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The Intricacies of MRI Magnets

26 May, 2011 | MRI

mri magnet

mri magnet

The MRI magnet is the largest and the most expensive component of the Magnetic Resonance Imaging System.

The Three Main MRI Magnet Types

In general, there are three main types of magnets used in MRI systems: resistive, permanent and superconducting.

Resistive MRI magnets are wound around a cylinder and create a magnetic field when electricity runs through them. They are cheaper, but use more energy to create the required strength for the magnetic field.

The permanent magnet is very large and heavy, and is always charged. Electricity is not needed to generate the magnetic field in the permanent MRI magnet.

Most MRI magnets are of the superconducting type.  A superconducting MRI magnet is an electromagnet made of superconducting wire. Superconducting wire has a resistance approximately equal to zero when it is cooled to a temperature close to absolute zero, which is accomplished by immersing it in liquid helium. Once current is caused to flow in the coil, it will continue to flow as long as the coil is kept immersed in liquid helium during the MRI scan. Although some losses do occur over time, due to infinitely small resistance of the coil; in modern, well-designed MRI magnets, these losses are usually only a few ppm (parts per million) of the main magnetic field per year.

The Role of Superconducting MRI Magnets

The superconducting MRI magnets work together in a very controlled environment, to make sure the magnetic field is equal throughout the machine. The amount of electricity and the number and strength of the magnets control how strong or weak the field is.

There are additionally, three gradient MRI magnets used in the MRI machine to help this process. They are very weak magnets, unlike the others in the system. They create a variable field after the other magnets have been activated, for the purpose of generating a stable field first and are turned off and on very quickly to create different pictures or “slices” for a more thorough and in-depth examination of the patient.

The length of wire in the superconducting MRI magnet is typically several kilometers long. This long coil of wire is maintained at a temperature of 4.2 K by immersing it in the liquid helium. The coil and liquid helium are kept in a large Dewar vacuum flask, with the typical volume of liquid helium in an MRI magnet being around 1500 liters.

In earlier MRI magnet designs, this Dewar was typically surrounded by an additional liquid nitrogen Dewar, which acted as a thermal buffer between the outer room temperature and the liquid helium.

In later MRI magnet designs, the liquid nitrogen region was replaced by a Dewar which is cooled by a cryocooler or refrigerator. There is a refrigerator outside the MRI magnet with cooling lines going to a coldhead in the liquid helium. This special design eliminates the need to add liquid nitrogen to the MRI magnet, and increases the liquid nitrogen hold time to 3-4 years. Currently, researchers are working on an MRI magnet that requires no liquid helium.

Properties of Materials Used

The superconducting portions of most current MRI magnets are composed of niobium-titanium. This material has a critical temperature of 10 kelvins and can superconduct at up to about 15 teslas. More expensive MRI magnets can be made of niobium-tin, which have a critical temperature of 18 kelvins. When operating at 4.2 kelvins, they are able to withstand a much higher magnetic field intensity: as high as 25 to 30 teslas. Unfortunately, it is far more difficult to make the required filaments from this material, which is the reason that sometimes a combination of niobium-tin for the high-field sections and niobium-titanium for the lower-field sections is used.  Vanadium-gallium is yet another material sometimes used for the high-field inserts.

The coil windings of a superconducting MRI magnet are made of wires or tapes of superconductors (niobium-titanium or niobium-tin). The wire or tape itself may be made of tiny filaments of superconductor in a copper matrix. The copper is needed to add mechanical stability and to provide a low-resistance path for the large currents, in case the temperature or the current rises and superconductivity is lost. The coil must be carefully designed to withstand magnetic pressure and Lorentz forces that could otherwise cause wire fracture or crushing of insulation between adjacent turns.

Shielded MRI Magnet and Fringe Field

An important advance in MRI magnet technology is the shielded magnet. This MRI magnet has a smaller fringe field. The fringe field drops to 0.5 mT by four meters from the magnet. This is important for safety reasons and makes it easier to site the MRI magnet. The shielding is achieved by a second set of superconducting windings, outside of the main ones and with opposite current, which reduce the fringe field.

Quench and Ramifications

A quench is an abnormal termination of MRI magnet operation that occurs when part of the superconducting coil enters the resistive state. This can occur because the field inside the magnet is too large, the rate of change of field is too large, or a combination of the two. More rarely, a defect inside the MRI magnet can cause a quench.  When  this happens, that particular spot is subject to rapid Joule heating, which raises the temperature if the surrounding regions. This pushes those regions into the resistive state as well, which leads to more heating in a chain reaction. The entire MRI magnet then becomes resistive.

This is accompanied by a loud bang, as the energy in the magnetic field is converted to heat and rapid boil-off of the cryogenic fluid. The abrupt decrease of current can result in kilovolt inductive voltage spikes and arcing. Permanent damage to the magnet is rare, but components can be damaged by localized heating or large mechanical forces. In practice, MRI magnets usually have safety devices to stop or limit the current when the beginning of a quench is detected. If a large magnet undergoes a quench, the inert vapor formed by the evaporating cryogenic fluid can present a significant asphyxiation hazard to operators, by displacing breathable air. Rooms built in support of superconducting MRI magnet equipment should be equipped with pressure-relief mechanisms 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 MRI 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.

MRI Magnet Special Safety Considerations

Most forms of medical or biostimulation implants are generally considered contraindications for MRI scanning. These include: pacemakers, vagus nerve stimulators, implantable cardioverter-defribrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and capsules retained from capsule endoscopy. Patients are therefore always asked for complete information about all implants before an MRI scan. To reduce risks, implants are increasingly being developed to make them able to facilitate the safe scanning of selected implants and pacing devices. Cardiovascular stents are considered safe.

Ferromagnetic foreign bodies, such as shell fragments or metallic implants such as surgical prostheses and aneurysm clips are potential risks. Interaction of the magnetic MRI magnet fields with such objects can lead to trauma due to movement of the object in the magnetic field. Titanium and its alloys are safe from movement from the magnetic field.

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