O15 - Understanding the Mechanism of Water-induced Fingertip Wrinkling via High-resolution 3D Ultrasound and Photoacoustic Imaging

Introduction: : Fingertip wrinkling after prolonged immersion in water is a well-known phenomenon. It has been proposed as a noninvasive indicator of autonomic nervous system function and dermatological health. Although recognized for centuries, the exact physiological mechanism underlying this process remains incompletely understood. The prevailing hypothesis attributes water-induced fingertip wrinkling to vasoconstriction, where sympathetic nervous system activation leads to blood vessel constriction, resulting in a reduction in skin volume and surface deformation.

This hypothesis originates from a 2003 study in which the authors measured blood flow velocity in the ulnar and digital arteries and superficial skin vasculature using 2D ultrasound (US) and laser Doppler imaging before and after 30 minutes of water immersion. The study reported a significant decrease in blood flow velocity in fingers with wrinkles. However, its conclusions are limited by the lack of continuous temporal monitoring, sensitivity to imaging plane selection, and absence of spatially resolved analysis across distinct skin layers. Recent advances in photoacoustic (PA) imaging offer new opportunities to study this phenomenon. PA imaging provides high-resolution visualization of hemoglobin distribution, enabling real-time detection of vascular changes.

In this study, we employed longitudinal high-resolution 3D US/PA imaging to monitor the fingertip wrinkling process and conducted quantitative spatial-temporal analysis across different tissue layers to better understand the mechanism of water-induced fingertip wrinkling.

Materials and

Methods: : The left fourth finger of a healthy adult volunteer was submerged in a water tank maintained at 40°C for 30 minutes. 3D US/PA imaging was performed on the fingertip using a US/PA imaging system (Vevo LAZR, FUJIFILM VisualSonics) equipped with a high-frequency linear array transducer (LZ-550, center frequency = 40 MHz). The laser was operated at a wavelength of 1064 nm. A total of 49 volumetric US/PA datasets were acquired from a 14 mm × 20 mm × 15 mm region throughout the immersion period. Two experimental conditions were studied: (1) a reduced sympathetic activity condition, in which imaging was performed at midnight while the volunteer was in a relaxed state to suppress fingertip wrinkling; and (2) a normal condition to induce typical fingertip wrinkling.

The acquired 3D US/PA raw data were exported and processed in MATLAB. The fingertip surface was automatically segmented from the US images, and the epidermis, dermis, and hypodermis regions were defined for each slice within the 3D volume. Since the US and PA images were naturally co-registered, the average PA signal amplitude within each tissue region was calculated at each time point. Temporal trends and spatial differences in PA signal were analyzed to characterize vascular dynamics associated with the wrinkling process.

Results, Conclusions, and Discussions:: Examples of 3D US/PA image renderings under normal and reduced sympathetic activity conditions are shown in Fig. 1a. Representative 2D US and PA images from a selected slice at 0 minutes and 30 minutes are presented in Fig. 1b, with the segmented epidermis, dermis, and hypodermis regions marked in blue, red, and green, respectively. Wrinkles are indicated by arrows in the US images. Several wrinkles were observed in the normal condition, whereas no obvious wrinkling was found under the reduced sympathetic activity condition.

The normalized PA amplitude over time for different regions in both experimental conditions is shown in Fig. 1c. Under the reduced sympathetic activity condition, the epidermis, dermis, and hypodermis exhibited less than a 3% change in average PA amplitude over 30 minutes, indicating relatively stable vascular dynamics. In contrast, under normal conditions, the epidermis demonstrated a gradual increase in PA amplitude of 5.62% over the first 12 minutes, with no significant change thereafter. The dermis exhibited a rapid decrease in PA amplitude by 7.87% within the first 5 minutes following immersion, after which the signal remained relatively stable. The hypodermis showed minimal change throughout the entire period.

Given that the epidermis contains no blood vessels, the major source of the PA signal in this layer is melanin, whose absorption properties remain unchanged during immersion. Therefore, the observed increase in epidermal PA amplitude is attributed to an increase in surface area associated with the wrinkling process. The dermis is rich in vasculature which is responsive to sympathetic nervous system activation. The exhibited reduction in PA amplitude indicates a decrease in blood volume, providing strong support for the vasoconstriction hypothesis. However, the decrease in dermal PA amplitude occurred primarily within the first 5 minutes, before visible wrinkles had formed. This suggests that additional mechanisms following vasoconstriction must contribute to the subsequent development of fingertip wrinkles.
Overall, our findings demonstrate that 3D US/PA imaging enables spatial-temporal monitoring of vascular and structural changes during the fingertip wrinkling process. These results provide new evidence supporting the vasoconstriction model and highlight the complex, multi-phase nature of water-induced fingertip wrinkling.

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