How should I prepare for my MRI procedure?

The magnetic field used for MRA will pull on any iron-containing object in the body, such as a heart pacemaker, intrauterine device, vascular access port, metal plate, pins, screws or staples. You will be given a questionnaire to answer regarding these issues. The radiologist or technologist should know about any such item and also whether you have ever had a bullet in your body, whether you ever worked with metals, or if you have had a joint replacement. If there is any question, an x-ray can be taken to detect metal objects.

The radiologist should also know if you have fillings in your teeth, which could distort images of the facial region or brain. Braces make it harder to properly adjust the MRI unit. You will be asked to remove hairpins, jewelry, eyeglasses, hearing aids and any dental work that can be taken out. Some wigs contain metal and must be removed. Red dyes used in tattoos and permanent eyeliner may contain metallic iron, but this is rarely a problem. You should report any drug allergies to the radiologist or technologist and should mention if there's any possibility that you might be pregnant.

You can eat normally before the exam (unless told differently), but a young child should not eat or drink for about four hours if they will receive a sedative. The rules vary at different MRI facilities, so be sure to check with your medical center about eating and drinking before the exam. Medications may be taken as usual. Some patients will feel uncomfortably confined (claustrophobic) when enclosed in an MRI unit. If necessary, you will be given a sedative to help put you at ease, though probably fewer than one in every 20 patients will need this. You will wear a lightweight medical gown for the exam.

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.

Example of the MRI equipment that may be used is shown to the left.

How does the procedure work?

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.

What are the benefits vs. risks?

Benefits

• 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

• 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.

Further Reading

Atoms to Visualization FASEB

MRI History / How is Works University of Colorado

MRI Prescan Checklist MIT