MRI

What Is Magnetic Resonance Imaging (MRI)?

Magnetic Resonance Imaging, or MRI, is a painless and safe diagnostic procedure that uses a powerful magnet and radio waves to produce detailed images of the body’s organs and structures, without the use of X-rays or other radiation.

A computer converts signals from the MRI scan into extremely clear, cross-sectional images of the part of the body that has been scanned. Each image is a slice of the body area scanned, and numerous images are created that clearly show all the features of that particular part of the body.

The images produced by MRI can be compared to a sliced loaf of bread. Just as you can lift each individual slice from the loaf and see both the slice and the inside of the bread, so too the image “slices” produced by the MRI show the exact details of the inside of the body.

The computer is able to reconstruct all the images into a single image resembling an X-ray. This reconstruction also can be made into three-dimensional images, allowing complete and remarkable visualization of the body area scanned from all angles

MRI is arguably the greatest advance in diagnostic medical techniques over the past century.

How Does Magnetic Resonance Imaging Work?

Unlike CT, or computed tomography-another type of imaging-MRI uses no radiation. Instead, MRI uses a powerful magnet and radio waves to produce high-quality, cross-sectional pictures of the part of the body being studied. Each picture represents a virtual slice through the part of the body being imaged.

• The MRI scanning machine is a large donut-shaped magnet with a sliding scanning table. A person lies on this table, which then slides into the desired position in the MRI magnet. The machine produces loud, repetitive noises, like banging, during the procedure. But these noises, while unpleasant at times, aren’t dangerous or indicative of a problem.
• In our bodies, the nuclei of hydrogen atoms (called protons) normally point randomly in different directions. However, when exposed to the magnetic field in an MRI chamber, the nuclei line up in parallel formation, like rows of tiny magnets. Nearly two-thirds of the body’s hydrogen atoms are found in water and fat molecules.
• When the nuclei are subjected to a strong but harmless (and painless) pulse of radio waves from the MRI machine, they are knocked out of their parallel alignment. As they fall back into alignment, they produce a detectable radio signal.
• The signal is recorded by the machine and transferred to a computer.
• The computer uses these signals to calculate an image that is based on the strength of signal produced by different types of tissue. For example, tissues that contain little or no hydrogen (such as bone) appear black. Those that contain large amounts of hydrogen (such as the brain) produce a bright image.

How Safe is an MRI scan? What are the risks of an MRI scan?

Because MRI does not involve the use of x-rays, it is safe for the majority of people.

Certain people, however, may be unable to undergo the procedure. These include:

• Those who have implanted medical devices, including heart pacemakers and inner ear implants
• Those with metal close to or in an important organ, for example a piece of metal in the eye possibly from an old injury, or metal clips in the brain following treatment for a brain aneurysm

The reason for these limitations is that the powerful magnetic field of the scanner may interfere with the internal mechanism of some of these medical devices, which may be dangerous, as well as the fact that the presence of metal will cause poor quality pictures if it is near the part being scanned.

While there are no known hazards, MRI is not proven to be safe during pregnancy.

What Are The Risks Of An MRI

• Because MRI uses low-energy, non-ionizing radio waves, there are no known risks or side effects. In fact, since the technique uses no radiation, it can be repeated with no known adverse effects.
• While there are no known hazards, MRI is not proven to be safe during pregnancy. If a pregnant woman must undergo an MRI, she will be asked to sign a special consent form.
• The magnet at the center of the procedure may affect, or be affected by, any person fitted with a pacemaker, hearing aid, or other electrical device. People with such devices should advise the physician or technician. They are generally advised not to have an MRI.

When Is An MRI Requested?

MRI is used for a variety of diagnostic purposes. It is most often used to obtain information that hasn’t been provided by other imaging techniques, including ultrasound, conventional x-ray, or computed tomography.

In general, MRI is used to:

• Determine exactly what the problem is inside the body
• Show exactly where the problem is
• Rule out certain diseases

Because MRI produces images in any plane, it is particularly valuable in studying the brain and spinal cord and pinpointing even the smallest abnormality there. Because the water and fat content of tumors is different from surrounding normal tissue, MRI can reveal the precise location and size of tumors.

MRI also:

• Provides images of the internal structure of the eye and ear
• Produces detailed images of the heart and major blood vessels
• Provides images of blood flow in the circulatory system
• Produces detailed images of joints and soft tissues, particularly of cartilage, ligaments and tendons within joints such as the knee

Some additional diseases and medical conditions identified by MRI include:

• Disorders of chest and lungs
• Disorders of abdominal organs and the digestive tract
• Disorders of the kidneys, urinary tract and pelvic organs
• Infections
• Inflammatory conditions
• Trauma and other injury

How Do You Prepare For An MRI?

For an “ordinary” MRI, no special preparation is required, but leave any jewelry or other metal objects at home.

Under certain conditions, specialized MRIs require dietary restrictions provided by the physician. These include:

• When the MRI requires injection or ingestion of a contrast agent (a dye used for image enhancement)
• When sedation or anesthesia is necessary
• Other special considerations

It is important to notify the person performing the test of any illness, allergy, or previous drug reaction.

Open Scanners

Special scanners with open magnets exist for those people who are too claustrophobic and/or too large to fit in the regular machines. The image quality of the images obtained with some of the older open magnets is not quite as good as those obtained with regular magnets (but they’re still quite exceptional).

What Happens During the Procedure?

• When you arrive at the hospital, clinic, or laboratory where the test is to take place, you are usually asked to fill out the MRI screening questionnaire (unless this has been done previously).
• You can wear regular clothing as long as it is free of metal (zippers, buttons, etc).
• You must remove all metal objects, including jewelry, and electronic devices (such as a watch), as these will interfere with the test and may be damaged by the strong magnetic field within the MRI scanner.
• The technician or nurse will prepare you for the procedure, which may include oral or intravenous sedation by the radiologist, or anesthesia by an anesthesiologist.
• The technician will take you to the scanning table, which resembles a narrow bed, and position you for the test.
• Before the MRI begins, the table is moved to the doughnut-shaped magnet (it looks like a large box with an opening in the center). After the machine is adjusted, the MRI begins.
• The usual procedure is that the technologist will perform the MRI with the radiologist, and they will be in contact at all times. During the procedure, you will hear several series of loud, repetitive pulsing noises. In many instances, you will be given protective headphones or ear plugs. These noises may be initially be frightening, but they are harmless and indicate that the machine is working. It is particularly important to remain completely still during these sequences of noises, as the MRI machine is obtaining images at these times.
• If the first pictures are motion-free, it will shorten the total exam time and prevent delays.
• An entire MRI exam may take from 20 minutes to 1.5 hours, depending on the type of information required by the radiologist and your physician, and the quality of the images they need to make a diagnosis.
• All people who have sedation or anesthesia will receive continuous monitoring and support during this time.

What Should You Do After the Test?

After the images are obtained, you can resume normal activities immediately.

MRI: Frequently Asked Questions

Here are some frequently asked questions related to MRI.

Q: Are the “radio waves” used in MRI like regular radio waves?

A: No. While the MRI does use a radio wave antenna to send signals to the body and receive signals back during the procedure, the “radio wave signals” are actually a changing magnetic field that is much weaker than the strong magnetic field of the main magnet in the machine.

Q: Why not just get an x-ray?

A: MRIs are particularly useful for looking at the non-bony parts or “soft tissues” of the body-the same types of body parts and tissues that x-ray machines are not designed to pick up

Q: Are there any disadvantages to MRI?

A: Aside from those who suffer from claustrophobia, or who have implanted medical devices that prohibit the use of MRI, there are no known medical disadvantages. One financial disadvantage is that the MRI costs more than a regular x-ray or CT scan.

Q: Why are “stress tests” ordered?

A: Standard stress tests such as treadmill exercise tests, in which the patient walks on a treadmill while being “hooked up” to an electrocardiogram machine, can indicate how well the heart handles increased physical exertion. Stress tests also help physicians to find a blockage or other problem in the blood vessels of the heart.

Q: What is the main difference between x-ray and MRI?

A: Aside from the fact that MRI does not use radiation to obtain images, the biggest difference is that MRI can “see through” bone and define fluid-filled soft-tissue, while x-rays can only define bone.

MRI: Putting It All Together

Here is a summary of the important facts and information related to MRI.

• Magnetic Resonance Imaging, or MRI, is a painless diagnostic procedure that uses a powerful magnet and radio waves to produce high-quality, cross-sectional images of organs and body structures without the use of X-rays or other ionizing radiation.
• Because MRI does not involve the use of x-rays, it is safe for the majority of people. MRI uses low-energy, non-ionizing radio waves, so there are no known risks or side effects, even with repeated imaging.
• The MRI scanning machine is a large donut-shaped magnet with a movable scanning table on which a person lies. The machine produces loud noises, like banging, during the procedure. The noises, while unpleasant at times, aren’t dangerous or indicative of a problem, but are a signal that images are being taken.
• The computer uses the MRI signals to create an image that is based on the strength of signal produced by different types of tissue. For example, tissues that contain little or no hydrogen (such as bone) appear black. Those that contain a large amount of hydrogen (such as the brain) produce a bright image.
• In general, MRI is used to rule out certain diseases and to facilitate treatment for medical conditions.
• For an “ordinary” MRI, no special preparation is required.
• An entire MRI exam may take from 20 minutes to 1.5 hours, depending on the type of information required by the radiologist and physician, and the quality of the images needed to make a particular diagnosis.
• Normal activities can be immediately resumed after the images are obtained.
• Because the machine has a tunnel-like appearance, some people feel uncomfortable, or even claustrophobic (fearful of being closed in). Most people can overcome this feeling, but for those who need assistance, the doctor can prescribe a tranquilizer for use before the procedure.
• Special scanners with open magnets exist for those people who are too claustrophobic and/or too large to fit in the regular machines. Unfortunately, the image quality of these scanners is generally not as good as it is from those with closed magnets.

MRI: Glossary

Here are definitions of medical terms related to MRI.

Echocardiogram: A method of obtaining an image of the heart structure by using ultrasound (inaudible, high-frequency, sound waves). The sound is reflected differently by each part of the heart. The result is a complex series of echoes that are recorded and analyzed.

Electrocardiogram (ECG, or EKG): A record of the electrical impulses that immediately precede contraction of the heart muscle. The waves produced are known as the P, Q, R, S, and T waves. The EKG is used to diagnose heart disorders, many of which produce deviations from normal electrical patterns.

Ischemia: Insufficient blood supply to a specific organ or tissue, usually caused by a blood vessel disease, but can also result from vessel injury, constriction, or inadequate blood flow due to inefficient action of the heart.

Radio waves (MRI): The “radio wave signals” used in MRI are actually a changing magnetic field that is much weaker than the strong magnetic field of the main magnet in the machine.

Stroke: Damage to part of the brain caused by the interruption of the blood supply, or by leakage of blood outside the vessel walls. Stroke can cause damage to sensation, movement, or function, depending on which area of the brain is affected.

Thallium stress test: A specialized kind of exercise stress test which is done after the patient receives a injection of a harmless radioactive substance (thallium) into the bloodstream. Its purpose is to show how well blood flows to the heart muscle during exercise. The source of exercise can be either a treadmill or a stationary bicycle.

Ventricle: There are two ventricles in the heart, the right and the left ventricle. They are the lower, pumping chambers, which receive blood from the upper heart chambers (atria) and pump it to the lungs and the rest of the body.

MRI: Additional Sources Of Information

Here are some reliable sources that can provide more information on MRI.

An excellent review of the MRI procedure and the physics and mathematics that make it possible was written by Joseph P. Hornack, professor of chemistry and imaging science at the Rochester Institute of Technology. The entire text is available at: http://www.cis.rit.edu/htbooks/mri/inside.htm