(cuts, fractures, slips, falls, crush and lifting injuries), 9% from projectiles, and 6% acoustic (hearing loss) [3] (Figure 1.2).
Figure 1.2 MRI adverse events reported to the FDA. Data from [3].
A significant source of risk from MRI arises when patients have implants, particularly active implanted medical devices (AIMDs), such as cardiac pacemakers or neuro‐stimulators. However, whereas a decade ago, custom might have been pre‐cautionary – not to scan these patients, modern practice is moving towards finding ways to scan whenever there may be significant benefit to the patient. This requires that all MR practitioners have a deeper understanding of the possible interactions between the device, human tissues, and the scanner, and of MR safety in general. That is the purpose of this book, to ensure all MR practitioners have sufficient knowledge to practise safely for the benefit of their patients.
OVERVIEW OF MRI OPERATION
MRI relies upon the properties of nuclear magnetism. The nucleus of an atom consists of subatomic particles: electrically neutral neutrons and positively charged protons. In an atom the electrical charge of the protons is usually balanced by the negative charge of the surrounding electron cloud. MRI concerns the nucleus of hydrogen, mainly as it occurs in water and fat molecules.
Nuclear magnetic resonance
Hydrogen is the simplest element in the universe with the atomic number of one, meaning its nucleus possesses a single proton. The proton is said to exhibit a property known as spin. The consequences of spin only become observable in an externally applied magnetic field (denoted B0) in which the proton spins precess, like spinning tops or gyroscopes, around the direction of B0. In the external field the proton spins must adhere to specific energy levels or quantum basis states (Figure 1.3a). A slight imbalance between the populations of these results in a net magnetization, M0 (Figure 1.3b). M0 can be manipulated by applying the appropriate frequency (or energy) of electromagnetic radiation. This is the Larmor or resonance frequency:
(1.1)
where the subscript “0” means “at resonance”. γ (“gamma bar”) is the gyromagnetic ratio of the hydrogen nucleus. When frequency f is expressed in MHz and B0 in tesla, γ has a value of approximately 42.58 MHz T‐1. This simple relationship underpins all of MRI.
Figure 1.3 Nuclear magnetism: (a) basis state energy differences; (b) formation of macroscopic magnetization M0 from the sum of basis state spin vectors.
The radiofrequency energy is applied as a magnetic field B1 orthogonal to the direction of B0 (Figure 1.4). Whilst B1 is present the magnetization precesses around both B0 and B1 directions, tipping away from the z‐axis (usually head–foot) of the scanner. B1 is applied in a short burst as a RF pulse. The angle of deflection away from the z‐axis is known as the flip angle α. For a simple rectangular shaped RF pulse this is
(1.2)
where tp is the duration of the pulse (in seconds), B1 is the amplitude of the “excitation” pulse (in tesla), and γ = 2π × γ (2.68 × 108 radians s‐1).
Example 1.1 B1 amplitude
What B1 amplitude is required for a 1 ms rectangular shaped RF pulse to produce a flip angle of 90°?
Express α in radians (=
Figure 1.4 Excitation of the macroscopic magnetization M by the B1 RF field.
Once excited, the magnetization recovers towards its initial equilibrium value M0 by two independent relaxation processes: T1 relaxation restores the longitudinal or z‐component of magnetization towards M0; T2 relaxation causes the transverse component, the signal, to decay to zero. T1 and T2 relaxation times vary by tissue type and exhibit changes due to pathology, often increasing where disease or injury is present.
Image formation
Image formation is achieved by varying the value of magnetic field in the z‐ or B0 direction. The field variation is applied by passing electrical pulses through one or more sets of gradient coils, forming the gradient pulses. The gradients, known as Gx, Gy, Gz, are designed to produce linear variations in the z‐component of the magnetic field with respect to the x, y, and z axes (Figure 1.5). In terms of their function in image formation they are known as slice‐select (GSS), phase‐encode (GPE), or frequency‐encode (GFE).
Figure 1.5 Bz from magnetic field gradients Gx and Gy.
Slice selection
By applying a narrow bandwidth B1 pulse, shaped to include a limited range of frequencies, simultaneously with the slice select gradient Gss, the excitation region is restricted to a narrow slice of the patient’s anatomy with a width or thickness:
(1.3)
The slice thickness can be controlled by changing the amplitude of the slice‐select gradient or by changing the bandwidth of the RF pulse. The slice orientation can be selected by using different gradient coils (or a combination of coils for oblique
