The terms radiofrequency (RF) pulse sequence and pulse sequence are often used interchangeably. However, RF pulse sequence correctly applies only to the first line of the musical score. Therefore the term pulse sequence is used here to represent not only the RF pulses but also the pulsed gradient magnetic fields and the received MR signals. The basic SE pulse sequence is shown in Figure 16-3.

Often the lines of the musical score are identified as RF_t, G_Z, RF_S, G_Y(_Φ), and G_X(ω) (Figure 16-4, A). These symbols identify images that are acquired in the transverse plane. Other orthogonal and oblique planes are now frequently obtained, so the symbols G_Z, G_Y, and G_X are less meaningful.

In this and successive chapters, the following symbols are used: RF_t, G_SS, RF_s, B_ф, and G_R (Figure 16-4, B). In practice, any of the physical (x, y, or z) gradient coils can perform the functions of slice selection, phase encoding, or freqeuncy encoding. Thus, these symbols are more appropriate for indicating the functional utility of gradient magnetic fields.

The slice selection gradient is represented by G_SS, which can define any of the three orthogonal planes. Oblique images are obtained when G_SS represents any combination of the three gradients energized simultaneously. The phase-encoding gradient magnetic field is G_ф. The gradient magnetic field that is energized while a signal is acquired is G_R, the read gradient. Sometimes G_R is identified as the frequency-encoding gradient magnetic field.

The G_SS is energized to establish a difference in resonance frequency along one axis. Usually, it is the long axis of the patient lying supine if transverse images are required. An RF pulse covering a specified frequency range excites proton spins in a slice of the patient.

The transverse magnetization of the patient (M_XY) is then phase encoded, using G_ф, in a direction perpendicular to the long axis of the patient, along either the anterior-posterior axis (Y) or the lateral axis (X). This phase encoding is accomplished by briefly energizing either the X or Y gradient coils. When the phase-encoding gradient magnetic field is off, all spins precess with the same frequency but now have different phase shifts that depend on position along the phase-encoding gradient.

Next, the remaining gradient magnetic field, G_R, is switched on for a well-defined interval, during which time the MR signal is received. During this time interval, a difference in resonance frequencies along the direction of frequency encoding occurs, causing the transverse magnetization to rotate with different frequency depending on location along that axis (Figure 16-5). This is often termed frequency encoding.

The commonly used reconstruction algorithms can distinguish between phase positions of opposite sign. The phase-encoding gradient has to be of a defined amplitude and time duration to distinguish between adjacent voxels (Figure 16-6).

Unfortunately, with just one phase gradient amplitude, an ambiguous signal is received. The solution, as discussed in Chapters 12 through 14, requires that this pulse sequence be repeated with a number of different phase-encoding amplitudes equal to the required matrix size.

During the time between the 90° and 180° RF pulses, the spin-spin or transverse relaxation causes a dephasing of the transverse magnetization. Other mechanisms, such as magnetic field inhomogeneities, varying magnetic susceptibility of different tissues, and chemical shift, also contribute to the dephasing of M_XY during this time.

The 180° RF refocusing pulse inverts the phase of the spins by putting the faster component of the transverse magnetization behind the slower. This effect should persist over time, except the dephasing caused by nonrandom processes, such as static magnetic field, B₀ inhomogeneity. Inhomogeneity is refocused after the 180° RF pulse to form the SE.

In SE imaging, the spatial frequency domain (k-space) is filled line by line in sequential order from bottom to top (Figure 16-7). This process normally starts with large, negative phase-encoding gradient amplitudes going through zero phase-encoding gradient amplitude and then to a large, positive phase-encoding gradient amplitude.