By Dr. Ir. Marinus T. Vlaardingerbroek, Dr. Ir. Jacques A. den Boer (auth.)
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B ~rf' Fig. 22. a A Maxwell coil pair as a z-gradient coil. 16) where L is the self-inductance and R the (very low) resistance of the conductor. When a steep current ramp is required, so a large dI jdt (low "rise time"), we must choose L to be as low as possible (the voltage U is restricted by practical design problems). However, this implies a small coil with high non-linearity. So in gradient-coil design we have a trade-off between the coil self-inductance and the linearity of the gradient field [17].
Directly following the excitation pulse all spins precess in the same phase with their magnetic moments parallel, adding up to the maximum induced signal. After some time they start to loose their phase coherence due to two effects. First, other spins and the molecular magnetic fields, due to the macro-molecules in the tissue, bB m perturb the total magnetic field experienced by the different free water protons during their fast random (diffusion) motion through these fields. This causes the precession frequency to change per proton in a random way, resulting in dephasing of the proton spins and thus in a decay of the total magnetization.
Of course in the long run some heat still leaks into the magnet, which causes helium evaporation. Another problem is that the magnet current decreases slowly, due to non-zero ohmic losses. This means that the gyromagnetic frequency also decreases slowly and the synthesizer must be tuned to ever lower frequencies in the "initialization phase". This, of course, cannot go on forever, since the frequency can come outside the tuning range of the electronic components, such as RF coils. As a result the magnet current must be brought back to its original value, which is often combined with helium 28 1.
