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| Fig. 1: A single stage 2 MeV linear Van de Graaff particle accelerator. A linear particle accelerator is required to accelerate incident ions to high energies. (Source: Wikimedia Commons) |
Nuclear microscopy is a combination of imaging and analytical techniques. Based on a focused beam of accelerated particles, nuclear microscopy has the ability of imaging the morphology of the tissue and of producing the correspondent elemental maps, whether in major, minor, or trace concentrations. [1] Nuclear microscopy consists of three ion beam techniques - Scanning Transmission Ion Microscopy (STIM), Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) - that can be applied simultaneously. Ion beam techniques allows users to modify materials and to obtain elemental information about the studied samples. STIM provides information on the density and structure of the sample; PIXE measures the elemental concentrations of inorganic elements; RBS characterizes the organic matrix. [2] These techniques ability to map and measure quantitative trace element concentrations at the cellular level in unstained cells make it a powerful tool for biomedical studies.
A nuclear microprobe (NMP) is implemented in nuclear microscopy. An NMP is composed of a focusing and scanning system, an irradiation chamber, and adequate detection modules. In a nuclear microprobe, charged particles - typically protons or He+ - are focused using magnetic or electric fields. The charged particles are generated in a small accelerator with accelerating voltages typically between 1.5 and 3 MV. [1] NMP use the ion beam analysis techniques described below as microspies to visualize samples in the micro- to nanometer ranges.
STIM is best suited to study relatively thin samples (30 µm or less). [2,3] It is based on the transmission of incident protons from a 1.5 to 3 MeV proton beam that have not suffered nuclear backscattering collisions. [1] Information on the density structure of the sample is determined through measurement of the energy, or energy loss, of the transmitted proton. The energy loss of transmitted protons depends on the density variations of the sample. Morphology of samples can be obtained by measuring density variations of high-resolution images. Beam spatial resolutions below 100 nm are particularly helpful in the study of tissue morphology and structure since it can identify cell boundaries, being particularly proficient in stratified organs such as the skin. [1]
PIXE is a non-destructive analytical procedure that allows the detection of numerous elements. PIXE analysis uses the detection of characteristic X-rays emitted by the sample elements after the ionization of atomic inner-shells by the incident protons to produce maps of minor and trace elements. When a fast incoming proton knocks out an inner-shell electron from an atom, the inner-shell vacancy will be filled by an electron from the next shell accompanying a single X-ray emitted with energy equal to the difference between the two levels; the emitted X-ray, which has an energy characteristic of the parent atom, can be detected and its energy can be measured. [1] The energy of the X-rays determines the element, and the number of X- rays detected determines the concentration of the sample. PIXE has the capacity to measure elements from sodium onward in the periodic table with high quantitative accuracy. However, detection of X-rays below sodium is not practical because of absorption in the detector window. [1] Overall, PIXE has excellent quantitative precision and analytical sensitivity (110 μg/g on a dry weight basis) for most of the elements detected. [1]
RBS measures the number and energy of elastically backscattered ions, usually H or He ions with energies in the range 14 MeV, to determine the sample stoichiometry and elemental depth distributions. [3] When a fast incoming proton collides with the nucleus of an atom, the backscattered particle rebounds from the sample and is detected by a silicon detector (Fig. 1). In biological tissue, the density and composition of the matrix elements (e.g. C, N, and O) can be estimated by measuring of the energy of the backscattered protons. [2]
The determination of trace elements in biological cells and tissue may aid in disease diagnosis. The techniques outlined above are useful for measuring any imbalances in trace elements in localized regions of biological tissue and can provide unique information on many diseases. [2] Nuclear microscopy has been used for various applications in life science and biomedical research. More recently, it has been used to analyze tissues and cells related to various diseases such as Alzheimers Diseases, Atherosclerosis, and Epilepsy. [2]
© Gigi Nwagbo. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Verissimo et al., Nuclear Microscopy: A Tool for Imaging Elemental Distribution and Percutaneous Absorption In Vivo," Microsc. Res. Techniq. 70, 302 (2007).
[2] M. Q. Ren et al., "Nuclear Microscopy in the Life Sciences at the National University of Singapore," Biol. Trace Elem. Res. 71, 65 (1999).
[3] S. J. Mulware, "The Review of Nuclear Microscopy Techniques: An Approach for Nondestructive Trace Elemental Analysis and Mapping of Biological Materials," J Biophys. 2015, 740751 (2015).
