Magnetic resonance imaging (MRI) scanner

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"Magnetic resonance imaging (MRI) is a noninvasive medical test that helps physicians diagnose and treat medical conditions. MRI can give different information about structures in the body than can be obtained using a standard x-ray, ultrasound, or computed tomography (CT) exam. [1]MRI uses a powerful magnetic field, radio frequency pulses and a computer to produce detailed pictures of organs, soft tissues, bone and virtually all other internal body structures. The images can then be examined on a computer monitor, printed or copied to CD. MRI does not use ionizing radiation (x-rays)." [2] Also comes in a mobile trailer system for those without in-house MRI capabilities. Magnetic resonance imaging relies upon signals derived from water molecules, which comprise between 70% and 80% of the average human brain. This ubiquitous biological molecule has two protons, which by virtue of their positive charge act as small magnets on a subatomic scale. Positioned within the large magnetic field of an MR scanner, typically 30 to 60 thousand times stronger than the magnetic field of the earth, these microscopic magnets collectively produce a tiny net magnetization that can be measured outside of the body and used to generate very high-resolution images that reveal information about water molecules in the brain and their local environment.

Protons placed in a magnetic field have the interesting property that they will absorb energy at specific frequencies, and then re-emit the energy at the same frequency. To measure the net magnetization, a coil placed around the head is used to both to generate electromagnetic waves and measure the electromagnetic waves that are emitted from the head in response. Unlike CT, which uses x-rays with very high frequency energy, MRI uses electromagnetic waves in the same portion of the electromagnetic spectrum as broadcast FM radio.

MRI is also a tomographic imaging modality, in that it produces two-dimensional images that consist of individual slices of the brain. Images in MRI need not be acquired transaxially, and the table or scanner does not move to cover different slices in the brain. Rather, images can be obtained in any plane through the head by electronically “steering” the plane of the scan. Precise spatial localization is achieved through a process termed gradient encoding. [3] The switching on and off of these magnetic field gradients are the source of the loud clicking and whirring noises that are heard during an MRI scan. While this process requires more time than CT scanning, imaging can be performed relatively rapidly using modern gradient systems. [4] Lastly, a contrast media, or “dyes” are used both in brain CT and MRI to provide another mechanism for modulating image intensity beyond what is possible using intrinsic tissue contrast. The device operates on 60Hz, 480 VAC, 3-phase Delta-Wye, 200 amps power circuit. The maximum allowable line voltage variation is +/- 10 percent (432-528 VAC) and 3-phase balance should be with 2%. The room requires data lines for telephone (RJ11) and work station (RJ45) communications.


A number of features of MRI scanning can give rise to risks.

These include:

  • Powerful magnetic fields
  • Cryogenic liquids
  • Noise
  • Claustrophobia

In addition, in cases where MRI contrast agents are used, these also typically have associated risks.

Magnetic field[]

Most forms of medical or biostimulation implants are generally considered contraindications for MRI scanning. These include pacemakers, vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators. Patients are therefore always asked for complete information about all implants before entering the room for an MRI scan. Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. To reduce such risks, implants are increasingly being developed to make them able to be safely scanned,[5] and specialized protocols have been developed to permit the safe scanning of selected implants and pacing devices.

Ferromagnetic (Iron) foreign bodies such as shell fragments, or metallic implants such as surgical prostheses and aneurysm clips are also potential risks. Interaction of the magnetic and radio frequency fields with such objects can lead to trauma due to movement of the object in the magnetic field or thermal injury from radio-frequency induction heating of the object.

Titanium and its alloys are safe from movement from the magnetic field.

In the United States, a classification system for implants and ancillary clinical devices has been developed by ASTM International and is now the standard supported by the US Food and Drug Administration:

  • MR-Safe — The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure.
  • MR-Conditional — A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1.5 teslas' or 'safe in magnetic fields below 500 gauss in strength').
  • MR-Unsafe — Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.

The very high strength of the magnetic field can also cause "missile-effect" accidents, where ferromagnetic objects are attracted to the center of the magnet, and there have been incidences of injury and death.[6][7] To reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner and patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs and ferromagnetic detection devices are used by some sites. [8]

There is no evidence for biological harm from even very powerful static magnetic fields[9]

Magnetic Shielding[]

Gauss line

Gauss line in a mobile MRI trailor. Magnetic Shielding is validated through EMI/EMC Testing.

Magnetic shielding is contained within the side walls. Image quality is directly related to stationary objects within a certain range of the magnet center. For example, vehicle traffic, high power lines, etc. should not be within direct sight of the gauss line.

Radio frequency energy[]

A powerful radio transmitter is needed for excitation of proton spins. This can heat the body to the point of risk of hyperthermia in patients, particularly in obese patients or those with thermoregulation disorders. Several countries have issued restrictions on the maximum specific absorption rate that a scanner may produce.

Peripheral nerve stimulation (PNS)[]

The rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation. Volunteers report a twitching sensation when exposed to rapidly switched fields, particularly in their extremities. The reason the peripheral nerves are stimulated is that the changing field increases with distance from the center of the gradient coils (which more or less coincides with the center of the magnet. Note however that when imaging the head, the heart is far off-center and induction of even a tiny current into the heart must be avoided at all costs. Although PNS was not a problem for the slow, weak gradients used in the early days of MRI, the strong, rapidly switched gradients used in techniques such as EPI, fMRI, diffusion MRI, etc. are indeed capable of inducing PNS. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in field per unit time) or else prove that no PNS is induced for any imaging sequence. As a result of dB/dt limitation, commercial MRI systems cannot use the full rated power of their gradient amplifiers.

Acoustic noise[]

Switching of field gradients causes a change in the Lorentz force experienced by the gradient coils, producing minute expansions and contractions of the coil itself. As the switching is typically in the audible frequency range, the resulting vibration produces loud noises (clicking or beeping). This is most marked with high-field machines[10] and rapid-imaging techniques in which sound intensity can reach 120 dB(A) (equivalent to a jet engine at take-off),[11] and therefore appropriate ear protection is essential for anyone inside the MRI scanner room during the examination.[12]


As described in Physics of Magnetic Resonance Imaging, many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids used are non-toxic, their physical properties present specific hazards.

An unintentional shut-down of a superconducting electromagnet, an event known as "quench", involves the rapid boiling of liquid helium from the device. If the rapidly expanding helium cannot be dissipated through an external vent, sometimes referred to as 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.[13]

Liquid helium, the most commonly used cryogen, a byproduct, in MRI, undergoes near explosive expansion as it changes from liquid to a gaseous state. Rooms built in support of superconducting MRI equipment should be equipped with pressure relief mechanisms[14] and an exhaust fan, in addition to the required quench pipe.

Since a quench results in rapid loss of all cryogens in the magnet, recommissioning the magnet is expensive and time-consuming. Spontaneous quenches are uncommon, but may also be triggered by equipment malfunction, improper cryogen fill technique, contaminants inside the cryostat, or extreme magnetic or vibrational disturbances.

Contrast agents[]

The most commonly used intravenous contrast agents are based on chelates of gadolinium. In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare, occurring in approx. 0.03–0.1%.[15] Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo radiocontrast.[16]

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents. The most frequently linked is gadodiamide, but other agents have been linked too.[17] Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.[18] In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released.


No effects of MRI on the fetus have been demonstrated.[19] In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided.

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound and it lacks the ionizing radiation of CT. MRI without contrast agents is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratomas, facilitating open fetal surgery, other fetal interventions, and planning for procedures (such as the EXIT procedure) to safely deliver and treat babies whose defects would otherwise be fatal.


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 must 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 (fear of tight spaces) 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 and discomfort 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:

  • Advance preparation
    • visiting the scanner to see the room and practice lying on the table
    • visualization techniques
    • chemical sedation
    • general anesthesia
  • 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

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.

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.


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.

The European Physical Agents Directive[]

The European Physical Agents (Electromagnetic Fields) Directive is legislation adopted in European legislature. Originally scheduled to be required by the end of 2008, each individual state within the European Union must include this directive in its own law by the end of 2012. Some member nations passed complying legislation and are now attempting to repeal their state laws in expectation that the final version of the EU Physical Agents Directive will be substantially revised prior to the revised adoption date.

The directive applies to occupational exposure to electromagnetic fields (not medical exposure) and was intended to limit workers’ acute exposure to strong electromagnetic fields, as may be found near electricity substations, radio or television transmitters or industrial equipment. However, the regulations impact significantly on MRI, with separate sections of the regulations limiting exposure to static magnetic fields, changing magnetic fields and radio frequency energy. Field strength limits are given, which may not be exceeded. An employer may commit a criminal offense by allowing a worker to exceed an exposure limit, if that is how the Directive is implemented in a particular member state.

The Directive is based on the international consensus of established effects of exposure to electromagnetic fields, and in particular the advice of the European Commissions's advisor, the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The aims of the Directive, and the ICNIRP guidelines it is based on, are to prevent exposure to potentially harmful fields. The actual limits in the Directive are very similar to the limits advised by the Institute of Electrical and Electronics Engineers, with the exception of the frequencies produced by the gradient coils, where the IEEE limits are significantly higher.

Many Member States of the EU already have either specific EMF regulations or (as in the UK) a general requirement under workplace health and safety legislation to protect workers against electromagnetic fields. In almost all cases the existing regulations are aligned with the ICNIRP limits so that the Directive should, in theory, have little impact on any employer already meeting their legal responsibilities.

The introduction of the Directive has brought to light an existing potential issue with occupational exposures to MRI fields. There are at present very few data on the number or types of MRI practice that might lead to exposures in excess of the levels of the Directive.[20] There is a justifiable concern amongst MRI practitioners that if the Directive were to be enforced more vigorously than existing legislation, the use of MRI might be restricted, or working practices of MRI personnel might have to change.

In the initial draft a limit of static field strength to 2 T was given. This has since been removed from the regulations, and whilst it is unlikely to be restored as it was without a strong justification, some restriction on static fields may be reintroduced after the matter has been considered more fully by ICNIRP. The effect of such a limit might be to restrict the installation, operation and maintenance of MRI scanners with magnets of 2 T and stronger. As the increase in field strength has been instrumental in developing higher resolution and higher performance scanners, this would be a significant step back. This is why it is unlikely to happen without strong justification.

Individual government agencies and the European Commission have now formed a working group to examine the implications on MRI and to try to address the issue of occupational exposures to electromagnetic fields from MRI.


Nikola Tesla discovered the Rotating Magnetic Field in 1882 in Budapest, Hungary. This was a fundamental discovery in physics.

In 1946, Felix Bloch proposed in a Nobel Prize winning paper some rather new properties for the atomic nucleus. He stated that the nucleus behaves like a magnet. He realized that a charged particle, such as a proton, spinning around its own axis has a magnetic field, known as a magnetic momentum.[21]

In 1960, Raymond Damadian discovered that malignant tissue had different NMR parameters than normal tissue. He mused that, based on these differences, it should be possible to do tissue characterization.

In 1956, the "Tesla Unit" was proclaimed in the Rathaus of Munich, Germany by the International Electro-technical Commission-Committee of Action. All MRI machines are calibrated in "Tesla Units". The strength of a magnetic field is measured in Tesla or Gauss Units. The stronger the magnetic field, the stronger the amount of radio signals which can be elicited from the body's atoms and therefore the higher the quality of MRI images.

In 1937, Columbia University Professor Isidor I. Rabi working in the Pupin Physic Laboratory in Columbia University, New York City, observed the quantum phenomenon dubbed nuclear magnetic resonance (NMR). He recognized that the atomic nuclei show their presence by absorbing or emitting radio waves when exposed to a sufficiently strong magnetic field.

Professor Isidor I. Rabi received the Nobel Prize for his work. He is one of 28 Nobel Laureates from the Pupin Physics Laboratory in New York City.

Raymond Damadian, a physician and experimenter working at Brooklyn's Downstate Medical Center discovered that hydrogen signal in cancerous tissue is different from that of healthy tissue because tumors contain more water. More water means more hydrogen atoms. When the MRI machine was switched off, the bath of radio waves from cancerous tissue will linger longer then those from the healthy tissue.

In 1973, Paul Lauterbur, a chemist and an NMR pioneer at the State University of New York, Stony Brook, produced the first NMR image.

Mike Goldsmith, one of the graduate students cobbled a wearable antenna coil to monitor the hydrogen broadcast detected by the coil.

On July 3, 1977, nearly five hours after the start of the first MRI test, the first human scan was made as the first MRI prototype. [22]

MRI versus CT[]

MRI Stroke

MRI of a patient who has had a stroke of the left hemisphere of the brain. The arrow indicates the area that was affected.

Basically, a MRI Scan does not use ionizing radiation, and is thus preferred over CT in children and patients requiring multiple imaging examinations. MRI allows the evaluation of structures that may be obscured by artifacts from bone in CT images. It has a much greater range of available soft tissue contrast, depicts anatomy in greater detail, and is more sensitive and specific for abnormalities On the other hand, Magnetic resonance imaging (MRI) is a medical imaging scan that costs the organization or patient more money to produce per image versus a CT scan. All tests to be carried out in accordance with written procedures and ACR MRI Accreditation methods.[23]


Hydrogen orbits

If we look at a bunch of hydrogen protons (as in a molecule) we see, in fact, a lot of tiny bar magnets spinning around their own axes. When we put a person in a magnet some interesting things happen to the hydrogen protons: 1. They align with the magnetic field. This is done in two ways, parallel or anti-parallel. 2. They precess or “wobble” due to the magnetic momentum of the atom

Atoms have everything to do with MRI, because we use them to generate our MR image. Our body consists of 80% water. The hydrogen atom has 1 proton, and 1 electron. This proton is electrically charged and it rotates around its axis. Also the hydrogen proton can be looked at as if it were a tiny bar magnet with a north and a south pole.

In quantum physics there is a thing called “Gyro Magnetic Ratio.” Hydrogen is the largest with a 42.57 MHz/Tesla gyroscopic spin. If we look at a bunch of hydrogen protons we see, in fact, a lot of tiny magnets spinning around their own axes. The magnetic field strength of a 1.5 Tesla magnet is ± 30.000 times stronger than the earth gravitational field! When we put a person in a magnet some interesting things happen to the hydrogen protons: 1. They align with the magnetic field. This is done in two ways, parallel or anti-parallel. They precess (spin) or “wobble” due to the magnetic momentum of the atom.

Before the system starts to acquire the data it will perform a quick measurement (also called prescan) to determine (amongst others) at which frequency the protons are spinning (the Larmor frequency alias center frequency). Let us assume we work with a 1.5 Tesla system. The centre or operating frequency of the system is 63.855 MHz. In order to manipulate the net magnetization we will therefore have to send an Radio Frequency (RF) pulse with a frequency that matches the centre frequency of the system: 63.855 MHz. This is where the Resonance comes from in the name Magnetic Resonance Imaging.

We rotate the spinning hydrogen atom 90º into the X-Y plane lifting the protons into a higher energy state. The protons want to go back to their original situation, called equilibrium called T1 relaxation. T2 happens in the X-Y plane and is much faster than T1.

T2Echo spin

T2 Spin echo

During the relaxation phase, the atom spins and shed their excess energy, which they acquired from the 90º RF pulse, in the shape of radio frequency waves. In order to produce an image we need to pick up these waves before they disappear into space. This can be done with a Receive coil. The receive coil can be the same as the Transmit coil or a different one. The received signal is then fed into a computer and, amazingly, a quarter of a second later an image appears on the screen.

Gradient coils are a set of wires in the magnet, which enable us to create additional magnetic

fields, which are, in a way, superimposed on the main magnetic field B0.


There are 3 sets of wires. Each set can create a magnetic field in a specific direction: Z, X or Y. When a current is fed into the Z gradient, then a magnetic field is generated in the Z direction.

Everyone knows that MRI can make a lot of noise during acquisition. The magnetic field, which is generated, is very strong. Although the gradient coils are very tightly fixed in a kind of resin, the forces, exhibited by the gradient coil, are enough to make them vibrate, hence the noise.



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