Chapter 5 The Imaging Process Perry Sprawls, Ph.D.
Link to Book Table of Contents	Chapter Contents Shown Below

Introduction And Overview

 The MR imaging process consists of two major functions as shown in Figure 5-1. The first is the acquisition of RF signals from the patient’s body and the second is the mathematical reconstruction of an image from the acquired signals.

Figure 5-1. The two functions, acquisition and reconstruction, that make up the MR image production process.

            In this chapter we will develop a general overview of the imaging process and set the stage for considering the different methods and techniques that are used to produce optimum images for various clinical needs

k Space

 During the acquisition process the signals are collected, digitized, and stored in computer memory in a configuration known as k space. The k space is divided into lines of data that are filled one at a time. One of the general requirements is that the k space must be completely filled before the image reconstruction can be completed. The size of k space (number of lines) is determined by the requirements for image detail and will be discussed in Chapters 9 thru 11.

 Acquisition

 The acquisition process consists of an imaging cycle that is repeated many times. The time required for a complete acquisition is determined by the duration of the cycle multiplied by the number of cycles. The duration of a cycle is TR (Time of Repetition), the adjustable protocol factor that is used to select the different types of image contrast. Also, the number of cycles used in an acquisition is adjustable. The number of cycles depends on the quality of the image that is required. The complete relationship between number of imaging cycles and image quality characteristics is described in Chapter 10.

Reconstruction

The image reconstruction process is usually fast compared to the acquisition process and generally does not require any decisions or adjustments by the operator.

Imaging Protocol  

Each imaging procedure is controlled by a protocol that has been entered into the computer. Issues that must be considered in selecting, modifying, or developing a protocol for a specific clinical procedure include:

• The imaging method to be used
• The image types (PD, T1, T2, etc.)
• Spatial characteristics (slice thickness, number, etc.)
• Detail and visual noise requirements
• Use of selective signal suppression techniques
• Use of artifact reduction techniques

            In the following chapters we will address each of these issues and the specific protocol factors that are used to produce the desired image characteristics.

 Imaging Methods

 There are several different imaging methods that can be used to create MR images. The principal difference among these methods is the sequence in which the RF pulses and gradients are applied during the acquisition process. Therefore, the different methods are often referred to as the different pulse sequences. An overview of the most common methods is shown in Figure 5-2. As we see, the different methods are organized in a hierarchy structure. For each imaging method there is a set of factors that must be adjusted by the user to produce specific image characteristics.

Figure 5-2. The principal spin echo and gradient echo imaging methods. GRandSE, or GRASE, is a combination of the two methods.

            The selection of a specific imaging method and factor values is generally based on requirements for contrast sensitivity to a specific tissue characteristic (PD, T1, T2) and acquisition speed. However, other characteristics such as visual noise and the sensitivity to specific artifacts might vary from method to method.

            All of the imaging methods belong to one or both of the two major families, spin echo or gradient echo. The difference between the two families of methods is the process that is used to create the echo event at the end of each imaging cycle. For the spin echo methods, the echo event is produced by the application of a 180° RF pulse, as will be described in Chapter 6. For the gradient echo methods the event is produced by applying a magnetic field gradient, as described in Chapter 7. Each method has very specific characteristics and applications.

The Imaging Cycle

A common characteristic of all methods is that there are two distinct phases of the image acquisition cycle, as shown in Figure 5-3.

Figure 5-3. The longitudinal and transverse magnetization phases of an imaging cycle. T1 and PD contrast are produced during the longitudinal phase and T2 contrast is produced during the transverse phase.

One phase is associated with longitudinal magnetization and the other with transverse magnetization. In general, T1 contrast is developed during the longitudinal magnetization phase and T2 contrast is developed during the transverse magnetization phase. PD contrast is always present, but becomes most visible when it is not overshadowed by either T1 or T2 contrast. The predominant type of contrast that ultimately appears in the image is determined by the duration of the two phases and the transfer of contrast from the longitudinal phase to the transverse phase.

            The duration of the two phases (longitudinal and transverse) is determined by the selected values of the protocol factors, TR (Time of Repetition) and TE (Time to Echo).

TR

TR is the time interval between the beginning of the longitudinal relaxation, following saturation, and the time at which the longitudinal magnetization is converted to transverse magnetization by the excitation pulse. This is when the picture is snapped relative to the longitudinal magnetization.

            Because the longitudinal relaxation takes a relatively long time, TR is also the duration of the image acquisition cycle or the cycle repetition time (Time of Repetition).

TE

 TE is the time interval between the beginning of transverse relaxation following excitation and when the magnetization is measured to produce image contrast. This happens at the echo event and is when the picture is snapped relative to the transverse magnetization. Therefore, TE is the Time to Echo event.

Excitation

 The transition from the longitudinal magnetization phase to the transverse magnetization phase is produced by applying an RF pulse. This is generally known as the excitation process because the transverse magnetization represents a more unstable or “excited” state than longitudinal magnetization.

The excitation pulse is characterized by a flip angle. A 90˚ excitation pulse converts all of the existing longitudinal magnetization into transverse magnetization. This type of pulse is used in the spin echo methods. However, there are methods that use excitation pulses with flip angles that are less than 90˚. Small flip angles (<90˚) convert only a fraction of the existing longitudinal magnetization into transverse magnetization and are used primarily to reduce acquisition time with the gradient echo methods described in Chapter 7.

The Echo Event and Signals

The transverse magnetization phase terminates with the echo event, which produces the RF signal. This is the signal that is emitted by the tissue and used to form the image. The echo event is produced by applying either an RF pulse or a gradient pulse to the tissue, as will be described in Chapters 6 and 7

Contrast Sensitivity

 In MRI the usual procedure is to select one of the tissue characteristics (PD, T1, T2) and then adjust the imaging process so that it has maximum, or at least adequate, contrast sensitivity for that specific characteristic. This produces an image that is heavily weighted by that characteristic. The contrast sensitivity of the imaging process and the resulting image contrast is determined by the specific imaging method and the combination of imaging protocol factor values, which we will consider in much more detail in later chapters. The discussion in this chapter will be based on the conventional spin echo method that uses only two factors, TR and TE, to control contrast sensitivity. However, it establishes some principles that apply to all methods.

T1 Contrast

During the relaxation (regrowth) of longitudinal magnetization, different tissues will have different levels of magnetization because of their different growth rates, or T1 values. Figure 5-4 compares two tissues with different T1 values.

Figure 5-4. The amount of T1 contrast captured during the longitudinal magnetization phase is determined by the value of TR that is selected by the operator.

            The tissue with the shorter T1 value experiences a faster regrowth of longitudinal magnetization. Therefore, during this period of time it will have a higher level of magnetization, produce a more intense signal, and appear brighter in the image. In T1-weighted images brightness or high signal intensity is associated with short T1 values.

            t the beginning of each imaging cycle, the longitudinal magnetization is reduced to zero (saturation) by an RF pulse, and then allowed to regrow, or relax. This is what happens in the spin echo method. In some other imaging methods, as we will see in the next two chapters, the cycle might begin with either partially saturated or inverted longitudinal magnetization. In all cases, T1 contrast is formed during the regrowth process. At a time determined by the selected TR value, the cycle is terminated and the magnetization value is converted to transverse, measured and displayed as a pixel intensity, or brightness, and a T1-weighted image is produced.

            In principle, at the beginning of each imaging cycle all tissues are dark. As the tissues regain longitudinal magnetization, they become brighter. The brightness, or intensity, with which they appear in the image depends on when during the regrowth process the cycle is terminated and the picture is snapped. This is determined by the selected TR value. When a short TR is used, the regrowth of the longitudinal magnetization is interrupted before it reaches its maximum. This reduces signal intensity and tissue brightness within the image but produces T1 contrast.

            Increasing TR increases signal intensity and brightness up to the point at which magnetization is fully recovered, which is determined by the PD of each tissue. For practical purposes, this occurs when the TR exceeds approximately three times the T1 value for the specific tissues. Although it takes many cycles to form a complete image, the longitudinal magnetization is always measured at the same time in each cycle as determined by the setting of TR.

            To produce a T1-weighted image, a value for TR must be selected to correspond with the time at which T1 contrast is significant between the two tissues. Several factors must be considered in selecting TR. If T1 contrast is represented by the ratio of the tissue magnetization levels, it is at its maximum very early in the relaxation process. However, the low magnetization levels present at that time do not generally produce adequate RF signal levels for many clinical applications. The selection of a longer TR produces greater signal strength but less T1 contrast.

            The selection of TR must be appropriate for the T1 values of the tissues being imaged. If a TR value is selected that is equal to the T1 value of a tissue, the picture will be snapped when the tissue has regained 63% of its magnetization. This represents the time when there is maximum contrast between tissues with small differences in T1 values.

Proton Density (PD) Contrast

 The density, or concentration, of protons in each tissue voxel determines the maximum level of magnetization that can be obtained. Differences in PD among tissues can be used to produce image contrast, as illustrated in Figure 5-5.

Figure 5-5. Proton density (PD) contrast is captured by setting TR to relatively long values. At that time the magnetization is determined by PD and is not T1 as in the earlier part of the cycle.

 Here we see the growth of longitudinal magnetization for two tissues with the same T1 values but different relative PDs. The tissue with the lowest PD (80) reaches a maximum magnetization level that is only 80% that of the other tissue. The difference in magnetization levels at any point in time is because of the difference in PD and is therefore the source of PD contrast.

            Although there is some PD contrast early in the cycle, it is generally quite small in comparison to the T1 contrast.

            The basic difference between T1 contrast and PD contrast is that T1 contrast is produced by the rate of growth (relaxation), and PD contrast is produced by the maximum level to which the magnetization grows. In general, T1 contrast predominates in the early part of the relaxation phase, and PD contrast predominates in the later portion. T1 contrast gradually gives way to PD contrast as magnetization approaches the maximum value. A PD-weighted image is produced by selecting a relatively long TR value so that the image is created or “the picture is snapped” in the later portion of the relaxation phase, where tissue magnetizations approach their maximum values. The TR values at which this occurs depend on the T1 values of the tissues being imaged.

            It was shown earlier that tissue reaches 95% of its magnetization in three T1s. Therefore, a TR value that is at least three times the T1 values for the tissues being imaged produces almost pure PD contrast.

T2 Contrast  

Now let us turn our attention to the transverse phase. During the decay of transverse magnetization, different tissues will have different levels of magnetization because of different decay rates, or T2 values. As shown in Figure 56, tissue with a relatively long T2 value will have a higher level of magnetization, produce a more intense signal, and appear brighter in the image than a tissue with a shorter T2 value.

            Figure 5-6 shows the decay of transverse magnetization for tissues with different T2 values. The tissue with the shortest T2 value loses its magnetization faster than the other tissues.

Figure 5-6. The formation of T2 contrast during the transverse magnetization phase. The amount of T2 contrast captured depends on the selected value of TE, the Time to Echo event.

            The difference in T2 values of tissue can be translated into image contrast. For the purpose of this illustration we assume that the two tissues begin their transverse relaxation with the levels of magnetization determined by the PD. This is the usual case where the PD contrast present at the end of the longitudinal phase carries over to the beginning of the transverse phase. In effect, the transverse phase begins with PD contrast but adds T2 contrast as time elapses. The decay of the magnetization proceeds at different rates because of the different T2 values. The tissue with the longer T2 value maintains a higher level of magnetization than the other tissue and will remain bright longer. The difference in the tissue magnetizations at any point in time represents contrast.

            At the beginning of the cycle there is no T2 contrast, but it develops and increases throughout the relaxation process. At the echo event the magnetization levels are converted into RF signals that are displayed as image pixel brightness; this is the time to echo event (TE) and is selected by the operator. Maximum T2 contrast is generally obtained by using a relatively long TE. However, when a very long TE value is used, the magnetization and the RF signals might be too low to form a useful image. In selecting TE values, a compromise must often be made between T2 contrast and good signal intensity.

            The transverse magnetization characteristics of tissue (T2 values) are, in principle, added to the longitudinal characteristics carried over from the longitudinal phase (e.g., T1 and PD) to form the MR image. Usually we do not want to add T2 contrast to T1 contrast. That is because these two types of contrast oppose each other. Remember in Chapter 1 we saw that tissues that are bright in T1 images are dark in T2 images. This means that if we were to mix T1 and T2 contrast in the same image, one would cancel the other. When setting up a protocol for a T2 image it is necessary to use a long TR (in addition to a long TE) so that no, or very little, T1 contrast carries over to the transverse phase.

Mind Map Summary

The Imaging Process

             The MR imaging process is one of creating contrast among tissues based on their magnetic characteristics. The primary characteristics are proton density (PD), T1, and T2. It is a dynamic activity in which the magnetization levels of the various tissues are undergoing almost constant change. During each imaging cycle there are two distinct magnetization phases: longitudinal and transverse. Different types of contrast are developed in each of these phases.

            After application of a saturation pulse, which reduces the longitudinal magnetization to zero, the magnetization begins to regrow, a process known as relaxation. The rate of regrowth for a specific tissue is determined by that tissue’s T1 value. Tissues with short T1 values grow faster than tissues with long T1 values. During this regrowth, T1 contrast in the form of different levels of magnetization is created among the tissues. This is the contrast that will be displayed in an image if the protocol parameters are set to produce a T1-weighted image. In a T1-weighted image, tissues with short T1 values will be bright. Tissues and fluid with long T1 values will be darker.

            When an RF pulse is applied to longitudinal magnetization, it converts (flips) it to transverse magnetization, an unstable excited magnetic condition that decays with time. This decay process is the transverse magnetization relaxation process. The rate of decay of a specific tissue depends on that tissue’s T2 value. Tissues with short T2 values decay faster than tissues with longer T2 values. When the imaging protocol factors are set to produce a T2-weighted image, tissues with short T2 values will be dark and tissues and fluids with longer T2 values will be bright.