1 Magnetic resonance
Principles and application to diagnostic imaging
Introduction
Nuclear magnetic resonance (NMR), the principle on which magnetic resonance (MR) imaging is based, was discovered independently in 1946 by Felix Bloch and Edward Purcell; they were jointly awarded the Nobel Prize for physics in 1952.1 The first actual image was produced by Paul Lauterbur; although it was grainy and fuzzy, it nevertheless showed the difference between ordinary and ‘heavy’ water (D2O). The importance of MR imaging has been such that he too went on to win a Nobel Prize of his own in 2003.2 The first human image was captured in 1977.3
The name ‘nuclear magnetic resonance’ was altered to ‘magnetic resonance imaging’ in the 1980s. The official reason given was the public’s negative connotation with the word ‘nuclear’, particularly after the accident at the Three Mile Island power station in 1979. The unofficial explanation was that hospital workers feared that NMR might stand for ‘No More Radiographers’, whereas MRI could only mean ‘More Radiographers Immediately’.4
Magnetic resonance imaging uses powerful computer algorithms to generate multiplanar imaging slices through the body.5 Because it does not utilize ionizing radiation and provides a greater degree of contrast between the different soft tissues of the body than does computed tomography (CT), it is particularly useful in cardiovascular, musculoskeletal, neurological and oncological imaging.6 The relative lack of attendant safety issues also means that it is a modality that can be used to monitor the course of a condition or the response to treatment by repeat imaging.6 Although MR imaging is generally safe and non-invasive, there are circumstances in which it is contraindicated; these are detailed in Box 1.01.7
Box 1.01 MRI contraindications or relative contraindications
In place of x-rays, MR imaging uses a powerful magnetic field to align the magnetic moments of certain atoms within the body. Radiofrequency fields are then used to alter the alignment of this magnetization, which causes the atomic nuclei to produce a rotating magnetic field, specific to each tissue type, which is detectable by a receptor inside the scanner. This signal can then be manipulated to build sufficient information to construct an image of the body.4,8
Magnetic resonance
Subatomic particles, such as protons, neutrons, positrons and electrons, have the property of spin, causing the particle to act as a magnetic dipole; that is, having north and south poles.4,9 If two such particles pair up, the laws of magnetic attraction and repulsion mean that they will point in opposite directions and cancel each other out, so that the resultant particle has no overall magnetism. Certain nuclei, such as 1H, which consists of a single proton, 3He, 13C, 23Na and 31P, have an uneven number of protons and neutrons, and therefore have an unpaired particle, giving the nucleus an overall magnetism, known as the magnetic moment.3,4
This effect is particularly strong in hydrogen, which does not have other particles to ‘dilute’ the relative strength of its magnetic moment; it is this nucleus, therefore, that is of primary importance in MR imaging.4,10 Hydrogen has two spin states, sometimes referred to as ‘up’ and ‘down’.9 When these spins are placed in a strong external magnetic field, such as that found in an MRI scanner, they precess around an axis along the direction of the field (Figure 1.01).11 Most protons have low energies and will align with the magnetic field; however, a small number have sufficient energy to have an anti-parallel alignment.11
Figure 1.01 • The magnetic moment of the proton precesses about the flux lines of the applied magnetic field.
The frequency with which the protons precess (ω0) is directly proportional to the strength of the magnetic field (B). In a magnetic field of 1 tesla, of the order found in MR scanners, hydrogen atoms will precess at precisely 42.6 MHz, which is in the radio wave band of the electromagnetic spectrum. This value, which varies from nucleus to nucleus as well as with magnetic field strength, is known as the Larmor frequency.4,11
Energy absorption
Applying a radiofrequency pulse of this value at 90° to the magnetic field causes the hydrogen nuclei, and only the hydrogen nuclei, to resonate; other nuclei – even if they have dipoles – will have their own unique resonance frequency for a given magnetic field. The hydrogen nuclei absorb energy from the pulse, which increases their net energy and, therefore, the number of high-energy anti-parallel protons. If exactly the right amount of energy is applied, this will result in the magnetic fields of the combined hydrogen magnetic moments cancelling each other out; effectively, the net magnetization vector of the hydrogen nuclei lies at 90° to the applied field.4,12
Phase coherence
The applied radiofrequency pulse also causes the rotating magnetic moments of the hydrogen nuclei to move into phase with each other; this is known as phase coherence. It affects all of the nuclei, so the low-energy ‘spin up’ protons are in phase both with each other and with the high-energy ‘spin down’ protons. This means that the net magnetization vector now precesses in exactly the same way as the individual protons at the Lamor frequency.4,12
The MR signal
The image detector in MR imaging is an electromagnetic coil, which acts as a receiver. It is placed at 90° to the applied magnetic field in what is known as the transverse plane; the direction of the magnetic field is the longitudinal plane. Whilst the net magnetization vector lies in the transverse plane, its precession means that it passes across the receiver, inducing a voltage in the coil. This is the MR signal. Because the motion of precession is circular, it induces an alternating current in exactly the same way that a spinning magnet induces an alternating current in a power-generating coil.3,12,13
When the radiofrequency pulse is removed, the protons begin to lose energy, and the difference between the numbers of spin up and spin down protons increases again until the net magnetization vector lies in the longitudinal plane. As this happens, the MR signal decays: electromotive force is only induced by the component of the magnetic field lying perpendicular to it – the longitudinal component of the vector induces no current in the receptor coil. This decrease in the magnitude of the MR signal is called free induction decay (free because it happens when the hydrogen nuclei are ‘free’ of the radiofrequency pulse).12
The removal of the radiofrequency pulse also causes the net magnetization vector to stop precessing. This loss of phase coherence is called transverse (or T2) relaxation whereas the recovery of longitudinal magnetization is called longitudinal (or T1) relaxation.4
