An MRI of the Circulatory System (Blood Vessels)
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 radio 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. The procedure is performed in the MRI machine so it uses MRI technology to detect, diagnose and aid the treatment of heart disorders, stroke and blood vessel diseases. For most MRA procedures a contrast material is injected into a vein to provide more detailed and clearer images. 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?
- 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 now will do the necessary operation after confirmation by MRA, doing away 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 have a more invasive catheter study.
- MRA also is 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 by 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.
How should I prepare for the procedure?
The magnetic field used for MRA will pull on any ferromagnetic metal object in the body, such as a heart pacemaker, intrauterine device, vascular access port, metal plate, or 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 also should 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 you are instructed otherwise. Medications may be taken as usual. Some patients will feel uncomfortably confined (claustrophobic) when enclosed in an MRI unit. You should discuss this with your physician prior to your exam. If necessary, a mild sedative may be prescribed to help ease your anxiety.
What does the equipment look like?
The conventional MRI unit is a closed cylindrical magnet in which the patient must lie still, and consequently may feel "closed-in" or truly claustrophobic. However more "patient-friendly" designs are rapidly coming into routine use. FMH offers the "short-bore" systems which are wider and shorter and do not fully enclose the patient.The traditional MRI unit is a large tube surrounded by a circular magnet, in which the patient lies without moving for the duration of the scan.
An MRI uses high field-strength magnets and special coils to produce detailed images of body tissues without the use of x-rays. A moving table will position you inside a hollow tube that is open at both ends at all times. You will hear a series of loud, “knocking” sounds. This is the instrument positioning its internal components to get the very best view. You can listen to music or use the earplugs provided, and the technologist will communicate with you throughout the 30-45 minute procedure.
How does the procedure work?
| Hepatic Vasculature
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 called protons in the tissues, which are then spun by a beam of radio waves and produce signals that are picked up by a receiver in the scanner. It is these signals that are processed by the computer to produce images. The resulting images are very sharp and detailed, and so are able to detect 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 one to five 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.
What will I experience during the procedure?
The technologist will make you as comfortable as possible. For those who become very uncomfortable when enclosed in a small space, a mild sedative is nearly always effective. Speak to your physician about your discomfort in small spaces before you go into the exam. You may notice a warm feeling in the area being studied. This is normal, but do not hesitate to report it if it bothers you. The loud tapping or knocking noises that are heard during certain parts of the exam disturb some patients; earplugs are provided to reduce this noise.
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.