Gradient amplifiers

NMR microscopy systems are built as add-ons to NMR spectrometers. The general technology of an NMR spectrometer is not the subject of this chapter, only the parts that are of particular importance in NMR microscopy applications, such as the magnets, the shim systems, the gradient systems, the NMR microscopy probes and the gradient amplifiers, will be described in some detail. 

The hardware required for PEG work, although commercially available, is not usually included in the basic version of spectrometers. It consists of a gradient accessory, a gradient amplifier, a shielded probehead equipped with a z-gradient coil and a pulse shaper (which in itself is an integral part of the gradient accessory). 

Equipment typical for use in material imaging is depicted in Fig. 1.1.8. It consists of a solid-state NMR spectrometer console, a high-field wide-bore magnet (4—I4 l ), high-power rf amplifiers (/ = I kW) for H and for other nuclei, also called X nuclei, three gradient amplifiers, one for each space direction, and an rf receiver. One computer... 

Fig. 2.3.1 Block diagram of an NMR imaging spectrometer. The main components are the magnet with gradient coils, transmitter TX, receiver RX, gradient amplifier, and a computer for control of the experiment and for data processing and display. Adapted from [Krel] with permission from Publicis MCD.

The principal instrumental limitations are gradient heating and restrictions on gradient amplifier duty cycle. Many modem gradient coils are, however, actively cooled by either water or cold air and driven by amplifiers that can approach a 100% duty cycle for extended periods. ... 

Figure 3.61. Calibration of the gradient amplifier linearity using the sequence of Fig. 3.59b. Assuming system linearity over the full output range and extrapolating the calibration, the maximum gradient strength provided by this system is 0.51 T m (51 G cm ).

This is what a current state-of-the-art gradient amplifier can produce in terms of current. In a typical self-diffusion probe produced by leading manufacturers, this would then give 10-20 T/m as a field gradient strength. 

For self-diffusion measurements, the HT PFG NMR probe is a 10 mm liquid probe (Bruker design) equipped with a gradient coil providing 5.5G/cm/A and with a gradient amplifier of 10 A. The thermal shielding of the radiofrequency and gradient coils is insured by several ceramic barriers. The NMR pulse sequence used here combines stimulated echoes and bipolar gradient pulses [17]. 

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