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Magnetic resonance imaging (MRI)
is a method of producing extremely detailed pictures of body tissues and organs without the need for x-rays. The electromagnetic energy that is released when exposing a patient to radiofrequency waves in a strong magnetic field is measured and analyzed by a computer, which forms two- or three-dimensional images that may be viewed on a TV monitor. MR angiography (MRA) is an MRI study of the blood vessels. It utilizes MRI technology to detect, diagnose and aid the treatment of heart disorders, stroke, and blood vessel diseases. MRA provides detailed images of blood vessels without using any contrast material, although a special form of contrast material is often given to make the MRI images even clearer. The procedure is painless, and the magnetic field is not known to cause tissue damage of any kind.What are some common uses of the procedure?
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Many patients with arterial disease now have it treated in the radiology department rather than undergoing surgery in an operating room. MRA is a very useful way of finding problems with blood vessels and determining how to best to treat those problems.§
The carotid arteries in the neck that conduct blood to the brain are a common site of atherosclerosis, which may severely narrow or block off an artery, reducing blood flow to the brain and even causing a stroke. If an ultrasound study shows that such disease is present, many surgeons will perform the necessary operation after confirmation with MRA, dispensing with the need for catheter angiography.§
MRA has found wide use in checking patients for diseased intracranial (in the head) arteries, so that only those with positive findings will need to undergo a more invasive catheter study.§
MRA is also used to detect disease in the aorta and in blood vessels supplying the kidneys, lungs and legs.§
Patients with a family history of arterial aneurysm, a ballooning out of a segment of the vessel wall, can be screened with MRA to see if they have a similar disorder that has not produced symptoms. If an aneurysm is found, it may be eliminated surgically, possibly avoiding serious or fatal bleeding.What does the equipment look like?
The traditional
MRI unit is a large tube surrounded by a circular magnet, in which the patient lies without moving for several seconds at a time. The patient is placed on a wheeled bed that is moved into the magnet. In recent years, patient-friendly units have been designed, and examination in such units is becoming increasingly available. These machines are both shorter and wider than a conventional MRI unit and do not fully enclose the patient. Some of the newer C-shaped units are even open on all sides and are thus very attractive to patients who tend to be claustrophobic. A drawback is that image quality is not as consistently good.Examples of the MRI equipment that may be used are shown at the top of this page.
Exposing the patient to radio waves in a strong magnetic field generates data that are used by a computer to create images of tissue slices that may be viewed in any plane or from any direction. The magnetic field lines up atomic particles in the tissues called protons, which are then spun by a beam of radiofrequency waves and produce signals that are picked up by a receiver in the imager. It is these signals that are processed by the computer to produce images. The resulting images are very sharp and detailed and are thus able to demonstrate tiny changes from the normal pattern that are caused by disease or injury. Special settings are used to image various structures, such as arteries in the case of MRA.
How is the procedure performed?
The patient is placed on a special table and positioned inside the opening of the
MRI unit. A typical exam consists of two to six imaging sequences, each taking two to 15 minutes. Each sequence provides a specific image orientation and a specified degree of image clarity or contrast. Depending on the type of exam being done, the total time needed can range from 10 to 60 minutes, not counting the time needed to change clothing, have an IV put in and answer questions. When contrast material is needed, a substance called gadolinium is given by IV injection during one of the imaging sequences. It highlights blood vessels, making them stand out from surrounding tissues.The radiologist and technologist leave the examining room during the actual imaging process, but the patient can communicate with them at any time using an intercom. Some centers permit a friend to stay nearby, or a parent if a child is being examined. When the exam is completed you will be asked to wait to make sure that more images are not needed.
Who interprets the results and how do I get them?
A radiologist experienced in
MRI will analyze the results and send a report to your physician, along with an interpretation of the findings. Your physician in turn will discuss the MRA findings with you. Some centers now send diagnostic reports and images over the Internet, speeding up the process.What are the benefits vs. risks?
Benefits
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Detailed images of blood vessels and blood flow are obtained without having to insert a catheter directly into the area of interest, so that there is no risk of damaging an artery.§
The procedure itself and the time needed to recover are shorter than after a traditional catheter angiogram.§
MRA is less costly than catheter angiography.§
There is no exposure to x-rays during an MRI study.§
Even without using contrast material, MRA can provide high-quality images of many blood vessels, making it very useful for patients prone to allergic reactions.§
As with catheter-based angiography or CT angiography, it frequently is possible to defer surgery after getting the results of an MRA study. If surgery remains necessary, it can be performed more accurately.Risks
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There are no definite side effects from any type of MRI study, including MR angiography. Claustrophobia may be a problem, however. When it is severe and not relieved by giving a sedative, an alternative imaging method may have to be tried. If a metal implant is present but goes undetected, it may be affected by the strong magnetic field to which the patient is exposed. In addition, if the implant is close to the examination site it may be hard to get high-quality images.§
MRI is generally avoided during the first three months of pregnancy. Ultrasound is preferred at this time, unless the woman might have a very serious condition that is best detected with MRA. The effects of MRI on the fetus, if any, remain to be determined. The general rule for MRI and other diagnostic studies in pregnancy is that they should be avoided unless there is substantial risk from missing the correct diagnosis because the procedure is not done. Women who are breast-feeding should inform the radiologist and ask how to proceed. They may pump breast milk before the exam for use until the gadolinium contrast material has cleared from the body.What are the limitations of MR Angiography?
MRA does not image calcium, as does
CT angiography. The procedure should be avoided in any patient with an implanted neurostimulator, metallic ear implant or metallic object within the eye socket. It should also be avoided if there is a bullet fragment or if the patient has a port for delivering insulin or chemotherapy. Patients with pacemakers should consult with their doctor. For patients who are very claustrophobic, adequate nursing staff must be on hand to monitor sedation.The clearness of MRA images does not yet match those obtained with conventional angiography. MRI of small vessels, in particular, may not be adequate for diagnosis and treatment planning. Sometimes it may be difficult to separate images of arteries from veins with MRA.