Chapter 2 agnetic Resonance Imaging System Components
agnetic Resonance Imaging System Components
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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
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.
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.
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.
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.
High homogeneity is obtained by the process of shimming, as described later.
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.
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.
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.
Figure 2-4. A magnetic field gradient produced by a current in the gradient cell.
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.
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.
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.
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.
Figure 2-6. The three types of RF coils (body,
head, and surface) that are the antennae
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.
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.
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.
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.
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.
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.
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.