All You Ever Wanted To Know About MRI Kim Eriks and Katy Koukouras

MRI=Magnetic Resonance Imaging

Allows the clinician to see high quality images of the inside of the body: Brain Heart Lungs Spine Knees Wrist Etc.

MRI is a very close relative of NMR, which allows clinicians to obtain chemical and physical information about certain molecules. In the 1970’s the name was changed from NMRI to MRI due to the negative connotations associated with the word “nuclear”. Many patients thought that the exam would expose them to radiation.

In 1952 Felix Bloch and Edward Purcell were awarded the Nobel Prize when they discovered the concepts surrounding NMR/MRI. During the time between 1950-1970, the idea was used for chemical and physical analysis of molecules.

In 1971, Raymond Damadian discovered that NMR could be used in the detection of diseases. In 1974, Damadian received a patent for the design of his MRI machine. In 1977, Damadian did his first scan on a human, his assistant, Larry Minkoff. He couldn’t go in himself due to his enormous size.

Damadian’s first prototype was called “Indomitable”, due to criticism and the seven years that it took to complete. In 1978, Damadian established a new corporation called FONAR, which introduced the first commercial MRI scanner in 1980.

MRI machines have come a long way since Slide shows which area of the brain is responsible for touch. MRI machines have come a long way since Indomitable. Previously, it took up to five hours to get an image, whereas today, it takes minutes. In 1992, functional magnetic resonance imaging (fMRI) was discovered, which allowed clinicians to see various regions of the brain, their functions, and their specific locations.

MRI machines look like a large block with a tube running through the middle of the machine, called the bore of the magnet. The bore is where the patient is located for the duration of the scan.

The MRI machine picks points in the patients body, decides what type of tissue the points define, then compiles the points into 2 dimensional and 3 dimensional images. Once the 3 dimensional image is created, the MRI machine creates a model of the tissue. This allows the clinician to diagnose without the use of invasive surgery.

The largest and most important components of the MRI machine are the magnets. The magnet strength is measured in units of Tesla or Gauss (1 Tesla = 10,000 Gauss). Today’s MRI machines have magnets with strengths from 5000 to 20,000 Gauss. To give perspective on the strength of these magnets, the earth’s magnetic field is about .5 Gauss, making the MRI machine 10,000 to 30,000 times stronger.

There are three types of magnets: Resistive Magnets Permanent Magnets Superconducting Magnets

The resistive magnet has many coils of wire that wrap around the bore, through which electrical currents are passed, creating a magnetic field. This particular magnet requires a large amount of electricity to run, but are quite cheap to produce. The permanent magnet is one that delivers a magnetic field, which is always on at full strength and therefore, does not require electricity. The cost to run the machine is low due to the constant magnetic force. However, the major drawback of these magnets is the weight in relation to the magnetic field they produce.

The superconducting magnets are very similar to the design of the resistive magnets, in that they too have coils through which electricity is passed creating a magnetic field. However, the major difference between the resistive magnet and the superconducting magnet is the fact that the coils are constantly bathed in liquid helium at -452.4ºC. This cold temperature causes the resistance of the wire to be near zero, therefore reducing the electrical requirement of the system. All of these factors allow for the machine to remain a manageable size, have the ability to create high quality images, and still operate at a reasonable cost.

The superconducting magnet is the most commonly used in machines today, giving the highest quality images of all three magnet types.

There is another type of magnet that is found in all MRI machines, called gradient magnets These magnets are responsible for altering the magnetic field in the area to be scanned and can magnetically “slice” the tissue to be examined from every angle.

MRI’s of the heart can be done to look at many different areas including: vessels, chambers, and valves. The MRI can detect problems associated with different heart diseases including plaque build up and other blockages in blood vessels due to coronary artery disease or heart attacks.

MRI’s of the brain can evaluate how the brain is working, whether normal or abnormal. Brain MRI’s can show damage resulting from different problems such as: damage due to stroke, abnormalities associated with dementia and/or Alzheimer’s, seizures, and tumors. fMRI are done prior to brain surgery, to give a map of the brain, and help plan the procedure.

MRI’s can be done on the knee to evaluate damage to the meniscus, ligaments, and tendons. Tears in the ligaments are given a grade 1-3 depending on their severity: 1-fluid around the ligament 2-fluid around the ligament with partial disruption of the ligament fibers 3-complete disruption of the ligament fibers

Often prior to a MRI scan, a patient would need to have a contrast dye, either injected or taken orally, usually gadolinium as seen here.

The Procedure…

Once the contrast dye has been injected, the patient enters the bore of the MRI machine on their back lying on a special table. The patient will enter the machine head first or feet first, depending on the area to be scanned. Once the target is centered, the scan can begin.

The scan can last anywhere from 20-30 minutes. The patient has a coil that is placed in the target area, to be scanned. A radio frequency is passed through the coils that excites the hydrogen protons in the target area. The gradient magnets are then activated in the main magnet and alter the magnetic field in the area that is being scanned.

The patient must hold completely still in order to get a high quality image. (This is hard for patients with claustrophobia, and often times a sedative will be given, if appropriate.) The radio frequency is then turned-off and the hydrogen protons slowly begin to return to their natural state.

The magnetic field runs down the center of the patient, causing the slowing hydrogen protons to line-up. The protons either align themselves pointed towards the head or the feet of the patient, and most cancel each other out. The protons that are not cancelled create a signal and are the ones responsible for the image.

The contrast dye is what makes the target area stand out and show any irregularities that are present. The dye blocks the X-Ray photons from reaching the film, showing different densities in the tissue. The tissue is classified as normal or abnormal based on its response to the magnetic field.

The tissues with the help of the magnetic field send a signal to the computer. The different signals are sent and modified into images that the clinician can evaluate, and label as normal or abnormal. If the tissue is considered abnormal, the clinician can often detect the abnormality, and monitor progress and treatment of the abnormality.

The MRI has allowed clinicians to treat, monitor, and learn about many different diseases and problems. As well as, to learn how the body functions, normally, without needing to resort to more invasive methods like surgery.

MRI treatment is a wonderful option for most patients, but there are some people who are not candidates. Those include: Patients with pacemakers cannot have the scan done as the magnet from the MRI interferes with the signal sent from the pacemaker, and deactivates it. 2) Patients who are too tall, or too obese 3) Patients who have orthopedic hardware can get distortion in the image, and the scan quality is not as high.

THE FUTURE OF MRI: The possibility of having very small machines that scan specific parts of the body. The continuing improvements on seeing the venous and arterial systems. Brain mapping while the patient does specific tasks, allowing clinician’s to see what part of the brain is responsible for that task/activity. Improvements on the ability to do MRI’s of the lungs. ETC.