NMR Hardware

This educational session lecture introduces the elements of the generic MR system hardware required to obtain images or spectra. The design criteria and function of the magnets, gradients, radiofrequency spectrometer and RF coils are examined. Examples of each, how they are designed and optimized is given (with examples) in the lecture. As far as possible examples related to the topics covered by ICMRM 11 will be given.

Magnets It is most usual for high field, homogeneity and stability magnets to be based on superconductive technology with axially symmetric coil windings. These magnets deliver the highest performance but require cryogenic cooling and are generally not portable (a 7T whole body system is 35 tons). In microscopy and materials applications the desirable parameters may be compromised in order to give portability, light weight or to fit with other constraints. These magnetic fields are then generated by a combination of electromagnet, permanent magnets and other magnetic materials. Magnetic fields may be optimized to give the best homo-geneity or gradient at a particular sweet-spot (which could be outside the magnet itself).

Gradients In order to spatially resolve the benchtop NMR signal then electromagnets are employed to give a (usually) linear profile in Bz with all three (or fewer) spatial axes. Gradient design techniques based on target field and boundary element methods are discussed. Examples of conventional cylindrical gradient design are shown. These design methods can be further exploited to give gradient designs on non-cylindrical geometries or arbitrary former shapes. Gradient design methods can be used to design both shim and pure electromagnet based field profiles.

RF Systems: Spectrometer The radiofrequency (RF) spectrometer is the central control component for the NMR analyzer system and provides system master clock and timings of gradient and RF pulses. The phase and timing stability in a high resolution system is critical and should exceed the magnet in its performance. This level of performance can be achieved if certain design specifications and criteria are followed. The modern spectrometer system is predominantly digital, with analogue components only making up the final parts nearest to the RF coils (i.e. power amplifiers, low-noise pre-amplifiers and transmit-receive switching et el). For example, in the most recent Ingenia body systems from Philips the entire acquisition system is placed on the receive coil itself within the magnet.

RF Systems: Coils After the main field strength, it is the quality and ability of the coil to faithfully pick up the NMR signal from the sample which defines the overall quality of our information. Signals and noise in an NMR experiment is discussed and the importance of optimum noise matching is introduced. Examples of coils which satisfy a range of demands in various geometries are discussed. The role of finite element RF simulation in coil design is demonstrated. This educational session lecture introduces the elements of the generic MR system hardware required to obtain images or spectra. The design criteria and function of the magnets, gradients, radiofrequency spectrometer and RF coils are examined. Examples of each, how they are designed and optimized is given (with examples) in the lecture. As far as possible examples related to the topics covered by ICMRM 11 will be given.

Magnets It is most usual for high field, homogeneity and stability magnets to be based on superconductive technology with axially symmetric coil windings. These magnets deliver the highest performance but require cryogenic cooling and are generally not portable (a 7T whole body system is 35 tons). In microscopy and materials applications the desirable parameters may be compromised in order to give portability, light weight or to fit with other constraints. These magnetic fields are then generated by a combination of electromagnet, permanent magnets and other magnetic materials. Magnetic fields may be optimized to give the best homo-geneity or gradient at a particular sweet-spot (which could be outside the magnet itself).

Gradients In order to spatially resolve the NMR signal then electromagnets are employed to give a (usually) linear profile in Bz with all three (or fewer) spatial axes. Gradient design techniques based on target field and boundary element methods are discussed. Examples of conventional cylindrical gradient design are shown. These design methods can be further exploited to give gradient designs on non-cylindrical geometries or arbitrary former shapes. Gradient design methods can be used to design both shim and pure electromagnet based field profiles.

RF Systems: Spectrometer The radiofrequency (RF) spectrometer is the central control component for the NMR system and provides system master clock and timings of gradient and RF pulses. The phase and timing stability in a high resolution system is critical and should exceed the magnet in its performance. This level of performance can be achieved if certain design specifications and criteria are followed. The modern spectrometer system is predominantly digital, with analogue components only making up the final parts nearest to the RF coils (i.e. power amplifiers, low-noise pre-amplifiers and transmit-receive switching et el). For example, in the most recent Ingenia body systems from Philips the entire acquisition system is placed on the receive coil itself within the magnet.

RF Systems: Coils After the main field strength, it is the quality and ability of the coil to faithfully pick up the NMR signal from the sample which defines the overall quality of our information. Signals and noise in an NMR experiment is discussed and the importance of optimum noise matching is introduced. Examples of coils which satisfy a range of demands in various geometries are discussed. The role of finite element RF simulation in coil design is demonstrated.

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