Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field temporarily aligns all the water molecules in your body. Radio waves cause these aligned particles to produce very faint signals, which are used to create cross-sectional MRI images – like slices in a loaf of bread. The MRI machine can combine these slices to produce 3-D images that may be viewed from many different angles.
A special sequence called “diffusion-weighted imaging” creates a special signal in brain tissue that has been damaged recently. This sequence is very important for the diagnosis of stroke.
MRI cannot be used in patients who have older aneurysm or sugical clips or in those who have pacemaker.
The same machine can also be used to obtain information about blood vessels in the brain. It can use the flow of blood to create this information or can be performed with a contrast called gadolinium.
MRI is another imaging technology used in stroke diagnosis. Exactly how MRI works is somewhat difficult to explain, but it uses magnetic fields and radio technology to produce images of the brain. MRi is capable of demonstrating both ishemic and hemorrhagic stroke, and may show ischemic stroke as early as minutes after it starts. It can demostrate even very small strokes anywhere in the brain, inculding areas where CT may be negative. MRI uses no radiation. The disadvatages of MRI include the need for a patient to remain still for a period of 20 minutes in a relatively confined space, and the fact that it cannot be used in patients with pacemakers. MRI of the brain, when used for the diagnosis of stroke, is performed without IV contrast.
MRA, or magnetic resonance angiography, is a technique which also is able to demonstrate blood vessels. This technique uses a standard MRI machine with special software designed for this purpose. MRA of the head, which may demostrate the intracranial circulation, is performed without contrast. MRA of the neck, which demonstrates the blood vessels leading from the heart to the brain is generally performed with contrast. The advanages of this test is that it does not expose the patient to radiation, and an image of the blood vessels inside the head may be obtained without contrast. The disadvantages of MRA is that it requires the patient to remain immobile for up to twenty minutes in a confined space. MRA may at times overestimate narrowing or occulsion of blood vessels.