MRI (Perfusion/Diffusion Weighted)

Diffusion-weighted magnetic resonance imaging (DWI) and perfusion-weighted magnetic resonance imaging (PWI) are two kinds of magnetic resonance imaging (MRI) tests. MRI uses a large magnetic field, pulses of radio waves, and a computer to produce detailed, cross-sectional images of internal organs and blood vessels. They each create images that can show the difference between healthy and unhealthy tissue. DWI shows the diffusion (movement) of water through the brain. An increase in water diffusion suggests cell death; a decrease in diffusion could be due to swelling of cells before they die. The test is used for early, rapid detection of stroke. (The whole brain can be imaged in less than 1 minute.) PWI uses MR contrast (dye) to show blood flow through the blood vessels of the brain; it can show a decrease in cerebral blood flow.

Physicians use DWI and PWI to diagnose:

Stroke;
Brain or pituitary gland tumor;
Pituitary disease;
Brain stem or cerebellum abnormalities; and
Multiple sclerosis.

DWI and PWI used together provide information about the location and extent of cell death within minutes of stroke, and have been shown to be superior to conventional MRI. DWI and PWI are also useful in helping a neurosurgeon or neurointerventionalist plan treatments for conditions such as brain tumor. DWI has the potential to provide early indication of response to treatment and predict response to treatment.

PRE-TEST GUIDELINES

Typically, there is no preparation for DWI or PWI. Because motion can blur DWI and PWI images, some patients who are anxious about the test may receive a sedative beforehand. If they do, they will need to arrange a ride home.

RISK FACTORS FOR POSSIBLE COMPLICATIONS

Patients who may be unsuited for DWI or PWI include those who:

Have implanted metal devices (pacemaker, artificial heart valves, cochlear implant);
Require life support;
Are claustrophobic; and
Are pregnant.

WHAT TO EXPECT

DWI and PWI are usually administered in a hospital or an outpatient healthcare facility and take 30 to 90 minutes to complete.

The patient changes into a hospital gown, removes any wig, dentures, or any metal items, and lies on an MRI table. The technician may offer the patient a sedative and earplugs or headphones to insulate the patient from noise. If a contrast agent is being used, it will be injected into the patient's vein.

The technician will slide the MRI table with the patient on it through a hollow, donut-shaped machine called a gantry. It houses the MRI scanner, which is comprised of coils and large, cylindrical-shaped magnets. The machine creates a magnetic field and directs radio waves at the area of the body to be scanned. This exposure is painless, but some patients with dental fillings feel a mild tingling. Throughout the procedure, the scanner produces humming and thumping sounds.

The only discomfort associated with the test is stiffness in the back caused by having to lie still on a hard surface for an extended period.

POST-TEST GUIDELINES

If patients are given contrast dye as part of the test, they will be asked to drink liquids to flush the dye from their body. Otherwise, patients can resume normal activities immediately.

Main menu