Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 41

Figure 1.26 shows simulations of the electric and magnetic fields generated around an eight‐rung birdcage transmit coil [5]. The magnetic B₁‐field is highly uniform, whilst the electric field (E) is concentrated around the rungs. In air B₁ decreases rapidly beyond the limits of the transmit coil.² B₁ is produced as a pulse consisting of a “carrier” frequency (at the Larmor frequency) multiplied by a shape or envelope (Figure 1.27). The simple rectangular pulses of Equation 1.2 are seldom used in practice and a more general expression for flip angle is

(1.8)

Figure 1.26 Simulated electric (L) and magnetic fields (R) from an eight‐rung birdcage coil. Scale in dB. Source [5], licensee BioMed Central Ltd.

Figure 1.27 RF pulse consisting of the carrier (Larmor) frequency multiplied by a shape function or pulse envelope. The example shown is a truncated sinc (sinx/x) function.

This is equal to the area under the curve of the pulse envelope. Three important points arise:

1 for the same pulse shape and duration, the B1 amplitude is proportional to the flip angle;

2 for the same pulse shape and duration, the B1amplitude required to produce a given flip angle is independent of B0;

3 the peak amplitude of B1 alone is not sufficient to characterize the RF exposure.