Keywords
dentistry - metabolism - multiple sclerosis - spectroscopy
Introduction
The electromagnetic spectra have been routinely used in the field of medicine and
dentistry to detect abnormalities and fractures and to observe healing tissues, but
this worthy detection tool comes with a risk of exposing the patients to excessive
radiations. Although X-rays are swift and painless, long-term exposure to their radiations
could cause harmful effects including cellular damage. Many new powerful analytical
tools have been developed in recent years which can deliver precise results with minimal
potential damage to the body tissues.[1] Nuclear magnetic resonance (NMR) was first discovered in the 1940s.[2] The NMR uses the magnetic properties of assured atomic nuclei and is widely being
used in physics and chemistry. In dentistry, this technique is predominantly beneficial
to explore the structure of amorphous glasses and dental cements, bioactive glasses
interaction with oral tissues, identification of salivary metabolites for disease
detection,[3]
[4] and understanding the periodontal diseases by gingival crevicular fluid biomarkers
analysis.[5]
[6] It is also commonly used to review the fluoridation of apatite surfaces in the tooth
structure.[7] Therefore, this review is aimed at providing a general overview of the main principles
of NMR, types of this technique, and the advantages and disadvantages of NMR spectroscopy.
In addition, an insight into the current uses of NMR in the field of medicine and
dentistry and ongoing developments of NMR spectroscopy for future applications is
discussed.
Basic Mechanism of Action of Nuclear Magnetic Resonance
Basic Mechanism of Action of Nuclear Magnetic Resonance
The basic principles of NMR are that the structural and chemical composition of different
substances can be determined by their nuclei, which have their distinctive magnetic
field. The basic NMR spectrometer analyzes using a magnetic field and a special detector
to assess the changes ([Fig. 1]). The strength of the external magnetic field causes electrically charged nucleus
to move from a lower energy level (E1) to a higher energy level (E2) and the difference
between E2 and E1 is symbolized as ΔE which is dependent on the power of the magnetic
field and size of the nuclear field moment.[8]
Fig. 1 Schematic presentation of a typical nuclear magnetic resonance spectrometer showing
the relationship of various components (magnet, magnetic field, and detector).
The electromagnetic radiation rhythm attains the NMR signal with a frequency (v) causing the nuclei to move to a higher energy level (E1/E2). When this electromagnetic
radiation is stopped, it causes the nuclei to relax and accomplish thermal equilibrium.
This release of energy from the nuclei is recorded in the form of spectra on the computer,
and these spectra are exclusive for every nucleus and are equivalent to the energy
levels between the two states (E2/E1)[8] ([Fig. 2]).
Fig. 2 A line in the spectrum is related to a transition between two energy levels (E2 and
E1).[9]
Nuclear Spin
The protons and neutrons of an atom exhibit spin. In some materials, the protons and
neutrons exhibit paired spin such as carbon 12C, oxygen 16O, and sulfur 32S, and in
these cases, they cancel each other causing the nucleus not to spin. In some materials,
the number of protons and neutrons in an atom are unpaired such as in proton 1H, phosphorus 31P, and fluoride 19F.
Applied Magnetic Field and the Nuclear Moment
The spin of the protons and neutrons creates a magnetic moment in the nucleus of an
atom. This magnetic moment could be toward the externally applied magnetic field or
away from it. B0 denotes this applied magnetic field. The more desired direction of
the nuclear moment is toward the magnetic field which is at lower energy level denoted
as α compared with the path opposite to the magnetic field denoted as β5. The magnetic field is accountable for retaining the distinction in the energy levels.
If it does not happen, all the orientations of the spin of 21 + 1 would be of equal
energy.[2]
Magnetic Field Strength
NMR requires a magnetic field that is both strong and uniform. The magnetic field
strength is measured in Tesla or MHz. The NMR requires a reference nucleus to represent
the strength of the magnetic field.
Chemical Shift
The movement of the electrons creates a magnetic field in and around the nucleus.
This magnetic field created is different in the direction as compared with the outer
magnetic field. Any change in the magnetic field causes a similar change in the spectrum
of the NMR. This sum of the shift is controlled by nature of the nucleus and nature
of the motion of electrons in its surrounding atoms and molecules. This phenomenon
is called “chemical shift (CS).” A reference compound is needed to measure CS[9] and to determine and differentiate magnetically inequivalent nuclei present in a
molecule.
Spin–Spin Coupling
The nuclei close to each other induce an incident called spin–spin coupling (SS) due
to the difference in nuclei's magnetic field direction. This direction could be either
toward or opposing the magnetic field, causing the splitting of NMR signals. This
magnetic field direction could either strengthen or fade the signals of NMR signals
that can split into two or more components depending upon the specific nuclei having
characteristic distance and relative potency.[10]
Spin Relaxation (SR)
Spin relaxation is the comeback of energy levels to stability. This occurs due to
the loss of resonance signals with the passage of time after releasing the resonance
frequency. There are two relaxation processes allowing the nuclear spin to return
to stability, which are spin–lattice relaxation and spin–spin relaxation.[11] T
1 represents the time for a specific nucleus to come back to its thermal stability.
Several the elements such as structure, molecular mass, temperature, and the solution
could influence this procedure.[12] SS relaxation happens when energy is lost from the nuclei/loss of signals. The energy
lost is conveyed to nearby spin-active nuclei. SS relaxation time (T
2) is the half-life of this procedure.
Types of Nuclear Magnetic Resonance Spectroscopy
Types of Nuclear Magnetic Resonance Spectroscopy
Solid-state NMRs are used for chemical analysis to recognize any changes in the structure
during phase transitions and different transformations in a solid state. The main
technique frequently used in a solid-state NMR is magic angle spinning (MAS). This
magic angle makes the resolution of the sample more precise by making the broader
lines of the NMR narrower,[13] resulting in narrower signals giving isotropic values and spinning sidebands to
identify the CS of the nuclei for structural determination of solid materials.
Phosphorus Nuclear Magnetic Resonance
In the solid-state NMR, phosphorus is one of the isotopes used to study the molecules
and structures of different samples. Compound classes of phosphorus were identified
which included orthophosphate diesters, polyphosphate, phosphonates, orthophosphate
monoesters, and orthophosphates.[14]
Proton Nuclear Magnetic Resonance
Proton is the initial and the most frequent atom to be used in NMR spectroscopy. It
is also called hydrogen-NMR (1H-NMR) that provides information about the different varieties of hydrogen present
in the molecule and also gives information about its adjacent surroundings.[15] 1H-NMR spectrum of main materials shows small CS range for usual compound is being
studied. This CS ranges from +14 to –14 ppm and a broad difference in extent of coupling
constant was observed.[16]
27Silicon Magic Angle Spinning Nuclear Magnetic Resonance
Silicon is an essential element, and its 29Si isotope, which is used in 29Si-NMR, has 4.70% natural occurrence with the half spin nucleus. It is another spectroscopic
technique used to investigate the structures of organic compounds. Its value of the
magnetic moment is a little low causing a low resonance frequency. The predominance
of 29Si-NMR shifts is present in a range from +50 to –200 ppm.[17]
19Fluorine Magic angle Spinning Nuclear Magnetic Resonance
Isotopes of fluoride are naturally present in very less quantities except for the
19F isotope. F-19 is the only constant isotope of fluorine found in large quantities.
Due to its good nuclear qualities and a great quantity, it is used in 19F MAS NMR technique.19F-NMR technique is very rapid when compared with 1H-NMR technique, and without a doubt, 19F nucleus is one of the most amenable NMR nuclei. Fluorine has a spin of half-nucleus.
Its nucleus in molecules is usually encircled by nine electrons, and its binding energy
is 147,801 keV. The sensitivity of 19F-NMR spectroscopy to its CS (to study the fine details of the local surrounding)
is much high for fluorine, thus making it very reactive to NMR measurements with an
extremely broad CS range.[18] Yesinowski and Mobley[19] verified the capacity of this NMR to differentiate between fluorapatite, fluorohydroxyapatite,
and calcium fluoride in massive phase and also on hydroxyapatite surfaces. 19F-NMR can identify fluoride even in minimal concentrations starting from 0.1%. 19F-NMR technique has also been used to study the metabolism of drugs containing fluoride.[20]
13Carbon Nuclear Magnetic Resonance Spectroscopy
This technique is a significant tool to recognize carbon atoms in any organic material.
It also gives detailed information regarding the chemical structure of the organic
compound being studied. 13C is an isotope of carbon which has a spin quantum number of ½ and is only 1.1% naturally
present, and this isotope can be detected by 13C-NMR. 13C-NMR is less responsive to carbon in view of the fact that the main isotope of carbon
is 12C, which is not magnetically active; therefore, it cannot be detected through this
technique. The intensities of the signals in carbon-NMR are not usually comparative
to the number of corresponding 13C atoms. They are strongly reliant on the numerals of adjacent spins.[21] Magnets utilized in C-NMR have a usual diameter of 10 mm and its usual range of
CS is much larger compared with proton NMR. 13C-NMR can be used to find out the composition of different molecules and is also used
in the drug industry to verify drug purity.
27Aluminum Magic Angle Spinning Nuclear Magnetic Resonance
27Aluminum MAS-NMR has a natural abundance of 100%, and it has a 5/2 nuclear spin. The
nucleus of aluminum is very responsive giving wide lines over the broad range of CS.
The main application of this NMR is to identify the existence of aluminum and to observe
the probable structural changes of the different varieties of aluminum. In a previous
study, 27Al-NMR has been used to observe alteration of Al (IV) into Al (VI) in the setting
glass carbomer cement.[22]
Advantages of Nuclear Magnetic Resonance Spectroscopy
Advantages of Nuclear Magnetic Resonance Spectroscopy
Noninvasiveness
The fundamental quality of NMR is its noninvasiveness. Due to NMR, the studies of
biological cells and tissues are now possible without damaging the sample. The fact
that both spectrum and imaging can be obtained without destroying the sample is noticeably
the greatest advantage of NMR as in in vivo studies.[23]
Lack of Ionizing Radiation
Another major advantage of NMR is its lack of ionizing radiation. Many techniques
are being used in vivo studies involving ionizing radiations. NMR has made it possible
to avoid the exposure from radiation, which could be harmful to both the researcher
and the subject. NMR utilizes isotopes which are stable such as carbon-13 to measure
metabolic fluxes instead of radioactive compounds. NMR not only minimizes the exposure
to observer and subject from the harmful rays but also eliminates the need to dispose
of the radioactive tissues and other materials that might be contaminated during the
study.[1] Thus, the application of NMR can ensure the safety of the employees as well as reduce
experimental cost due to the removal of discarded radioactive substances.
Adjustability
Extensive variety of processes can be investigated through NMR due to the flexibility
of the particular technique that can be applied. This technique not only gives information
about the physiology of the tissues, but it also gives great images of those tissues.
With NMR, a single study can be performed with the same basic technique in both humans
and animals, which is important to increase the translation of information. The computed
tomography (CT) scan technique can only provide the imaging but not the metabolic
or anatomic details,[24] whereas NMR spectroscopy has the capability of acquiring a wide variety of information.
Detailed Structural Analysis
Over the period, NMR has played a major responsibility in determining the mechanisms
and chemical connections at a molecular level. This technique has helped to obtain
information regarding the minute details about the physical and chemical characteristics
of structures.[25] NMR can also analyze the parameters of CS, and it can give details on the local
bonding environment around a particular atom, which could be calculated for the extended
period of times with NMR.[26] It utilizes the pseudo wave function to get information about large compound structures.[27] NMR has the capability to assist studies of biochemical processes conducted in vivo,
which is not efficiently achieved with other imaging techniques. Lee et al[28] proposed that NMR is a better-quality technique as compared with X-ray diffraction
in determining the archaeological bone structure.
Disadvantages of Nuclear Magnetic Resonance Spectroscopy
Disadvantages of Nuclear Magnetic Resonance Spectroscopy
There are a few limitations.
Presence of High Magnetic Field Surroundings Is Essential
An unavoidable outcome when performing the NMR technique is the requirement to perform
in a surrounding which has a high magnetic field. The presence of the magnetic field
can affect the proper functioning of monitors and computer-controlled devices. If
any sharp objects such as a scalpel, scissors, or stapler are present in the NMR magnetic
field area, it can get attracted to the magnet field which can cause severe injuries
to the workers. Nowadays, monitoring devices used in the magnetic field area are being
designed to function properly under the magnetic field.[29] Furthermore, instruments being used in NMR studies are made nonferromagnetic to
reduce the problems encountered with high magnetic field surroundings.[30] NMR system cannot be purchased by a single investigator or for single research because
of its high cost.
Uses in Medicine and Dentistry
Uses in Medicine and Dentistry
CT scan images of the cranium are restricted by artifacts, but this limitation does
not occur with NMR. In the field of medicine, NMR gives the benefit of identifying
pediatric tumors, hematomas, and other pathologies.[31] Since multiple sclerosis is a very tricky disease to identify, NMR has become the
prime diagnostic device for multiple sclerosis.[17] NMR has particular use for certain body areas such as brain where it produces very
detailed and definite images showing delineation between gray and white matter,[32] whereas some tissues such as bone, having low water percentage cannot emit strong
signals to create images for NMR.[33] Moreover, NMR is apparently victorious in identifying breast cancer at an early
stage. According to a radiologist at Cleveland, a mammogram cannot differentiate between
small cancer and a spot, when there are multiple cysts in the breast; however, with
NMR, this distinction is possible.[34] NMR technique also gives good images of fatty tissues, and a large quantity of fat
creates wonderful images. In addition, the diagnosis of vascular diseases is promising
with the NMR[35] as it enables the detailed structural analysis of the surfaces of blood vessels
and their abnormalities.
In dentistry, the aim of treatment is to preserve natural tissue and reconstruct the
loss tissue with the help of biomaterials. These dental biomaterials are studied by
many characterizing machines such as mechanical tester, physical testing, rheological
testing, and biocompatibility testing. For that, NMR spectroscopy is a miracle machine
to understand in-depth chemical reaction of materials ingredients and their effect
with natural tissues. Extensive research on gas ionomer cement (GIC), resin composites,
dental bone cements, and periodontal membranes materials has been conducted using
the NMR spectroscopy. Prosser et al used NMR spectroscopy and reported the role of
tartaric acid in the setting reaction of GIC, was “The fluid cement pastes have shown
that tartaric acid reacts more readily than the polyacid with the glass, and hence
suppresses the premature gelation of the cement.”[36]
The cross-linking of Al in the setting of GIC is very crucial, and upon cement formation,
Al ions in the glass are leached out from the surface layer of the glass, which is
revealed by solid-state NMR spectroscopy.[37] A novel antimicrobial polymeric dental restorative material was experimentally synthesized
to see the biocompatibility, strength, and remineralization property by NMR (1H- and 13C-NMR) spectroscopy.[38] The advancements of proteomics in dentistry have brought a revolution in the management
of oral diseases and analysis of molecular changes during the reconstruction or rehabilitation
of oral tissues (soft and hard) with dental materials.[39] To observe the orthodontically induced external apical root resorption biomarkers,
Zhou et al studied the 1H-NMR-based metabolomics and detected the inflammatory metabolites from saliva samples.[40] This study brings an importance of NMR spectroscopy in the field of clinical dentistry
and dental early diagnosis. Another study on salivary metabolomics has reported the
identification of several metabolic signatures from the control and sarcoidosis patients.[41] The noninvasive, easy, and low-cost sampling of the human saliva attracted it as
a diagnostic oral fluid, and by these omic devices such as NMR, more biomarkers can
be explored.[42]
Conclusions
Considering the potential advantages of the NMR technique, it can be concluded with
authority that it has become a preferred choice of technique for any diagnosis, treatment
planning, maintenance of treatment and also to see the behavior of foreign materials
interaction with the human body. NMR is still a growing technology, and it is being
anticipated that few discoveries are now just around the corner.
Financial Support and Sponsorship
Nil.
