Magnetic Resonance Imaging Unit
An MRI unit has several diagnostic clinical applications, including:
- diagnosing diseases of the central nervous system, brain, and spine
- detecting musculoskeletal disorders and injuries
- identifying infectious diseases such as those associated with acquired immunodeficiency syndrome (AIDS)
- detecting metastatic liver disease
- imaging the cardiovascular system
- staging prostate, bladder, and uterine cancers
- studying bone marrow diseases
- imaging the breast adjunctive to conventional mammography
Some MRI units can perform magnetic resonance angiography (MRA), which is used to image vascular and arteriovenous malformations, thromboses, stenoses, and other vascular abnormalities. In particular, MRA is used for evaluating the carotid artery and cerebral vasculature in patients with suspected or known stroke. An MRI unit can also be used in conjunction with other imaging modalities such as computed tomography (CT) for localizing the treatment target for radiation treatment planning and prior to surgical treatment of tumors, including stereotactic radiosurgery and image-guided surgery. It is also possible to evaluate brain function associated with certain tasks such as language or vision using functional MRI.
MRI provides images with excellent contrast that allow clinicians to clearly see details of soft tissue, bone, joints, and ligaments. Because MRI does not use ionizing radiation to produce images, like radiography and CT, it is often the examination of choice for imaging the male and female reproductive systems, pelvis and hips, and urinary tract and bladder.
MRI units are used in the radiology department and outpatient imaging centers for diagnostic imaging, in the emergency care and critical care settings to diagnose acute conditions such as stroke in the clinical research setting (especially for brain research), and in orthopedic practices. Large hospitals usually have one or more MRI units that are typically located in the radiology department or in a separate annex near the radiology and emergency departments.
An MRI unit consists of a magnet system, a radiofrequency (RF) transmitter/receiver system, a gradient system, a patient table, a computer workstation, and operator console. The magnetic strength of the magnet is measured in teslas (T), a unit of magnetic field strength, and ranges from 0.064–4 T, depending on the type of system. The magnetic field generated during an MRI examination is approximately 8,000 times stronger than the Earth's magnetic field. Principles of image production are based on the magnetic spin properties of hydrogen atoms in the body's tissues and fluids and how they behave in a magnetic field. Basically, hydrogen protons (particles located in the atom's nucleus) will align with an applied magnetic field and will spin perpendicular to the magnetic field when a radiofrequency pulse is added. When the pulse is terminated, protons relax back into alignment with the magnetic field, and this generates a radiofrequency signal that is received by the antenna coil. Different tissues such as those high in water and in fat will produce different signals that are then processed by the computer and converted into anatomical images. MRI protocols and imaging sequences are based on the different signals produced by different types and physiologic states of tissue.
The magnet system is contained in the gantry, which is a large square or round unit with a hole in the center (the bore) through which the patient table is moved. Magnets may be of three types: permanent magnet, resistive or superconducting electromagnet, and iron-core electromagnet. Permanent magnets are extremely heavy and thus require special construction; however, they do not require electrical power or cooling because they are constructed of magnetic alloys. They also have almost no fringe field (the magnetic field outside the magnet itself). Permanent magnets are limited to field strengths of 0.3 T or less. Resistive electromagnets use electrical coils to generate a magnetic field and thus require cooling water. Resistive magnets are limited to field strengths of 0.5 T.
An MRI unit with a field strength less than 0.2 T is considered low field, an MRI unit with a field strength of0.2 T to 1 T is considered mid field, and an MRI unit with a field strength greater than 1 T is considered high field. In general, high-field MRI units are capable of shorter imaging times and higher image quality and are preferred for many clinical applications.
The radiofrequency system transmits and receives signals using a coil that acts as an antenna. Separate coils are used for head and body imaging, and specially designed coils are used for imaging the spine, face, knee, breast, shoulder, and extremities. The gradient system produces magnetic fields in the direction of the primary field and perpendicular to the primary field in order to select the area for imaging and to register the location of signals received from the area imaged. The radiofrequency and gradient systems are turned on and off (pulsed) to control image contrast; these pulse patterns are called a pulse sequence. There are several different types of pulse sequences used, and they vary according to the duration, frequency, and timing of the pulses. Different pulse sequences are used to image different anatomic areas, and the pulse sequence is chosen based on the characteristics of the tissue being imaged such as fat content, water content, and anatomic area.
There are several different types of MRI units and MRI imaging methods:
- Conventional MRI units have long, closed bores that surround most of the patient's body during imaging.
- Short-bore MRI units were developed in response to patient claustrophobia and to retain the image-quality benefits of conventional systems, and have bore lengths that allow patients of average height to have much of their body outside the bore during imaging. The patient's head can then be outside the bore for exams not involving the brain and neck, thereby reducing claustrophobic reactions.
- Open MRI units were also developed in response to patient claustrophobia and to facilitate interventional procedures. They have bores that are open on most sides (sometimes columns are used to support the gantry). Open MRI units usually have low-field strengths.
- Dedicated extremity/head/breast MRI units have very small bores designed to accommodate imaging of limbs, joints, or the head, and are primarily used for orthopedic applications. A dedicated breast MRI system is also available.
- Mobile MRI units are installed in a specially designed trailer and driven to hospitals that do not have an MRI unit. Mobile MRI services are used frequently in rural areas.
- Functional MRI is an imaging technique that rapidly acquires images that display changes in cerebral blood flow in response to visual or auditory stimuli or motor tasks. This technique is used primarily for research to map the functional organization of the brain.
- Interventional and intraoperative MRI is a developing field that involves performing interventional procedures such as catheterization or guidewire insertion, and intraoperative guidance such as during neuro-surgery, using a specially designed MRI unit. Open MRI units are being used for these applications due to their open-bore design, which facilitates patient access.
- MRI spectroscopy is an imaging technique used primarily in research that measures metabolites in the brain to evaluate brain tissue.
- Echoplanar MRI is an imaging technique that uses rapidly oscillating magnetic field gradients for image acquisition in less than 30 milliseconds. It is used to evaluate real-time cardiac and brain function, as well as muscle activity.
- MRI angiography is an imaging technique used to evaluate the blood vessels, for example, to detect aneurysms or atherosclerosis. Injection of a contrast agent is required.
- Diffusion tensor MRI is a relatively new imaging technique that tracks water molecules in the brain to detect abnormalities associated with stroke, multiple sclerosis, and other conditions.
An MRI unit is operated by the MRI technologist who prepares the patient for the examination, including administering any necessary intravenous contrast agents and positioning the patient on the table. Some examinations require the use of special surface coils (e.g., for head, knee, etc.) to focus the radiofrequency pulses on the area of interest. The MRI technologist places or attaches the appropriate coil and helps the patient onto the table. After the patient is properly positioned, the MRI technologist goes to the control room, which is adjacent to but separated from the MRI unit by a window, and initiates the imaging sequences selected by the radiologist. Usually, two to six imaging sequences are performed,
Image artifacts may occur during image acquisition, and the technologist and radiologist should monitor acquired images for artifacts. Patient motion, respiratory motion, implants, signal loss, and improper unit settings can all cause artifacts to occur. Constantly occurring artifacts related to the unit's operation or magnetic field may require a service call from the manufacturer or testing by a medical physicist.
With regard to patient safety, there are no side effects associated with the magnetic field during an MRI examination, but, in general, MRI is not recommended for pregnant women. Patients with a pacemaker, cochlear implants, aneurysm clips, and other metallic implants must check with a physician before undergoing MRI due to the possible effects of the magnetic field on the implants. Patients who have been exposed to shrapnel or metal shavings (especially in the eye) may not be able to have an MRI; instances where the magnetic field caused movement of metal fragments in the body and subsequent patient injury have been reported. Because eye-shadow may contain metallic substances, patients undergoing MRI should not wear make-up during the examination.
Several incidents have occurred where patients undergoing MRI examinations received serious skin burns from contact with surface coils or monitoring cables. Therefore, the United States Food and Drug Administration (FDA) has issued precautions to prevent burns during MRI, including removal of unnecessary coils, cables, and leads before the scan is begun; frequent checking of coils, cables, and leads for frayed insulation or exposed wires; and a thorough check that cables and leads do not form loops, touch the sides of the magnet bore, or directly touch the patient.
The magnetic field requires that all medical equipment used in the MRI suite be MRI-compatible. For example, patient monitoring equipment, intravenous poles, ventilators, and contrast media injectors should have been tested and certified by the manufacturer as MRI-compatible. If interventional procedures are performed in the MRI suite, anesthesia units, surgical instrumentation, patient monitoring systems, and resuscitation equipment should all be MRI-compatible. The operation and performance of equipment that is not MRI-compatible may be affected by the magnetic field, or if the equipment contains certain metals, it may be attracted to the magnet, causing equipment damage and presenting a safety problem. Because patients may be brought into the MRI suite on wheelchairs or with oxygen canisters, MRI staff should be sure that the magnetic field is not on during patient transfer. There have been several hazard reports of injury to patients and staff by oxygen canisters, wheelchairs, and other metal items when they were rapidly drawn to the magnet.
During the MRI examination, all patients, but particularly those under sedation or anesthesia or in critical condition, should be monitored using physiologic monitoring equipment, intercom systems, and video. Some patients may be claustrophobic during the examination or may experience anxiety. To alleviate these discomforts, an MRI-compatible music system and increased ventilation in the magnet bore can be installed.
Depending on the type of magnet, different types of shielding are required for the MRI suite. The performance of the MRI unit depends on the homogeneity, or uniformity, of the magnetic field, which may be disturbed by surrounding hospital equipment, metallic structures, and environmental factors. A process called shimming is used to improve the uniformity of the magnetic field, and is accomplished by using shim coils or ferromagnetic materials around the magnet. Shimming is usually done during installation or testing by physicists. The entire MRI suite may need to be shielded with different materials to insulate the magnet from outside interference or to prevent the magnet's fringe field from interfering with the operation of medical equipment in adjacent areas.
Because of the complexity of an MRI unit, a service contract covering parts replacement, preventive maintenance, and emergency repairs is usually purchased from the manufacturer or a third-party service organization. The biomedical engineering staff and/or the MRI technologist conduct periodic performance testing of image quality and other parameters. Surface coils should be cleaned and maintained according to manufacturer instructions. Many MRI units have special cooling system requirements, and storage and replenishment of cryogens (chemicals used for cooling the magnet system) is necessary. This is generally performed by the service provider or biomedical engineering staff.
Health care team roles
The MRI examination is conducted by an MRI technologist and a radiologist. The MRI technologist is responsible for preparing the patient for the examination
MRI technologists have completed special education programs in MRI physics, operation, and safety. All manufacturers of MRI units provide on-site, and sometimes off-site, training on the technical features and clinical applications of their systems. The American College of Radiology has developed an MRI site accreditation program, which requires that the MRI system, quality control procedures, MRI technologists, and radiologists be evaluated according to certain standards of performance. As of 2001, this accreditation was not mandatory, but many facilities undergo the process to demonstrate quality performance.
Aneurysm—Localized enlargement of an artery or vein.
Artifact—An artificial feature in an acquired MRI image.
Atherosclerosis—Hardening of the arteries, a form of heart disease.
Metabolites—Byproducts of metabolism that accumulate in brain tissue that is measured by MRI spectroscopy.
Teslas (T)—A unit of magnetic field strength.
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American College of Radiology. 1891 Preston White Drive, Reston, VA 20191-4397. (800)227-5463. <http://www.acr.org>.
American Society of Radiologic Technologists (ASRT). 15000 Central Avenue SE, Albuquerque, NM 87123-2778. (800) 444-2778. <http://www.asrt.org>.
Clinical Magnetic Resonance Society. 2825 Burnet Avenue, Suite 2, Cincinnati, OH 45219. (800) 823-2677. <http://www.cmrs.com>.
International Society for Magnetic Resonance in Medicine. 2118 Milvia Street, Suite 201, Berkeley, CA 94704. (510) 841-1899. <http://www.ismrm.org>.
Radiological Society of North America. 820 Jorie Boulevard, Oak Brook, IL 60523-2251. (630) 571-2670. <http://www.rsna.org>.
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Jennifer E. Sisk, M.A.