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Systems and safety: MR hardware and fields

INTRODUCTION

Magnetic resonance imaging (MRI) has grown, from its initial development in the late 1970s to early 1980s, to become one of the most utilized diagnostic imaging modalities. In 2015 there were 103 million MR examinations performed in hospitals from a population of 1.1 billion people in 29 developed countries. A total of 33 000 scanners were in use in 36 countries serving a combined population of 1.7 billion [1].

The two greatest advantages of MRI are its superior soft tissue discrimination compared to X-rays or CT, and a lack of exposure to ionizing radiation. MRI uses a combination of magnetic fields of varying frequencies: radiofrequency in the megahertz (MHz) region; audio or "very low frequencies" (VLF) up to tens of kilohertz (kHz); and a static field (zero hertz). None of these possesses sufficiently localized concentrations of electromagnetic (EM) energy to damage atoms, molecules, or cells (Figure 1.1). The risk of cancer induction from magnetic field exposures encountered in MRI is quite possibly zero - unlike X-rays, CT, mammography, or the radioactive tracers used in nuclear medicine. This makes MRI very attractive for serial examinations, for scans of children whose tissues are more sensitive to the ionizing radiation used in alternative modalities, or for research studies on groups of healthy volunteers.

Figure 1.1 The electromagnetic spectrum showing frequency and wavelength of radiations, relative scale, and applications.

So, is MRI safe?

Obviously not, or there would be no need for this book. Whilst later chapters will show that MRI is relatively benign from a biological point of view, the practice of MRI may involve significant risk to the patient and to others present during the examination. The MRI examination room is potentially the most hazardous environment within the radiology department because of the possibility of catastrophic and fatal accidents where practice is poor or where safety protocols are not fully observed or understood.

Nowhere is this better illustrated than in the tragic case of a six-year old boy who in 2001 was struck by an oxygen tank which had flown into the scanner, later dying from his injuries. This prompted a root and branches review of MR safety practice within the USA by the American College of Radiology [2] leading to a series of recommendations. It is concerning, that even today, not all these recommendations are routinely followed in every institution. In a 10-year review of MRI-related incidents reported to the US Food and Drug Authority (FDA) 59% were thermal (excessive heating, burns), 11% mechanical (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:

where the subscript "0" means "at resonance". ? ("gamma bar") is the gyromagnetic ratio of the hydrogen nucleus. When frequency f is expressed in MHz and B0 in tesla, ? 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 a. For a simple rectangular shaped RF pulse this is

where tp is the duration of the pulse (in seconds), B1 is the amplitude of the "excitation" pulse (in tesla), and ? = 2p × ? (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 a in radians (= ). From Equation 1.2

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:

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 views). By changing the RF frequency images may be acquired as a series of 2D multiple slices at different locations (Figure 1.6).

Figure 1.6 Multiple slice imaging. Changing the frequency of each RF pulse whilst a gradient Gss is applied selects a different slice position.

In-plane localization

The localization of the MR signal within a slice is usually achieved by two processes: phase-encoding (PE) and frequency-encoding (FE), each using gradient pulses along orthogonal directions. These pulses encode the MR signal in terms of spatial frequencies. Image acquisition requires multiple repetitions of the basic block of a pulse sequence using a different amplitude of PE pulse each time (Figure 1.7). TR is the time interval between successive repetitions. Image reconstruction is achieved by the mathematical operation of a two-dimensional (2D) Fourier transform.

Figure 1.7 Simple 2D pulse gradient echo (GRE) sequence showing pulse amplitudes and timings of the...