The Measurement of Magnetic Fields

The application of magnetism to medicine covers a rather large range of fields, and as a consequence a great variety of methods has been developed suitable for the measurement of magnetic field strength. However, within the scope of this book only those methods significant to medical applications will be treated in detail. Each method offers advantages as well as drawbacks which, in general, are more or less pronounced for certain ranges of field strength. Therefore, the instruments used must be chosen according to the field range of the specific application in order to provide optimum sensitivity. Whilst a number of different definitions of the term "sensitivity" have been reported in the literature, within the context of this chapter the term ''detection limit'' is preferred. This denotes the lowest value of field strength which can be reliably detected. Of course the meaning of ''reliably'' must also be defined, with one condition being that the signal:noise ratio is greater than 2. A compilation of typical field ranges, together with some selected principles of measurement, is provided in Table 1.2. Many more methods and corresponding instruments - especially for applications in other technical fields - are available, though only few of these have been used for medical techniques. The reason for this is that, in many cases, the detection limit does not meet the corresponding demand, while in other cases the handling may be too complicated. On occasion, potential users may not be familiar with the respective method, and consequently only about five principles of the methods listed in Table 1.2 are usually applied in medicine.

For many years, the primary position with regard to the greatest sensitivity or lowest value of detection limit was held by the SQUID principle; this type of field sensor is described in detail in Section 2.2. More recently, the atomic magnetometer was developed with slightly higher sensitivity (Kominis et al., 2003; Schwindt et al., 2004). In addition, the optical pumped magnetometer could be used to map the human cardiomagnetic field (Bison et al., 2003). This new measuring principle is based on the detection of the so-called Larmor-spin precession of atoms which are excited by optical radiation. The sensitivity of the technique is essentially determined by the relaxation time which passes until the increased energy of the atoms is delivered to the surroundings.

The secondary position with regard to detection limit is held by three principles. The first of these, nuclear precession magnetometry, is used to detect small local variations of an otherwise strong constant magnetic field; details of the basic principle are provided in Section 3.2. The other principles are rotating-coil magnetome-try, which is not used widely in medicine, and flux-gate magnetometers, which are used in different fields of application.

Table 1.2. Examples of field ranges and the corresponding measuring principles. The detection limits are given for quasistatic fields.

Field source

m0H

Measuring principle

Detection limit

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