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 radio-frequency (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 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 radio-frequency pulse is added. When the pulse is terminated, protons relax back into alignment with the magnetic ﬁeld, and this generates a radio-frequency 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. Superconducting magnets use titanium alloy coils that require cooling with liquid helium or liquid nitrogen (cryogens). They can have ﬁeld strengths of up to 2 T. Iron-core electromagnets use a combination of permanent and electromagnet technology, and require cooling water for operation. Field strengths are usually 0.3 or 0.4 T.
An MRI unit with a ﬁeld strength less than 0.2 T is considered low ﬁeld, an MRI unit with a ﬁeld strength of 0.2 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 radio-frequency 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 radio-frequency 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 radio-frequency 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, each lasting approximately two to 15 minutes. The technologist instructs the patient via an intercom system when the scanning sequence is to begin and whether holding of breath or stillness is required. While the images are being acquired, the MRI technologist and radiologist review them on the computer workstation to make sure the image quality is sufficient for diagnosis.
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 by making sure that all metallic objects have been removed and that the patient does not have any metallic implants that will be affected by the examination. It is recommended that a prescreening MRI questionnaire be developed and that all patients be required to complete the form prior to having an MRI. If necessary for the area being imaged, an intravenous contrast agent will be administered by either the technologist or a nurse. Nursing staff may also be present during the examination, depending on the medical condition of the patient. The radiologist oversees the selection of MRI imaging sequences and protocols and reviews the acquired images to be sure image quality is appropriate for diagnosis. The radiologist also provides the final interpretation of images and provides a report for any referring physicians. For cancer cases, oncologists may also be involved in image review for treatment planning purposes. If interventional MRI procedures are performed, specialists such as a gastroenterologist, orthopedic surgeon, neurologist, or neurosurgeon may perform the procedures while the MRI unit is operated by the technologist.
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.