Magnetic resonance imaging is a relatively new technology. The first MR image was published in 1973 and the first cross-sectional image of a living mouse was published in January 1974. The first studies performed on humans were published in 1977. By comparison, the first human X-ray image was taken in 1895.
Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, because the word nuclear was associated in the public mind with ionizing radiation exposure it is generally now referred to simply as MRI. Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. The term magnetic resonance tomography (MRT) is also sometimes used.
The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments of some of these protons align with the direction of the field.
A radio frequency transmitter is then briefly turned on, producing an electromagnetic field. In simple terms, the photons of this field have just the right energy, known as the resonance frequency, to flip the spin of the aligned protons. As the intensity and duration of the field increases, more aligned spins are affected. After the field is turned off, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. It is these photons that produce the signal which can be detected by the scanner. The frequency at which the protons resonate depends on the strength of the magnetic field. As a result of conservation of energy, this also dictates the frequency of the released photons.
It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. Additional magnetic fields are applied during the scan in order to make the magnetic field strength depend on the position within the patient, providing a straightforward method to control where the protons are excited by the radio photons. These fields are created by passing electric currents through solenoids, known as gradient coils. Since these coils are within the bore of the scanner, there will be large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to dampen this noise, it can approach 130 decibels (the human pain threshold) with strong fields.
An image can be constructed because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. In the case of cardiac pacemakers, the results can sometimes be lethal, so patients with such implants are generally not eligible for MRI.
MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, muscle, connective tissue and most tumors.
Death and injuries have occurred from projectiles created by the magnetic field, although few compared to the millions of examinations administered. MRI makes use of powerful magnetic fields which, though they have not been demonstrated to cause direct biological damage, can interfere with metallic and electromechanical devices. Additional (small) risks are presented by the radio frequency systems, components or elements of the MRI system's operation, elements of the scanning procedure and medications that may be administered to facilitate MRI imaging.
Of great concern is the dramatic increase in the number of reported MRI accidents to the U.S. Food and Drug Administration (FDA). Since 2004, the last year in which a decline in the number of MRI accidents was reported, the full spectrum of MRI accidents has increased significantly in the following years. The 2008 FDA accident report data culminates in a 277% increase over the 2004 rate.
There are many steps that the MRI patient and referring physician can take to help reduce the remaining risks, including providing a full, accurate and thorough medical history to the MRI provider.
Claustrophobia and discomfort
Due to the construction of some MRI scanners, they can be potentially unpleasant to lie in. Older models of closed bore MRI systems feature a fairly long tube or tunnel. The part of the body being imaged needs to lie at the center of the magnet which is at the absolute center of the tunnel. Because scan times on these older scanners may be long (occasionally up to 40 minutes for the entire procedure), people with even mild claustrophobia are sometimes unable to tolerate an MRI scan without management. Modern scanners may have larger bores (up to 70 cm) and scan times are shorter. This means that claustrophobia is less of an issue, and many patients now find MRI an innocuous and easily tolerated procedure.
Nervous patients may still find the following strategies helpful:
Alternative scanner designs, such as open or upright systems, can also be helpful where these are available. Though open scanners have increased in popularity, they produce inferior scan quality because they operate at lower magnetic fields than closed scanners. However, commercial 1.5 tesla open systems have recently become available, providing much better image quality than previous lower field strength open models.
visiting the scanner to see the room and practice lying on the table
Coping while inside the scanner
holding a "panic button"
closing eyes as well as covering them (e.g. washcloth, eye mask)
listening to music on headphones or watching a movie with a Head-mounted display while in the machine
Scan Rooms with lighting, sound and images on the wall. Some rooms come with images on the walls or ceiling.
For babies and young children chemical sedation or general anesthesia are the norm, as these subjects cannot be instructed to hold still during the scanning session. Obese patients and pregnant women may find the MRI machine to be a tight fit. Pregnant women may also have difficulty lying on their backs for an hour or more without moving.
Acoustic noise associated with the operation of an MRI scanner can also exacerbate the discomfort associated with the procedure.
Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies, and incidental localized heating, have been addressed in the American College of Radiology's White Paper on MR Safety which was originally published in 2002 and expanded in 2004. The ACR White Paper on MR Safety has been rewritten and was released early in 2007 under the new title ACR Guidance Document for Safe MR Practices.
In December 2007, the Medicines in Healthcare product Regulation Agency (MHRA), a UK healthcare regulatory body, issued their Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use.
In February 2008, the Joint Commission, a US healthcare accrediting organization, issued a Sentinel Event Alert #38, their highest patient safety advisory, on MRI safety issues.
In July 2008, the United States Veterans Administration, a federal governmental agency serving the healthcare needs of former military personnel, issued a substantial revision to their MRI Design Guide which includes physical or facility safety considerations.