AcknowledgementsUCLA Center for Excellence in Engineering and Diversity (CEED) • Howard Hughes Medical Institute • Center for Advanced Surgical and Interventional Technology (CASIT) • UCLA Engineering Henry Samueli School of Engineering and Applied Science • Special Thanks to the Grundfest Lab and members

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Investigation of Rough Surface Scattering in Terahertz ImagingAlondra Rivas, Jun Y. Jang, Shijun Sung, Zachary D. Taylor, Warren S. Grundfest

Department of Electrical Engineering, Department of Bioengineering, University of California, Los Angeles, Depart of Surgery, DGSOM, Los Angeles

Abstract: The Terahertz (THz) imaging maps tissue hydration from its acute sensitivity to water. In THz sensing of biological tissue, surface roughness is of critical importance because scattering from surface roughness changes and confounds the accurate measurement of THz signature of tissue. When applied to samples with varying degrees of surface roughness, variation in signal power and deviation was observed. The experiment was prepared by using grinded brass plates as replicas of human skin. Each brass plate is designed with distinct degree of roughness in order to evaluate the fluctuations in reflected THz signals. In 2-dimensional image, such variations manifests as noise. Statistical analysis was performed on the THz imaging signal from the various brass plate samples. The images were specially fitted in a graphical manner in relation to the THz signal received by the imaging detector and the wavelength of the signal. Ultimately, the effect roughness of the surface in the collected THz image signals was characterized.

ObjectiveTo characterize the effects of surface roughness in THz imaging using targets polished with known roughness profile (15µm, 36 µm, 80 µm, 265 µm), and apply to remote sensing of biological tissues.*

Motivation

•Terahertz (THz) frequency (100GHz – 10 THz, Tera = 1012) has high sensitivity to water, and low light energy •Has a potential to become highly useful tool in medical imaging to look at tissue’s hydration•In reflection THz sensing of tissues, scattering of THz waves due to the roughness of skin needs to be understood

•Tissue is highly inhomogeneous• May or may not have regular features of different sizes• Depends on condition, subjects, location, temperature, etc.

Goal: To see if roughness dominates the THz reflection imaging system signal

THz Image Results

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Evaluation

Electromagnetic waves scatters upon hitting an object or surface. Waves hitting a random, rough surface will scatter to random direction. Strength and directivity of reflected(scattered) waves are affected by:

•Property of the material (what we are interested in seeing)•Roughness of interface•Property of probing electromagnetic waves (THz radiation)

• At THz region,

Scattering of Electromagnetic Waves

Transition from specular reflection to diffuse scattering. The surfaces are: (a) smooth, (b) slightly rough, (3) moderately rough, (d) very rough.

Method of AnalysisRoughness

•Many ways to define “rough” surface•Roughness may be measured and characterized by:

• Surface height distribution• Statistical description of surface

• Deviation from mean height• Correlation

A random surface with (a) large, (b) small correlation distance.

Conclusion• Brass plate polished with 15µm sandpaper revealed minimal

signal variance (about 0.06V variation over the signal strength of ~5V).

• Brass plate polished with 36µm sandpaper showed similar SNR to that of 15 μm, suggesting either the system’s SNR is limited around 53 dB, or roughness does not have heavy impact until 36 μm or higher.

• Brass plate polished with 80µm sandpaper is most similar to that of smooth human skin. Signal

• Brass plate polished with 265µm sandpaper showed greatest signal variation (as much as 0.25 V).

• The degree of surface roughness on a sample is apparent in THz signal variance revealed in the image past 80 um.

Future Work• Calibration of the THz imaging plane, for accurate

measurement of signal strength is difficult. Optical design of the THz system remains an important task

• Test our result against appropriate theoretical prediction, and simulations of scattering from rough surface

• Noise contribution of the electronics will be improved to provide cleaner images.

• Apply what we have observed in this experiment in clinical images, and determine possible need to flatten the tissue

References1.)Tchvialevaa, Lioudmila, et al. "Skin roughness assessment." (2010). 2.)COOK, THOMAS H., et al. "Quantification of the skin's topography by skin profilometry." Itnl’ Journal of Cosmetic Science” 3.)Zimnyakov, Dmitry A., et al. "Speckle pattern statistics analysis in human skin structure investigations." Europto Biomedical Optics‘. 4.)Darvin, Maxim, et al. "Cutaneous concentration of lycopene correlates significantly with the roughness of the skin." European Journal of Pharmaceutics and Biopharmaceutics. 5.)Manuskiatti, W. D. A. H. I., D. A. Schwindt, and H. I. Maibach. "Influence of age, anatomic site and race on skin roughness and scaliness. " Dermatology “. 6.) Beckmann and Spizzinchino, et al. “The Scattering of Electromagnetic Waves from Rough Surfaces.” (1969).Fielder etl, Texture Analysis of the Surface of the Human Skin, Skin Pharacol.

• Look at the variations in signal in a 2-D THz image, caused by surface roughness.

1.Polish a flat metallic surface with known roughness 2.Level the imaging target to imaging plane as close as

possible3.Take THz images of target 4.Analyze variations in signal pixel-by-pixel

• Signal to Noise Ratio by Polishing parameters

Polishing Parameters Signal-to-Noise Ratio (dB)Height Deviation 10-40 um (Tchvialeva)

30 um (Cook)>100 um (Ma; optically measured)

Correlation Length 80 um (Zimnyakov)180 um in Psoriatic skin

Polishing Parameters Signal-to-Noise Ratio (dB)15 μm 54 dB36 μm 53dB78 μm 52 dB

265 μm 44 dB

• Estimate of Typical Roughness of Human Skin • Many ways to measure it:

• Line Profiling• Areal Topography• Optical Measurements

Signal to Noise Ratio Evaluation

Visible THz Images

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