Chapter 6 Spin Echo Imaging Methods Perry Sprawls, Ph.D.
Link to Book Table of Contents	Chapter Contents Shown Below

    Introduction And Overview  

Spin echo is the name of the process that uses an RF pulse to produce the echo event. It is also the name for one of the specific imaging methods within the spin echo family of imaging methods; all of which use the spin echo process. We will first discuss the spin echo process and see how an RF pulse can produce an echo event and signal and then consider the spin echo methods.

The Spin Echo Process  

The decay of transverse magnetization (i.e., relaxation) occurs because of dephasing among individual nuclei, as described in Chapter 4.

            Let us recall that 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 moments must be in the same direction within the transverse plane. When a nucleus has a transverse orientation, it is actually precessing or rotating around an axis that is parallel to the magnetic field.

            After the application of a 90˚ excitation pulse, the nuclei have a transverse orientation and are precessing together, or in-phase, around the magnetic field axis. This is the normal precession discussed earlier but flipped into the transverse plane. However, within an individual voxel some nuclei precess or spin faster than others. After a short period of time, the nuclei are not spinning in-phase. As the directions of the nuclei begin to spread, the magnetization of the tissue decreases. A short time later, the nuclei are randomly oriented in the transverse plane; there is no transverse magnetization.

            The two factors that contribute to the de-phasing of the nuclei and the resulting transverse relaxation will now be reviewed again here. One is an exchange 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 and the source of contrast that we want to capture in T2 images. A second factor, which produces relatively rapid dephasing of the nuclei and loss of transverse magnetization, is the inhomogeneity of the magnetic field. Even within a small volume of tissue, the field inhomogeneities are sufficient to produce rapid dephasing. This effect, which is generally unrelated to the 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. We remember that this real relaxation time is designated as T2*. The value of T2* is always much less than the tissue T2 value. As a result, the transverse magnetization disappears before T2 contrast can be formed.

            We are about to discover that spin echo is a process for recovering the lost transverse magnetization and making it possible to produce images of the three tissue characteristics, including T2.

            An RF signal is produced whenever there is transverse magnetization. Immediately after an excitation pulse, a so-called free induction decay (FID) signal is produced. The intensity of this signal is proportional to the level of transverse magnetization. Both decay rather rapidly because of the magnetic field inhomogeneities just described. The FID signal is not used in the spin echo methods. It is used in the gradient echo methods to be described in Chapter 7.

            The spin echo process is used to compensate for the dephasing and rapid relaxation caused by the field inhomogeneities and to restore the magnetization to the level that depends only on the tissue T2 characteristics. The sequence of events in the spin echo process is illustrated in Figure 6-1.

Transverse magnetization is produced with a 90˚ RF excitation pulse that flips the longitudinal magnetization into the transverse plane. Immediately following the RF pulse, each voxel is magnetized in the transverse direction. However, because of the local magnetic field inhomogeneities within each voxel, the protons precess at different rates and quickly slip out of phase. This produces the rapid decay characterized by T2* and the associated FID signal. At this time the protons are still rotating in the transverse plane, but they are out of phase.

            If a 180° pulse is applied to the tissue containing these protons, it flips the protons around an axis in the transverse plane; this reverses their direction of rotation as illustrated in Figure 6-2.

This causes the fast protons to be located behind the slower ones. As the faster protons begin to catch up with the slower ones, they regain a common alignment, or come back into phase. This, in turn, causes the transverse magnetization to reappear and form the echo event. However, the magnetization does not grow to the initial value because the relaxation (dephasing) produced by the tissue is not reversible. The rephasing of the protons causes the magnetization to build up to a level determined by the T2 characteristics of the tissue. As soon as the magnetization reaches this maximum, the protons begin to move out of phase again, and the transverse magnetization dissipates. Another 180˚ pulse can be used to produce another rephasing. In fact, this is what is done in multi-echo imaging and will be described later in this chapter.

RF Pulse Sequence  

The different imaging methods are produced by the type (flip angle) and time intervals between the applied RF pulses. The basic pulse sequence for the spin echo method is shown in Figure 6-3. Each cycle begins with a 90° excitation pulse that produces the initial transverse magnetization and a later 180° pulse that rephases the protons to produce the echo event.

            The time between the initial excitation and the echo signal is TE. This is controlled by adjusting the time interval between the 90˚ and the 180˚ pulses, which is 1/2 TE.

The Spin Echo Method  

This method can be used to produce images of the three basic tissue characteristics: PD, T1, and T2. The sensitivity to a specific characteristic is determined by the values selected for the two time intervals or imaging factors, TR and TE.

            The process of creating images with the three types of contrast (PD, T1, and T2) described in the last chapter was a description of the spin echo method. There we saw that the type of image that was produced depended on the values selected for the two protocol factors, TR and TE. We will now review that process with a few more details specifically as it applies to the spin echo method.

Proton Density (PD) Contrast  

PD contrast develops as the longitudinal magnetization approaches its maximum, which is determined by the PD of each specific tissue. Therefore, relatively long TR values are required to produce a PD-weighted image. Short TE values are generally used to reduce T2 contrast contamination and to maintain a relatively high signal intensity.

T1 Contrast  

To produce image contrast based on T1 differences between tissues, two factors must be considered. Since T1 contrast develops during the early growth phase of longitudinal magnetization, relatively short TR values must be used to capture the contrast. The second factor is to preserve the T1 contrast during the time of transverse relaxation. The basic problem is that if T2 contrast is allowed to develop, it generally counteracts T1 contrast. This is because tissues with short T1 values usually have short T2 values. The problem arises because tissues with short T1s are generally bright, whereas tissues with short T2s have reduced brightness when T2 contrast is present. T2 contrast develops during the TE time interval. Therefore, a T1-weighted image is produced by using short TR values and short TE values.

T2 Contrast  

The first step in producing an image with significant T2 contrast is to select a relatively long TR value. This minimizes T1 contrast contamination and the transverse relaxation process begins at a relatively high level of magnetization. Long TE values are then used to allow T2 contrast time to develop.

The spin echo method is the only method that produces true T2 contrast. That is because it is able to rephase the protons and remove the T2* effect.

Multiple Spin Echo  

It is possible to produce a series of echo events within one cycle as illustrated in Figure 6-4.

This is done by applying several 180° pulses after each 90° excitation pulse. The advantage is that echo events with different TE values are produced in one acquisition cycle. Separate images are formed for each TE value. This makes it possible to create both a PD image (short TE) and a T2 image (long T2) in the same acquisition.

            Table 6-1 summarizes the combination of TR and TE values used to produce the three basic image types with the spin echo method. Optimum values of TR and TE for a specific protocol might vary because of considerations for other factors such as image acquisition time, number of slices, etc.

Table 6-1. Selection of TR and TE values to produce the three image types with spin echo method. Values shown are typical but can be varied to some extent to accommodate specific imaging conditions.

Inversion Recovery  

Inversion recovery is a spin echo imaging method used for several specific purposes. One application is to produce a high level of T1 contrast and a second application is to suppress the signals and resulting brightness of fat and fluids. The inversion recovery pulse sequence is obtained by adding an additional 180˚ pulse to the conventional spin echo sequence, as shown in Figure 6-5.

 The pulse is added at the beginning of each cycle where it is applied to the longitudinal magnetization carried over from the previous cycle. Each cycle begins as the 180˚ pulse inverts the direction of the longitudinal magnetization. The regrowth (recovery) of the magnetization starts from a negative (inverted) value, rather than from zero, as in the spin echo method.

            The inversion recovery method, like the spin echo method, uses a 90° excitation pulse to produce transverse magnetization and a final 180° pulse to produce a spin echo signal. That is why it is classified as one of the spin echo, rather than gradient echo, methods. An additional time interval is associated with the inversion recovery pulse sequence. The time between the initial 180˚ pulse and the 90˚ pulse is designated the Time after Inversion (TI). It can be varied by the operator and used as a contrast control.

T1 Contrast 

The principal characteristic of many inversion recovery images is high T1 contrast. This occurs because the total longitudinal relaxation time is increased because it starts from the inverted state. There is more time for the T1 contrast to develop. A T1 image produced by the inversion recovery method is compared to one produced by the spin echo method in Figure 6-6.

Notice the significant difference in contrast. The use of the inversion method for other applications will be discussed in Chapter 8.

 Mind Map Summary

Spin Echo Imaging Methods

            Spin echo is a technique used to produce an echo event by applying a 180˚ RF pulse to the dephased transverse magnetization. This compensates for the dephasing produced by field inhomogeneities and makes it possible to produce images that show the T2 characteristics of tissue. The time to the echo event, TE, is a protocol factor that can be adjusted to produce different weightings to the T2 contrast. When a short TE value is selected, the T2 effect is reduced, and the resulting image will be either a PD or T1-weighted image, depending on the selected TR value.

            It is possible to use a series of 180˚ RF pulses within one cycle to produce multiple echo events, each with a different TE value. Both PD and T2-weighted images can be acquired in the same acquisition.