In vivo THz imaging of human skin: Accounting for occlusion effects

Abstract

In vivo terahertz (THz) imaging of human skin needs to be done in reflection geometry due to the high attenuation of THz light by water in the skin. To aid the measurement procedure, there is typically an imaging window onto which the patient places the area of interest. The window enables better pulse alignment and helps keep the patient correctly positioned during the measurement. In this paper, we demonstrate how the occlusion caused by the skin contact with the imaging window during the measurement affects the THz response. By studying both rapid point measurements and imaging over an area of a human volar forearm, we find that even 5 seconds of occlusion affects the THz response. As the occlusion time increases, the skin surface water content increases, resulting in the reduction of the amplitude of the reflected THz pulse, especially in the first 3 minutes. Furthermore, it was found that the refractive index of the volar forearm increased by 10% to 15% after 20 minutes of occlusion. In this work, we examine and propose a model for the occlusion effects due to the quartz window with a view to compensating for its influence.

THz in vivo measurements: the effects of pressure on skin reflectivity

“…This is due to occlusion by the imaging window, as each image is acquired by raster scanning the THz spot in lines going from up to down and measuring the next line to the right. The duration of the acquisition process for each image was 36 seconds, therefore the left-side of the images are occluded for less time than the right sides, and with increasing occlusion time the peak to peak decreases [17]. Fig.…”

“…To increase the signal to noise of our results, we averaged the ten consecutive THz pulses after the skin has been in contact with the imaging window for 30 seconds. The ten THz pulses were acquired in 2.5 seconds and since the skin had already been in contact for 30 seconds, the occlusion affect over these pulses is minimal [17]. Further, each pressure measurement was repeated 3 times to ensure there were no errors caused by the individual's behavior, ie.…”

“…In this work, we obtain a reflection coefficient ratio between the sample and a reference (air). The key equations relevant to our work are given here and more details can be found in references [16,17].…”

“…However, when the skin contacts the window, occlusion and mechanical deformation will occur to the skin's surface. Sun et al investigated the effects of occlusion [17]: their results showed that the THz response of skin changed most rapidly during the first 5 s of contact with the quartz window due to the induced occlusion and therefore water accumulation in the skin. However, to date there have been no studies of how the contact pressure affects the in-vivo THz response of skin.…”

Exploiting Total Internal Reflection Geometry for Terahertz Devices and Enhanced Sample Characterization

“…The resonant frequencies of some intermolecular bonds are located in the THz frequency range which helps analyze the dynamic vibrations in the materials . The nonionizing nature of THz light (4.14 meV at 1 THz) and low power (around 100 µW) characteristics of the commercially available systems make THz imaging suitable even for in vivo measurements …”