Magnetic Resonance Imaging Scanner
Magnetic Resonance Imaging Scanner
Magnetic Resonance Imaging Scanner
A computer controls all parts of the MRI scanner. It processes all incoming information and then forms images of the body part being scanned.
A magnetic resonance imaging (MRI) scanner is a machine that uses magnetic fields and radiofrequency (RF) radiation, or radio waves, to take detailed pictures of tissues and organs within the body. MRI is noninvasive, which means that the patient is not subjected to any surgery or X rays. MRI is very valuable in diagnosing (identifying the nature and cause of) disorders of the brain and spinal cord, heart disease, cancers, and injuries and diseases affecting bones, ligaments, and cartilage.
The MRI scanner was originally called the NMR (nuclear magnetic resonance) scanner. The "nuclear" part of the term was changed in the 1980s because it was feared people might think dangerous radiation was involved in the process. Actually, "nuclear" refers to the nucleus, or the center of an atom.
Under ordinary conditions, components of the nucleus called protons spin around like toy tops. If a powerful, steady magnetic field is applied to the protons, they line up with the magnetic field in an orderly formation. When radio frequency (RF) radiation is directed into the magnetic field, the protons take energy from it and go out of formation. Once the RF signal stops, the protons resume their former positions and emit their own energy, or RF signals, giving away their location. This phenomenon is called nuclear magnetic resonance (NMR).
It is due to NMR phenomenon that magnetic resonance imaging is possible. Each kind of tissue sends out its own characteristic NMR signal. A computer is able to process the signals emitted by the disturbed protons, as well as the time it took them to get back into formation. The computer can also figure out the type of tissue in which the protons are located. It can also search for signals given out by specific types of protons, such as cancer cells. The computer converts this information into three-dimensional pictures on a television screen. Copies of the images are then made.
In 1938, American physicist Isidor Isaac Rabi (1898–1988) developed the first basic NMR device, by which he measured the magnetic properties of atoms. He was able to do so by measuring the spin of the protons in the atom's nucleus. Spin refers to the property of the protons that gives them a nuclear magnetic moment, that is, the protons behave like small magnets.
In 1945, two groups of scientists improved on Rabi's NMR device. Many applications of NMR in such fields as biology and chemistry have since resulted from the devices independently developed by Swiss-born Felix Bloch (1905–1983) and his colleagues at Stanford University (California) and the Edward Purcell (1912–1997) research team at Harvard University (Massachusetts).
Two men and a single vision
Raymond V. Damadian (1936–), an American medical doctor and researcher, first proposed the application of NMR in scanning the human body in 1969. In laboratory animal experiments, Damadian found that different body tissues emitted NMR signals that vary in length. He also found that cancerous tissues differ from normal tissues in the quality and length of their NMR signals. In 1977, Damadian and his colleagues, Drs. Larry Minkoff and Michael Goldsmith, formed the first MRI scan of a human being. Actually, Minkoff was the "patient," who had to stay still for nearly five hours during the imaging of his whole body.
At about the same time, British engineer Godfrey Hounsfield (1919–) was also experimenting with nuclear magnetic resonance. Hounsfield had just invented the CAT (computerized axial tomography) scanner, which became commercially available in 1975. Like Damadian, Hounsfield believed that NMR signals emitted by cancerous cells are different from those emitted by healthy cells.
The primary functioning parts of an MRI scanner include an external magnet, gradient coils, RF (radio frequency) equipment, and a computer. Other components include an RF shield, a power supply, NMR probe, display unit, and a refrigeration unit.
The external magnet, also called the imaging magnet, is the largest component of the MRI scanner. It is responsible for creating the powerful, steady magnetic field that penetrates throughout the body part in which a possible disorder is being identified.
Three types of magnets are currently used—the superconducting magnet, the resistive magnet, and the permanent magnet. Superconducting magnets, made of superconducting wire, are the most commonly used in MRI scanners. The superconducting wire is so called because it conducts, or transmits, electricity with no resistance at temperatures near absolute zero. The wire is made of a niobium-titanium alloy embedded in copper and is supercooled with liquid helium and liquid nitrogen. This type of magnet carries electricity without energy loss and, therefore, generates larger magnetic fields for higher-quality imaging.
A resistive magnet, as its name implies, creates a magnetic field by resistance to electric current in aluminum wire that is wrapped around an iron core. The magnet requires a great amount of electricity. It also requires water cooling since the resistance generates heat.
The third type of magnet used is a permanent magnet. It is made of ferromagnetic material, which generally contains iron, nickel, or cobalt. It is quite large and heavy and does not require electricity to run. Although the magnetic field in this magnet is always present, the stability of the magnetic field is not reliable.
THE NUCLEUS IS A SMALL MAGNET
In the early 1900s, scientists figured out that atoms, the tiniest particles of matter, are made up of protons and neutrons found within a central nucleus and surrounded by electrons. In 1924, Austrian physicist Wolfgang Pauli (1900–1958) discovered that the nucleus is a small magnet. This magnetic property is the basis for nuclear magnetic resonance (NMR), now called magnetic resonance imaging (MRI).
The three gradient coils used in the MRI scanner are also magnets. These resistive magnets are positioned inside the main magnet and are kept at room temperature. While the main magnet subjects the patient to an intense, steady magnetic field, the gradient magnets provide varying strengths of magnetic fields to produce gradients, or variations, in the main magnetic field. The gradient coils help obtain the three-dimensional information needed for accurate images.
The RF system plays many roles in an MRI scanner. First, it transmits the RF radiation that causes the atoms in the tissue to emit a signal. Next, it receives the emitted signal and makes it louder so that a computer can use the information to construct an image on a television screen. RF coils are the primary pieces of hardware in the RF system. Different coils are available for the different body parts. The coil is usually positioned along the body part that is being scanned. Several types of RF coils are used. In newer models of MRI scanners, an RF shield is part of the machine. The shield is an aluminum sheet that reduces RF interferences.
The computer controls all parts of the MRI scanner. It controls the RF signals sent and stores the signals received. The computer processes all the information and then forms images of the body part being scanned.
The Manufacturing Process
The individual parts of an MRI scanner are typically manufactured separately and then assembled into a large unit. These units are extremely heavy, sometimes weighing over 100 tons (102 metric tons).
1 The superconducting magnet is the type of magnet most commonly used in an MRI scanner. The basic design consists of coils of conductive (capable of transmitting electricity) wire, a cooling system, and a power supply. The coils are made by wrapping wire around a cylindrical tube through which electric current is passed. The wire is made from threads of a niobium-titanium alloy embedded in copper. To create the necessary magnetic field, several coils are used.
2 The coils are immersed in a vessel of liquid helium. This lowers their temperature to where they would not resist the flow of electricity, which can cause loss of energy. To help maintain a stable low temperature, the vessel is surrounded by two more vessels containing other coolants, such as liquid nitrogen. This whole assembly is then suspended with thin rods in a vacuum-sealed container. A power source is hooked up to the magnetic coils. The power source is used only when the magnets need to be energized. The cylindrical magnet is attached to a patient support, which is a movable table that brings the patient into the magnetic field.
3 The three gradient coils are resistive magnets, each of which is made by winding thin strips of copper or aluminum in a specific pattern. The coils are strengthened by adding epoxy into their structure. The width of the coil is designed so that it is large enough to prevent claustrophobia (fear of being in an enclosed space) in the patient, but not so large that it requires significant energy to operate. The gradient coils are typically shielded to prevent interfering eddy currents (electrical current set by alternating magnetic fields).
4 The electrical components of the RF system may be produced by an outside manufacturer and then assembled by the MRI manufacturer. These components are attached to the RF transmitter and receiver coils. The coils are made of conducting copper that can create a vibrating magnetic field.
5 Different types of RF coils are designed for different parts of the body. A surface coil has a simple design, consisting of a loop of wire that may be circular or rectangular. It is used for imaging the shoulder or the spine and is placed directly on the patient. A bird cage coil, which is placed inside the gradient coils, is used for imaging the head and brain, while a paired saddle coil is used for the knees. A single turn solenoid coil is used for imaging the extremities, such as the wrists. Each type of coil is attached to a power source.
6 The computer, which is supplied by a computer manufacturer, is modified and programmed for the MRI scanner. Attached to the computer are the user interface, the Fourier transformer, the signal converter, and a preamplifier. A display device and a laser printer also come with the computer.
7 Each MRI component is assembled together and placed into an appropriate frame. The whole scanner is put together at the hospital or other medical facility where it will be used. The magnet is typically transported in an air-suspended vehicle for a smoother ride.
Throughout the manufacturing process, visual and electrical inspections of the MRI scanner are performed. The finished scanner is then tested to make sure it is working properly. The testing is done under different environmental conditions, such as excessive heat and humidity. Most manufacturers establish their own quality specifications for their products. Various government agencies, such as the Department of Health and Human Services, and medical organizations have also proposed standards and performance recommendations.
MRI technology is young and has plenty of room for development. New models are not only shorter but also lighter. Some companies are looking into smaller-sized machines just for certain body parts, such as the foot or the hand. Open MRI scanners have solved the problem of patients who experience claustrophobia when placed in the conventional narrow tube. A vertically open MRI machine called the Magnetic Resonance Therapy comes with a tracker that can be attached to a patient's joint. The physician, who can stand on either side of the patient, can see, within seconds, what happens to the problem joint as the patient moves.
Dr. Raymond Damadian, the inventor of the MRI scanner and founder of FONAR, the first MRI manufacturing company, has several machines under development. These include a standup model, a breast scanner, and a room scanner with enough space for a team of surgeons and other equipment.
Researchers are addressing other factors that contribute to patient discomfort, such as the loud noises accompanying the scanning process. Toshiba Corporation of Japan has developed a noise-reduction technology in which the gradient coils, the source of the noise, are placed within a sealed vacuum chamber.
WHAT'S ALL THAT RACKET?
Most MRI scanners make a loud, hammering noise during the imaging process. When electricity passes through the gradient coil, which is inside the main magnet, a force acts on the passing electric current. This force switches the electrical current on and off, causing vibration, which in turn causes the gradient coil to produce a high-level noise. Patients are usually given earplugs or stereo headphones to muffle the noise.
An area of interest in MRI application involves brain functional imaging. This is imaging that relates body function or thought to specific locations in the brain. When an area of the brain is active, blood flow to that area increases. MRI scanning, when done at a rapid speed, can see blood moving through the organs. Researchers believe MRI's future role in functional imaging may include helping evaluate the appropriateness and effectiveness of certain treatments for diseases, such as Alzheimer's and Parkinson's diseases.
- A mixture of a metal and a nonmetal or a mixture of two or more metals.
- functional imaging:
- An imaging technique used to determine brain function by relating thought or a body function to certain areas in the brain.
- gradient coils:
- The three magnetic coils located within the main magnet, which are designed to produce desired variations in the main magnetic field to help obtain the three-dimensional information needed for creating precise images of a body part.
- nuclear magnetic resonance:
- A phenomenon in which magnetic fields and radio waves cause the nucleus of an atom to emit small radio signals.
- nuclear spin:
- The property of an atom's nucleus that makes it behave like a magnet.
- permanent magnet:
- A large, heavy magnet that does not require electricity to run and in which the magnetic field is always present.
- radio frequency (RF) radiation:
- Also called radio wave, an electromagnetic wave with a frequency used for the transmission of radio and television signals.
- resistive magnet:
- A magnet that creates a magnetic field by the resistance to electric current in aluminum wire that is wrapped around an iron core. It requires a great amount of electricity and water cooling.
- superconducting magnet:
- A magnet whose magnetic field is produced by electric current in wires made of a superconducting material, such as niobium-titanium. Such material transmits electricity with no resistance at temperatures near absolute zero.
For More Information
Commission on Physical Sciences, Mathematics, and Applications. "Magnetic Resonance Imaging." In Mathematics and Physics of Emerging Biomedical Imaging. Washington, D.C.: National Academy Press, 1996.
Yount, Lisa. "The Better to See You." In Medical Technology. New York, NY: Facts On File, Inc., 1998.
"Raymond V. Damadian: Scanning the Horizon." Scientific American (June 1997): pp. 32–33.
Dalton, Louisa. "Two Magnets are Cheaper Than One: Stanford Engineers Construct an Inexpensive MRI Scanner." Stanford University.http://www.stanford.edu/dept/news/pr/01/mri321.html (accessed on July 22, 2002).
Gould, Todd A. "How Magnetic Resonance Imaging (MRI) Works." How Stuff Works.http://www.howstuffworks.com/mri.htm (accessed on July 22, 2002).
Hornak, Joseph P. "Imaging Hardware." The Basics of MRI.http://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm (accessed on July 22, 2002).