Chapter 4 Tissue Magnetization And Relaxation Perry Sprawls, Ph.D.
Link to Book Table of Contents	Chapter Contents Shown Below

Introduction And Overview

 We have considered the behavior of individual nuclei when placed in a magnetic field. MRI depends on the collective, or net, magnetic effect of a large number of nuclei within a specific voxel of tissue. If a voxel of tissue contains more nuclei aligned in one direction than in other directions, the tissue will be temporarily magnetized in that particular direction. This process is illustrated in Figure 4-1. In the absence of a magnetic field, the nuclei are randomly oriented and produce no net magnetic effect. This is the normal state of tissue before being placed in a magnetic field. When the tissue is placed in a magnetic field, and some of the nuclei align with the field, their combined effect is to magnetize the tissue in the direction of the magnetic field. A large arrow, the magnetization vector, is used to indicate the amount and direction of the magnetization. When tissue is placed in a magnetic field, the maximum magnetization that can be produced depends on three factors: (1) the concentration (density) of magnetic nuclei, typically protons, in the tissue voxel; (2) the magnetic sensitivity of the nuclide; and (3) the strength of the magnetic field. Since an imaging magnetic field aligns a very small fraction of the magnetic nuclei, the tissues are never fully magnetized. The amount of tissue magnetization determines the strength of the RF signals emitted by the tissue during an imaging or analytical procedure. This, in turn, affects image quality and imaging time, as explained in Chapter 10.

Figure 4-1. The magnetization of tissue produced by the alignment of magnetic nuclei (protons) in a magnetic field.

            Let us recall that an MR image is an image of magnetized tissue and that the contrast we see is produced by different levels of magnetization that exist in the different tissues at the time when “the picture is snapped.” As we will see in this chapter the level of magnetization at specific times during the imaging process is determined by the three tissue characteristics: proton density (PD), T1, and T2.

            We will now see how these characteristics produce image contrast.

 Tissue Magnetization  

When tissue is placed in a magnetic field, it reaches its maximum magnetization within a few seconds and remains at that level unless it is disturbed by a change in the magnetic field or by pulses of RF energy applied at the resonant frequency. The MRI procedure is a dynamic process in which tissue is cycled through changes in its magnetization during each imaging cycle.

 Magnetic Direction

 The direction of tissue magnetization is specified in reference to the direction of the applied magnetic field, as shown in Figure 4-2. There are two principle directions that tissue is magnetized during the imaging process. Longitudinal magnetization is when the tissue is magnetized in a direction parallel to the direction of the field. Transverse magnetization is when the direction of tissue magnetization is at a 90¡ angle with respect to the direction of the magnetic field and is in the transverse plane.

Figure 4-2. Longitudinal and transverse magnetization.

Magnetic Flipping

 The direction of tissue magnetization can be changed or flipped by applying a pulse of RF energy. This is done many times throughout the imaging process.

 Flip Angle

 The angle the magnetization is flipped is determined by the duration and strength of the RF pulse. Pulses are characterized by their flip angles.

 Pulses with 90° and 180° flip angles are the most common but smaller flip angle pulses are also used in some imaging methods, such as gradient echo imaging.

 The 90° Pulse, Saturation and Excitation

 When a 90° pulse is applied to longitudinal magnetization, it flips it into the transverse plane as shown in Figure 4-3. This has two effects. First, it reduces the longitudinal magnetization to zero, a condition called saturation. It also produces transverse magnetization. As we will soon learn, transverse magnetization is an unstable or excited condition. Therefore, when a 90° pulse is applied to longitudinal magnetization, it produces both saturation of the longitudinal magnetization and a condition of excitation (transverse magnetization).

 

Figure 4-3. The application of a 90° RF pulse to longitudinal magnetization produces saturation of the longitudinal magnetization and creates transverse magnetization, an excited condition.

            The actual direction of magnetization is not limited to longitudinal or transverse. It can exist in any direction. In principle, magnetization can have both longitudinal and transverse components. Since the two components have distinctly different characteristics, we consider them independently.

 Longitudinal Magnetization And Relaxation

 As we have seen, when tissue is placed in a magnetic field, it becomes magnetized in the longitudinal direction. It will remain in this state until the magnetic field is changed or until the magnetization is redirected by the application of an RF pulse. If the magnetization is temporarily redirected by an RF pulse, it will then, over a period of time, return to its original longitudinal position. If we consider only the longitudinal magnetization, it regrows after it has been reduced to zero, or saturated. This regrowth, or recovery, of longitudinal magnetization is the relaxation process, which occurs after saturation. The time required for the longitudinal magnetization to regrow, or relax, depends on characteristics of the material and the strength of the magnetic field.

            Longitudinal magnetization does not grow at a constant rate, but at an exponential rate, as shown in Figure 4-4. An important concept to remember is that the MR image is an image of magnetized tissue with brightness indicating the level of magnetization. During the relaxation process, the level of magnetization is changing. Therefore, the brightness of tissue (if we could see it) is also changing as indicated by the scale on the right of the illustration. Saturation turns the tissue dark and then it recovers brightness during the relaxation period.

Figure 4-4. The growth of longitudinal magnetization (and tissue brightness) during the relaxation process following saturation.

            The characteristic that varies from one type of tissue to another, and can be used to produce image contrast, is the time required for the magnetization to re-grow, or the relaxation time. Because of its exponential nature, it is difficult to determine exactly when the magnetization has reached its maximum. The convention is to specify the relaxation time in terms of the time required for the magnetization to reach 63% of its maximum. This time, the longitudinal relaxation time, is designated T1. The 63% value is used because of mathematical, rather than clinical, considerations. Longitudinal magnetization continues to grow with time, and reaches 87% of its maximum after two T1 intervals, and 95% after three T1 intervals. For practical purposes, the magnetization can be considered fully recovered after approximately three times the T1 value of the specific tissue. We will see later that this must be taken into consideration when setting up an imaging procedure.

 T1 Contrast

The time required for a specific level of longitudinal magnetization regrowth varies from tissue to tissue. Figure 4-5 shows the regrowth of two tissues with different T1 values. In this illustration we watch the intensity of brightness of a voxel of tissue during the relaxation process. Let us recall that the brightness of a tissue (RF signal intensity) is determined by the level of magnetization existing in a voxel of tissue at any instant in time. What we see in an image depends on when we “snap the picture” during the relaxation process. The important thing to notice is that the tissue with the shortest T1 has the highest level of magnetization at any particular time. The clinical significance of this is that tissues with short T1 values will be bright in T1-weighted images.

Figure 4-5. The formation of contrast between two tissues with different T1 values.

            Table 4-1 lists typical T1 values for various tissues. Two materials establish the lower and upper values for the T1 range: fat has a short T1, and fluid falls at the other extreme (long T1). Therefore, in T1-weighted images, fat is generally bright, and fluid [cerebrospinal fluid (CSF), cyst, etc.] is dark. Most other body tissues are within the range between fat and fluid.

            The longitudinal relaxation process involves an interaction between the protons and their immediate molecular environment. The rate of relaxation (T1 value) is related to the naturally occurring molecular motion. The molecular motion is determined by the physical state of the material and the size of the molecules. The relatively rigid structure of solids does not provide an environment for rapid relaxation, which results in long T1 values. Molecular motion in fluids, and fluid-like substances, is more inducible to the relaxation process. In this environment molecular size becomes an important characteristic.

            Relaxation is enhanced by a general matching of the proton resonant frequency and the frequency associated with the molecular motions. Therefore, factors that change either of these two frequencies will generally have an effect on T1 values

 Molecular Size

 Small molecules, such as water, have faster molecular motions than large molecules, such as lipids. The frequencies associated with the molecular motion of water molecules are both higher and more dispersed over a larger range for the larger molecules. This reduces the match between the frequencies of the protons and the frequencies of the molecular environment. This is why water and similar fluids have relatively long T1 values. Larger molecules, which have slower and less dispersed molecular movement, have a better frequency match with the proton resonant frequencies. This enhances the relaxation process and produces short T1 values. Fat is an excellent example of a large molecular structure that exhibits this characteristic.

 Table 4-1. T2 and T1 Values for Various Tissues 

            Tissues generally contain a combination of water and a variety of larger molecules. Some of the water can be in a relatively free state while other water is bound to some of the larger molecules. In general, the T1 value of the tissue is probably affected by the exchange of water between the free and the bound states. When the water is bound to larger molecular structures, it takes on the motion characteristics of the larger molecule. Factors such as a pathologic process, which alters the water composition of tissue, will generally alter the T1 values.

 Magnetic Field Strength Effect

 T1 values depend on the strength of the magnetic field. This is because the field strength affects the resonant frequency of the protons. As field strength is increased, the resonant frequency also increases and becomes less matched to the molecular motion frequencies. This results in an increase in T1 values, as indicated in Table 4-1.

            Let us now combine two factors to create a T1 image as illustrated in Figure 4-6. One factor is that different tissues have different T1 values and rates of regrowth of longitudinal magnetization. This then causes the different tissues to be at different levels of magnetization (brightness) when the picture is snapped during the relaxation period. Here we see the order of tissue brightness is inversely related to T1 values. In principle, the tissues with short T1 values get brighter faster and are at a higher level when the picture is snapped.

Figure 4-6. A T1 image showing the relationship of tissue brightness (signal intensity) to T1 values and level of magnetization during the longitudinal relaxation process.

 Transverse Magnetization And Relaxation

 Transverse magnetization is produced by applying a pulse of RF energy to the magnetized tissue. This is typically done with a 90˚ pulse, which converts longitudinal magnetization into transverse magnetization. Transverse magnetization is an unstable, or excited, condition and quickly decays after the termination of the excitation pulse. The decay of transverse magnetization is also a relaxation process, which can be characterized by specific relaxation times, or T2 values. Different types of tissue have different T2 values that can be used to discriminate among tissues and contribute to image contrast.

            Transverse magnetization is used during the image formation process for two reasons: (1) to develop image contrast based on differences in T2 values; and (2) to generate the RF signals emitted by the tissue. Longitudinal magnetization is an RF silent condition and does not produce any signal. However, transverse magnetization is a spinning magnetic condition within each tissue voxel, and that generates an RF signal. As we will see in the next chapter, each imaging cycle must conclude with transverse magnetization to produce the RF signal used to form the image.

            The characteristics of transverse magnetization and relaxation are quite different from those for the longitudinal direction. A major difference is that transverse magnetization is an unstable condition and the relaxation process results in the decay, or decrease, in magnetization, as shown in Figure 4-7. The T2 value is the time required for 63% of the initial magnetization to dissipate. After one T2, 37% of the initial magnetization is present.

Figure 4-7. The decay of transverse magnetization during the relaxation process and the associated tissue brightness.

T2 Contrast

 The difference in T2 values of tissues is the source of contrast in T2-weighted images. This is illustrated in Figure 4-8. Here we watch two tissues, with different T2 values, during the relaxation process. We see that they are both getting darker with time as the magnetization decays. However, they are not getting darker at the same rate. The tissue with the shorter T2 becomes darker faster leaving the tissue with the longer T2 to be bright at times during the relaxation time.

Figure 4-8. The formation of T2 contrast during the decay of transverse magnetization.

            What we will actually see in a T2-weighted image, as shown in Figure 4-9, depends on the level of magnetization at the time when we snap the picture. The important thing to observe here is that the tissues with long T2 values are bright in T2 images.

Figure 4-9. A T2 image showing the relationship of tissue brightness (signal intensity) to T2 values.

            In general, a T2-weighted image appears to be a reversal of a T1-weighted image. Tissues that are bright in one image are dark in the other image. This is because of a combination of two factors. One factor is that T1 and T2 values are generally related. Even though T2 values are much shorter than T1 values, as shown in Table 4-1, they are somewhat proportional. Tissues with long T1 values usually have long T2 values. The other factor is that the order of brightness in a T2 image is in the same direction as the T2 values. Remember, it was a reversed relationship for T1 images.

            The decay of transverse magnetization (i.e., relaxation) occurs because of a dephasing among individual nuclei (protons) within the individual voxels, as shown in Figure 4-10.

Figure 4-10. The dephasing of protons that produces  transverse magnetization decay.

            Two basic conditions are required for transverse magnetization: (1) the magnetic moments of the nuclei must be oriented in the transverse direction, or plane; and (2) a majority of the magnetic moments must be in the same direction, or in phase, within the transverse plane. When a nucleus has a transverse orientation, it is actually spinning around an axis that is parallel to the magnetic field.

            After the application of a 90˚ pulse, the nuclei have a transverse orientation and are rotating together, or in phase, around the magnetic field axis. This rotation or spin is a result of the normal precession discussed earlier. The precession rate, or resonant frequency, depends on the strength of the magnetic field where the nuclei are located. Nuclei located in field areas with different strengths spin (precess) at different rates. Even within a very small volume of tissue, nuclei are in slightly different magnetic field strengths. As a result, some nuclei spin faster than others. Also, there are interactions (spin-spin interactions) among the spinning nuclei. After a short period of time, the nuclei are not spinning in phase. As the directions of the nuclei begin to spread and they dephase, the magnetization of the tissue decreases. A short time later, the nuclei are randomly oriented in the transverse plane, and there is no transverse magnetization.

Proton Dephasing

 Two major effects contribute to the dephasing of the nuclei and the resulting transverse relaxation. In the imaging process the spin echo technique is used to separate the two sources of dephasing, as we will see in Chapter 6.

T2 Tissue Characteristics

 One effect is the exchange of energy among the spinning nuclei (spin-spin interactions), which results in relatively slow dephasing and loss of magnetization. The rate at which this occurs is determined by characteristics of the tissue. It is this dephasing activity that is characterized by the T2 values as shown in Table 4-1.

 T2* Magnetic Field Effects

 A second effect, which produces relatively rapid dephasing of the nuclei and loss of transverse magnetization, is the inherent inhomogeneity of the magnetic field within each individual voxel. The field inhomogeneities are sufficient to produce rapid dephasing. This effect, which is different from the basic T2 characteristics of the tissue, tends to mask the true relaxation characteristics of the tissue. In other words, the actual transverse magnetization relaxes much faster than the tissue characteristics would indicate. This real relaxation time is designated as T2*. The value of T2* is usually much less than the tissue T2 value, as illustrated in Figure 4-11. Several factors can contribute to field inhomogeneities and to T2* decay. One is the general condition of the magnetic field. Some fields are more homogeneous than others. Another factor is that different tissues or materials in the body might have different magnetic susceptibilities. Susceptibility is a characteristic of a material that determines its ability to become magnetized when it is in a magnetic field. If a region of tissue contains materials with different susceptibilities, this results in a reduction of field homogeneity.

Figure 4-11. Comparison of relaxation produced by the T2 characteristics of tissue and the T2* effects associated with magnetic field inhomogeneities.

Magnetic Susceptibility

 The magnetization of tissue that we have been discussing is a nuclear magnetic effect produced by the alignment of magnetic nuclei in a magnetic field. Other materials can become magnetized by other, non-nuclear effects.

            Many materials are susceptible to magnetic fields and become magnetized when located in fields. The susceptibility of a material is determined by the orbital electrons in the atom rather than the magnetic properties of the nucleus. Significant susceptibility is present only when there are unpaired electrons in the outer orbit.

            There are three general types of materials with respect to magnetic susceptibility: diamagnetic, paramagnetic, and ferromagnetic. The primary characteristic of each type is the amount and direction of magnetization that the material develops when placed in a magnetic field. There are situations when each type plays a role in the MR imaging process.

 Contrast Agents

 The inherent tissue characteristics (PD, T1, and T2) do not always produce adequate contrast for some clinical objectives. It is possible to administer materials (i.e., contrast agents) that will alter the magnetic characteristics within specific tissues or anatomical regions. There are several different types of contrast agents, which will now be considered. Contrast agents used in MRI are generally based on relaxation effects.

 Diamagnetic Materials

 Diamagnetic materials have negative and relatively low magnetic susceptibility. This means that they develop only low levels of magnetization and it is in a direction opposite to the direction of the magnetic field. Although many biological molecules are diamagnetic, this is not a significant factor in MR imaging.

 Paramagnetic Materials

 Paramagnetic materials play an important role in contrast enhancement. They are materials with unpaired electrons that give each atom a permanent magnetic property. In paramagnetic materials each atom is magnetically independent, which distinguishes it from other materials to be discussed later.

            Paramagnetic substances include metal ions such as gadolinium, manganese, iron, and chromium. Other substances such as nitroxide free radicals and molecular oxygen also have paramagnetic properties.

            Gadolinium has seven unpaired electrons in its orbit, which give it a very strong magnetic property. It must be chelated to reduce its toxicity. An example is gadolinium chelated to diethylene triamine penta-acetic acid (GaDTPA).

            When a paramagnetic substance, such as gadolinium, enters an aqueous solution, it affects the relaxation rate of the existing protons. It does not produce a signal itself. In relatively low concentrations, the primary effect is to increase the rate of longitudinal relaxation and shorten the value of T1. In principle, the fluctuating magnetic field from the individual paramagnetic molecules enhances the relaxation rate. The primary result is an increase in signal intensity with T1-weighted images. It is classified as a positive contrast agent.

            Signal intensity will generally increase with the concentration of the paramagnetic agents until a maximum intensity is reached. This intensity is very dependent on the imaging parameters. Higher concentrations will generally produce a reduction of signal intensity. This occurs because the transverse relaxation rate is also increased, which results in a shortening of the T2 value.

 Superparamagnetic Materials

 When materials with unpaired electrons are contained in a crystalline structure, they produce a stronger magnetic effect (susceptibility) in comparison with the independent molecules of a paramagnetic substance. The susceptibility of superparamagnetic materials is several orders of magnitude greater than that of paramagnetic materials. These materials are in the form of small particles. Iron oxide particles are an example.

            The particles produce inhomogeneities in the magnetic field, which results in rapid de-phasing of the protons in the transverse plane and a shortening of T2.

            Superparamagnetic materials in the form of large particles generally reduce signal intensity and are classified as negative contrast agents. When in the form of very small particles, they reduce T1 and increase signal intensity.

 Ferromagnetic Materials

Ferromagnetic is the name applied to iron and only a few other materials that have magnetic properties like iron. These materials have a very high susceptibility and develop a high level of magnetism when placed in a magnetic field.

Mind Map Summary

Tissue Magnetization And Relaxation

            When tissue containing magnetic nuclei, i.e., protons, is placed in a strong magnetic field, the tissue becomes magnetized. It is initially magnetized in the longitudinal direction. However, by applying a pulse of RF energy the magnetization can be flipped into the transverse plane. Both longitudinal and transverse magnetization have characteristics that can be used to develop image contrast. An imaging procedure can be adjusted to display the different types of contrasts.

            When a 90° RF pulse is applied to longitudinal magnetization, it produces two effects. First, it temporarily destroys the longitudinal magnetization, a condition known as saturation. It also produces transverse magnetization, a condition known as excitation because transverse magnetization is an unstable excited state.

            After a saturation pulse is applied, the longitudinal magnetization will recover or regrow, a process known as relaxation. The rate of regrowth is a characteristic of each specific tissue and is described by its T1 value, the longitudinal relaxation time. A tissue with a short T1 will recover its magnetization fast and will appear bright in a T1-weighted image. Tissues with longer T1 values will recover magnetization somewhat slower and will be relatively dark in T1-weighted images.

            Following the production of transverse magnetization by the RF pulse the magnetization begins to decay or relax. The rate of relaxation is a characteristic of each specific tissue and is expressed by the T2 values, the transverse relaxation time. A tissue with a short T2 will lose its transverse magnetization rapidly and will appear relatively dark in T2-weighted images. Tissues and body fluids with long T2 values will retain their transverse magnetization longer and will appear bright in T2-weighted images.