Pulsed-field gradient nuclear magnetic resonance as a tool for ...

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PRICE
Figure 4 An example of a shielded magnetic gradient coil system in an NMR probe head.
Only the coil formers are shown, and the wires can be imagined to be wound around the
slots on the formers. The primary gradient coil produces the constant gradient over the
sample volume which is contained within the rf coils. The shield coil is designed to prevent
the gradient pulse from affecting outside the gradient coils, thereby preventing the generation
of eddy currents adapted from Price et al. Ž 6 ..
In very high gradient systems, the actual
gradient coils must be air or water cooled. The position of the thermocouple is critical for
the accuracy and stability of the temperature control. The inclusion of gradient coils in the
probe head normally makes the probe more difficult to shim.
to obtain the diffusion tensor Žsee
Part 1,
Anisotropic Diffusion . .
Amplifiers
Ideally, we desire infinitely fast rise and fall times
of the gradient pulses. In practice, there are two
factors which limit the maximum current switching
speed; the first is that the power supply voltage
must equal RI LdIdt, where I is the
current and L and R are the load Ži.e.,
gradient
coils plus leads. inductance and resistance, respectively,
and the second is the slew rate Ži.e.,
the maximum rate of change of the output voltage.
of the power supply. Thus, the amplifier used
must have suitable current and voltage parameters
to drive the gradient coil used. Typically rise
and fall times of the gradient pulses are on the
order of 50 s.
Since the current through a gradient coil induces
heating, which in turn results in a change in
gradient coil resistance, the amplitude of the gra-