Terahertz endoscopic imaging for colorectal cancer detection: Current status and future perspectives

Abstract

Terahertz (THz) imaging is progressing as a robust platform for myriad applications in the field of security, health, and material science. The THz regime, which comprises wavelengths spanning from microns to millimeters, is non-ionizing and has very low photon energy: Making it inherently safe for biological imaging. Colorectal cancer is one of the most common causes of death in the world, while the conventional screening and standard of care yet relies exclusively on the physician's experience. Researchers have been working on the development of a flexible THz endoscope, as a potential tool to aid in colorectal cancer screening. This involves building a single-channel THz endoscope, and profiling the THz response from colorectal tissue, and demonstrating endogenous contrast levels between normal and diseased tissue when imaging in reflection modality. The current level of contrast provided by the prototype THz endoscopic system represents a significant step towards clinical endoscopic application of THz technology for in-vivo colorectal cancer screening. The aim of this paper is to provide a short review of the recent advances in THz endoscopic technology and cancer imaging. In particular, the potential of single-channel THz endoscopic imaging for colonic cancer screening will be highlighted.

Keywords: Cancer detection; Colon; Colonoscopy; Cross-pol; Endoscopy; Flexible waveguides; Metal-coated; Polarization; Polarization-sensitive; Terahertz imaging.

Figure 1

Enface histology[19,20] sections of hyperplastic mucosa (HP), normal (inset: Mucus secreting colon cell) (N), sporadic juvenile benign polyp (P), low grade stage I (S1), intermediate stage II (S2), and high grade stage III colon (S3). Mu: Mucin; E: Epithelium; M: Mitoses.

Figure 2

Terahertz spectroscopic results for the absorption coefficient (A) and refractive index (B) of fresh excisions of normal (blue) and cancerous (red) colon tissues[38] (Printed with permission). THz: Terahertz.

Figure 3

Absorption (A) and reflectance (B) measurements of paraffin embedded dehydrated fixed specimens of normal and cancerous colon tissue[39] (printed with permission). THz: Terahertz.

Figure 4

Photographs, absorption (transmittance) and reflection images of formalin fixed dehydrated colon tissue[39] (printed with permission).

Figure 5

An example terahertz image of excised cancerous, dysplastic and healthy colonic tissues. A: Example terahertz (THz) image of tissue containing healthy regions, dysplasia and cancerous tissue; B: The histology results (drawn onto a photographic image of the tissue samples); C: The histology results are overlaid on the THz image. In this example, regions a and b are normal tissue, c is dysplastic tissue and d is cancerous tissue[38] (printed with permission).

Figure 6

Schematic of continuous-wave terahertz reflection imaging system[42].

Figure 7

Digital photograph (A) and corresponding terahertz reflectance images (B) of normal (N) and cancerous (C) colon tissue[42].

Figure 8

Experimental setup for the transmission loss measurement in metal and metal dielectric. Inset: A: 4 mm Ag (top), 3 mm Ag, 2 mm Au, and 2 mm Ag; B: 3 mm Ag/PS (top), 2 mm Au/PS, and 4 mm Ag/PS) coated terahertz waveguides[54].

Figure 9

Schematic of waveguide based terahertz near-field transmission imaging system[41] (printed with permission). PMMA: Polymethyl methacrylate.

Figure 10

Terahertz transmittance images and stained histology sections showing cancerous and normal colon tissue[41] (printed with permission).

Figure 11

Schematic of single-channel prototype terahertz endoscopic imaging setup. Inset: Terahertz (A) transmission imaging of a small 10 mm leaf, and (B) reflection imaging of a 25-cent coin. THz: Terahertz.

Figure 12

Digital photograph, cross-polarized terahertz reflection images of normal N vs cancerous C human colonic formalin fixed (A and B) and fresh (C and D) tissue sets[58].