Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 43

The parameter B₁₊rms is used to characterize the average B₁ exposure. The “+” refers to the rotating component of B₁ responsible for excitation of the magnetization. An efficient coil should not generate a B₁₋. RMS stands for root‐mean‐square and is a type of averaging used for time‐varying waveforms. For example, the RMS value for a sinusoidal waveform is 1/√2 or approximately 0.71 of the peak amplitude (Figure 1.28a). B₁₊rms is defined as

(1.9)

Figure 1.28 (a) the RMS value of a sinusoid is the peak amplitude divided by √2; (b) B₁₊ RMS for a train of N RF pulses of amplitude B₁₊, duration τ within time T.

calculated over 10 second intervals (T=10 s). The easiest way to visualize this is to consider a regular train of N rectangular RF pulses (Figure 1.28b), each of amplitude B₁₊ and duration t_p. In this case

(1.10)

Consequently B₁₊rms depends upon the

 flip angle

 number of RF pulses (echoes, slices, etc.)

 RF pulse shape

 TR.

B₁ is also a time‐varying magnetic field. We can calculate the magnitude of dB/dt for a circulating field B₁₊e^−iωt as³

(1.11)

The rate of change is proportional both to the frequency and the amplitude. As B₁₊ is typically μT and f in MHz, RF dB/dt is of the order of a few tesla per second.