Friday, May 14, 2021

Abstract-Study on an artificial phenomenon observed in terahertz biological imaging

 

Zhongbo Yang, Muyang Zhang, Dandan Li, Ligang Chen, Ailing Fu, Yanmei Liang,  Huabin Wang, 

The THz-TDS system and the determination of the radius of the THz beam by the knife-edge method. (a) Schematic of the THz path of the THz-TDS system. (1) and (7) Photoconductive antennae; (2) and (6) High-resistance hyper hemisphere Si lenses; (3) and (5) High-density polyethylene plano convex lenses; (4) Sample. The sample was positioned in the optical path with the aid of a miniature optical clamping device installed on a two-dimensional motorized precision translation stage. A laser level was used in the installation of the sample to ensure that the sample was perpendicular to the incident direction of the terahertz beam. The focus plane is indicated by a dashed line accompanied with the character ‘F’; the green bar with the character ‘S’ indicates the sample position in the Z axis. ‘Z1’ indicates the distance between the front surface of the sample and the focus plane, namely, the defocus distance. (b) A typical time-domain spectrum and (c) the corresponding frequency-domain spectrum of the system free of samples. (d) The line profile extracted from the THz amplitude image at 1.2 THz. (e) The first derivative (black square dot) of the THz amplitude data in (d), fitted by a Gaussian function (red curve).

https://www.osapublishing.org/boe/fulltext.cfm?uri=boe-12-6-3133&id=450836

Terahertz (THz) wave-based imaging of biological samples is an emerging but promising field. In the present work, we report an artificial phenomenon observed in imaging melanoma slices, which can lead to mistakenly interpretation of the experimental results. It was observed that a structure similar to but smaller than the sample contour appeared inside the melanoma slice image. The underlying mechanism of this phenomenon was then investigated both experimentally and theoretically. By imaging a regular standard sample (vinyl coverslip) with a THz time domain spectroscopy (THz-TDS) system and reconstructing its images at 0.8 and 1.2 THz, we can clearly observe the afore-mentioned artifacts. The experimental results are highly consistent with the simulations based on the Fresnel-Kirchhoff diffraction theory in which possible optical aberrations were incorporated. It can be concluded that this artifact was caused by the frequency-dependent diffraction of the sample edge. The work demonstrated here is essential for correct interpretation of the images obtained by the THz-TDS technique.

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