MRI (Magnetic Resonance Imaging)

Introduction

On July 3, 1977, the first MRI exam was performed on a human being.  It took almost five hours to produce one image. Dr. Raymond Damadian, a physician and scientist, along with colleagues Dr. Larry Minkoff and Dr. Michael Goldsmith, labored for seven years to reach that point. They named their original machine "Indomitable." This machine is now in the Smithsonian Institution. As late as 1982, there were a handful of MRI scanners in the United States. Today there are thousands, and images can be created in seconds what used to take hours.

The basic design of an MRI machine resembles a cube, typically measuring 7 feet tall by 7 feet wide by 10 feet long, although new models are rapidly shrinking.  There is a horizontal tube running from front to back through the center of the machine which houses an extraordinary strong magnet. This tube is known as the bore of the magnet. The patient, lying on his or her back, slides into the bore on a special table.  Whether or not the patient goes in head first or feet first, as well as how far in the magnet they will go, is determined by the type of exam to be performed.  MRI scanners vary in size and shape, and newer or specially designed models have some degree of openness around the sides, but the basic design is the same.  Once the body part to be scanned is in the exact center or isocenter of the magnetic field, the scan can begin.

In conjunction with radio wave pulses of energy, 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 point might be a cube that is half a millimeter on each side. The MRI system goes through the patient's body point by point, building up a 2-D or 3-D map of tissue types.  It then integrates all of this information together to create 2-D images or 3-D models.

MRI provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other imaging modality.  MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked.  By changing exam parameters, the MRI system can cause tissues in the body to assume different appearances.  This is very helpful to radiologists who read MRIs in determining if something seen is normal or not.  MRI systems can also image flowing blood in virtually any part of the body. This allows us to perform studies that show the arterial system in the body, but not the tissue around it.  In many cases, the MRI system can do this without a contrast injection, which is required in vascular radiology.

Magnetic Intensity

The biggest and most important component in an MRI system is the magnet. The magnet in an MRI system is rated using a unit of measure known as a tesla. The magnets in use today in MRI are generally in the 0.5-tesla to 3.0-tesla range.

Safety

Prior to allowing a patient or support staff member into the scan room, he or she is thoroughly screened for metal objects.  Often however, patients have implants inside them that make it very dangerous for them to be in the presence of a strong magnetic field. People with pacemakers cannot be scanned or even go near the scanner because the magnet can cause the pacemaker to malfunction. Aneurysm clips in the brain can be very dangerous as the magnet can move them, causing them to tear the very artery they were placed on to repair. Some dental implants are magnetic.  Most orthopedic implants, even though they may be ferromagnetic, are fine because they are firmly embedded in bone.  Even metal staples in most parts of the body are fine -- once they have been in a patient for a few weeks, enough scar tissue has formed to hold them in place.  Each time we encounter patients with an implant or metallic object inside their body, we investigate thoroughly to make sure it is safe to scan them. There are no known biological hazards to humans from being exposed to magnetic fields of the strength used in medical imaging today. Most facilities prefer not to image pregnant women.  This is due to the fact that there has not been much research done in the area of biological effects 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.

The Magnets

There are three basic types of magnets used in MRI systems:

  • Resistive magnets consist of many windings or coils of wire wrapped around a cylinder or bore through which an electric current is passed. This causes a magnetic field to be generated. If the electricity is turned off, the magnetic field dies out. These magnets are lower in cost to construct than a superconducting magnet (see below), but require huge amounts of electricity (up to 50 kilowatts) to operate because of the natural resistance in the wire.
  • A permanent magnet's magnetic field is always there and always on full strength, so it costs nothing to maintain the field. The major drawback is that these magnets are extremely heavy. They weigh many, many tons at the 0.4-tesla level. A stronger field would require a magnet so heavy it would be difficult to construct. Permanent magnets are getting smaller, but are still limited to low field strengths.
  • Superconducting magnets are by far the most commonly used. A superconducting magnet is somewhat similar to a resistive magnet -- coils or windings of wire through which a current of electricity is passed create the magnetic field. The important difference is that the wire is continually bathed in liquid helium at 452.4 degrees below zero. This almost unimaginable cold causes the resistance in the wire to drop to zero, reducing the electrical requirement for the system dramatically and making it much more economical to operate. Superconductive systems are still very expensive, but they can easily generate 0.5-tesla to 3.0-tesla fields, allowing for much higher-quality imaging.

A very uniform, or homogeneous, magnetic field of incredible strength and stability is critical for high-quality imaging.  It forms the main magnetic field. Magnets like those described above make this field possible.

Another type of magnet found in every MRI system is called a gradient magnet. There are three gradient magnets inside the MRI machine.  These magnets are very, very low strength compared to the main magnetic field; they may range in strength from 180 gauss to 270 gauss, or 18 to 27 millitesla (thousandths of a tesla).

The main magnet immerses the patient in a stable and very intense magnetic field, and the gradient magnets create a variable field.  The rest of an MRI system consists of a very powerful computer system, some equipment that allows us to transmit RF (radio frequency) pulses into the patient's body while they are in the scanner, and many other secondary components

Understanding the Technology

The MRI machine applies an RF (radio frequency) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine.  The pulse causes the protons in that area to absorb the energy required to make them spin, or precess, in a different direction.  This is the "resonance" part of MRI. The RF pulse forces them (only the one or two extra unmatched protons per million) 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.

These RF pulses are usually applied through a coil.  MRI machines come with many different coils designed 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.  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 are turned on and off very rapidly in a specific manner, they alter the main magnetic field on a very local level.  What this means is that we can pick exactly which area we want a picture of.  In MRI we speak of "slices." Think of a loaf of bread with slices as thin as a few millimeters -- the slices in MRI are that precise. We can "slice" any part of the body in any direction, giving us 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.

When the RF pulse is turned off, the hydrogen protons begin to slowly return to their natural alignment within the magnetic field and release their excess stored energy.  When they do this, they give off a signal that the coil now picks up and sends to the computer system.  What the system receives is mathematical data that is converted into a picture that we can put on film. That is the "imaging" part of MRI.

Visualization

Most imaging modalities use injectable contrast, or dyes, for certain procedures.  MRI is no different.

MRI contrast works by altering the local magnetic field in the tissue being examined.  Normal and abnormal tissue will respond differently to this slight alteration, giving us differing signals.  These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and disease processes better than we could without the contrast.

The fact that MRI systems do not use ionizing radiation is a comfort to many patients, as is the fact that MRI contrast materials have a very low incidence of side effects. Another major advantage of MRI is its ability to image in any plane.  CT 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 images in the sagitall plane (slicing the bread side-to-side lengthwise) and coronally (think of the layers of a layer cake) or any degree in between, without the patient ever moving.  If you have ever had an X-ray, you know that every time they take a different picture, you have to move.  The three gradient magnets discussed earlier allow the MRI system to choose exactly where in the body to acquire an image and how the slices are oriented.

Advantages

MRI is ideal for:

  • Diagnosing multiple sclerosis (MS);
  • Diagnosing tumors of the pituitary gland and brain;
  • Diagnosing infections in the brain, spine or joints ;
  • Visualizing torn ligaments in the wrist, knee and ankle;
  • Visualizing shoulder injuries ;
  • Diagnosing tendonitis ;
  • Evaluating masses in the soft tissues of the body ;
  • Evaluating bone tumors, cysts and bulging or herniated discs in the spine; and
  • Diagnosing strokes in their earliest stages.

Disadvantages

Although MRI scans are ideal for diagnosing and evaluating a number of conditions, it does have drawbacks as follows:

  • There are many people who cannot safely be scanned with MRI (for example, because they have pacemakers);
  • The machine makes a lot of noise during a scan.  The noise sounds like a continual, rapid hammering.  Patients are given earplugs or stereo headphones to muffle the noise (in most MRI centers you can even bring your own cassette or CD to listen to).  The noise results from 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;
  • MRI scans require patients to hold very still for extended periods of time.  MRI exams can range in length from 20 minutes to 90 minutes or more.  Even very slight movement of the part being scanned can cause very distorted images that will have to be repeated; and
  • Orthopedic hardware (screws, plates, artificial joints) in the area of a scan can cause severe artifacts (distortions) on the images.  The hardware causes a significant alteration in the main magnetic field.

The Future of MRI

The future of MRI seems limited only by our imagination. This technology is still in its infancy, comparatively speaking. It has been in widespread use for less than 20 years (compared with over 100 years for X-rays).

Very small scanners for imaging specific body parts are being developed.  Functional brain mapping (scanning a person's brain while he or she is performing a certain physical task such as squeezing a ball, or looking at a particular type of picture) is helping researchers better understand how the brain works.  Research is under way in a few institutions to image the ventilation dynamics of the lungs through the use of hyperpolarized helium-3 gas. The development of new, improved ways to image strokes in their earliest stages is ongoing.