TwinTree Insert

03-02 Magnet Types

he magnetic field of an MR system can be generated by different magnet sys­tems: permanent, resistive, and superconductive. They have different dis­tinc­ti­ve fea­tures and are employed in different ways. A fourth, hybrid, magnet type is a blend of permanent and resistive magnets.

03-02-01 Permanent Systems

Certain alloys possess ferromagnetic properties. A magnet built of such ma­te­ri­als has the advantage of needing no power to main­tain the field strength. It needs no cooling because there is no power dissipa­tion and it has a small fringe (stray) field when compared to the other mag­net systems. Capital and ope­ra­tio­nal costs of permanent magnets are low.

The disadvantages are the weight of these systems for whole body imaging, al­though new alloys developed during recent years have cut down the weight of per­ma­nent systems from 100 to less than 20 tons. Another draw­back of per­­ma­nent mag­net systems are the field-strength li­mi­ta­tions that 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 03-05). The field direction has an im­pact on the use of certain transmitter and re­cei­ver coils.

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.

Figure 03-05:
Top: Schematic drawings of a permanent magnet. Bottom: Commercial version of a low field per­ma­­nent MR imaging equipment.

03-02-02 Resistive Systems

Resistive (electromagnetic) 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 03-06.

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.

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 (see Figure 18-04). 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.

Figure 03-06:
Cuts through two different kinds of air core electro­magnets. The common four loops of wire creat­ing the sta­tic magnetic field can be arranged (a) parallel or (b) perpendicular to the pa­ti­ent table; the per­pen­di­cu­­lar (head to foot) orientation is more com­mon.

03-02-03 Hybrid Systems

Some companies have developed whole-body 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 03-07). These systems reach field strengths up to 0.4 T. Their weight is between 10 and 15 tons. Ultrahigh field research system can be hy­brids between superconductive and resistive systems.

Figure 03-07:
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 super­­con­­duc­­tive. Thus, when superconductive 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 original design for superconducting magnets involved 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 03-08). These systems were replaced by single-dewars using 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 in­creased. 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; whole-body sys­tem are being developed.

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

Figure 03-08:
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 ultrahigh field (3 Tesla) superconductive MR imaging equipment. A cir­cu­lar bore of 70 cm in diameter is minimum common stan­dard.

The physical field-strength limitations for super­conducting magnets are not yet esta­­bli­sh­ed. For imaging purposes, small and whole-body sys­tems up to 9.4 T have been used; scientific machines for spectroscopy and imaging are being developed for fields of up to 14.1 T — field strength is permanently pushed further up. Only super­­con­duct­ing magnets can be used for such purposes [⇒ Moser 2017].

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.

Material for wires and coils. Until re­cently, coils for super­con­duct­ive magnet sys­tem were commonly made with nio­bium-titanium (NbTi). 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, eliminating the need for liquid helium and pos­sib­le quenches [⇒ Bud’ko 2015].

These new conductors 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 [⇒ Marabotto 2006]. 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 already described as com­pet­ing with that of high field systems even before the introduction of high-temperature super­con­duct­ive coils (see also Diagnostic accuracy).

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

Table 03-02:
Properties of different magnet types.

03-02-05 Hybrid PET-MRI Systems

Magnetic reso­nance imaging equipment can be com­bined with positron emission tomography (PET) into one machine [⇒ Shao 1997]. Do not confuse hybrid magnets with hybrid PET-MRI systems.

Hy­brid PET-MRI systems 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 2015].

The technical demands on such a hybrid systems are high, in particular ar­rang­ing PET and MRI detectors into a single gantry.