Instrumentation

9
Instrumentation




After reading this chapter, you will be able to:



  • Differentiate between the different types of magnetism.
  • Understand the differences in MRI scanner design and form-factor.
  • Explain the function of the technical components found inside an MRI scanner.

INTRODUCTION


This chapter critically evaluates equipment design in MRI and examines the components that make up a modern MRI system. As stated in previous chapters, MRI scanning requires a homogenous, powerful magnetic field and a system to transmit and receive pulses of electromagnetic radiation in the RF spectrum (see Chapters 1 and 5). In addition, spatial encoding requires sophisticated manipulation of the static field in three orthogonal planes (see Chapter 5). All MRI scanners must therefore incorporate the following:



  • A powerful magnet to create magnetic field over a 40–50 cm spherical volume
  • A shim system to improve the homogeneity of the magnetic field
  • A gradient system to create linear slopes in field strength in any direction
  • An RF transmission system to generate and transmit pulses of electromagnetic radiation
  • A set of RF receiver coils to detect signal from the patient
  • A computer system to allow input of parameters and displaying images
  • A computer subsystem capable of coordinating the application of RF pulses and gradients and reconstructing the acquired data into images and storing them.

The configuration of the scanner is largely dictated by the clinical or research requirements that it is required to address. Figure 9.1 shows a schematic diagram of the main components of a closed-bore MRI system. The components are arranged in layers, forming concentric cylinders around the magnet bore. Each of these components is evaluated in this chapter, but first let’s examine the meaning of magnetism.

Diagram shows closed-bore MRI scanner having receive coil, patient transport system, to body coil, from receive coil, RF transmitter, RF amplifier, gradient amplifier with main magnet, active shielding, quench pipe, and shim coil all from computer.

Figure 9.1 Closed-bore MRI scanner in axial cross-section revealing the principal components to be arranged in concentric circles, most of them being cylindrical electromagnets.


MAGNETISM


Magnetism is the second most powerful fundamental force of nature [1]. However, it is difficult to determine when humans were first aware of this phenomenon. The prehistoric iron age occurred over 4000 years ago, but there are naturally occurring magnetic minerals such as magnetite that are likely to have been known to earlier civilizations. The writings of Aristotle suggest that Thales of Miletus (600 BCE) was one of the first Greek philosophers to examine ferromagnetism from a scientific viewpoint. An understanding of electromagnetism is first attributed to Hans Christian Øersted in 1820. Øersted accidentally discovered that a compass-needle aligned to an electrical conductor in his laboratory, anticipating Michael Faraday’s work on electromagnetic induction in the 1830s. In the modern world, we know that many chemical elements exhibit magnetic properties, and these are categorized according to a phenomenon known as magnetic susceptibility (see Table 9.1). Magnetic susceptibility, as the name suggests, refers to how susceptible (responsive) a material is to an applied magnetic field, to what extent it is magnetized, and whether it is attracted or repelled by the external field. Differences occur due to the atomic or molecular structure of the material in question, specifically the number of electrons surrounding the atomic nucleus and how they move and spin (Equation (9.1)). The types of magnetism are defined in the following sections [2]:










Equation 9.1
B0 = H0 (1 + x)

B0 is the magnetic field in teslas (T)


H0 is the magnetic intensity in amperes/m

This equation shows the apparent magnetization of an atom. A substance is diamagnetic when x < 0. A substance is paramagnetic when x > 0

Table 9.1 Things to remember – magnetism.

















Paramagnetic substances add to (increase) the applied magnetic field
Super-paramagnetic substances have a magnetic susceptibility that is greater than that of paramagnetic substances but less than that of ferromagnetic materials
Diamagnetic substances slightly oppose (decrease) the applied magnetic field
Diamagnetic effects appear in all substances. However, in materials that possess both diamagnetic and paramagnetic properties, the positive paramagnetic effect is greater than the negative diamagnetic effect, and so the substance appears paramagnetic
Ferromagnetic substances are strongly attracted to, and align with the applied magnetic field. They are permanently magnetized even when the applied field is removed
Moving a conductor through a magnetic field induces an electrical charge in it
Moving electrical charge in a conductor induces a magnetic field around it

Diamagnetism


Diamagnetic compounds are characterized by the fact that they exhibit a weak repulsion to an external magnetic field. This is known as having a small negative magnetic susceptibility. Diamagnetic elements have atoms in which all electrons are evenly paired. As a fast-spinning negatively charged particle, a single unpaired electron induces a powerful magnetic moment. However, when the spins are paired, their magnetic fields cancel each other. This is because the electrons spin in opposite directions. As a result, a diamagnetic material does not retain any net magnetism when removed from the external field and does not possess any magnetic moment of its own. When placed in an external field, the lines of magnetic flux diverge around a diamagnetic material as shown in Figure 9.2. A total of 31 elements are identified as being diamagnetic in the periodic table, including hydrogen and helium, and quite a few metals such as gold, silver, and lead.

Diagram shows effect of diamagnetic substance on homogenous magnetic field with diamagnetic substance, and diamagnetic substance in magnetic field.

Figure 9.2 Effect of a diamagnetic substance on a homogenous magnetic field.


Paramagnetism


Paramagnetic compounds are characterized by the fact that they exhibit a weak attraction to an external magnetic field. This is known as having a small positive magnetic susceptibility. Paramagnetic elements increase the strength of an external magnetic field into which they are introduced. The effect is due to the presence of unpaired electrons, which, as stated above, generates a net magnetic moment. However, on removal from an external field, the electron paths lose alignment; the paramagnetic material does not retain any net magnetism, and does not possess any magnetic moment of its own. When placed in an external field, the lines of magnetic flux converge toward a paramagnetic object as shown in Figure 9.3. The periodic table lists 29 paramagnetic elements including calcium, oxygen, and many metals including aluminum, titanium, and platinum.

Diagram shows effect of diamagnetic substance on homogenous magnetic field with paramagnetic substance, and paramagnetic substance in magnetic field.

Figure 9.3 Effect of a paramagnetic substance on a homogenous magnetic field.


Ferromagnetism


Ferromagnetic compounds are said to have a large positive magnetic susceptibility and are powerfully attracted to an external magnetic field. This is a safety concern in MRI from a projectile hazard perspective (see Chapter 10). Magnetic domains (whereby the atomic magnetic moments are aligned parallel by an external magnetic field) cause ferromagnetic elements to retain their magnetic moments when removed from an external field. As can be seen in Figure 9.4, the flux lines of an external magnetic field are powerfully distorted by a ferromagnetic object, and this causes geometric distortion of images in MRI patients with ferromagnetic implants. There are just four naturally occurring elements that are ferromagnetic at a normal ambient temperature; iron, nickel, cobalt, and gadolinium.

Diagram shows ferromagnetic substance in homogenous magnetic field having ferromagnetic substance, substance in magnetic field, and substance on removal from external magnetic field.

Figure 9.4 Ferromagnetic substance in a homogenous magnetic field.


SCANNER CONFIGURATIONS


There are currently three main types of scanner configuration in clinical use [3]:



  • Closed-bore systems
  • Open systems
  • Extremity systems.

Closed-bore systems


Closed-bore systems are the most popular type of MRI scanner worldwide. They feature the familiar tunnel-shaped magnet bore and resemble, in shape, a larger version of a computed tomography (CT) scanner. Longitudinal table movement allows the patient to be positioned with the region of interest lying at the center of the magnet bore. This encloses the patient to the front, back, and sides, but still allows limited access. Patients having lower extremity scans may be positioned feet first, which permits most of the body to remain outside of the bore. Closed-bore scanners generate the main magnetic field using toroidal superconducting solenoid electromagnets positioned in circumference to the cylindrical bore. This type of scanner can generate very high magnetic field strengths, typically between 1 and 3 T for clinical use and up to 8 T (and above) for research studies. The highest field currently generated by this type of magnet for live-animal research is 21 T.


Open systems


Open systems have a different design, whereby the patient is positioned on a wider imaging table that is maneuvered between two magnetic poles that are located above and below the imaging volume. This only encloses the patient above and below, leaving a relatively unobstructed view from all sides. This is advantageous when scanning large animals, humans having a large habitus (broad or obese), and nervous/claustrophobic patients (such as small children), who may find the open access more tolerable. The design also facilitates easy side access to the patient by clinicians when undertaking interventions such as biopsies. Importantly, these scanners also permit a degree of sideways table movement. This is very useful when imaging lateral body structures such as the shoulder or elbow, as it allows the region of interest to be positioned closer to the isocenter of the magnet rather than at the edge of the imaging volume where there may be poorer field homogeneity. Flexion and extension views of the spine are also possible as patients have the space to adopt positions that are not possible in the confines of a closed-bore scanner. At least one manufacturer offers upright open MRI systems that permit weight-bearing examinations. Open scanners use large permanent magnets or superconducting solenoids to generate the main magnetic field. The maximum currently available field strength for an open superconducting MRI system is 1.2 T.


imagesRefer to animation 9.1 on the supporting companion website for this book: www.wiley.com/go/westbrook/mriinpractice


Extremity systems


Extremity scanners, as the name suggests, are designed to scan limbs and are smaller in size than their whole-body counterparts. The typical design is approximately the size and shape of a domestic washing machine having a narrow aperture in the center that is large enough to accommodate an arm or leg. Slightly larger models are the size of a fluoroscopy unit and may be angled to allow weight-bearing views of the spine, hips, and knees. The magnetic field is typically generated by permanent magnets and is therefore restricted to below 1 T. This has certain negative trade-offs in terms of scan time and image quality but offers advantages too. The small physical size of the scanner and reduced magnetic fringe field means that they can be located in small rooms and offices. They are also cheaper to purchase, and running costs may be lower, as permanent magnets do not require electrical power or liquid helium fills to maintain the magnetic field.


The following sections of this chapter examine the components of closed-bore superconducting MRI scanners, but be aware that many of the same principles apply to open systems, particularly those using solenoid electromagnets. The primary difference is that in open systems the solenoids are positioned horizontally above and below the patient (anteroposteriorly) rather than inferosuperiorly (see Figure 9.5).

Image described by caption and surrounding text.

Figure 9.5 Differences in solenoid configuration in (a) closed-bore and (b) open MRI scanners.


MAGNET SYSTEM


Creating the magnetic field required for high-quality anatomical imaging is a demanding task. There are six main requirements, each of which presents technological challenges:



  • The field strength (flux density) must be high, typically between 1.0 and 8.0 T.
  • The fringe field having a strength of 0.5 mT (5 G) or greater must not extend outside of safety Zones III and IV, and should ideally be contained within the magnet room (see Chapter 10).
  • The field must be spatially homogenous to a very high degree.
  • The homogeneity must extend over a large spherical imaging volume (40 cm) to accommodate the required anatomical FOVs.
  • The field must be temporally stable. This means that the flux density must not vary over time (for example, if the environmental temperature fluctuates).
  • The weight and bulk of the magnet must be kept at a level that does not pose any problems with installation in a normal imaging department.

Fortunately, modern MRI systems have features that address all these requirements. They are now covered in more detail [4].


Permanent magnets


Permanent magnet MRI scanners do not employ electromagnets. Instead, they are equipped with large discs of a ferromagnetic alloy such as neodymium, boron, and iron, or aluminum, nickel, and cobalt (alnico). Neodymium magnets are also known as rare-earth magnets (despite neodymium being neither “rare” nor “earth”) and are some of the most powerful permanent magnets. The ferromagnetic discs are known as pole shoes and are typically mounted on a yoke that positions them directly above and below the imaging volume (Figure 9.6). The magnetic field is created by the inherent ferromagnetism of the alloy, namely the combined forces of unpaired electrons in the atoms of the metal that create a macroscopic magnetic field. The advantages of this type of magnet are that it does not require electrical power or cryogenic cooling. These advantages are somewhat offset by the fact that these magnets are unable to generate a flux density of more than 0.5 T, are typically very heavy (17 US tons), and cannot be switched off in an emergency. Furthermore, the flux density of a permanent magnet is not stable and may change with the environmental temperature of the magnet room.

Diagram shows design of permanent-magnet open scanner having N and S poles with B0 arrow pointing toward top with flux lines.

Figure 9.6 Typical design of a permanent-magnet open scanner. The flux lines of the static field run vertically in this type of scanner.


Resistive electromagnets


The phenomenon of electromagnetism was first discovered by Hans Christian Øersted in 1820. Øersted observed that a direct current flowing through a conductive wire caused a magnetic field to be induced around the conductor. Together with Michael Faraday’s law of electromagnetic induction, Øersted’s law provides an explanation for the mechanism of electromagnets such as those used in MRI scanners. The direction of the induced lines of magnetic flux is visualized by using what is known as the “right-hand grip rule” as shown in Figure 9.7. This analogy supposes that a conductor, such as the length of a wire, is gripped in the right hand. The thumb indicates the conventional direction of current flow along the wire (+ to − ). The direction of the fingers as they curl around the wire indicates the direction of the induced magnetic field. This model can also be adapted for solenoid magnets. In this case, the fingers represent the direction of the current as it flows through the windings of the solenoid, and the direction of the thumb indicates the direction of the induced magnetic field. If we assume that current flows around the solenoid windings in a clockwise direction, and we stand at the front end of a closed-bore MRI scanner, the flux lines run parallel to the bore, with the north pole of the magnet at the far end. The flux density of these magnets is determined by the number of windings in the solenoids and the magnitude of the current flowing through them (see Equation (1.6)).

Diagram shows right-hand grip rule where human hand holds magnetic field in direction of fingers and current in direction of thumb along with conductor pointing on one side.

Figure 9.7 Right-hand grip rule.


Resistive MRI scanners employ copper-wound solenoids that operate just below normal room temperature. The principal advantage of this type of system is that the field strength can be adjusted and the magnet switched off safely after use. Industrial resistive magnets can achieve an ultra-high field strength; however, they typically feature very narrow magnet bores. To attain a maximum flux density of around 0.4 T, in a solenoid of a size required for human scanning, a current of over 10 kilowatts (kW) is required. Like an electric bar fire, the resistivity in the windings produces significant heat, and water cooling is required to prevent damage to the system (which would otherwise become incandescently hot). This is achieved by siting the solenoid magnets inside a water-filled vessel through which chilled water is constantly circulated. Superconducting magnets were introduced to avoid resistivity issues. These devices use cryogens (coolants) to reduce the temperature of the windings to within 4° of absolute zero (4 kelvin (K)). This enables a substantially higher flux density using a solenoid large enough to fit a patient inside.


Superconducting electromagnets


Superconducting electromagnets create a magnetic field in the same way as a resistive magnet; however, the windings of the solenoid are spun from a type of metal alloy that is superconductive [5] (typically niobium/titanium). This means that the resistivity of the metal decreases to zero when the metal is cooled below a certain critical temperature (known as the transition temperature). To understand how this affects the design of the MRI scanner, it is first necessary to describe how the cooling system works.


Cryostat


The term cryostat is derived from the Greek words meaning “cold” and “stable.” The cryostat is a somewhat larger version of a thermal vacuum flask that you might use to keep your wine chilled. The cryostat contains the cryogen liquid helium, which has a boiling point of just 4.2 K (−268.9 °C). The primary function of the cryostat is to prevent heat transfer from the adjacent system components (particularly the gradient coils) to the cryogen. This thermal insulation reduces the rate at which the liquid helium boils off to the atmosphere.


The physical construction of a cryostat is shown in Figure 9.8. The outer structure consists of a hollow cylindrical steel tank. This is almost entirely seal-welded except for an aperture through which the helium is filled and through which also passes an atmospheric exhaust vent (quench pipe). The entire outer tank is evacuated of air, which largely reduces heat transfer by thermal convection. On top of the outer shell of the cryostat is a refrigeration unit that chills the metal superstructure of the cryostat, helping to prevent heat transfer by conduction. The area inside the cylinder of the cryostat is known as the warm bore. This contains not only the patient bore but also the components of the MRI system that operate at room temperature.

Diagram shows construction of MRI cryostat with inner heat shield, main magnet solenoid, liquid helium, bucking coil, aluminum cryogen chamber, wide-bore cryogen vent, et cetera.

Figure 9.8 Construction of an MRI cryostat.


Inside this outer tank is a similarly shaped secondary cryogen chamber constructed from aluminum. In Figure 9.8, the cryogen chamber is shown half removed from the steel cryostat for clarity. The outside wall of the cryogen chamber is swathed in layers of aluminized polyester sheeting with insulating spacers. This highly reflective insulating material is familiar to anyone who has seen a space-blanket used to protect patients (or marathon runners) from hypothermia. The term “space-blanket” refers to the fact that the material was originally developed by NASA (National Aeronautics Space Administration) to provide an insulation layer in space-suits. This was necessary to protect the astronauts from extremes in temperature during the moon landings. Its highly reflective surface forms a very efficient heat shield that prevents heat transfer by thermal radiation. This combination of features considerably reduces heat transfer, thereby the helium boil-off rate. Many modern scanners also feature a helium recondensing or recycling system that further reduces helium loss to a negligible amount [6]. Such scanners are unlikely to require a helium refill during their operational lifetime.


Cryogens – liquid helium


Liquid helium is the cryogen of choice for superconducting magnets because of its extremely low temperature. Achieving a temperature lower than 4 K is very difficult outside a specialized plasma laboratory. Helium is readily available because in some geographical areas, it forms a (small) percentage of natural gas. However, as a finite and diminishing resource, it is important not to waste it. There are currently large extraction plants operating in several countries including the United States of America (USA) and Qatar, where there are sizable underground reserves of fossil fuels. Due to the likely diminishing availability of helium and the inherent risks of quench, current research is exploring superconducting magnets that can operate at higher temperatures (20 K). These use alloys such as yttrium barium copper oxide rather than niobium/titanium [7].


Solenoid magnets


As previously discussed, the main magnetic field is induced using a solenoid magnet. In practice, several separate solenoid segments are employed. The reason for this is that a single, evenly wound, cylindrical solenoid of a suitable length for an MRI scanner is not capable of creating a large enough homogenous imaging volume to achieve the required FOV. To achieve a 40 cm spherical imaging volume, it is necessary to segment the solenoid into sections, each having a certain number of windings (also known as turns). Figure 9.9 shows a model representation of a segmented solenoid magnet of the type used in a closed-bore superconducting MRI scanner. Solenoid magnets are typically wound onto a former (or bobbin). In the model, there are two large circular solenoids at each end of the former with four smaller solenoids positioned along the length of the structure. The end solenoids are responsible for generating the bulk of the main magnetic field, while the four evenly spaced solenoids ensure that the homogenous imaging volume is large enough to cover the requisite 40 cm sphere.

Diagram shows bobbin used to create segmented solenoid electromagnet with bucking coil, main magnet solenoid, solenoid segments.

Figure 9.9 Bobbin used to create a segmented solenoid electromagnet.


Ramping a magnet


Bringing an MRI magnet up to the required field strength involves a simple parallel circuit featuring what is known as a “persistent switch.” The parallel winding is a superconductor with a heating coil positioned around the outside. When heated, the terminals of the solenoid are connected to an external power supply to power up the magnet. When the required field strength is achieved, the heater is turned off, and the persistent switch becomes superconductive. This creates a closed loop of superconductive wire and effectively bypasses the external power supply because the flowing electrons show a preference for the nonresistive circuit (Figure 9.10). The process of energizing an MRI solenoid is called ramping, reflecting the fact that the current is gradually increased during the process. To ramp down the magnet when decommissioning the system, the heater is reactivated on the persistent switch. This diverts the current from the solenoids through a resistor to dissipate the energy. If a large ferromagnetic object becomes lodged inside the MRI scanner, this technique is used to safely remove it without a quench of the system. However, this is only done in a non-emergency event where there is no danger to life or limb (see Chapter 10).

Diagram shows parallel circuit and persistent switch for ramping magnet with power supply ON with room temperature, dump resistor, heater, superconductor, persistent switch on ramping up and ramping down circuits.

Figure 9.10 Parallel circuit and persistent switch used for ramping a magnet.


Field strength (flux density)


Magnetic fields are measured in two main ways. Strictly speaking, magnetic field strength is measured in amperes (A) per m and is given the symbol H. A magnetic field is also visualized as having lines of flux that are seen when iron-filings are sprinkled around a bar magnet. Flux density describes the number of flux lines passing through a given area. In the context of MRI, this is the spherical imaging volume at the isocenter of the bore. Flux density is affected by the magnetic permeability of the medium through which the lines pass. This is seen in passive shielding where the flux lines preferentially pass through a steel plate rather than air (see next section). Although the terms “field strength” and “flux density” are not strictly interchangeable, for the purposes of MRI, they can be considered to have a close relationship. This is because the permeability of the patient and the air in the magnet bore remain constant (see Equation (9.1)). Flux density is measured in teslas and is given the symbol B, and for this reason the main magnetic field of the scanner is known as B0. The field strength (flux density) of an MRI magnet varies according to the scanner design and typically ranges between 0.15 and 8 T. In SI units, the tesla is used to quantify flux density. In the older CGS system, the gauss is used (where 1 T equals 10 000 G – see Chapter 1). Although CGS units are generally being superseded by SI units, it is useful to keep the gauss unit, because it is sometimes considered a more meaningful way to measure lower magnetic field strengths. The earth’s magnetic field is usually around 0.5 G depending on one’s position in relation to the equator (this is also stated as 50 μT or 0.000 05 T).

Mar 9, 2019 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Instrumentation
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