Recent advances in terahertz technology for biomedical applications

Terahertz Imaging and Characterization Protocol for Freshly Excised Breast Cancer Tumors

This manuscript presents a protocol to handle, characterize, and image freshly excised human breast tumors using pulsed terahertz imaging and spectroscopy techniques. The protocol involves terahertz transmission mode at normal incidence and terahertz reflection mode at an oblique angle of 30°. The collected experimental data represent time domain pulses of the electric field. The terahertz electric field signal transmitted through a fixed point on the excised tissue is processed, through an analytical model, to extract the refractive index and absorption coefficient of the tissue. Utilizing a stepper motor scanner, the terahertz emitted pulse is reflected from each pixel on the tumor providing a planar image of different tissue regions. The image can be presented in time or frequency domain. Furthermore, the extracted data of the refractive index and absorption coefficient at each pixel are utilized to provide a tomographic terahertz image of the tumor. The protocol demonstrates clear differentiation between cancerous and healthy tissues. On the other hand, not adhering to the protocol can result in noisy or inaccurate images due to the presence of air bubbles and fluid remains on the tumor surface. The protocol provides a method for surgical margins assessment of breast tumors.

Corneal hydration assessment indicator based on terahertz time domain spectroscopy

Terahertz technology has shown broad prospects for measuring corneal water content, which is an important parameter of ocular health. Based on terahertz time-domain spectroscopy, a new indicator named characteristic ratio (CR) of the sum of low (0.2-0.7 THz) and high (0.7-1.0 THz) frequency spectral intensities, for characterizing corneal hydration is introduced in this work. CR is calculated from the real-time reflection spectra after error elimination of ex vivo human corneal stroma samples which is collected during dehydration under natural conditions (temperature: 22.4 ± 0.3°C; humidity: 20.0 ± 3%). The corresponding relationships between CR and corneal water content are reported. Comparing the linear fitting results with the published similar study, the coefficients of variation of the fitting slope and intercept are 39.4% and 27.6% lower, respectively. This indicates that this approach has the potential to achieve corneal water content in-vivo detection in the future.

Study on glycoprotein terahertz time-domain spectroscopy based on composite multiscale entropy feature extraction method

Tumor genesis is accompanied by glycosylation of related proteins. Glycoprotein is usually regarded as a tumor marker since glycoproteins are consumed remarkably more by the cancer cells than the normal ones. In this paper, the terahertz time-domain attenuated total reflection (ATR) technique is applied to inspect the glycoprotein solution from a concentration gradient of 0.2 mg/ml to 50 mg/ml. A significant nonlinear relationship between the absorption coefficient and the concentrations has been discovered. The influence of the dynamical hydration shell around glycoprotein molecules on the absorption coefficient is discussed and the phenomenon is explained by the concepts of THz excess and THz defect. In order to identify glycoproteins, features are obtained by composite multiscale entropy (CMSE) method and clustered by the K-means algorithm. The results indicate that features extracted by the CMSE method are better than the Principal Component Analysis (PCA) method in both specificity and sensitivity of recognition. Meanwhile, the absorption coefficient and dielectric loss angle tangent are more suitable for qualitative identification. Research shows that the CMSE method has important directive significance for analyzing glycoprotein terahertz spectroscopy. And it has the potential for glycoprotein related tumor markers identification using terahertz technology in medical applications.

Multi-resonance and ultra-wideband terahertz metasurface absorber based on micro-template-assisted self-assembly method

As a promising platform for multi-functional terahertz devices, metasurface absorbers have received widespread attention in recent years. However, due to the existence of manufacturing difficulties, high cost, fragility, single or narrow absorption and other disadvantages, their application ranges are severely limited. Therefore, to effectively solve these problems, we have designed a flexible and high-precision terahertz metasurface absorber based on the micro-template assisted self-assembly method. Free from high cost, complicated process and time-consumption, the sandwich structure terahertz metasurface absorber consisting of a ceramic microspheres layer, a dielectric spacer layer, and a metal copper film is fabricated economically. On the one hand, through assembling the microspheres on the dielectric spacer in a periodic pattern arrangement, multiple resonances can be observed with a maximum absorption rate of up to 92.5% at 0.745 THz and are insensitive to the polarization of incident light. On the other hand, by attaching the microspheres to the dielectric layer in a compact configuration, 90% absorption bandwidth beyond 1.2 THz can be observed with a central frequency of 1.8 THz. The theoretical model of multiple reflection and interference is employed to explain these absorption characteristics. Considering the flexible design and high-throughput manufacturing processes, this work provides a promising platform for the development of high-efficiency and multi-functional terahertz devices.

Terahertz Time-Domain Reconstruction of Coating Microstratigraphy on Gilded Surfaces

Here, a systematic study in order to assess the potential of THz time domain reflectometry for measuring the thicknesses of overpaint layers applied on original gilded surfaces was carried out. The work is part of a thorough characterization campaign, which is going on at the Rijksmuseum for addressing the conservation problems of a set of 19th century gilded picture frames on which heavy coatings were applied in previous undocumented restoration interventions. To perform such non-invasive thickness measurements, an analytical protocol based on Gaussian fits of the THz pulse-echo temporal profiles was optimized through the preparation of suitable technical samples and the comparison with direct thickness measurements. Finally, the methodology was validated by characterizing the microstratigraphy of an original sculptural element from a gilded picture frame in the Rijksmuseum collection. The results achieved show the effectiveness of the present approach in revealing multi-layered dielectric microstructures with a spatial resolution of about 30 µm when using a spectral range up to 1.5 THz.

Machine learning driven non-invasive approach of water content estimation in living plant leaves using terahertz waves

BACKGROUND

The demand for effective use of water resources has increased because of ongoing global climate transformations in the agriculture science sector. Cost-effective and timely distributions of the appropriate amount of water are vital not only to maintain a healthy status of plants leaves but to drive the productivity of the crops and achieve economic benefits. In this regard, employing a terahertz (THz) technology can be more reliable and progressive technique due to its distinctive features. This paper presents a novel, and non-invasive machine learning (ML) driven approach using terahertz waves with a swissto12 material characterization kit (MCK) in the frequency range of 0.75 to 1.1 THz in real-life digital agriculture interventions, aiming to develop a feasible and viable technique for the precise estimation of water content (WC) in plants leaves for 4 days. For this purpose, using measurements observations data, multi-domain features are extracted from frequency, time, time-frequency domains to incorporate three different machine learning algorithms such as support vector machine (SVM), K-nearest neighbour (KNN) and decision-tree (D-Tree).

RESULTS

The results demonstrated SVM outperformed other classifiers using tenfold and leave-one-observations-out cross-validation for different days classification with an overall accuracy of 98.8%, 97.15%, and 96.82% for Coffee, pea shoot, and baby spinach leaves respectively. In addition, using SFS technique, coffee leaf showed a significant improvement of 15%, 11.9%, 6.5% in computational time for SVM, KNN and D-tree. For pea-shoot, 21.28%, 10.01%, and 8.53% of improvement was noticed in operating time for SVM, KNN and D-Tree classifiers, respectively. Lastly, baby spinach leaf exhibited a further improvement of 21.28% in SVM, 10.01% in KNN, and 8.53% in D-tree in overall operating time for classifiers. These improvements in classifiers produced significant advancements in classification accuracy, indicating a more precise quantification of WC in leaves.

CONCLUSION

Thus, the proposed method incorporating ML using terahertz waves can be beneficial for precise estimation of WC in leaves and can provide prolific recommendations and insights for growers to take proactive actions in relations to plants health monitoring.

A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging

Food product safety is a public health concern. Most of the food safety analytical and detection methods are expensive, labor intensive, and time consuming. A safe, rapid, reliable, and nondestructive detection method is needed to assure consumers that food products are safe to consume. Terahertz (THz) radiation, which has properties of both microwave and infrared, can penetrate and interact with many commonly used materials. Owing to the technological developments in sources and detectors, THz spectroscopic imaging has transitioned from a laboratory-scale technique into a versatile imaging tool with many practical applications. In recent years, THz imaging has been shown to have great potential as an emerging nondestructive tool for food inspection. THz spectroscopy provides qualitative and quantitative information about food samples. The main applications of THz in food industries include detection of moisture, foreign bodies, inspection, and quality control. Other applications of THz technology in the food industry include detection of harmful compounds, antibiotics, and microorganisms. THz spectroscopy is a great tool for characterization of carbohydrates, amino acids, fatty acids, and vitamins. Despite its potential applications, THz technology has some limitations, such as limited penetration, scattering effect, limited sensitivity, and low limit of detection. THz technology is still expensive, and there is no available THz database library for food compounds. The scanning speed needs to be improved in the future generations of THz systems. Although many technological aspects need to be improved, THz technology has already been established in the food industry as a powerful tool with great detection and quantification ability. This paper reviews various applications of THz spectroscopy and imaging in the food industry.

Recognition of Pharmacological Bi-Heterocyclic Compounds by Using Terahertz Time Domain Spectroscopy and Chemometrics

In this study, we presented the concept and implementation of a fully functional system for the recognition of bi-heterocyclic compounds. We have conducted research into the application of machine learning methods to correctly recognize compounds based on THz spectra, and we have described the process of selecting optimal parameters for the kernel support vector machine (KSVM) with an additional `unknown' class. The chemical compounds used in the study contain a target molecule, used in pharmacy to combat inflammatory states formed in living organisms. Ready-made medical products with similar properties are commonly referred to as non-steroidal anti-inflammatory drugs (NSAIDs) once authorised on the pharmaceutical market. It was crucial to clearly determine whether the tested sample is a chemical compound known to researchers or is a completely new structure which should be additionally tested using other spectrometric methods. Our approach allows us to achieve 100% accuracy of the classification of the tested chemical compounds in the time of several milliseconds counted for 30 samples of the test set. It fits perfectly into the concept of rapid recognition of bi-heterocyclic compounds without the need to analyse the percentage composition of compound components, assuming that the sample is classified in a known group. The method allows us to minimize testing costs and significant reduction of the time of analysis.

Characterization and Water Content Estimation Method of Living Plant Leaves Using Terahertz Waves

An increasing global aridification due to climate change has made the health monitoring of vegetation indispensable to maintaining the food supply chain. Cost-effective and smart irrigation systems are required not only to ensure the efficient distribution of water, but also to track the moisture of plant leaves, which is an important marker of the overall health of the plant. This paper presents a novel electromagnetic method to monitor the water content (WC) and characterisation in plant leaves using the absorption spectra of water molecules in the terahertz (THz) frequency for four consecutive days. We extracted the material properties of leaves of eight types of pot herbs from the scattering parameters, measured using a material characterisation kit in the frequency range of 0.75 to 1.1 THz. From the computed permittivity, it is deduced that the leaf specimens increasingly become transparent to the THz waves as they dry out with the passage of days. Moreover, the loss in weight and thickness of leaves were observed due to the natural evaporation of leaf moisture cells and change occurred in the morphology of fresh and water-stressed leaves. It is also illustrated that loss observed in WC on day 1 was in the range of 5% to 22%, and increased from 83.12% to 99.33% on day 4. Furthermore, we observed an exponential decaying trend in the peaks of the real part of the permittivity from day 1 to 4, which was reminiscent of the trend observed in the weight of all leaves. Thus, results in paper demonstrated that timely detection of water stress in leaves can help to take proactive action in relation to plants health monitoring, and for precision agriculture applications, which is of high importance to improve the overall productivity.

In vivo terahertz imaging to evaluate scar treatment strategies: silicone gel sheeting

Silicone gel sheeting (SGS) is widely used for scar treatment; however, studies showing its interaction with skin and efficacy of scar treatment are still lacking. THz light is non-ionizing and highly sensitive to changes in water content and thus skin hydration. In this work, we use in-vivo THz imaging to monitor how SGS affects the THz response of human skin during occlusion, and the associated THz reflectivity and refractive index changes are presented. We find that SGS effectively hydrates the skin beneath it, with minimal lateral effects beyond the sheeting. Our work demonstrates that THz imaging is able to detect the subtle hydration changes on the surface of human skin caused by SGS, and it has the potential to be used to evaluate different scar treatment strategies.

Terahertz refractive index matching solution

We report on the fabrication and characterisation of a terahertz (THz) refractive index matching solution (TeraSol) based on barium titanate (BaTiO₃) particles and benzocyclobutene (BCB). The high refractive index of BaTiO₃ in the THz range makes this material ideal for tuning the effective refractive index of the solution over a wide range. Exploiting the effective medium approximation, we are able to determine the concentration of BaTiO₃ particles necessary to obtain target refractive index values between n = 1.8 and n = 5, optimised to match those of substrates widely used in the THz. TeraSol can dramatically reduce the reflections from the substrate during measurements with THz time domain spectroscopy at cryogenic and room temperature. These properties make TeraSol an appealing material for anti-reflective coatings.

Utilizing multilayer structures to enhance terahertz characterization of thin films ranging from aqueous solutions to histology slides

We propose a multilayer geometry to characterize thin-film samples in reflection terahertz time domain spectroscopy. Theory indicates that this geometry has higher sensitivity compared to ordinary transmission or reflection geometries when characterizing both low- and high-absorption samples. Pure water and water-ethanol mixtures are measured to verify the characterization accuracy of the proposed geometry and its capability to measure trace liquids. Paraffin-embedded oral cancer tissue is imaged to further show how the proposed geometry enhances the sensitivity for solid low-absorptive films.

Terahertz tomographic imaging of freshly excised human breast tissues

Terahertz imaging and spectroscopy characterization of freshly excised breast cancer tumors are presented in the range 0.15 to 3.5 THz. Cancerous breast tissues were obtained from partial or full removal of malignant tumors while healthy breast tissues were obtained from breast reduction surgeries. The reflection spectroscopy to obtain the refractive index and absorption coefficient is performed on experimental data at each pixel of the tissue, forming tomographic images. The transmission spectroscopy of the refractive index and absorption coefficient are retrieved from experimental data at few tissue points. The average refractive index and absorption coefficients for cancer, fat, and collagen tissue regions are compared between transmission and reflection modes. The reflection mode offers the advantage of retrieving the electrical properties across a significantly greater number of points without the need for sectioning or altering the freshly excised tissue as in the transmission mode. The terahertz spectral power images and the tomographic images demonstrated good qualitative comparison with pathology.

Organic Crystals for THz Photonics

Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.

Spatio-temporal and spatiospectral metrology of terahertz broadband uniformly topologically charged vortex beams

A comprehensive characterization of the diffraction properties of terahertz (THz) pulsed broadband vortex beams consisting of several electromagnetic field oscillations requires state-of-the-art techniques for studying the evolution of a wavefront as it propagates. For this purpose, we have applied the capabilities offered by THz pulse time domain holography. Accurate metrological study of pulsed single-period THz field propagation allowed us to reveal the spatio-temporal and spatiospectral couplings in broadband uniformly topologically charged vortex beams. Here, we reveal dynamics of such beam propagation in a free space as well as in the experiment with edge diffraction with 50% blocking of the beam focal waist. In this study, we compare the dynamics of freely propagating and edge-diffracted THz vortex. Despite the fact that in the amplitude representation one can observe the emergence of strong asymmetry, analysis of the spectral trajectory of the singular point at some distance from the obstacle and the visualization of phase distribution for individual spectral components testify to the conservation of transverse energy circulation. Similar to the edge diffraction of monochromatic optical vortices, it can be interpreted as self-reconstruction of vortex properties. The given term has not previously been used for the case of pulsed broadband THz beams, to the best of our knowledge.

THz Spectroscopy for a Rapid and Label-Free Cell Viability Assay in a Microfluidic Chip Based on an Optical Clearing Agent

Simple, rapid, and efficient cell viability assays play a fundamental role in much of biomedical research, including cell toxicology investigations and antitumor drug screening. Here, we demonstrate for the first time a rapid and label-free cell viability assay using THz spectroscopy in combination with a new optical clearing agent (OCA) and microfluidic technology. This strategy uses a considerably less absorptive OCA to replace the highly absorptive water molecules around the living cells and thus to decrease the background signal interference. Three low-viscosity oils were screened as potential OCA candidates, among which fluorinated oil was selected because of its lower absorption and lowest cytotoxicity. After the liquid medium was replaced with fluorinated oil in a microfluidic chip, an obvious THz spectral difference was observed between the fluorinated oils with and without living cells. This change in THz response was preliminarily attributed to the distinguishable signals between the cells and the fluorinated oil. In addition, we applied this method to cell viability assays of human breast cancer cells (MDA-MB-231) after treatment with different antitumor drugs. The results indicated that THz spectroscopy with the aid of the proposed water-replacement strategy presented excellent quantification of cell viability with the advantages of a rapid, label-free, nondestructive microassay, which offers significant potential to developing a convenient and practical cell analysis platform.

Rapid tissue histology using multichannel confocal fluorescence microscopy with focus tracking

BACKGROUND

Simplified hematoxylin and eosin (H&E) staining followed by cryo-sectioning enables rapid identification of cancerous tissue within the procedure room during Mohs micrographic surgery. Yet, a faster evaluation method is desirable as the staining protocol requires physically sectioning of the tissue after freezing, which leads to prolonged sectioning time along with the frozen artifacts that may occur in frozen sectioning.

METHODS

We present a multichannel confocal microscopy system to rapidly evaluate cancerous tissue. Using the optical sectioning capability of the confocal microscope, optically sectioned images of the freshly excised mouse tissue were acquired and converted into images resembling H&E histology. To show details of the nuclei and structure of the tissue, we applied a newly developed rapid tissue staining method using Hoechst 33342 and Eosin-Y. Line scanning and stitching was performed to overcome the limited field of view of the confocal microscope. Unlike previous confocal systems requiring an additional mechanical device to tilt the sample and match the focus of the objective lens, we developed a focus tracking method to rapidly scan large sample area. The focus tracking provides an effective means of keeping the image of the thick tissue in focus without additional devices. We then evaluated the performance of the confocal microscope to obtain optically sectioned images in thick tissue by comparing fluorescence stained slide images. We also obtained the corresponding H&E histology image to assess the potential of the system as a diagnostic tool.

RESULTS

We successfully imaged freshly excised mouse organs including stomach, tumor, and heart within a few minutes using the developed multichannel confocal microscopy and the tissue staining method. Using the pseudocolor method, colors of the acquired confocal grayscale images are converted to furthermore resemble Hematoxylin and Eosin histology. Due to the focus tracking and the line scanning, optically sectioned images were obtained over the large field of view. Comparisons with H&E histology have shown that the confocal images can acquire large details such as the ventricle as well as small details such as muscle fibers and nuclei.

CONCLUSIONS

This study confirms the use of confocal fluorescence microscopy technique to acquire rapid pathology results using optical sectioning, line scanning and focus tracking. We anticipate that the presented method will enable intraoperative histology and significantly reduce stress on patients undergoing surgery requiring repeated histology examinations.

Introduction to Photonics: Principles and the Most Recent Applications of Microstructures

Light has found applications in data transmission, such as optical fibers and waveguides and in optoelectronics. It consists of a series of electromagnetic waves, with particle behavior. Photonics involves the proper use of light as a tool for the benefit of humans. It is derived from the root word “photon”, which connotes the tiniest entity of light analogous to an electron in electricity. Photonics have a broad range of scientific and technological applications that are practically limitless and include medical diagnostics, organic synthesis, communications, as well as fusion energy. This will enhance the quality of life in many areas such as communications and information technology, advanced manufacturing, defense, health, medicine, and energy. The signal transmission methods used in wireless photonic systems are digital baseband and RoF (Radio-over-Fiber) optical communication. Microwave photonics is considered to be one of the emerging research fields. The mid infrared (mid-IR) spectroscopy offers a principal means for biological structure analysis as well as nonintrusive measurements. There is a lower loss in the propagations involving waveguides. Waveguides have simple structures and are cost-efficient in comparison with optical fibers. These are important components due to their compactness, low profile, and many advantages over conventional metallic waveguides. Among the waveguides, optofluidic waveguides have been found to provide a very powerful foundation for building optofluidic sensors. These can be used to fabricate the biosensors based on fluorescence. In an optical fiber, the evanescent field excitation is employed to sense the environmental refractive index changes. Optical fibers as waveguides can be used as sensors to measure strain, temperature, pressure, displacements, vibrations, and other quantities by modifying a fiber. For some application areas, however, fiber-optic sensors are increasingly recognized as a technology with very interesting possibilities. In this review, we present the most common and recent applications of the optical fiber-based sensors. These kinds of sensors can be fabricated by a modification of the waveguide structures to enhance the evanescent field; therefore, direct interactions of the measurand with electromagnetic waves can be performed. In this research, the most recent applications of photonics components are studied and discussed.

High-Speed Terahertz Waveform Measurement for Intense Terahertz Light Using 100-kHz Yb-Doped Fiber Laser

We demonstrate a high-speed terahertz (THz) waveform measurement system for intense THz light with a scan rate of 100 Hz. To realize the high scan rate, a loudspeaker vibrating at 50 Hz is employed to scan the delay time between THz light and electro-optic sampling light. Because the fast scan system requires a high data sampling rate, we develop an Yb-doped fiber laser with a repetition rate of 100 kHz optimized for effective THz light generation with the output electric field of 1 kV/cm. The present system drastically reduces the measurement time of the THz waveform from several minutes to 10 ms.

Composite multiscale entropy analysis of reflective terahertz signals for biological tissues

We demonstrate a composite multiscale entropy (CMSE) method of terahertz (THz) signal complexity analysis to distinguish different biological tissues. The THz signals reflected from fresh porcine skin and muscle tissues were measured and analyzed. The statistically significant difference and separation of the two tissues based on several parameters were analyzed and compared for THz spectroscopy and imaging, which verified the better performance of the CMSE method and further enhancement of the contrast among THz signals that interact with different tissues. This process provides a better analysis and discrimination method for THz spectroscopy and imaging in biomedical applications.