(AFP) Adiabatic fast passage is a NMR technique of producing rotation of the macroscopic magnetization vector by shifting the frequency of RF energy pulses (or the strength of the magnetic field) through resonance (the Larmor frequency) in a time short compared to the relaxation times. Particularly used for inversion of the spins between high and low energy states with an excess of spins in the higher energy level. A continuous wave NMR technique used in e.g., MR spectroscopy.

(MRS / MRSI - Magnetic Resonance Spectroscopic Imaging) A method using the NMR phenomenon to identify the chemical state of various elements without destroying the sample. MRS therefore provides information about the chemical composition of the tissues and the changes in chemical composition, which may occur with disease processes.
Although MRS is primarily employed as a research tool and has yet to achieve widespread acceptance in routine clinical practice, there is a growing realization that a noninvasive technique, which monitors disease biochemistry can provide important new information for the clinician.
The underlying principle of MRS is that atomic nuclei are surrounded by a cloud of electrons, which very slightly shield the nucleus from any external magnetic field. As the structure of the electron cloud is specific to an individual molecule or compound, then the magnitude of this screening effect is also a characteristic of the chemical environment of individual nuclei.
In view of the fact that the resonant frequency is proportional to the magnetic field that it experiences, it follows that the resonant frequency will be determined not only by the external applied field, but also by the small field shift generated by the electron cloud. This shift in frequency is called the chemical shift (see also Chemical Shift). It should be noted that chemical shift is a very small effect, usually expressed in ppm of the main frequency. In order to resolve the different chemical species, it is therefore necessary to achieve very high levels of homogeneity of the main magnetic field B0. Spectra from humans usually require shimming the magnet to approximately one part in 100. High resolution spectra of liquid samples demand a homogeneity of about one part in 1000.
In addition to the effects of factors such as relaxation times that can affect the NMR signal, as seen in magnetic resonance imaging, effects such as J-modulation or the transfer of magnetization after selective excitation of particular spectral lines can affect the relative strengths of spectral lines.
In the context of human MRS, two nuclei are of particular interest - H-1 and P-31. (PMRS - Proton Magnetic Resonance Spectroscopy) PMRS is mainly employed in studies of the brain where prominent peaks arise from NAA, choline containing compounds, creatine and creatine phosphate, myo-inositol and, if present, lactate; phosphorus 31 MR spectroscopy detects compounds involved in energy metabolism (creatine phosphate, adenosine triphosphate and inorganic phosphate) and certain compounds related to membrane synthesis and degradation. The frequencies of certain lines may also be affected by factors such as the local pH. It is also possible to determine intracellular pH because the inorganic phosphate peak position is pH sensitive.
If the field is uniform over the volume of the sample, "similar" nuclei will contribute a particular frequency component to the detected response signal irrespective of their individual positions in the sample. Since nuclei of different elements resonate at different frequencies, each element in the sample contributes a different frequency component. A chemical analysis can then be conducted by analyzing the MR response signal into its frequency components.

See also Spectroscopy.

Nitroxide radicals (or nitroxyl spin labels) are stable organic compounds with theoretical potential for use as a paramagnetic MRI contrast agent. Similar to gadolinium they have an unpaired electron, a property that provides enhancement in T1 based MRI, and a comparable pharmacokinetic. Depending on their structure and chemical bonding, different nitroxides formula may have the potential for use as cardiovascular imaging agents, to enhance the MR imaging on joints (e.g., dendrimer-linked nitroxides have a strong affinity for cartilage), to evaluate brain tumors and infarction, and as a contrast enhancement agent of body/abdominal NMR imaging. Nitroxides are rapidly enzymatically reduced in tissues to products that do not enhance the NMR signal, which can be a problem for MR imaging. In animal experiments with EPRI (electron paramagnetic resonance imaging), tissue redox studies show differences between tumors and normal tissues, which reflect their respective redox status consistent with the reduction/clearance of nitroxides.

(NMR) Nuclear Magnetic Resonance is a physical phenomenon of the magnetic property of nuclei, which have a positive nuclear spin quantum number.
Under the influence of an external static magnetic field this nuclei will precess about the direction of the magnetic field with an angular frequency (Larmor frequency). Through absorption and emission of RF energy (gradients, RF coils) at the resonance frequency (Larmor equation) and the processing of this raw data by the Fourier transformation - physical, chemical, electronic, and structural information about molecules can be obtained (NMR Magnetic Resonance Spectroscopy, Magnetic Resonance Imaging).

MR imaging techniques in which the image is built up from successive lines through the object. In various schemes, the lines are isolated by oscillating magnetic field gradients or selective excitation, and then the NMR signals from the selected line are encoded for position by detecting the FID or spin echo in the presence of a magnetic field gradient along the line; the Fourier transformation of the detected signal then yields the distribution of emitted NMR signal along the line.