Chapter 2 Magnetic Resonance Imaging System Components Perry Sprawls, Ph.D.
Link to Book Table of Contents	Chapter Contents Shown Below

Introduction And Overview  

The MRI system consists of several major components, as shown in Figure 2-1. At this time we will introduce the components and indicate how they work together to create the MR image. The more specific details of the image forming process will be explained in later chapters.

Figure 2-1. The major components of the Magnetic Resonance Imaging System.

            The heart of the MRI system is a large magnet that produces a very strong magnetic field. The patient’s body is placed in the magnetic field during the imaging procedure. The magnetic field produces two distinct effects that work together to create the image.

 Tissue Magnetization  

When the patient is placed in the magnetic field, the tissue becomes temporarily magnetized because of the alignment of the protons, as described previously. This is a very low-level effect that disappears when the patient is removed from the magnetic field. The ability of MRI to distinguish between different types of tissue is based on the fact that different tissues, both normal and pathologic, will become magnetized to different levels or will change their levels of magnetization (i.e., relax) at different rates.

Tissue Resonance

The magnetic field also causes the tissue to “tune in” or resonate at a very specific radio frequency. That is why the procedure is known as magnetic resonance imaging. It is actually certain nuclei, typically protons, within the tissue that resonate. Therefore, the more comprehensive name for the phenomenon that is the basis of both imaging and spectroscopy is nuclear magnetic resonance (NMR).

            In the presence of the strong magnetic field the tissue resonates in the RF range. This causes the tissue to function as a tuned radio receiver and transmitter during the imaging process. The production of an MR image involves two-way radio communication between the tissue in the patient’s body and the equipment.

 The Magnetic Field  

Figure 2-2 shows the general characteristics of a typical magnetic field. At any point within a magnetic field, the two primary characteristics are field direction and field strength.

Figure 2-2. The magnetic field produced by superconducting magnets.

Field Direction

 It will be easier to visualize a magnetic field if it is represented by a series of parallel lines, as shown in Figure 2-2. The arrow on each line indicates the direction of the field. On the surface of the earth, the direction of the earth’s magnetic field is specified with reference to the north and south poles. The north-south designation is generally not applied to magnetic fields used for imaging. Most of the electromagnets used for imaging produce a magnetic field that runs through the bore of the magnet and parallel to the major patient axis. As the magnetic field leaves the bore, it spreads out and encircles the magnet, creating an external fringe field. The external field can be a source of interference with other devices and is usually contained by some form of shielding.

 Field Strength

 Each point within a magnetic field has a particular intensity, or strength. Field strength is expressed either in the units of tesla (T) or gauss (G). The relationship between the two units is that 1.0 T is equal to 10,000 G or 10 kG. At the earth’s surface, the magnetic field is relatively weak and has a strength of less than 1 G. Magnetic field strengths in the range of 0.15 T to 1.5 T are used for imaging. The significance of field strength is considered as we explore the characteristics of MR images and image quality in later chapters.

 Homogeneity

 MRI requires a magnetic field that is very uniform, or homogeneous with respect to strength. Field homogeneity is affected by magnet design, adjustments, and environmental conditions. Imaging generally requires a homogeneity (field uniformity) on the order of a few parts per million (ppm) within the imaging area.

            High homogeneity is obtained by the process of shimming, as described later.

 Magnets

 There are several different types of magnets that can be used to produce the magnetic field. Each has its advantages and disadvantages.

 Superconducting

 Most MRI systems use superconducting magnets. The primary advantage is that a superconducting magnet is capable of producing a much stronger and stable magnetic field than the other two types (resistive and permanent) considered below. A superconducting magnetic is an electromagnet that operates in a superconducting state. A superconductor is an electrical conductor (wire) that has no resistance to the flow of an electrical current. This means that very small superconducting wires can carry very large currents without overheating, which is typical of more conventional conductors like copper. It is the combined ability to construct a magnet with many loops or turns of small wire and then use large currents that makes the strong magnetic fields possible.

            There are two requirements for superconductivity. The conductor or wire must be fabricated from a special alloy and then cooled to a very low temperature. The typical magnet consists of small niobium-titanium (Nb-Ti) wires imbedded in copper. The copper has electrical resistance and actually functions as an insulator around the Nb-Ti superconductors.

            During normal operation the electrical current flows through the superconductor without dissipating any energy or producing heat. If the temperature of the conductor should ever rise above the critical superconducting temperature, the current begins to produce heat and the current is rapidly reduced. This results in the collapse of the magnetic field. This is an undesirable event known as a quench. More details are given in Chapter 15 on safety. Superconducting magnets are cooled with liquid helium. A disadvantage of this magnet technology is that the coolant must be replenished periodically.

            A characteristic of most superconducting magnets is that they are in the form of cylindrical or solenoid coils with the strong field in the internal bore. A potential problem is that the relatively small diameter and the long bore produce claustrophobia in some patients. Superconducting magnetic design is evolving to more open patient environments to reduce this concern.

 Resistive

A resistive type magnet is made from a conventional electrical conductor such as copper. The name “resistive” refers to the inherent electrical resistance that is present in all materials except for superconductors. When a current is passed through a resistive conductor to produce a magnetic field, heat is also produced. This limits this type of magnet to relatively low field strengths.

 Permanent

 It is possible to do MRI with a non-electrical permanent magnet. An obvious advantage is that a permanent magnet does not require either electrical power or coolants for operation. However, this type of magnet is also limited to relatively low field strengths.

            Both resistive and permanent magnets are usually designed to produce vertical magnetic fields that run between the two magnetic poles, as shown in Figure 2-3. Possible advantages include a more open patient environment and less external field than superconducting magnets.

Figure 2-3. The magnetic field produced by typical resistive or permanent magnets.

Gradients

 When the MRI system is in a resting state and not actually producing an image, the magnetic field is quite uniform or homogeneous over the region of the patient’s body. However, during the imaging process the field must be distorted with gradients. A gradient is just a change in field strength from one point to another in the patient’s body. The gradients are produced by a set of gradient coils, which are contained within the magnet assembly. During an imaging procedure the gradients are turned on and off many times. This action produces the sound or noise that comes from the magnet.

             The effect of a gradient is illustrated in Figure 2-4. When a magnet is in a “resting state,” it produces a magnetic field that is uniform or homogenous over most of the patient’s body. In this condition there are no gradients in the field. However, when a gradient coil is turned on by applying an electric current, a gradient or variation in field strength is produced in the magnetic field.

Figure 2-4. A magnetic field gradient produced by a current in the gradient cell.

 Gradient Orientation

 The typical imaging magnet contains three separate sets of gradient coils. These are oriented so that gradients can be produced in the three orthogonal directions (often designated as the x, y, and z directions). Also, two or more of the gradient coils can be used together to produce a gradient in any desired direction.

 Gradient Functions

 The gradients are used to perform many different functions during the image acquisition process. It is the gradients that create the spatial characteristics by producing the slices and voxels that will be described in Chapter 9. The entire family of gradient echo imaging methods uses a gradient to produce the echo event and signal which will be described in Chapter 7. Gradients are also used to produce one type of image contrast (phase contrast angiography) for vascular imaging, as will be described in Chapter 12, and in the functional imaging methods described in Chapter 1.3 Gradients also are used as part of some of the techniques to reduce image artifacts, as will be described in Chapter 14.

 Gradient Strength

 The strength of a gradient is expressed in terms of the change in field strength per unit of distance. The typical units are millitesla per meter (mT/m). The maximum gradient strength that can be produced is a design characteristic of a specific imaging system. High gradient strengths of 20 mT/m or more are required for the optimum performance of some imaging methods.

 Risetime and Slew-Rate

 For certain functions it is necessary for the gradient to be capable of changing rapidly. The risetime is the time required for a gradient to reach its maximum strength. The slew-rate is the rate at which the gradient changes with time. For example, a specific gradient system might have a risetime of 0.20 milliseconds (msec) and a slew-rate of 100 mT/m/msec.

 Eddy Currents

 Eddy currents are electrical currents that are induced or generated in metal structures or conducting materials that are within a changing magnetic field. Since gradients are strong, rapidly changing magnetic fields, they are capable of producing undesirable eddy currents in some of the metal components of the magnet assembly. This is undesirable because the eddy currents create their own magnetic fields that interfere with the imaging process.

            Gradients are designed to minimize eddy currents either with special gradient shielding or electrical circuits that control the gradient currents in a way that compensates for the eddy-current effects.

 Shimming

 One of the requirements for good imaging is a homogeneous magnet field. This is a field in which there is a uniform field strength over the image area. Shimming is the process of adjusting the magnetic field to make it more uniform.

            Inhomogeneities are usually produced by magnetically susceptible materials located in the magnetic field. The presence of these materials produces distortions in the magnetic field that are in the form of inhomogeneities. This can occur in both the internal and external areas of the field. Each time a different patient is placed in the magnetic field, some inhomogeneities are produced. There are many things in the external field, such as building structures and equipment, that can produce inhomogeneities. The problem is that when the external field is distorted, these distortions are also transferred to the internal field where they interfere with the imaging process. Inhomogeneities produce a variety of problems that will be discussed later.

            It is not possible to eliminate all of the sources of inhomogeneities. Therefore, shimming must be used to reduce the inhomogeneities. This is done in several ways. When a magnet is manufactured and installed, some shimming might be done by placing metal shims in appropriate locations. Magnets also contain a set of shim coils. Shimming is produced by adjusting the electrical currents in these coils. General shimming is done by the engineers when a magnet is installed or serviced. Additional shimming is done for individual patients. This is often done automatically by the system.

 Magnetic Field Shielding

 The external magnetic field surrounding the magnet is the possible source of two types of problems. One problem is that the field is subject to distortions by metal objects (building structures, vehicles, etc.) as described previously. These distortions produce inhomogeneities in the internal field. The second problem is that the field can interfere with many types of electronic equipment such as imaging equipment and computers.

            It is a common practice to reduce the size of the external field by installing shielding as shown in Figure 2-5. The principle of magnetic field shielding is to provide a more attractive return path for the external field as it passes from one end of the magnetic field to the other. This is possible because air is not a good magnetic field conductor and can be replaced by more conductive materials, such as iron. There are two types of shielding: passive and active.

Figure 2-5. The principle of magnetic field shielding.

Passive Shielding

 Passive shielding is produced by surrounding the magnet with a structure consisting of relatively large pieces of ferromagnetic materials such as iron. The principle is that the ferromagnetic materials are a more attractive path for the magnetic field than the air. Rather than expanding out from the magnet, the magnetic field is concentrated through the shielding material located near the magnet as shown in Figure 2-5. This reduces the size of the field.

 Active Shielding

 Active shielding is produced by additional coils built into the magnet assembly. They are designed and oriented so that the electrical currents in the coils produce magnetic fields that oppose and reduce the external magnetic field.

 The Radio Frequency System

 The radio frequency (RF) system provides the communications link with the patient’s body for the purpose of producing an image. All medical imaging modalities use some form of radiation (e.g., x-ray, gamma-ray, etc.) or energy (e.g., ultrasound) to transfer the image from the patient’s body.

            The MRI process uses RF signals to transmit the image from the patient’s body. The RF energy used is a form of non-ionizing radiation. The RF pulses that are applied to the patient’s body are absorbed by the tissue and converted to heat. A small amount of the energy is emitted by the body as signals used to produce an image. Actually, the image itself is not formed within and transmitted from the body. The RF signals provide information (data) from which the image is reconstructed by the computer. However, the resulting image is a display of RF signal intensities produced by the different tissues.

 RF Coils

 The RF coils are located within the magnet assembly and relatively close to the patient’s body. These coils function as the antennae for both transmitting signals to and receiving signals from the tissue. There are different coil designs for different anatomical regions (shown in Figure 2-6). The three basic types are body, head, and surface coils. The factors leading to the selection of a specific coil will be considered in Chapter 10. In some applications the same coil is used for both transmitting and receiving; at other times, separate transmitting and receiving coils are used.

Figure 2-6. The three types of RF coils (body, head, and surface) that are the antennae  for transmitting pulses and receiving signals from the patient’s body.

Surface coils are used to receive signals from a relatively small anatomical region to produce better image quality than is possible with the body and head coils. Surface coils can be in the form of single coils or an array of several coils, each with its own receiver circuit operated in a phased array configuration. This configuration produces the high image quality obtained from small coils but with the added advantage of covering a larger anatomical region and faster imaging.

 Transmitter

 The RF transmitter generates the RF energy, which is applied to the coils and then transmitted to the patient’s body. The energy is generated as a series of discrete RF pulses. As we will see in Chapters 6, 7, and 8, the characteristics of an image are determined by the specific sequence of RF pulses.

            The transmitter actually consists of several components, such as RF modulators and power amplifiers, but for our purposes here we will consider it as a unit that produces pulses of RF energy. The transmitters must be capable of producing relatively high power outputs on the order of several thousand watts. The actual RF power required is determined by the strength of the magnetic field. It is actually proportional to the square of the field strength. Therefore, a 1.5 T system might require about nine times more RF power applied to the patient than a 0.5 T system. One important component of the transmitter is a power monitoring circuit. That is a safety feature to prevent excessive power being applied to the patient’s body, as described in Chapter 15.

 Receiver  

A short time after a sequence of RF pulses is transmitted to the patient’s body, the resonating tissue will respond by returning an RF signal. These signals are picked up by the coils and processed by the receiver. The signals are converted into a digital form and transferred to the computer where they are temporarily stored.

 RF Polarization

 The RF system can operate either in a linear or a circularly polarized mode. In the circularly polarized mode, quadrature coils are used. Quadrature coils consist of two coils with a 90˚ separation. This produces both improved excitation efficiency by producing the same effect with half of the RF energy (heating) to the patient, and a better signal-to-noise ratio for the received signals.

RF Shielding

 RF energy that might be in the environment could be picked up by the receiver and interfere with the production of high quality images. There are many sources of stray RF energy, such as fluorescent lights, electric motors, medical equipment, and radio communications devices. The area, or room, in which the patient’s body is located must be shielded against this interference.

            An area can be shielded against external RF signals by surrounding it with an electrically conducted enclosure. Sheet metal and copper screen wire are quite effective for this purpose.

            The principle of RF shielding is that RF signals cannot enter an electrically conductive enclosure. The thickness of the shielding is not a factor—even thin foil is a good shield. The important thing is that the room must be completely enclosed by the shielding material without any holes. The doors into imaging rooms are part of the shielding and should be closed during image acquisition.

 Computer Functions

 A digital computer is an integral part of an MRI system. The production and display of an MR image is a sequence of several specific steps that are controlled and performed by the computer.

 Acquisition Control

 The first step is the acquisition of the RF signals from the patient’s body. This acquisition process consists of many repetitions of an imaging cycle. During each cycle a sequence of RF pulses is transmitted to the body, the gradients are activated, and RF signals are collected. Unfortunately, one imaging cycle does not produce enough signal data to create an image. Therefore, the imaging cycle must be repeated many times to form an image. The time required to acquire images is determined by the duration of the imaging cycle or cycle repetition time—an adjustable factor known as TR—and the number of cycles. The number of cycles used is related to image quality. More cycles generally produce better images. This will be described in much more detail in Chapters 10 and 11.

            Protocols stored in the computer control the acquisition process. The operator can select from many preset protocols for specific clinical procedures or change protocol factors for special applications.

Image Reconstruction

The RF signal data collected during the acquisition phase is not in the form of an image. However, the computer can use the collected data to create or “reconstruct” an image. This is a mathematical process known as a Fourier transformation that is relatively fast and usually does not have a significant effect on total imaging time.

 Image Storage and Retrieval

 The reconstructed images are stored in the computer where they are available for additional processing and viewing. The number of images that can be stored—and available for immediate display—depends on the capacity of the storage media.

 Viewing Control and Post Processing

 The computer is the system component that controls the display of the images. It makes it possible for the user to select specific images and control viewing factors such as windowing (contrast) and zooming (magnification).

            In many applications it is desirable to process the reconstructed images to change their characteristics, to reformat an image or set of images, or to change the display of images to produce specific views of anatomical regions.

            These post-processing (after reconstruction) functions are performed by a computer. In some MRI systems some of the post processing is performed on a work-station computer that is in addition to the computer contained in the MRI system.

Mind Map Summary

Magnetic Resonance Imaging System Components 

The magnetic resonance imaging system consists of several major components that function together to produce images. During the image acquisition process the patient’s body is placed in a strong magnetic field. At each point, the magnetic field has a specific direction. This direction is used as a reference for expressing the direction of tissue magnetization. The strength of a magnetic field is determined by the type and design of the magnet. Superconducting magnets can produce strong magnetic fields. Resistive and permanent magnets are limited to relatively weak field strengths. The homogeneity, or uniformity of field strength is necessary for good imaging.  Homogeneity is reduced by magnetically susceptible materials that come into the field and produce distortions. This can occur in both the external field and within a patient’s body. Shimming is the process of adjusting the magnetic field to make it more homogeneous. This can be achieved by passive shims that are added when a magnet is installed and with active shimming produced by adjusting the currents in the shimming coils.

            Shielding of the magnetic field reduces the size and strength of the external magnetic field and also improves homogeneity by protecting from interference caused by objects in the external field area.

A gradient is an intentional variation in magnetic field strength that is produced by the gradient coils. There are three basic gradient coils that are oriented to produce gradients in the three orthogonal directions. Gradients perform several functions during the image acquisition process. An important characteristic of a gradient, especially for some advanced image procedures, is its strength and how fast it can be turned on and off.

            The MRI process consists of an exchange of RF pulses and signals between the equipment and the patient’s body. This is done through the RF coils that serve as the antenna for transmitting the pulses and receiving the signals. It is necessary to shield the imaging area by enclosing it in a conductive metal (copper) room to block external RF interference.

            The imaging process is controlled by information stored in a computer. The protocols programmed into the computer and selected by the operator guide the imaging process and determine the characteristics of the images. The RF signals collected from the patient’s body during the acquisition process are used by the computer to reconstruct the image.