Magnetic Resonance Imaging Un... Health Article

Definition

A magnetic resonance imaging (MRI) unit uses a magnetic field, radiofrequency waves, and computerized image processing to produce cross-sectional images of the anatomy.

Purpose

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.

Description

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. 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 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.