03-02 Magnet Types

The magnetic field of an MR system can be generated by different magnet sys­tems:

 permanent magnets;
 resistive magnets;
 superconducting magnets.

A forth, hybrid, magnet type is a blend of permanent and resistive magnets.

03-02-01 Permanent Magnets

Certain alloys possess ferromagnetic properties. A magnet built of such ma­te­ri­als has the advantage of needing no power to maintain the field strength. Like­wise, it needs no cooling because there is no power dissipation.

Such systems have small fringe (stray) fields when compared to the other mag­net systems. Capital and operational costs of permanent magnets are low.

The disadvantages are the weight of the currently produced systems for whole body imaging, although new alloys developed during recent years have cut down the weight of permanent systems from 100 tons to less than 20 tons. Another draw­back of permanent magnet systems are the field-strength li­mi­ta­tions, which pre­sent­ly seem to be about 0.3 T for magnetic resonance imaging. Most of them ope­ra­te at about 0.2 T.

Many permanent magnets have a vertical magnetic field which distinguishes them from some resistive and most superconducting systems with horizontal fields (Figure 03-04). The field direction has an impact on the use of certain transmitter and receiver coils.

Figure 03-04: Top: Schematic drawings of a permanent magnet. Such magnet systems can be designed in different ways, from a Greek temple shape to a C-shaped open system. In this case the field is produced by magnetized ce­ra­mic bricks; the outside consists of iron that provides structural support to the system, contains the stray field, and thus intensifies magnetic field strength. The field strength of permanent magnets can be influenced by the surrounding tem­pe­ra­tu­re, therefore temperature-stabilizing air conditioning is necessary for the magnet room. Bottom: Commercial version of a low field permanent MR imaging equipment.

03-02-02 Electromagnets or Resistive Systems

Resistive systems consist basically of a suitable coil or collection of coils through which a strong electric current is passed. If these coils are set up in a proper geo­me­try, a homogeneous magnetic field can be created, as shown in Fi­gu­re 03-01 and Fi­gu­re 03-05. Such systems have a high power consumption (e.g., a 0.1 T unit re­qui­res about 20 kW), create a lot of heat, and therefore need large ca­pa­ci­ty cooling systems.

Figure 03-05: Cuts through two different kinds of air core resistive electromagnets. As shown in Fi­gu­re 03-01, resistive magnets com­mon­ly consist of four loops of wire creat­ing the sta­tic magnetic field. They can be arranged (a) parallel or (b) perpendicular to the pa­ti­ent table; the perpendicular (head to foot) orientation is more com­mon.

The practical upper limit for large-bore magnets is about 0.7 T, but usually 0.3 T is considered the upper limit for commercially available machines. Fringe fields are present around such systems. The weight of these systems is typically below 5 tons. They are the lightest of all MR imaging systems.

Resistive magnets have the advantage that they can be switched off when the system is not being used or during emergencies.

03-02-03 Hybrid Magnets

Some companies have developed magnets which are hybrids between per­ma­nent and resistive systems. They are iron-cored electromagnets in which the magnetic energy of the resistive magnet is concentrated in the gap between the soft-iron pole pieces (Figure 03-06). These systems reach field strengths up to 0.4 T and are the most commonly used. Their weight is between 10 and 15 tons.

Figure 03-06: Hybrid magnets combine permanent mag­nets with electromagnets.Their power con­sump­tion is high, but field strength can be increased compared to a permanent or purely resistive magnet system. These mag­nets are also described as ‘iron core’ electromagnets.

03-02-04 Superconductive Systems

When certain alloys are cooled down to temperatures close to absolute zero, they show drastically reduced resistance to electric current: they become super­con­duc­ting. Thus, when superconducting alloys are placed in liquid helium (at tem­pe­ra­tu­res below a critical value of between -263° C and -269° C or 4 to 10 K), high cur­rents can be driven in a coil built of that alloy, and an extremely stable magnetic field of very high field strength can be produced.

The basic design for superconducting magnets involves a double cooling sys­tem using liquid nitrogen as cryogenic liquid in the first thermos container (cry­o­stat or dewar) and liquid helium in the second inner dewar (Figure 03-07).

Figure 03-07: Top: Schematic drawing of a su­per­con­duc­ti­ve MR imaging system. The magnetic field is produced by electric cur­rent flowing in wire loops cooled by the surrounding liquid helium. The power sup­ply is disconnected once the system is char­ged and running at the desired field strength. Recent machines do not require a nitrogen vessel any more. Bottom: Commercial version of an ultra- high field superconductive MR imaging equipment. A circular bore of 70 cm in diameter is minimum common standard.

These systems were replaced by single-dewars which use a re­fri­ge­ra­tor (cryo-cooler). When charged with current, the superconducting mag­net uses virtually no electrical power, but consumes cryogenic liquids. He­li­um must be re­ple­nish­ed by refilling, which is costly, or through a compressor con­nec­ted to the MR system which reliquifies cryogens. Wholesale costs of he­li­um more than qua­drupl­ed between 2008 and 2013; thus MRI running costs in­crea­se. Meanwhile, small-bore ultrahigh field animal equipment with magnets not requiring helium, but cooled solely using a standard low temperature cryo- cooler is being com­mer­ci­al­ly offered.

Superconducting magnets have large fringe fields and are usually shielded so that the environment is protected.

The field-strength limitations for superconducting magnets are not yet esta­bli­shed. For imaging purposes, small sys­tems up to 9.4 T and whole-body sys­tems up to 9.4 T have been used, and for spectroscopy fields of up to 14.1 T are in use – and field strength is always increasing. Only superconducting magnets can be used for in-vivo spectroscopy and functional imaging because of their high field strengths.

The magnetic field of a superconducting magnet can be discharged when the coil accidentally loses its superconductivity. This creates a sudden increase of tem­pe­ra­tu­re which, in turn, heats the liquefied coolant gases. They start boiling, in­creas­ing in volume, and helium is set free. Such an incident is described as a quench (see also Chapter 18).

Usually no permanent damage to the magnet is induced but the magnet has to be refilled with helium and cooled down to superconductivity, which may last se­ve­ral days.

 Until recently, coils for superconductive magnet system were commonly made with niobium-titanium. During the last few years, new superconducting materials have been developed which allow superconductivity to occur at higher tem­pe­ra­tu­res (up to 100 K). However, the majority of new materials were rather brittle and unsuited to wire (and hence magnet) production. In addition, many of the ma­te­ri­als lose their superconductivity in the presence of strong magnetic fields.

Meanwhile wires and coils using magnesium diboride (MgB2) could be com­mer­cial­ly created [review: ⇒ Bud’ko], eliminating the need for liquid helium and pos­sib­le quenches. These new conductors [⇒ Marabotto] working at 20 K allow the pro­duc­tion of superconducting easy-access open MR systems ope­ra­ting at 0.5 Tesla with an imaging performance equal to high-field equipment.

The advantages of these new systems are superior diagnostic quality, lower price, lower maintenance costs, the possibility to acquire images in any position (lying, standing, sitting, bending over), ease of installation and operation, eli­mi­na­tion of claustrophobia, little noise, and general patient friendliness.

This development is a major challenge for existing high field equipment, in par­ti­cu­lar because the diagnostic quality of mid-field systems was described equal to high field even before the introduction of high-temperature superconductive coils (see also: Diagnostic accuracy – Chapter 9).

Table 03-02 summarizes advantages and disadvantages of different magnet ty­pes.

Table 03-02:
Properties of different magnet types.

Hybrid PET-MRI systems. Magnetic resonance imaging equipment can be com­bin­ed with positron emission tomography (PET) into one machine [⇒ Shao]. Such a hy­brid system can deliver complementary functional and anatomical information about a specific organ or body system down to the cellular, perhaps to the mo­le­cu­lar level. At the time being, PET-MRI systems are research-focused work in pro­gress [⇒ Bashir]. The technical demands on such a hybrid systems are high, in particular ar­rang­ing PET and MRI detectors into a single gantry.