Magnetism

Chapter 8 Magnetism

Chapter contents

8.2 Magnetic fields 49

8.3 Bulk magnetic properties and permanent magnets 51

Further reading 52

8.1 Aim

In this chapter the key principles of magnetism are considered. Magnetism plays a vital role in the scanning technique called magnetic resonance imaging (MRI), which uses powerful magnetic fields to portray human tissues.

8.2 Magnetic fields

As long ago as the 11th century, the Chinese were employing magnetic compasses. These consisted of a length of permanently magnetized iron which will tend to align itself in the direction of the Earth’s magnetic field when free to pivot. The phenomenon that two like magnetic poles (such as two north poles) repel and two unlike magnetic poles (such as a north and a south pole) attract was demonstrated by Peter Peregrinus in the 13th century.

Whenever an electrical charge is in motion, a magnetic field is produced. This takes place when electrons flow in a conductor such as a metal wire. There are also electron orbital motions and spins in atoms, which produce tiny magnetic fields.

Whenever a field is produced in a volume of space, an energy gradient exists. In other words there is a change of energy within that volume. The energy gradient means that force may be exerted on a charge present within the field. For example, force may be exerted on a current-carrying wire, on a compass needle which experiences a twisting force, or on the magnetic spins of nuclei within the field of an MRI magnet. Materials which are capable of being temporarily magnetized experience a force when placed in a magnetic field. A vivid demonstration of this effect (not to be attempted!) is the sight of an object such as a pair of scissors accelerating through the air towards the bore of an MRI magnet. This is an example of the ‘projectile effect’ in which a magnetic field exerts a ‘torque’ (or twisting force) on a ferromagnetic object such as iron, tending to align it with the field and bring it closer to the strongest point in the field.

A magnetic north and magnetic south pole separated by a distance is known as a magnetic dipole. A dipole can comprise the two poles of a permanent magnet or the two ends of a current-carrying loop of conductor. If two magnetic poles of strength p are separated by a distance l, then the magnetic dipole moment of the system, m=p×l.

The force F between two magnetic poles is equal to the product of their individual strengths divided by the square of the distance between them.

F is proportional to (p₁×p₂) divided by d², where p₁ and p₂ are the individual magnetic pole strengths and d² is the square of the distance between them. Thus it can be seen that the force on an object entering a magnetic scanner room increases greatly as the distance to the scanner decreases. This is an example of an ‘inverse square law’. Further examples of the law, which is particularly important in radiography, are provided in Chapter 26.

The direction of a magnetic field is taken as the direction in which a hypothetical isolated magnetic north pole would move if placed in the field. Hence lines of magnetic field pass from the north to the south pole of a permanent magnet, as shown in Figure 8.1 (see page 50).

Figure 8.1 Lines of magnetic field surrounding a permanent bar magnet.

It isn’t correct to talk about the north and south poles of an MRI scanner. Such scanners produce a very powerful magnetic field in the z-axis (or long axis) of a patient lying within the scanner, and we can talk about a ‘spin-up’ orientation parallel to the field and a ‘spin-down’ orientation antiparallel to it. But the concept of an MRI scanner as a large bar magnet is misleading.

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Tags: Principles and Applications of Radiological Physics

Mar 6, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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