Robotically Controlled Terahertz Probe for In Vivo Skin Evaluation: Imaging with the PicoBot

Abstract

In this work, we demonstrate significant modifications to our robotically controlled terahertz (THz) sensing system, the "PicoBot," enabling it to perform in vivo imaging of skin rather than limiting it to single-point measurements. By integrating a robotic arm equipped with force-sensitive feedback control, we maintain consistent contact pressure between the probe and the skin surface throughout imaging. In conjunction with this hardware advancement, we introduce an accompanying image analysis pipeline that reduces noise and enhances repeatability across scans. These improvements allow for reliable intra- and inter-subject comparisons, a critical step toward the clinical utility of THz imaging. Our ultimate aim is to use THz imaging to detect skin cancer margins: this paper highlights progress towards this goal and skin evaluation in general.

Keywords: Imaging; In vivo; Skin cancer; Skin evaluation; Terahertz.

Fig. 1

Construction of the PicoBot imaging probe

Fig. 2

A diagram showing how the Menlo Terasmart is incorporated into the PicoBot probe

Fig. 3

a The XY stage, the Menlo THz reflection head and quartz window with the X and Y axes shown in green lines. b Raster scan path used to acquire an image. The stage moves in a continuous motion without stopping

Fig. 4

Flowchart of the PicoBot imaging protocol for an in vivo measurement of skin. The image acquisition process itself is outlined with a dashed box

Fig. 5

Side profile of the PicoBot probe. The base is attached to a robotic arm via a force sensor, this ensures accurate guidance of the PicoBot to the relevant area of skin, and consistent application of a mild force so the quartz imaging window makes good contact with the skin. The 3D camera used to guide the PicoBot can also be seen in the photo. The quartz imaging window of the PicoBot probe. The PicoBot imaging the right cheek of a volunteer

Fig. 6

a Example of the THz beam path through a quartz window reflected from a quartz-sample boundary. b The measured THz waveform consists of 2 pulses, $E_1(\tau ,t)$ and $E_{\mathit{sample}}(\tau ,t)$ and are shown in the time domain

Fig. 7

The change in the a amplitude and b phase of a measurement of air at a signal point over 90 min, with measurements made at a rate of 10Hz. c The change in amplitude and d of the corrected measurements of air made over 90 min, calculated using Eq. 8

Fig. 8

The change in the a amplitude and b phase at 0.5THz of an image of air taken with PicoBot. c The change in amplitude and d of the image formed of the corrected measurements of air, calculated using Eq. 9. The standard deviation for each of the parameters plotted is given by $\sigma$

Fig. 9

Images taken of the aluminium tape stuck to the quartz window of the PicoBot as shown in a. The images show the amplitude of the normalised corrected signal calculated using Eq. 6 at b 0.1THz, c 0.2THz, d 0.5THz, and e 0.8THz

Fig. 10

An example of the method used to calculate the resolution in the X direction. a An image of tape at 0.5THz with a black line indicating the profile used to find the X resolution at 0.5THz. b The amplitude at 0.55THz as a function of X along the black line. This can then be differentiated and fit with a Gaussian peak c the width of the fitted Gaussian can then be used to find the resolution of the image at 0.5THz in the X direction

Fig. 11

Resolution as a function of frequency in the X and Y direction a when the PicoBot setting dX=0.1mm is used and b when dX=1 mm is used, this setting is used during in vivo measurements

Fig. 12

The THz image showing the amplitude of the corrected signal $|M_c|$. The THz image showing the phase of the corrected signal $arg[M_c]$. In both images a dashed line indicates the boundary at which moisturiser was applied. A photo of the region being measured taken using an endoscopic camera mounted inside the PicoBot, moisturiser has only been applied on one side of the line as indicated. $|M_c|$ for an unmoisturised region and a moisturised region