Chapter Twelve
Contrast Agents: Fundamentals

12-01 More Magnetism

agnetic resonance contrast agents aim at changing signal intensity and thus image contrast. The main contrast parameters in MR imaging are proton density, the relaxation times, and magnetic susceptibility. It is rather difficult to alter the water content of tissues.

Therefore, the magnetic pro­per­ties have been the major target for the de­ve­lop­ment of contrast changing agents. Magnetic susceptibility describes the abi­li­ty of a material or substance to become magnetized by an external magnetic field.

Figure 12-01:
In which direction do we go? Where are the field lines?

All substances are diamagnetic. A strong external magnetic field speeds up or slows down the electrons orbiting in atoms in such a way as to oppose the action of the external field. These materials partly expel from their interior the magnet field in which they are placed.

Certain materials have ferro-magnetic properties, among them iron, nickel, co­balt, and their alloys. Ferromagnetic materials are strongly attracted by mag­nets. In ferromagnetic materials, there is a strong coupling of the individual mag­nets, resulting in their lining up parallel to one another (Figure 12-02a). They lose their magnetic properties when heated above a temperature known as the ‘Curie point’ (770° C for iron, 358° C for nickel, 1,120° C for cobalt).

Figure 12-02: (a) ferromagnetic material: atomic magnets are strongly coupled; (b) paramagnetic material in an external field: atomic magnets are weakly coupled; (c) ferrimagnetic material: weak overall magnetism; (d) antiferromagnetic material: atomic mag­nets coupled in an antiparallel manner, resulting in no magnetism.

Then they begin to show a kind of magnetic behavior which as called pa­ra­mag­ne­tic. Many other elements and compounds are paramagnetic at all tem­pe­ra­tu­res, among them oxygen, gadolinium, and manganese (see also Chapter 4). Paramagnetism is due to the presence of little colonies of atomic magnets, in which the individual magnets are weakly, if at all, bound to one another and there­fore capable only of random orientation in the absence of an external field (Figure 12-02b). Paramagnetic substances are feeble in their response to an ex­ter­nal magnetic field.

Superparamagnetic substances have a substantially higher susceptibility.

Ferrimagnetic materials, such as ferrites, are also coupled in an antiparallel fashion, but the overall effect of the individual magnets pointing in one direction exceeds. Thus, the net effect is that of weak overall magnetism (Figure 12-02c).

Antiferromagnetic materials consist of elementary magnets coupled together in opposite directions, resulting in zero net magnetization (Figure 12-02d).

Figure 12-03:
Different magnet compounds and outside magnetic fields.

Figure 12-03 shows the response of diamagnetic, paramagnetic, and fer­ro­mag­ne­tic substances to an outside magnetic field. For diamagnetic material, the value of susceptibility is always negative; for paramagnetic, su­per­pa­ra­mag­ne­tic, and ferromagnetic substances positive (Table 12-01).

Table 12-01:
Fundamentals of magnetic susceptibility χ.

12-02 Magnetic Resonance Contrast Agent Terms

To be able to develop and compare contrast agents there has to be agreement of how their properties should be measured. Table 12-02 summarizes the latest version (1997) of the Standard Nomenclature of Magnetic Resonance Contrast Agent Terms [⇒ EMRF 1997].

Table 12-02:
Recommendations for the Nomenclature of Magnetic Resonance Contrast Agent Terms.