathologies that involve them. Common examples of blood vessel abnormalities which can be demonstrated on a cerebral angiogram include cerebral aneurysm, arteriovenous malformation, and Moyamoya disease.
Wednesday, March 31, 2010
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