Terahertz endoscopy of hard-to-access objects in the context of neoplasms diagnosis

Terahertz (THz) technology was vigorously explored over the past decades. Today, it offers a variety of applications in:

• the non-destructive evaluation of composite materials, ceramics, and semiconductor devices,
• quality control in the pharmaceutical and food industries,
• public security,
• medical diagnosis and therapy.

The considerable interest to the THz biomedical applications is driven by the peculiarities of THz-wave–tissue interactions, as compared to other spectral ranges.

In the context of biomedicine, THz waves are strongly absorbed by tissue water. On the one hand, this makes THz waves sensitive to the content and state of tissue water, as an important endogenous marker for various pathologies, including neoplasms. On the other, this limits the depth of THz-wave penetration in tissues by only tens–hundreds of microns. Consequently, THz waves can only probe the superficial tissues.

Due to the limited probing depth, applications of THz spectroscopy and imaging in medical diagnosis can be classified into three modalities based on the diagnostic procedure:

• Non-invasive methods that do not compromise tissue integrity.
• Minimally-invasive methods that employ endoscopic or laparoscopic access to internal tissues and organs; they use the natural orifices of the body or minimize the size of incisions to reduce the patient trauma.
• Intraoperative methods that require open access to internal tissues and organs during surgical procedures, such as the intraoperative delineation of the tumor margins during its surgical resection.

Despite the growing ubiquity of THz technologies, their adoption in these practical fields is hampered by the lack of commercial THz endoscopes and related methods for studying hard-to-access objects. In contrast to the visible and infrared ranges, which benefit from diverse fiber optics technologies and a variety of endoscopic systems, the THz range suffers from an evident lack of such systems, attributed to the nascent state of THz endoscopy hardware and methods.

In a new paper published in Light: Advanced Manufacturing, a research group headed by Kirill Zaytsev (Prokhorov General Physics Institute RAS) reviews two distinct approaches to address the aforementioned problems of THz endoscopy, with an emphasis on medical diagnostics for neoplasms:

• The first uses a pair of fiber-coupled THz photoconductive antennas or a single THz transceiver to emit and detect THz waves in close proximity to a hard-to-access object. Here, optical fibers are used to flexibly deliver the near-infrared pump and probe laser beams to the THz emitter and detector, respectively. For such systems, data acquisition and processing—including the quantification of the THz optical properties of an analyte—are similar to those used in standard THz pulsed spectroscopy.
• The second exploits flexible THz optical fibers (or hard THz waveguides) as a key element to deliver THz waves from an emitter to an object and then to detect the reflected and back-propagated signal. This key technology relies on THz optical fibers and waveguides, which are still rare, expensive, and inefficient. Despite the current lack of commercially available THz fiber optic components and related methods for data collection and analysis, recent developments pave the way for solving this problem.

The authors also discuss notable examples of THz endoscopic systems, outlining their advantages and drawbacks. In conclusion, they consider the prospects for further research and development in this demanding branch of THz technology and the potential of THz endoscopy in medical applications.

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