THz Imaging of Skin Burn: Seeing the Unseen-An Overview

Countermeasure to cell dehydration caused terahertz near-field scanning image deterioration

Most biomedical applications of terahertz (THz) imaging are based on distinguishing the dielectric differences of tissues or cells in the THz band. But changes in bio-sample dehydration during the point-scanning process can lead to time-varying deviations in the imaging results. To eliminate the deviations, we proposed a time-varying dehydration kinetic model by analyzing the water diffusion process. The model is verified by experiments and applied to restore each point close to the initial imaging starting state of fresh cellular samples, compensating for the impact of slow speed point-scanning on image deterioration. This methodology has significantly improved the THz super-resolution imaging quality of fresh cellular samples, and successfully restored the cell contours that had been obscured by the cell dehydration over time. Although the dehydration model is developed in THz near-filed imaging, it also pertains to other spectral systems that operate in the raster-scan mode on fresh bio-samples.

Biomedical application of terahertz imaging technology: a narrative review

Background and Objective

Terahertz (THz) imaging has wide applications in biomedical research due to its properties, such as non-ionizing, non-invasive and distinctive spectral fingerprints. Over the past 6 years, the application of THz imaging in tumor tissue has made encouraging progress. However, due to the strong absorption of THz by water, the large size, high cost, and low sensitivity of THz devices, it is still difficult to be widely used in clinical practice. This paper provides ideas for researchers and promotes the development of THz imaging in clinical research.

Methods

The literature search was conducted in the Web of Science and PubMed databases using the keywords "Terahertz imaging", "Breast", "Brain", "Skin" and "Cancer". A total of 94 English language articles from 1 January, 2017 to 30 December, 2022 were reviewed.

Key Content and Findings

In this review, we briefly introduced the recent advances in THz near-field imaging, single-pixel imaging and real-time imaging, the applications of THz imaging for detecting breast, brain and skin tissues in the last 6 years were reviewed, and the advantages and existing challenges were identified. It is necessary to combine machine learning and metamaterials to develop real-time THz devices with small size, low cost and high sensitivity that can be widely used in clinical practice. More powerful THz detectors can be developed by combining graphene, designing structures and other methods to improve the sensitivity of the devices and obtain more accurate information. Establishing a THz database is one of the important methods to improve the repeatability and accuracy of imaging results.

Conclusions

THz technology is an effective method for tumor imaging. We believe that with the joint efforts of researchers and clinicians, accurate, real-time, and safe THz imaging will be widely applied in clinical practice in the future.

Full-field high-resolution terahertz imaging based on a high-resistance silicon solid immersion lens

The spatial resolution of the direct imaging system depends on the wavelength and the numerical aperture. In the terahertz (THz) waveband, the wavelength is relatively large, and the higher numerical aperture of the imaging system usually promises the possibility of achieving higher spatial resolution. Solid immersion technique is an effective method to expand the numerical aperture. We design and fabricate a hemisphere lens with high-resistance silicon to achieve the effect of solid immersion, and obtain full-field, high-resolution focal-plane imaging. The characteristics of the direct refraction imaging and the secondary reflection imaging are analyzed by ray-tracing calculations. And the field curvature of the equivalent object plane and the spot diagram on the vertical image plane of the lens are quantifiably evaluated. It is shown that the secondary reflection imaging can effectively reduce the geometric distortion and achieve more ideal imaging quality. The method of blocking different regions before and after the solid immersion lens is proposed to obtain a clear magnified image of a two-dimensional grating with the period of 300 µm. This method provides a powerful tool for THz full-field microscopic imaging.

Application of contrast-enhanced ultrasound in the diagnosis of burn depth

Background

The diagnosis of burn depth often relies on the subjective judgment of plastic surgeons. Contrast-enhanced ultrasound (CEUS) can visualize the microcirculation well and has potential value in diagnosing the depth of burn wounds. We assessed the depth of the burn wounds by CEUS, and compared the results with histological examination.

Methods

Two rhesus monkeys were used, and multiple burn wounds with different depths were made on their backs. The echo of the dermis and subcutaneous tissue were observed for each wound, and the thickness of the dermis was measured. CEUS was performed to evaluate the depth of burn wounds and compared with pathological results.

Results

(I) After scalding, dermal tissue edema occurred, and the thickness of the dermis measured by a US tended to increase gradually, related to the time of scalding and the order of measurement. (II) With the prolongation of the burn time, the depth of filling by contrast agent gradually increased, from the superficial dermis to the deep dermis and subcutaneous tissue, indicating that the depth of tissue damage gradually increased. This was consistent with the pathological observation. The thickness of the healthy dermis was about 1.3-1.8 mm, and 2.7-4.1 mm after scalding. The depth of the burn wounds was 0.9-4.1 mm, accounting for 32-100% of the full skin thickness.

Conclusions

CEUS is a convenient and fast examination method that is consistent with pathological diagnosis of the depth of burn wounds and could prove valuable for the accurate assessment of burn injuries.

A novel method for objectively, rapidly and accurately evaluating burn depth via near infrared spectroscopy

The accurate and objective evaluation of burn depth is a significant challenge in burn wound care. Herein, we used near infrared spectroscopy (NIRS) technology to measure the different depth of thermal burns in ex vivo porcine models. Based on the intensity of the spectral signals and the diffuse reflection theory, we extracted the optical parameters involved in functional (total hemoglobin and water content) and structural (tissue scattered size and scattered particles) features that reflect the changes in burn depth. Next, we applied support vector regression to construct a model including the optical property parameters and the burn depth. Finally, we histologically verified the burn depth data collected via NIRS. The results showed that our inversion model could achieve an average relative error of about 7.63%, while the NIRS technology diagnostic accuracy was in the range of 50 μm. For the first time, this novel technique provides physicians with real-time burn depth information objectively and accurately.

Effects of ginsenoside Rb1 on second-degree burn wound healing and FGF-2/PDGF-BB/PDGFR-β pathway modulation

Background

Panax notoginseng (Burk.) F. H. Chen (P. notoginseng) is a traditional Chinese medicine that has been used therapeutically for cardiovascular diseases, inflammatory diseases and traumatic injuries as well as for external and internal bleeding due to injury. Ginsenoside Rb1, a crucial monomeric active constituent extracted from P. notoginseng, has attracted widespread attention because of its potential anti-inflammatory, bacteriostatic, and cell growth-promoting effects. In this study, the therapeutic effects of ginsenoside Rb1 on second-degree burn in rats and the potential underlying mechanisms were explored.

Methods

A rat model of second-degree burn injury was established, and skin wound healing was monitored at different time points after ginsenoside Rb1 treatment. HE staining was performed to identify burn severity, and biological tissues were biopsied on days 0, 7, 14, and 24 after treatment. Skin wound healing at different time points was monitored by macroscopic observation. Furthermore, IHC, WB, and RT-PCR were utilized to determine the protein and mRNA expression levels of PDGF-BB, PDGFR-β, and FGF-2 in wound tissues after treatment.

Results

HE staining showed that after 24 days of ginsenoside Rb1 treatment, skin tissue morphology was significant improved. Macroscopic observation demonstrated that in ginsenoside Rb1-treated rats, the scab removal time and fur growth time were decreased, and the wound healing rate was increased. Collectively, the results of IHC, WB and RT-PCR showed that PDGF-BB, PDGFR-β, and FGF-2 expressions peaked earlier in ginsenoside Rb1-treated rats than in model rats, consistent with the macroscopic observations.

Conclusion

Collectively, these findings  indicated that ginsenoside Rb1 promotes burn wound healing via a mechanism possibly associated with upregulation of FGF-2/PDGF-BB/PDGFR-β gene and protein expressions.

Terahertz spectroscopy of human skin tissue models with different melanin content

Terahertz imaging has been proposed for burns and skin cancer identification. However, the role of melanocytes, melanosomes, melanin content and distribution in determining the terahertz optical properties of human skin has not been investigated. We use terahertz time domain spectroscopy to measure the optical properties of in vitro pigmented human skin tissue models from Asian, Black, and Caucasian donors. Spectra were collected at various time intervals and used to extract the absorption coefficient and index of refraction at terahertz frequencies. Our results indicate that the degree of cell differentiation and type of donor both contribute to the measured terahertz optical properties.

Glycerol dehydration of native and diabetic animal tissues studied by THz-TDS and NMR methods

The optical clearing method has been widely used for different spectral ranges where it provides tissue transparency. In this work, we observed the enhanced penetration of the terahertz waves inside biological samples (skin, kidney, and cornea) treated with glycerol solutions inducing changes of optical and dielectric properties. It was supported by the observed trend of free-to-bound water ratio measured by the nuclear magnetic resonance (NMR) method. The terahertz clearing efficiency was found to be less for diabetic samples than for normal ones. Results of the numerical simulation proved that pulse deformation is due to bigger penetration depth caused by the reduction of absorption and refraction at optical clearing.