Magnetic Resonance Imaging

Definition

Magnetic resonance imaging (MRI) is the newest, and perhaps most versatile, medical imaging technology available. Doctors can get highly refined images of the body's interior without surgery, using MRI. By using strong magnets and pulses of radio waves to manipulate the natural magnetic properties in the body, this technique makes better images of organs and soft tissues than those of other scanning technologies. MRI is particularly useful for imaging the brain and spine, as well as the soft tissues of joints and the interior structure of bones. The entire body is visible to the technique, which poses few known health risks.

Purpose

MRI was developed in the 1980s. The latest additions to MRI technology are angiography (MRA) and spectroscopy (MRS). MRA was developed to study blood flow, while MRS can identify the chemical composition of diseased tissue and produce color images of brain function. The many advantages of MRI include:

• Detail. MRI creates precise images of the body based on the varying proportions of magnetic elements in different tissues. Very minor fluctuations in chemical composition can be determined. MRI images have greater natural contrast than standard x rays, computed tomography scan (CT scan), or ultrasound, all of which depend on the differing physical properties of tissues. This sensitivity lets MRI distinguish fine variations in tissues deep within the body. It also is particularly useful for spotting and distinguishing diseased tissues (tumors and other lesions) early in their development. Often, doctors prescribe an MRI scan to more fully investigate earlier findings of the other imaging techniques.
• Scope. The entire body can be scanned, from head to toe and from the skin to the deepest recesses of the brain. Moreover, MRI scans are not obstructed by bone, gas, or body waste, which can hinder other imaging techniques. (Although the scans can be degraded by motion such as breathing, heartbeat, and normal bowel activity.) The MRI process produces cross-sectional images of the body that are as sharp in the middle as on the edges, even of the brain through the skull. A close series of these two-dimensional images can provide a three-dimensional view of a targeted area.
• Safety. MRI does not depend on potentially harmful ionizing radiation, as do standard x-ray and CT scans. There are no known risks specific to the procedure, other than for people who might have metal objects in their bodies.

MRI is being used increasingly during operations, particularly those involving very small structures in the head and neck, as well as for preoperative assessment and planning. Intraoperative MRIs have shown themselves to be safe as well as feasible, and to improve the surgeon's ability to remove the entire tumor or other abnormality.

Given all the advantages, doctors would undoubtedly prescribe MRI as frequently as ultrasound scanning, but the MRI process is complex and costly. The process requires large, expensive, and complicated equipment; a highly trained operator; and a doctor specializing in radiology. Generally, MRI is prescribed only when serious symptoms and/or negative results from other tests indicate a need. Many times another test is appropriate for the type of diagnosis needed.

Doctors may prescribe an MRI scan of different areas of the body.

• Brain and head. MRI technology was developed because of the need for brain imaging. It is one of the few imaging tools that can see through bone (the skull) and deliver high quality pictures of the brain's delicate soft tissue structures. MRI may be needed for patients with symptoms of a brain tumor, stroke, or infection (like meningitis ). MRI also may be needed when cognitive and/or psychological symptoms suggest brain disease (like Alzheimer's or Huntington's diseases, or multiple sclerosis ), or when developmental retardation suggests a birth defect. MRI can also provide pictures of the sinuses and other areas of the head beneath the face. Recent refinements in MRI technology may make this form of diagnostic imaging even more useful in evaluating patients with brain cancer, stroke, schizophrenia, or epilepsy. In particular, a new 3-D approach to MRI imaging known as diffusion tensor imaging, or DTI, measures the flow of water within brain tissue, allowing the radiologist to tell where the normal flow of fluid is disrupted, and to distinguish more clearly between cancerous and normal brain tissue. The introduction of DTI has led to a technique known as fiber tracking, which allows the neurosurgeon to tell whether a space-occupying brain tumor has damaged or displaced the nerve pathways in the white matter of the brain. This information in turn improves the surgeon's accuracy during the actual operation.
• Spine. Spinal problems can create a host of seemingly unrelated symptoms. MRI is particularly useful for identifying and evaluating degenerated or herniated spinal discs. It can also be used to determine the condition of nerve tissue within the spinal cord.
• Joint. MRI scanning is most commonly used to diagnose and assess joint problems. MRI can provide clear images of the bone, cartilage, ligament, and tendon that comprise a joint. MRI can be used to diagnose joint injuries due to sports, advancing age, or arthritis. MRI can also be used to diagnose shoulder problems, like a torn rotator cuff. MRI can also detect the presence of an otherwise hidden tumor or infection in a joint, and can be used to diagnose the nature of developmental joint abnormalities in children.
• Skeleton. The properties of MRI that allow it to see through the skull also allow it to view the inside of bones. It can be used to detect bone cancer, inspect the marrow for leukemia and other diseases, assess bone loss (osteoporosis ), and examine complex fractures.

• The rest of the body. While CT and ultrasound satisfy most chest, abdominal, and general body imaging needs, MRI may be needed in certain circumstances to provide better pictures or when repeated scanning is required. The progress of some therapies, like liver cancer therapy, needs to be monitored, and the effect of repeated x-ray exposure is a concern.

Precautions

MRI scanning should not be used when there is the potential for an interaction between the strong MRI magnet and metal objects that might be imbedded in a patient's body. The force of magnetic attraction on certain types of metal objects (including surgical steel) could move them within the body and cause serious injury. Metal may be imbedded in a person's body for several reasons.

• Medical. People with implanted cardiac pacemakers, metal aneurysm clips, or who have had broken bones repaired with metal pins, screws, rods, or plates must tell their radiologist prior to having an MRI scan. In some cases (like a metal rod in a reconstructed leg) the difficulty may be overcome.
• Injury. Patients must tell their doctors if they have bullet fragments or other metal pieces in their body from old wounds. The suspected presence of metal, whether from an old or recent wound, should be confirmed before scanning.
• Occupational. People with significant work exposure to metal particles (working with a metal grinder, for example) should discuss this with their doctor and radiologist. The patient may need prescan testing-usually a single, regular x ray of the eyes to see if any metal is present.

Chemical agents designed to improve the picture and/or allow for the imaging of blood or other fluid flow during MRA may be injected. In rare cases, patients may be allergic to or intolerant of these agents, and these patients should not receive them. If these chemical agents are to be used, patients should discuss any concerns they have with their doctor and radiologist.

The potential side effects of magnetic and electric fields on human health remain a source of debate. In particular, the possible effects on an unborn baby are not well known. Any woman who is, or may be, pregnant should carefully discuss this issue with her doctor and radiologist before undergoing a scan.

As with all medical imaging techniques, obesity greatly interferes with the quality of MRI.

Description

In essence, MRI produces a map of hydrogen distribution in the body. Hydrogen is the simplest element known, the most abundant in biological tissue, and one that can be magnetized. It will align itself within a strong magnetic field, like the needle of a compass. The earth's magnetic field is not strong enough to keep a person's hydrogen atoms pointing in the same direction, but the superconducting magnet of an MRI machine can. This comprises the "magnetic" part of MRI.

Once a patient's hydrogen atoms have been aligned in the magnet, pulses of very specific radio wave frequencies are used to knock them back out of alignment. The hydrogen atoms alternately absorb and emit radio wave energy, vibrating back and forth between their resting (magnetized) state and their agitated (radio pulse) state. This comprises the "resonance" part of MRI.

The MRI equipment records the duration, strength, and source location of the signals emitted by the atoms as they relax and translates the data into an image on a television monitor. The state of hydrogen in diseased tissue differs from healthy tissue of the same type, making MRI particularly good at identifying tumors and other lesions. In some cases, chemical agents such as gadolinium can be injected to improve the contrast between healthy and diseased tissue.

A single MRI exposure produces a two-dimensional image of a slice through the entire target area. A series of these image slices closely spaced (usually less than half an inch) makes a virtual three-dimensional view of the area.

Magnetic resonance spectroscopy (MRS) is different from MRI because MRS uses a continuous band of radio wave frequencies to excite hydrogen atoms in a variety of chemical compounds other than water. These compounds absorb and emit radio energy at characteristic frequencies, or spectra, which can be used to identify them. Generally, a color image is created by assigning a color to each distinctive spectral emission. This comprises the "spectroscopy" part of MRS. MRS is still experimental and is available in only a few research centers.

Doctors primarily use MRS to study the brain and disorders, like epilepsy, Alzheimer's disease, brain tumors, and the effects of drugs on brain growth and metabolism. The technique is also useful in evaluating metabolic disorders of the muscles and nervous system.

Magnetic resonance angiography (MRA) is another variation on standard MRI. MRA, like other types of angiography, looks specifically at fluid flow within the blood (vascular) system, but does so without the injection of dyes or radioactive tracers. Standard MRI cannot make a good picture of flowing blood, but MRA uses specific radio pulse sequences to capture usable signals. The technique is generally used in combination with MRI to obtain images that show both vascular structure and flow within the brain and head in cases of stroke, or when a blood clot or aneurysm is suspected.

Regardless of the exact type of MRI planned, or area of the body targeted, the procedure involved is basically the same and occurs in a special MRI suite. The patient lies back on a narrow table and is made as comfortable as possible. Transmitters are positioned on the body and the cushioned table that the patient is lying on moves into a long tube that houses the magnet. The tube is as long as an average adult lying down, and the tube is narrow and open at both ends. Once the area to be examined has been properly positioned, a radio pulse is applied. Then a two-dimensional image corresponding to one slice through the area is made. The table then moves a fraction of an inch and the next image is made. Each image exposure takes several seconds and the entire exam will last anywhere from 30-90 minutes. During this time, the patient is not allowed to move. If the patient moves during the scan, the picture will not be clear.

Depending on the area to be imaged, the radio-wave transmitters will be positioned in different locations.

• For the head and neck, a helmet-like hat is worn.
• For the spine, chest, and abdomen, the patient will be lying on the transmitters.
• For the knee, shoulder, or other joint, the transmitters will be applied directly to the joint.

Additional probes will monitor vital signs (like pulse, respiration, etc.).

The process is very noisy and confining. The patient hears a thumping sound for the duration of the procedure. Since the procedure is noisy, music supplied via earphones is often provided. Some patients get anxious or panic because they are in the small, enclosed tube. This is why vital signs are monitored and the patient and medical team can communicate between each other. If the chest or abdomen are to be imaged, the patient will be asked to hold his/her breath as each exposure is made. Other instructions may be given to the patient, as needed. In many cases, the entire examination will be performed by an MRI operator who is not a doctor. However, the supervising radiologist should be available to consult as necessary during the exam, and will view and interpret the results sometime later.

Preparation

In some cases (such as for MRI brain scanning or an MRA), a chemical designed to increase image contrast may be given by the radiologist immediately before the exam. If a patient suffers from anxiety or claustrophobia, drugs may be given to help the patient relax.

The patient must remove all metal objects (watches, jewelry, eye glasses, hair clips, etc). Any magnetized objects (like credit and bank machine cards, audio tapes, etc.) should be kept far away from the MRI equipment because they can be erased. The patient cannnot bring their wallet or keys into the MRI machine. The patient may be asked to wear clothing without metal snaps, buckles, or zippers, unless a medical gown is worn during the procedure. The patient may be asked to remove any hair spray, hair gel, or cosmetics that may interfere with the scan.

Aftercare

No aftercare is necessary, unless the patient received medication or had a reaction to a contrast agent. Normally, patients can immediately return to their daily activities. If the exam reveals a serious condition that requires more testing and/or treatment, appropriate information and counseling will be needed.

Risks

MRI poses no known health risks to the patient and produces no physical side effects. Again, the potential effects of MRI on an unborn baby are not well known. Any woman who is, or may be, pregnant, should carefully discuss this issue with her doctor and radiologist before undergoing a scan.

Normal results

A normal MRI, MRA, or MRS result is one that shows the patient's physical condition to fall within normal ranges for the target area scanned.

Abnormal results

Generally, MRI is prescribed only when serious symptoms and/or negative results from other tests indicate a need. There often exists strong evidence of a condition that the scan is designed to detect and assess. Thus, the results will often be abnormal, confirming the earlier diagnosis. At that point, further testing and appropriate medical treatment is needed. For example, if the MRI indicates the presence of a brain tumor, an MRS may be prescribed to determine the type of tumor so that aggressive treatment can begin immediately without the need for a surgical biopsy.

KEY TERMS

Angiography— Any of the different methods for investigating the condition of blood vessels, usually via a combination of radiological imaging and injections of chemical tracing and contrasting agents.

Diffusion tensor imaging (DTI)— A refinement of magnetic resonance imaging that allows the doctor to measure the flow of water and track the pathways of white matter in the brain. DTI is able to detect abnormalities in the brain that do not show up on standard MRI scans.

Gadolinium— A very rare metallic element useful for its sensitivity to electromagnetic resonance, among other things. Traces of it can be injected into the body to enhance the MRI pictures.

Hydrogen— The simplest, most common element known in the universe. It is composed of a single electron (negatively charged particle) circling a nucleus consisting of a single proton (positively charged particle). It is the nuclear proton of hydrogen that makes MRI possible by reacting resonantly to radio waves while aligned in a magnetic field.

Ionizing radiation— Electromagnetic radiation that can damage living tissue by disrupting and destroying individual cells. All types of nuclear decay radiation (including x rays) are potentially ionizing. Radio waves do not damage organic tissues they pass through.

Magnetic field— The three-dimensional area surrounding a magnet, in which its force is active. During MRI, the patient's body is permeated by the force field of a superconducting magnet.

Radio waves— Electromagnetic energy of the frequency range corresponding to that used in radio communications, usually 10,000 cycles per second to 300 billion cycles per second. Radio waves are the same as visible light, x rays, and all other types of electromagnetic radiation, but are of a higher frequency.

Resources

PERIODICALS

Clark, C. A., T. R. Barrick, M. M. Murphy, and B. A. Bell. "White Matter Fiber Tracking in Patients with Space-Occupying Lesions of the Brain: A New Technique for Neurosurgical Planning?" Neuroimage 20 (November 2003): 1601-1608.

Hendler, T., P. Pianka, M. Sigal, et al. "Delineating Gray and White Matter Involvement in Brain Lesions: Three-dimensional Alignment of Functional Magnetic Resonance and Diffusion-Tensor Imaging." Journal of Neurosurgery 99 (December 2003): 1018-1027.

Kubicki, M., C. F. Westin, P. G. Nestor, et al. "Cingulate Fasciculus Integrity Disruption in Schizophrenia: A Magnetic Resonance Diffusion Tensor Imaging Study." Biological Psychiatry 54 (December 1, 2003): 1171-1180.

Mahmoud-Ghoneim, D., G. Toussaint, J. M. Constans, and J. D. de Certaines. "Three-Dimensional Texture Analysis in MRI: A Preliminary Evaluation in Gliomas." Magnetic Resonance Imaging 21 (November 2003): 983-987.

Rees, J. "Advances in Magnetic Resonance Imaging of Brain Tumours." Current Opinion in Neurology 16 (December 2003): 643-650.

Satoh, T., K. Onoda, and S. Tsuchimoto. "Intraoperative Evaluation of Aneurysmal Architecture: Comparative Study with Transluminal Images of 3D MR and CT Angiograms." American Journal of Neuroradiology 24 (November-December 2003): 1975-1981.

ORGANIZATIONS

American College of Radiology. 1891 Preston White Drive, Reston, VA 22091. (800) 227-5463. 〈http://www.acr.org〉.

American Society of Radiologic Technologists. 15000 Central Ave. SE, Albuquerque, NM 87123-3917. (505) 298-4500. 〈http://www.asrt.org〉.

Center for Devices and Radiological Health. United States Food and Drug Administration. 1901 Chapman Ave., Rockville, MD 20857. (301) 443-4109. 〈http://www.fda.gov/cdrh〉.

Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is a medical device that uses a magnetic field and the natural resonance of atoms in the body to obtain images of human tissues. The basic device was first developed in 1945, and the technology has steadily improved since. With the introduction of high-powered computers, MRI has become an important diagnostic device. It is noninvasive and is capable of taking pictures of both soft and hard tissues, unlike other medical imaging tools. MRI is primarily used to examine the internal organs for abnormalities such as tumors or chemical imbalances.

History

The development of magnetic resonance imaging (MRI) began with discoveries in nuclear magnetic resonance (NMR) in the early 1900s. At this time, scientists had just started to figure out the structure of the atom and the nature of visible light and ultraviolet radiation emitted by certain substances. The magnetic properties of an atom's nucleus, which is the basis for NMR, were demonstrated by Wolfgang Pauli in 1924.

The first basic NMR device was developed by I. I. Rabi in 1938. This device was able to provide data related to the magnetic properties of certain substances. However, it suffered from two major limitations. Firstly, the device could analyze only gaseous materials, and secondly, it could only provide indirect measurements of these materials. These limitations were overcome in 1945, when two groups of scientists led by Felix Bloch and Edward Purcell independently developed improved NMR devices. These new devices proved useful to many researchers, allowing them to collect data on many different types of systems. After further technological improvements, scientists were able to use this technology to investigate biological tissues in the mid 1960s.

The use of NMR in medicine soon followed. The earliest experiments showed that NMR could distinguish between normal and cancerous tissue. Later experiments showed that many different body tissues could be distinguished by NMR scans. In 1973, an imaging method using NMR data and computer calculations of tomography was developed. It provided the first magnetic resonance image (MRI). This method was consequently used to examine a mouse and, while the testing time required was more than an hour, an image of the internal organs of the mouse resulted. Human imaging followed a few years later. Various technological improvements have been made since to reduce the scanning time required and improve the resolution of the images. Most notable improvements have been made in the three-dimensional application of MRI.

Background

The basic stages of an MRI reading are simple. First the patient is placed in a strong constant magnetic field and is surrounded by several coils. Radiofrequency (RF) radiation is then applied to the system, causing certain atoms within the patient to resonate. When the RF radiation is turned off, the atoms continue to resonate. Eventually, the resonating atoms return to their natural state and, in doing so, emit a radiofrequency radiation that is an NMR signal. The signal is then processed through a computer and converted into a visual image of patient.

The NMR signals that are emitted from the body's cells are primarily produced by the cells' protons. Early MR images were constructed based solely on the concentration of protons within a given tissue. These images, however, did not provide good resolution. MRI became much more useful for constructing an internal image of the body when a phenomena known as relaxation time, the time it takes for the protons to emit their signal, was taken into consideration. In all body tissues, there are two types of relaxation times, T1 and T2, that can be detected. Different types of tissues will exhibit different T1 and T2 values. For example, the gray matter in the brain has a different T1 and T2 value than blood. Using these three variables (proton density, T1, and T2 value), a highly resolved image can be constructed.

MRI is most used for creating images of the human brain. It is particularly useful for this area because it can distinguish between soft tissue and lesions. In addition to structural information, MRI allows brain functional imaging. Functional imaging is possible because when an area of the brain is active, blood flow to that region increases. When the scans are taken with sufficient speed, in fact, blood can be seen moving through organs. Another application for MRI is muscular skeletal imaging. Injuries to ligaments and cartilage in the joints of the knees, wrists, and shoulder can be readily seen with MRI. This eliminates the need for traditional invasive surgeries. A developing use for MRI is tracking chemicals through the body. In these scans NMR signals from molecules such as carbon 13 and phosphorus 31 are received and interpreted.

Raw Materials

The primary functioning parts of an MRI system include an external magnet, gradient coils, RF equipment, and a computer. Other components include an RF shield, a power supply, NMR probe, display unit, and a refrigeration unit.

The magnet used to create the constant external magnetic field is the largest piece of any MRI system. To be useful, the magnet must be able to produce a stable magnetic field that penetrates throughout a certain volume, or slice, of the body. There are three different kinds of magnets available. A resistive magnet is made up of thin aluminum bands wrapped in a loop. When electricity is conducted around the loop a magnetic field is created perpendicular to the loop. In an MRI system, four resistive magnets are placed perpendicular to each other to produce a consistent magnetic field. As electricity is conducted around the loop, the resistance of the loop generates heat, which must be dissipated by a cooling system.

Superconducting magnets do not have the same problems and limitations of the resistive type of magnet. Superconducting magnets are ring magnets, made out of a niobium-titanium alloy in a copper matrix, which are supercooled with liquid helium and liquid nitrogen. At these low temperatures, there is almost no resistance, so very low levels of electricity are needed. This magnet is less expensive to run than the resistive type, and larger field strengths can be generated. The other type of magnet used is a permanent magnet. It is constructed out of a ferromagnetic material, is quite large, and does not require electricity to run. It also provides more flexibility in the design of the MRI system. However, the stability of the magnetic field the permanent magnet generates is questionable, and its size and weight may be prohibitive. While each of these different kind of magnets can produce magnetic fields with varying strength, an optimum field strength has not been discovered.

To provide a method for decoding the NMR signal that is received from a sample, magnetic field gradients are used. Typically, three sets of gradient coils are used to provide data in each of the three dimensions. Like the primary magnets, these coils are made of a conducting loop that creates a magnetic field. In the MRI system, they are wrapped around the cylinder that surrounds the patient.

The RF system has various roles in an MRI machine. First, it is responsible for transmitting the RF radiation that induces the atoms to emit a signal. Next, it receives the emitted signal and amplifies it so it can be manipulated by the computer. RF coils are the primary pieces of hardware in the RF system. They are constructed to create an oscillating magnetic field. This field induces atoms in a defined area to absorb RF radiation and then emit a signal. In addition to sending the RF signal, the coils can also receive the signal from the patient. Depending on the type of MRI system, either a saddle RF coil or a solenoid RF coil is used. The coil is usually positioned alongside the subject and is designed to fit the patient. To reduce RF interferences, an aluminum sheet is used.

The final link in the MRI system is a computer, which controls the signals sent and processes and stores the signals received. Before the received signal can be analyzed by the computer, it is translated through an analog-digital convertor. When the computer receives signals, it performs various reconstruction algorithms, creating a matrix of numbers that are suitable for storage and building a visual display using a Fourier transformer.

The Manufacturing
Process

The individual components of an MRI system are typically manufactured separately and then assembled into a large unit. These units are extremely heavy, sometimes weighing over 100 tons (102 metric tons).

Magnet

• 1 The most frequently used magnets in an MRI system are superconducting electromagnets. These can be made using various materials, but the basic design involves a coil of conductive wire, a cooling system, and a power supply. The coils are made by wrapping wire, constructed from filaments of a niobium titanium alloy embedded in copper, in a large loop. To create the necessary magnetic field, a number of coils are used. In one type of system eight coils are used, six to create the primary magnetic field and two to compensate for the excess field.
• 2 The coils are immersed in a vessel containing liquid helium. This reduces the temperature to a level that makes them superconductive. To help keep the temperature stable, the vessel is surrounded by two more vessels containing other coolants like liquid nitrogen. This construction is then suspended with thin rods in a vacuum-sealed container. A power supply is hooked up to the magnetic coils and is used only when the magnet needs to be energized. The magnet is attached to the patient support, which is a sliding table that brings the patient into the magnetic field.

Gradient coils

• 3 The gradient coils are resistant type electromagnets. In an MRI system, there are typically three sets of gradient coils. Each coil is made by winding thin strips of copper or aluminum in a specific pattern. The coils are given strength by introducing an epoxy into their structure. The size of these coils determines the width of the opening into which the patient is placed. Since a smaller coil requires less energy, this width must be large enough to prevent claustrophobia in the patient but small enough to require a reasonable amount of electricity. These gradient coils are typically shielded to prevent interfering eddy currents.

RF system

• 4 The electronic components of the RF system may be provided by outside suppliers and assembled by the MRI manufacturer. These components are attached to the RF coils, which are made with varying designs. The transmitter and receiver coils are composed of the same type of materials as the gradient coils. They are also constructed much like the main magnet. However, they are made up of a loop of conducting material, such as copper, that can create an oscillating magnetic field. One type of RF coil is a surface coil, which is shaped in a circle and is applied directly on the patient. Another type is the saddle coil. These can either be fitted right into the magnet bore or shaped into a birdcage coil and placed just inside the gradient coils. Each type of coil is attached to a power source.

Computer

• 5 The computer is supplied by computer manufacturers and modified and programmed for use in an MRI system. Attached to it is the user interface, the Fourier transformer, the signal converter, and a preamplifier. A display device and a laser printer are also included.

Final assembly

• 6 Each of the components of the MRI are assembled together and placed into an appropriate frame. Assembly can take place at the plant or on-site, where the system will be used. In either case, the nature of the magnet typically requires special handling precautions, such as transporting it in an air-suspended vehicle.

Quality Control

The quality of each MRI system being manufactured is ensured by making visual and electrical inspections throughout the entire production process. The performance of the MRI is tested to be sure it is functioning properly. These tests are done under different environmental conditions, such as excessive heat and humidity. Most manufacturers set their own quality specifications for the MRI systems that they produce. Standards and performance recommendations have also been proposed by various medical organizations and governmental agencies.

The Future

The focus of current MRI research is in areas that include improving the scan resolution, reducing scan time, and improving MRI design. The methods for improving resolution and decreasing scan time involve reducing the signal to noise ratio. In an MRI system, noise is caused by randomly generated signals that interfere with the signal of interest. One method for reducing it is by using a high magnetic field strength. Improved designs for MRI systems will also help reduce this interference and decrease the noise associated with electromagnets. In the future, real time MRI scans should be available.

Where to Learn More

Books

Boer, Jacques and Marinus Vlaardingerbroek. Magnetic Resonance Imaging Theory and Practice. Springer, 1996.

Brown, J. and J. Heiken. Manual of Clinical Magnetic Resonance Imaging. Raven Press, 1991.

Rinck, P. Magnetic Resonance in Medicine. Blackwell Scientific Publications, 1993.

—PerryRomanowski

Magnetic resonance imaging (MRI)

Definition

Magnetic resonance imaging (MRI) scanners rely on the principles of atomic nuclear-spin resonance. Using strong magnetic fields and radio waves, MRI collects and correlates deflections caused by atoms into images. MRIs (magnetic resonance imaging tests) offer relatively sharp pictures and allow physicians to see internal bodily structures with great detail. Using MRI technology, physicians are increasingly able to make diagnosis of serious pathology (e.g., tumors) earlier, and earlier diagnosis often translates to a more favorable outcome for the patient.

Description

A varying (gradient) magnetic field exists in tissues in the body that can be used to produce an image of the tissue. The development of MRI was one of several powerful diagnostic imaging techniques that revolutionized medicine by allowing physicians to explore bodily structures and functions with a minimum of invasion to the patient.

In the last half of the twentieth century, dramatic advances in computer technologies, especially the development of mathematical algorithms powerful enough to allow difficult equations to be solved quickly, allowed

MRI to develop into an important diagnostic clinical tool. In particular, the ability of computer programs to eliminate "noise" (unwanted data) from sensitive measurements enhanced the development of accurate, accessible and relatively inexpensive noninvasive technologies.

Nuclear medicine is based upon the physics of excited atomic nuclei. Nuclear magnetic resonance (NMR) was one such early form of nuclear spectroscopy that eventually found widespread use in clinical laboratory and medical imaging. Because a proton in a magnetic field has two quantized spin states, NMR allowed the determination of the complex structure of organic molecules and, ultimately, the generation of pictures representing the larger structures of molecules and compounds (such as neural tissue, muscles, organs, bones, etc.). These pictures were obtained as a result of measuring differences between the expected and actual numbers of photons absorbed by a target tissue.

Groups of nuclei brought into resonance, that is, nuclei-absorbing and -emitting photons of similar electro-magnetic radiation (e.g., radio waves), make subtle yet distinguishable changes when the resonance is forced to change by altering the energy of impacting photons. The speed and extent of the resonance changes permit a nondestructive (because of the use of low energy photons) determination of anatomical structures. This form of NMR became the physical and chemical basis of the powerful diagnostic technique of MRI.

The resolution of MRI scanning is so high that they can be used to observe the individual plaques in multiple sclerosis . Ina clinical setting, a patient is exposed to short bursts of powerful magnetic fields and radio waves from electromagnets. MRI images do not utilize potentially harmful ionizing radiation generated by three-dimensional x-ray computed tomography (CT) scans, and there are no known harmful side effects. The magnetic and radio wave bursts stimulate signals from hydrogen atoms in the patient's tissues that, when subjected to computer analysis, create a cross-sectional image of internal structures and organs.

Healthy and diseased tissues produce different signal patterns and thus allow physicians to identify diseases and disorders.

American chemist and physicist Paul Lauterbur and British physicist Sir Peter Mansfield shared the 2003 Nobel Prize in Physiology or Medicine for their discoveries concerning the use of magnetic resonance to visualize different structures.

MRI tests, brain scans, and potential security issues

Studies of the potential of new brain wave scanners explore the possibility that MRI tests could be part of a more accurate form of polygraph (lie detector). Current polygraphs are of debatable accuracy (usually they are not admissible in court as evidence) and measure observable fluctuations in heart rate, breathing, perspiration, etc.

In a 2001 University of Pennsylvania experiment using MRI, 18 subjects were given objects to hide in their pockets, then shown a series of pictures and asked to deny that the object depicted was in their pockets. Included was a picture of the object they had pocketed and so subjects were "lying" (making a deliberate false statement) if they claimed that the object was not in their pocket. An MRI recorded an increase of activity in the anterior cinglate, a portion of the brain associated with inhibition of responses and monitoring of errors, as well as the right superior frontal gyrus, which is involved in the process of paying attention to particular stimuli.

After the September 11, 2001, terrorist attacks, a number of government agencies in the United States began to take a new look at brain scanning technology as a potential means of security screening. Such activity, along with an increase of interest in potential brain-wave scanning by the Federal Bureau of Investigation (FBI), has raised concerns among civil-liberties groups, which view brain-wave scanning as a particularly objectionable invasion of privacy.

Resources

PERIODICALS

Young, Emma. "Brain Scans Can Reveal Liars." New Scientist (November 12, 2001).

WEBSITES

Hornak, J. P. The Basics of MRI. May 9, 2004 (June 2, 2004). <http://www.cis.rit.edu/htbooks/mri/>.

Johnson, K. A., and J. A. Becker. The Whole Brain Atlas. May 9, 2004 (June 2, 2004). <http://www.med.harvard.edu/AANLIB/home.html>.

Paul Arthur

Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) is a tool of medical diagnosis used to examine tissues and organs inside a patient's body. MRI assembles a detailed, readable image based on the response of atoms placed within a strong magnetic field.

MRI Detects Cancer

The MRI can detect small tumors (growths) that might be cancerous. Because cancer cells have a resonance frequency different from healthy cells, they are readily identifiable. MRI can outline areas of soft tissue too thin to be picked up by X-rays. It can also see into large organs. MRI is used primarily to diagnose disorders and injuries of the head and the spine as well as congenital heart disease, cardiovascular disorders, and pelvic problems, and MRI can also be useful for examining the chest, joints, and the circulatory system.

How MRI Works

The MRI scanner is composed of a large, tube-like magnet, radio transmitters, and receivers, and a computer. The patient lies inside the tube, completely surrounded by an intense magnetic field. Inside the patient's body, the nuclei of certain atoms will spin, wobbling at precise frequencies. Using the radio signals, the computer searches for the frequencies associated with specific types of atoms (such as cancer cells). Once the radio waves are turned off, the atoms emit pulses of absorbed energy. The computer reads these pulses and uses them to draw a three-dimensional image of the scanned area.

MRI scanners, which are particularly expensive, are generally found only in large medical research centers. A specially trained radiologist must be present to supervise the procedure.

NMR

MRI technology is based on an earlier research technique known as nuclear magnetic resonance (NMR), which helps scientists to analyze the interactions between matter and electromagnetic radiation. In 1973, a paper published in Nature first explained how to use NMR on human bodies. The technique later came to be known as MRI, and since 1981, when it came into extensive use, MRI has proven its tremendous diagnostic value.

The MRI technique is also radically different from X-ray technology, because MRI uses no ionizing radiation.

MAGNETIC RESONANCE IMAGING

MAGNETIC RESONANCE IMAGING (MRI), first used for medical purposes in 1976, is based on the phenomenon of nuclear magnetic resonance reported in 1946 by Felix Bloch of Stanford University and Edward Purcell of Harvard University, physicists awarded the Nobel Prize in 1952. Using complicated equipment to measure resonating frequencies emitted by tissue components, images of those tissues are constructed in much the same way as in CAT (computerized axial tomography) scanning. Information can be obtained about the soft tissues of the body—such as the brain, spinal cord, heart, kidneys, and liver—but not bone. MRI does not involve ionizing radiations and is noninvasive.

Scientists developed the functional MRI (fMRI) in 1993. This technique allows for more accurate mapping of the human brain, especially those regions that control thought and motor control. Other developments include machines that have open sides, so that patients need not be enclosed in a chamber, as they are for an MRI; faster machines that can take full-body scans in minutes; machines that can provide real-time images during surgical procedures; machines with stronger, superconducting magnets; and finally, diffusion-weighted MRI, a scanning sequence that allows doctors to identify strokes by detecting the minute swelling of brain tissue that accompanies such attacks.

BIBLIOGRAPHY

Lee, Joseph K. T., et al., eds. Computed Body Tomography with MRI Correlation. 3d ed. Philadelphia: Lippincott-Williams, 1998.

Peter H.Wright/a. r.

magnetic resonance imaging (MRI), noninvasive diagnostic technique that uses nuclear magnetic resonance to produce cross-sectional images of organs and other internal body structures. The patient lies inside a large, hollow cylinder containing a strong electromagnet, which causes the nuclei of certain atoms in the body (especially those of hydrogen) to align magnetically. The patient is then subjected to radio waves, which cause the aligned nuclei to "flip" ; when the radio waves are withdrawn the nuclei return to their original positions, emitting radio waves that are then detected by a receiver and translated into a two-dimensional picture by computer. Unhampered by bone and capable of producing images in a variety of planes, MRI is used in the diagnosis of brain tumors and disorders, spinal disorders, multiple sclerosis, and cardiovascular disease. The procedure is considered to be without risk, but the scanner may interfere with pacemakers, hearing aids, or other mechanical devices. Although the images are similar in many ways to those of CAT scans , they are obtained without X rays or other radiation, and generally provide more contrast between normal and abnormal tissue.

magnetic resonance imaging (MRI) Diagnostic scanning system, based on the use of powerful magnets, which produces images of soft tissues in the body. Magnetic resonance imaging (MRI) is invaluable for producing images of the brain and spinal cord in particular. The scanner's magnet causes the nuclei within the atoms of the patient's body to line themselves up in one direction. A brief radio pulse is then beamed at the nuclei, causing them to spin. As they realign themselves to the magnet, they give off weak radio signals that can be recorded and converted electronically into images.

magnetic resonance imaging (MRI) (mag-net-ik) n. a diagnostic imaging technique based on the emission of electromagnetic waves from the body when the patient is placed in a strong magnetic field and exposed to radiofrequency radiation (see nuclear magnetic resonance). Most images rely on the signal from hydrogen in water, which is particularly strong, although other elements can be used. A major advantage over computerized tomography is the lack of X-rays, which reduces exposure to ionizing radiation.

mag·net·ic res·o·nance im·ag·ing (abbr.: MRI) • n. a form of medical imaging that measures the response of the atomic nuclei of body tissues to high-frequency radio waves when placed in a strong magnetic field, and that produces images of the internal organs.

magnetic resonance imaging (MRI) See nuclear magnetic resonance.