History
The first theoretical proposal was presented by Emil Wolf, and the first experimental demonstration was shown by Fercher et al. From 2000s, HT techniques had been extensively studied and applied to the field of biology and medicine, by several research groups including the MIT spectroscopy laboratory. Both the technical developments and applications of HT have been significantly advanced. In 2012 the first commercial HT company Nanolive was founded, later followed by Tomocube in 2014.Principles
The principle of HT is very similar toAdvantages and limitations
HT provides following advantages over conventional 3D microscopic techniques. # Label-free: Cellular membrane and subcellular organelles can clearly be imaged without using exogenous labeling agents. Thus, there are no issues of phototoxicity, photobleaching and photodamaging. # Quantitative imaging capability: HT directly measures cell’s 3D RI maps, which is intrinsic optical properties of materials. Because the measured RI can be translated into the mass density of a cell and using this information, mass of a cell can also be retrieved. # Precise and fast measurements: HT provides the spatial resolution down to approximately 100 nm and the temporal resolution of a few to a hundred frames per second, depending on the numerical apertures of used objective lenses and the speed of an image sensor. However, 3D RI tomography does not provide molecular specificity. Generally, the measured RI information cannot be directly related to information about molecules or proteins, except for notable cases such as gold nanoparticles or lipid droplets that exhibit distinctly high RI values compared to cell cytoplasm.Applications
The applications of HT include:Cell biology
HT provides 3D dynamic images of live cells and thin tissues without using exogenous labeling agents such as fluorescence proteins or dyes. HT enables quantitative live cell imaging, and also provides quantitative information such as cell volume, surface area, protein concentration. The label-free imaging and quantification of chromosomes were presented. The regulatory pathway of proteasome degradation by autophagy in cells were studies using HT.Correlative imaging
HT can be used with other imaging modalities for correlative imaging. For example, a combination of HT and fluorescence imaging enables a synergistic analytic approach. HT provides structural information whereas fluorescence signal provides molecular specific imaging, an optical analogous toLipid quantification
Intracellular lipid droplets play important roles in energy storage and metabolism, and are also related to various pathologies, including cancer, obesity, and diabetes mellitus. HT enables label-free and quantitative imaging and analysis for free or intracellular lipid droplets. Because lipid droplets have distinctly high RI (''n'' > 1.375) compared to other parts of cytoplasm, the measurements of RI tomograms provide information about the volume, concentration, and dry mass of lipid droplets. Recently, HT was used to evaluate the therapeutic effects of a nanodrug designed to affect the targeted delivery of lobeglitazone by measuring lipid droplets in foam cells.Experimental laboratory
HT provide various quantitative imaging capability, providing morphological, biochemical, and mechanical properties of individuals cells. 3D RI tomography directly provides morphological properties including volume, surface area, andInfectious diseases
The quantitative label-free imaging capability of HT have been exploited for the study of various infectious diseases. In particular, parasites-invaded host cells can be effectively imaged and studied using HT. This is because the staining or labeling of parasites requires complicated preparation process and the staining/labeling is not very effective in several parasites. The invasion of ''plasmodium falciparum'', or malaria inducing parasites, to individual red blood cells were measured using HT. The structural and biophysical alteration to host cells and parasites have been systematically analyzed. The invasion of babesia parasites to red blood cells were also studied. ''Toxoplasma gondii'', apicomplexan parasite causing toxoplasmosis, can infect nucleated cells. The alterations of 3D morphology and biophysical properties of ''T gondii'' infected cells were studied using HT.Biotechnology
The cell volume and dry mass of individual bacteria or micro algae can be effectively quantified using HT.{{Cite journal, title=Enhanced succinic acid production by Mannheimia employing optimal malate dehydrogenase, journal=Nature Communications, year=2020, doi=10.1038/s41467-020-15839-z, last1=Ahn, first1=Jung Ho, last2=Seo, first2=Hogyun, last3=Park, first3=Woojin, last4=Seok, first4=Jihye, last5=Lee, first5=Jong An, last6=Kim, first6=Won Jun, last7=Kim, first7=Gi Bae, last8=Kim, first8=Kyung-Jin, last9=Lee, first9=Sang Yup, volume=11, issue=1, page=1970, pmid=32327663, pmc=7181634 Because it does not requires the staining process while providing the precise quantification values, HT can be used for testing the efficacy of engineered stains.Scientific community
The following are active scientific conferences on HT, as a part of quantitative phase imaging techniques:See also
* Quantitative phase imaging *References