Various electronics circuits and human-made or natural phenomena and atomic configurations spontaneously exhibit or can be configured to exhibit resonance. What this means is that when they oscillate, the associated waveform is at a stable fundamental frequency. Nuclear magnetic resonance (NMR) takes place when the isotope of an atom is located within a magnetic field. It oscillates at a specific resonant frequency that depends upon the strength of the magnetic field and physical properties of the nucleus. Varying the strength of the magnetic field will adjust the frequency of the output. The nuclei alternately emit and absorb electromagnetic radiation conforming to characteristic wave phenomena. Measuring the radiation and analyzing the waveforms have permitted theoreticians to observe quantum properties of a number of atomic nuclei.
Nuclear magnetic resonance can be used in imaging techniques because the frequency of the emitted electromagnetic radiation varies with strength of the applied magnetic field. Magnetic resonant imaging (MRI) can be achieved by making a magnetic gradient, then observing and analyzing the changing frequency spectrum of the emitted electromagnetic radiation. Resolution is improved at higher energy levels of the magnetic field, often using superconductors.
Specifically, individual nuclei/protons begin to precess (that is, spin such that the rotation axis describes a cone, with the motion of the rotating body perpendicular to the direction of the torque) at what’s called the Larmor frequency (fo = γBo) when placed in an external magnetic field (typically denoted Bo). At thermal equilibrium, the nuclei/protons (when viewed statistically) tend to prefer existence in lower energy states. This results in a net magnetization (M) aligned with Bo. Hitting the nuclei/protons with energy from a transverse magnetic field (typically denoted B1) at the Larmor frequency causes nuclear magnetic resonance. The B1 field is typically turned on for only a few milliseconds and is called an RF-pulse.
During stimulation into nuclear magnetic resonance, the net magnetization M, originally aligned with Bo, precesses around the Bo direction and develops transverse components (Mxy). After the B1 field goes off, M continues to precess around Bo. A receiver coil sensitive to magnetic flux changes in the transverse plane can then detect the Mxy components as they oscillate at the Larmor frequency. A small current is generated in this coil via the Faraday-Lenz induction principle.
Of course the earth has its own magnetic field. When it is exploited in order to perform NMR, there is a pronounced contrast between data that can be resolved in earth’s field NMR (EFNMR) and traditional instrumentation assembled in the lab without benefit of the earth’s magnetic field. To perform EFNMR, the usual procedure is to briefly apply a strong magnetic pulse, then let the earth’s magnetic field take over. It turns out that low-intensity magnetic fields with reduced frequency output are actually more suitable for certain types of observations whereas instruments not dependent on earth’s magnetic field work better in other applications.
These phenomena have provided researchers a great amount of information regarding the nature and behavior of atomic nuclei. The intensity of the magnetic field can be adjusted and measured and the resulting electromagnetic radiation can be displayed, especially in the frequency domain. The interaction of these metrics provides insight into the composition and behavor of the nuclei that are at the heart of the experiment.
An atom’s intramolecular field influences the resonant frequency in terms of spectral location and distribution. From this data researchers can obtain knowledge of the molecules’ electronic structures. The unique and characteristic magnetic fields associated with individual compounds serve as markers, so they can be used to identify an organic compound that may be obscured within a complex mixture.
NMR spectroscopy is used extensively in biological investigations and for medical diagnosis in lab settings. NMR spectroscopy still has limited usage due to the expense of these huge machines. Because resolution depends upon intensity of the magnetic field, NMR spectroscopy involves liquid helium-cooled superconducting magnets. Lower resolution units work with permanent magnets, and they suffice for ball-park measurement. For serious research, however, very high-strength magnetic fields are required. Magnetic resonance imaging (MRI) is a specialized application of NMR. It is a more benign variant of CAT scan, because it does not use X-rays or ionizing radiation. Instead, magnetic fields, electric field gradiants and RF radiant energy combine to create graphic images of the human body as a diagnostic procedure. (MRI is also applied to inanimate objects, making use of NMR spectroscopy.)
The MR signal is an electrical current induced in a receiver coil following stimulation of the sample by an RF-pulse. The recorded signal takes the form of oscillations at the Larmor frequency but with amplitudes modulated by magnetic field inhomogeneities and other effects. The signals in MR imaging arise from multiple volume elements (called voxels), often from an entire slice (or slab of tissue containing many slices) recorded simultaneously. Each volume element contains different materials with different magnetic properties. The resulting total MR signal is thus the sum of thousands of signals and echoes arising from individual voxels with different amplitudes, frequencies, and phases.
MRI machines sort out the resulting huge array of overlapping and interfering signals using techniques such as frequency-encoding, phase-encoding, variations of signal timing, and by exploiting knowledge of coil locations and sensitivity. Fourier transform methods also allow the data to be sorted out according to differences in frequency and accumulated phase.
The baseline frequency of the MR signal is on the order of 60 MHz, but the spatial and spectral modulation typically spans less than 50-100 kHz. The baseline oscillation near the Larmor frequency acts only as a carrier wave for the much lower frequency imaging information; phase and amplitude modulation carry the information about the sample at hand.
Patients undergoing an MRI procedure invariably comment on the loud noise. This is produced by the magnetic gradient coils as they are continually switched on and off for the purpose of encoding position information. Different tissues are represented by excited atoms returning between pulses to their equilibrium states.
The MRI scanner consists of a main magnet, shim coils that correct for main magnetic field fluctuations and the gradient system, which reveals the resonant signal. Needless to say, the entire ensemble from main magnet to final display is computer controlled with an elaborate user interface. Most MRI magnets, which generate powerful magnetic fields, are superconducting devices, cooled by liquid helium. Lower-strength MRI machines use less powerful permanent magnets.
A variant, magnetic resonance spectroscopy imaging (MRSI), produces not a single frequency as in conventional MRI, but a spectrum of resonant signals. Each of these frequencies excites a different isotope within the individual or tissue sample being examined. Consequently, an extensive chemical profile is developed. One advantage in this procedure is that specific metabolic disorders, especially in the brain, may be diagnosed. Additionally, MRS is valuable in accumulating data on tumor metabolism.
Within a patient or in a tissue sample, MRSI uses spectroscopic techniques to develop local spectra. It provides less resolution than an MRI, but information regarding a wide range of metabolites is made available, greatly enhancing diagnostic capabilities. MRSI equipment requires high magnetic field strengths, so the instrumentation is exceptionally costly, limiting availability.
A new technique is real-time MRI, where moving objects such as the beating heart and pulsing arterial-vascular system are imaged. This activity may be analyzed by the medical professional and actually be observed by the patient. A temporal resolution of 20 msec is possible with linear resolution as fine as 1.5 mm.
Interventional MRI refers to the practice of using this type of imaging simultaneously while undertaking a surgical procedure. (Ferromagnetic surgical tools cannot be used.) Specialized imaging equipment is necessary. Alternately, the surgical procedure is interrupted so that MRI can be performed.
Magnetic resonance guided focused ultrasound is still another technique that has recently emerged. Guided by MRI, ultrasonic energy is focused on a diseased tissue or tumor that is to be eliminated. Localized heat exceeding 150°F is applied, destroying the targeted tissue. Real-time MRI permits the medical professional to accurately focus the heat-producing sonic beam without harming adjacent healthy tissue.
MRI has limitations. For example, sensitivity is deficient in some applications because the population difference between contrasting spin states, which determines magnetic activity, is small at room temperature. One strategy to improve sensitivity is to increase magnetic field strength. This involves greater reliance on superconductivity, more cooling and greater cost. A solution on the horizon is the possibility of higher-temperature superconductivity, which is an objective of intense and ongoing research. If this materializes, the cost of MRI may decline.