How MRI Works

MRI scanners vary in size and shape, and some newer models have a greater degree of openness around the sides. Still, the basic design is the same, and the patient is pushed into a tube that's only about 24 inches (60 centimeters) in diameter [source: Hornak]. But what's in there?

The biggest and most important component of an MRI system is the magnet. There is a horizontal tube -- the same one the patient enters -- running through the magnet from front to back. This tube is known as the bore. But this isn't just any magnet -- we're dealing with an incredibly strong system here, one capable of producing a large, stable magnetic field.

The strength of a magnet in an MRI system is rated using a unit of measure known as a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). The magnets in use today in MRI systems create a magnetic field of 1.5-tesla to 7.0-tesla, or 15,000 to 70,000 gauss. When you realize that the Earth's magnetic field measures 0.5 gauss, you can see how powerful these magnets are.

Most MRI systems use a superconducting magnet, which consists of many coils or windings of wire through which a current of electricity is passed, creating a magnetic field of up to 2.0 tesla. Maintaining such a large magnetic field requires a good deal of energy, which is accomplished by superconductivity, or reducing the resistance in the wires to almost zero. To do this, the wires are continually bathed in liquid helium at 452.4 degrees below zero Fahrenheit (269.1 below zero degrees Celsius) [source: Coyne]. This cold is insulated by a vacuum. While superconductive magnets are expensive, the strong magnetic field allows for the highest-quality imaging, and superconductivity keeps the system economical to operate.

Two other magnets are used in MRI systems to a much lesser extent. Resistive magnets are structurally like superconducting magnets, but they lack the liquid helium. This difference means they require a huge amount of electricity, making it prohibitively expensive to operate above a 0.3 tesla level. Permanent magnets have a constant magnetic field, but they're so heavy that it would be difficult to construct one that could sustain a large magnetic field.

There are also three gradient magnets inside the MRI machine. These magnets are much lower strength compared to the main magnetic field; they may range in strength from 180 gauss to 270 gauss. While the main magnet creates an intense, stable magnetic field around the patient, the gradient magnets create a variable field, which allows different parts of the body to be scanned.

Another part of the MRI system is a set of coils that transmit radiofrequency waves into the patient's body. There are different coils for different parts of the body: knees, shoulders, wrists, heads, necks and so on. These coils usually conform to the contour of the body part being imaged, or at least reside very close to it during the exam. Other parts of the machine include a very powerful computer system and a patient table, which slides the patient into the bore. Whether the patient goes in head or feet first is determined by what part of the body needs examining. Once the body part to be scanned is in the exact center, or isocenter, of the magnetic field, the scan can begin.

What goes on during a scan? Find out next.

When patients slide into an MRI machine, they take with them the billions of atoms that make up the human body. For the purposes of an MRI scan, we're only concerned with the hydrogen atom, which is abundant since the body is mostly made up of water and fat. These atoms are randomly spinning, or precessing, on their axis, like a child's top. All of the atoms are going in various directions, but when placed in a magnetic field, the atoms line up in the direction of the field.

These hydrogen atoms have a strong magnetic moment, which means that in a magnetic field, they line up in the direction of the field. Since the magnetic field runs straight down the center of the machine, the hydrogen protons line up so that they're pointing to either the patient's feet or the head. About half go each way, so that the vast majority of the protons cancel each other out -- that is, for each atom lined up toward the feet, one is lined up toward the head. Only a couple of protons out of every million aren't canceled out. This doesn't sound like much, but the sheer number of hydrogen atoms in the body is enough to create extremely detailed images. It's these unmatched atoms that we're concerned with now.

Next, the MRI machine applies a radio frequency (RF) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. When the pulse is applied, the unmatched protons absorb the energy and spin again in a different direction. This is the "resonance" part of MRI. The RF pulse forces them to spin at a particular frequency, in a particular direction. The specific frequency of resonance is called the Larmour frequency and is calculated based on the particular tissue being imaged and the strength of the main magnetic field.

At approximately the same time, the three gradient magnets jump into the act. They are arranged in such a manner inside the main magnet that when they're turned on and off rapidly in a specific manner, they alter the main magnetic field on a local level. What this means is that we can pick exactly which area we want a picture of; this area is referred to as the "slice." Think of a loaf of bread with slices as thin as a few millimeters -- the slices in MRI are that precise. Slices can be taken of any part of the body in any direction, giving doctors a huge advantage over any other imaging modality. That also means that you don't have to move for the machine to get an image from a different direction -- the machine can manipulate everything with the gradient magnets.

But the machine makes a tremendous amount of noise during a scan, which sounds like a continual rapid hammering. That's due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise. In most MRI centers, you can bring a music player to drown out the racket, and patients are given earplugs.

When the RF pulse is turned off, the hydrogen protons slowly return to their natural alignment within the magnetic field and release the energy absorbed from the RF pulses. When they do this, they give off a signal that the coils pick up and send to the computer system. But how is this signal converted into a picture that means anything?

The MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The system goes through the patient's body point by point, building up a map of tissue types. It then integrates all of this information to create 2-D images or 3-D models with a mathematical formula known as the Fourier transform. The computer receives the signal from the spinning protons as mathematical data; the data is converted into a picture. That’s the "imaging" part of MRI.

The MRI system uses injectable contrast, or dyes, to alter the local magnetic field in the tissue being examined. Normal and abnormal tissue respond differently to this slight alteration, giving us differing signals. These signals are transferred to the images; an MRI system can display more 250 shades of gray to depict the varying tissue [source: Coyne]. The images allow doctors to visualize different types of tissue abnormalities better than they could without the contrast. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality.

An X-ray is very effective for showing doctors a broken bone, but if they want a look at a patient's soft tissue, including organs, ligaments and the circulatory system, then they'll likely want an MRI. And, as we mentioned on the last page, another major advantage of MRI is its ability to image in any plane. Computer tomography (CT), for example, is limited to one plane, the axial plane (in the loaf-of-bread analogy, the axial plane would be how a loaf of bread is normally sliced). An MRI system can create axial images as well as sagitall (slicing the bread side-to-side lengthwise) and coronal (think of the layers in a layer cake) images, or any degree in between, without the patient ever moving.

But for these high-quality images, the patient can't move very much at all. MRI scans require patients to hold still for 20 to 90 minutes or more. Even very slight movement of the part being scanned can cause distorted images that will have to be repeated. And there's a high cost to this kind of quality; MRI systems are very expensive to purchase, and therefore the exams are also very expensive.

But are there any other costs? What about the patient's safety?

Maybe you're concerned about the long-term impact of having all your atoms mixed about, but once you're out of the magnetic field, your body and its chemistry return to normal. There are no known biological hazards to humans from being exposed to magnetic fields of the strength used in medical imaging today. The fact that MRI systems don’t use ionizing radiation, as other imaging devices do, is a comfort to many patients, as is the fact that MRI contrast materials have a very low incidence of side effects. Most facilities prefer not to image pregnant women, due to limited research of the biological effects of magnetic fields on a developing fetus. The decision of whether or not to scan a pregnant patient is made on a case-by-case basis with consultation between the MRI radiologist and the patient's obstetrician.

However, the MRI suite can be a very dangerous place if strict precautions are not observed. Credit cards or anything else with magnetic encoding will be erased. Metal objects can become dangerous projectiles if they are taken into the scan room. For example, paperclips, pens, keys, scissors, jewelry, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet at very high speeds.

Big objects pose a risk, too -- mop buckets, vacuum cleaners, IV poles, patient stretchers, heart monitors and countless other objects have all been pulled into the magnetic fields of the MRI. In 2001, a young boy undergoing a scan was killed when an oxygen tank was pulled into the magnetic bore [source: McNeil]. Once, a pistol flew out of a policeman's holster, the force causing the gun to fire. No one was injured.

To ensure safety, patients and support staff should be thoroughly screened for metal objects prior to entering the scan room. Often, however, patients have implants inside them that make it very dangerous for them to be in the presence of a strong magnetic field. These include:

Most modern surgical implants, including staples, artificial joints and stents are made of non-magnetic materials, and even if they're not, they may be approved for scanning. But let your doctor know, as some orthopedic hardware in the area of a scan can cause distortions in the image.