A device that amplifies very low-level signals. A preamplifier is generally placed close to its signal source and has a very low noise figure as it is the principal determinant of electronic noise within the system. Preamplifiers used in NMR systems usually have a 50 ohm input impedance, and require a matching network to interface to the RF coil, although preamplifiers with high input impedance may be used with surface coils. Such devices typically use a field effect transistor (FET) as their input stage.
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The process of adjusting the transmitter and receiver circuitry so that it provides optimal signal performance at the Larmor frequency. A properly tuned scanner will produce images with a higher signal to noise ratio, and therefore improved diagnostic versatility.
More generally, tuning is the process of adjusting the components for optimal performance, including matchingimpedances.
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