A wide variety of instruments are used for studies of nuclear magnetic resonance. Most of these instruments contain the following three major components: 1) Magnet 2) Probe 3) RF electronics. In a typical NMR spectrometer the magnet is in the shape of a hollow cylinder and the probe is placed within it; the sample is placed in a location such that the probe surrounds the sample. However, the geometry and placement of magnet, probe and sample may be changed if required. For example, in the case of NMR logging, the magnet is inside, the probe surrounds it and a hollow cylindrical region surrounding it is sampled for NMR signals.
For any system at equilibrium, in the absence of a magnetic field, all the nuclear spin states are equally populated and hence there is no net polarization due to the nuclear spins. Therefore, it is necessary to introduce an external magnetic field which leads to preferential population of the lower energy nuclear spin states. The energy differences between the different nuclear spin states are proportional to the strength of the magnetic field. Therefore, higher magnetic fields lead to greater separation between the energy levels and greater polarization at equilibrium. The required magnetic field is usually provided by an external magnet.
For high resolution NMR spectroscopy the magnetic field homogeneity has to be better than 1 ppb (part per billion) over the volume of interest. The homogeneity requirements are usually lower for MRI and NMR relaxometry. The homogeneity of the magnetic field created by the primary magnet is improved by using a set of shims.
High magnetic fields (1 Tesla to 17 Tesla) are generally preferred for high resolution, high sensitivity NMR spectroscopic experiments. In general, higher magnetic fields provide higher signal to noise ratio as well as higher resolution. Most high resolution NMR spectrometers used by chemists and biologists use superconducting magnets. However, NMR spectrometers with lower resolution may use permanent magnets or electromagnets. It is also feasible to carry out certain types of NMR experiments in much weaker fields - in fact many NMR spectroscopic experiments have been conducted using the earth's Magnetic field.
The probe in an NMR spectrometer is responsible for coupling the radio frequency electromagnetic field generated by the RF transmitter to the sample. It is also responsible for detecting the NMR signal (through induction) and passing it to the receiver for amplification and digitization.
In the earliest NMR spectrometers, the probe consisted of a pair of orthogonal coils. One coil was coupled to the RF transmitter and was used to generate a homogeneous RF electromagnetic field over the sample area of interest. Continuous RF or pulses of coherent RF radiation were used to create transverse magnetization in the sample. The precession of this transverse magnetization would cause induction in a second coil known as the receiver coil. The receiver coil was orthogonal to the transmission coil. However, at present, most NMR spectrometers and MRI instruments use a single coil for both transmission and reception.
MR electronics consists of a subsystems for controlling the MR transceiver, magnetic field gradients, field frequency lock, spinner speed, temperature of the sample (VT), and an interface to a computer for receiving controlling instructions and to transmit the digitized data.
Recently it has become possible to consolidate all the essential electronic functions as well as the probe into a single integrated circuit.
Magnetic field gradients
Magnetic field gradients are controlled by an independent subsystem. Magnetic field gradients are generated by passing current through coils of appropriate geometry. Static magnetic field gradients are used for shimming. In addition pulsed magnetic field gradients can be produced based on the instructions in the pulse program.
The MR transceiver contains two major subsystems: The transmitter and the receiver subsystems
Transmitter subsystem: It consists of the RF synthesizers and amplifiers. This subsystem is responsible for generating pulse sequences containing RF pulses of specified frequency, amplitude, phase, shape and duration at specified times. Multiple RF synthesizers are required because many MR experiments require simultaneous application of RF pulses of different frequencies. Earlier MR systems used waveform generators for RF synthesis with subsequent phase modulation. However, more recent systems rely on DDS (direct digital synthesis).
RF switch is responsible for coupling either the Transmitter or the Receiver subsystem to the probe. This ensures that the sensitive receiver subsystem is not overloaded with the high powered RF signal generated by the transmitter system. Also, the receiver is 'blanked' during the transmission and for a short duration afterward.
Receiver subsystem: This consists of the components: Preamplifier, Amplifier, Mixer and the Analog-to-Digital converter (ADC). The Mixer is used to subtract a reference frequency of specified phase from the observed signal, resulting a signal of lower frequency that can be easily digitized.
Field frequency lock
The field frequency lock consists of a negative feedback loop that keeps the magnetic field constant. The superconducting magnets that are commonly used in high resolution NMR spectrometers usually have a drift of a few ppb per day (ppb = parts per billion of the total magnetic field). Many high resolution multidimensional NMR experiments often require more than 1 day of measurement. In order to the ensure that the magnetic field at the sample stays constant, during this time, an additional small magnetic field is created by passing current through a loop of appropriate shape to compensate for the change in magnetic field due to the main (superconducting) magnet. Accurate determination of the required additional magnetic field is carried out by measuring the frequency of a reference nucleus either present in the sample or added to it for the purpose of enabling the field frequency lock. The resonance frequency of the reference nucleus (usually deuterium) is measured; this is a highly sensitive indicator of the effective magnetic field in the sample because the resonance frequency is proportional to the strength of the magnetic field and because frequencies can be measured with very high accuracy. The frequency of the reference nucleus is repeatedly measured at fixed time intervals, and any small decrease in frequency is offset by a small increase in the current in the coils responsible for creating the compensating magnetic field.