Biomedical Engineering Reference
In-Depth Information
3
The Imaging of a Magnetic Source
H. Kado, H. Ogata, Y. Haruta, M. Higuchi, M. Shimogawara, J. Kawai,
Y. Adachi, C. Bertrand, and G. Uehara
3.1
The Principle of Magnetic Field Measurement
The purpose of this section is to provide information about magnetic field
measurement, especially of very weak magnetic signals, and about the ana-
lysis of the source of the measured signal. It is not our intention to provide
full information on magneticism and the related area, which is too wide and
beyond our capability. To realize the purpose, limited discussions will be pro-
vided on topics from basic principles of measurement to several examples
of the state-of-the-art issue. The discussions will include the definition of a
magnetic field, an example of a magnetic field source, the concept of measure-
ment of the field, practical technology and measuring devices and an example
of source analysis. Most of those items are also described in the following subs-
ections in more detail. Some of the details are basic knowledge, while others
have to do with contemporary studies on applications of the technology. We
have tried to arrange the description as self-contained as possible and to keep
reference citations to the necessary minimum.
A knowledge of magnetism is assumed to be already established at a level
such as a university undergraduate course on electromagnetism, including
basic vector concepts. If you are not familiar with these subjects, we stron-
gly recommend that you refer to textbooks on electromagnetism that are
accessible at most public or university libraries. Two physical quantities in
magnetism - the magnetic field and magnetic flux - are important concepts
in the context of this chapter. Two systems - SI and cgs - are commonly used
to express electrical and magnetic quantities. In this chapter, we use the SI
system.
3.1.1
The Magnetic Field
A magnetic field is a result of the existence of magnetic sources. One simple
magnetic source would be a magnet. To observe the magnetic field generated
by a magnet, small compass magnets, which are one of the most simple
magnetic measurement devices, can be used. Figure 3.1 shows the pattern
of the compass magnets placed around the magnetic source. The pattern
indicates the existence of an invisible force distribution between the compass
magnets and the magnetic source. The strength and direction of the force
