Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 17

The localization of the MR signal within a slice is usually achieved by two processes: phase‐encoding (PE) and frequency‐encoding (FE), each using gradient pulses along orthogonal directions. These pulses encode the MR signal in terms of spatial frequencies. Image acquisition requires multiple repetitions of the basic block of a pulse sequence using a different amplitude of PE pulse each time (Figure 1.7). TR is the time interval between successive repetitions. Image reconstruction is achieved by the mathematical operation of a two‐dimensional (2D) Fourier transform.

Figure 1.7 Simple 2D pulse gradient echo (GRE) sequence showing pulse amplitudes and timings of the components.

The application of a second set of PE gradients in conjunction with the selection of a thicker slab of tissue makes three‐dimensional (3D) imaging possible (Figure 1.8).

Figure 1.8 3D imaging sequence with a second phase‐encode gradient in the slab direction.