Characterization of Porcine Skin Using a Portable Time-Domain Optical Coherence Tomography System ^†

Abstract

Optical coherence tomography (OCT) is an imaging tool used to visualize the cross-section of a sample. Additionally, this device can measure the sample’s physical properties. This experiment used a portable version to measure the epidermal thickness and dermal extinction coefficient of porcine skin obtained from different anatomical sites. The thinnest epidermis was found to be from the ear region, while the thickest is from the leg. Meanwhile, the lowest dermal extinction coefficient was from the ear, while the highest was from the belly. These measured properties can be used as aids for diagnosing various skin conditions in humans and animals.

1. Introduction

The skin is the outermost organ of the body. As such, it is the first line of defense of the organism from the environment and dehydration. Therefore, the skin needs to maintain its structural integrity to maintain homeostasis within the organism.

In an individual, skin properties can vary depending on anatomic location, age, sex, occupation, and many other factors [1,2]. The person’s health condition may also alter the properties of the skin. Epidermal thickness has been used to study skin conditions. Increased epidermal thickness can be seen in patients with actinic keratosis, which is a condition that may lead to skin cancer [3]. On the other hand, decreased epidermal thickness can result from skin aging due to exposure to ultraviolet radiation [4]. Skin thickness can give a clue about the proliferation of epidermal cells; hence, it can be used to measure the degree of healing [5] and the effectiveness of drug delivery [6].

Traditionally, epidermal thickness was measured by excising a small skin region and viewing it under a light microscope [7]. While biopsy remains the gold standard in diagnostics, it is invasive and requires a significant amount of time for tissue processing. Newer methods of measuring epidermal thickness include high-resolution ultrasound [8] and optical coherence tomography (OCT) [9]. Both methods are non-invasive and can thus be used in vivo.

Studies have also shown that the extinction coefficient (EC) of tissue changes along with chemical and structural changes of the skin during disease states. Edematous areas are caused by an increased water content, leading to reduced EC. Similarly, oral squamous cell carcinoma tissues showed lower extinction coefficient measurements than that of normal oral tissues [10], while pustular lesions have increased signal scattering, thus increasing the EC [11]. Due to ethical considerations and sample availability, animal tissue is the most suitable subject before testing on human volunteers, especially when using a diagnostic modality still under development.

The animal whose skin is found to have the closest resemblance to human skin regarding hair and lipid distribution, and immunogenicity is that of the pig [12,13]. Porcine skin is a possible model for studies on UV protection and epidermal morphology [14]. The wound-healing process and cell proliferation of human and porcine skin are similar [15]. It makes porcine skin the ideal candidate for our OCT study in human skin modeling. The first published OCT paper was in 1991, showing an image of the human retina [16]. OCT are vastly applied in dentistry [17], dermatology [18], and agriculture [19]. There are two types of OCT which are dependent on frequency and time. The absence of a movable reflector allows frequency domain (FD-OCT) to acquire signals faster than time domain (TD-OCT). FD-OCT is associated with a rapid scan speed and a higher resolution in contrast to TD-OCT. It also increases the signal-to-noise ratio [20,21]. However, the simple design and cheaper components make TD-OCT a viable type today [22]. It can also penetrate deeper when compared to FD-OCT. In this study, a time-domain OCT was developed.

This study aims to characterize porcine skin in terms of epidermal thickness and extinction coefficient of the dermis since these properties can easily be extracted from a single A-scan. Information derived from studies like these is useful in modeling human skin conditions, which can be later utilized for rapid, non-invasive screening and diagnosis of diseases. Furthermore, this study can also extend to veterinary medicine to benefit non-human species.

2. Materials and Methods

TD-OCT is based on the Michelson interferometer as shown in Figure 1a.

The reference mirror is designed as a rotating retroreflector instead of moving in translation motion [24]. The advantage of a rotating mechanism over a translational one is a more extended scanning range, which can be easily adjusted by changing the retroreflector’s rotation radius. Using a rotating reflector, the repeatability has improved with an optical path difference at angles less than ±20 degrees. A 1310 nm SLD (Anritsu Co. Ltd., Kanagawa, Japan) with a spectral width of 106 nm and an average axial resolution of 7 μm in air. The detailed discussion on the mechanism was discussed by Shiina et al. [24]. This system has been used for gelatin-skin-based phantoms [23,25] and leaf structures [26]. The specifications of the TD-OCT system are summarized in

Table 1. Specifications of the TD-OCT system [22].

Specification	Value
Center wavelength	1310 nm
Spectral width	106 nm
Axial resolution	7 μm
Lateral resolution, spot size	6 μm
Numerical aperture	0.14
Scanning rate	25 scans/s
Scanning depth in air	12–14 mm

.

Porcine skin was bought from a local market. Skin samples were obtained from different parts, namely the belly, buttocks, leg, cheek, and ear. A piece of skin was mounted onto a glass slide, as shown in Figure 1b. The thickness can also be measured since the entire epidermal layer can be visualized from the A-scan. Light from the probe cannot penetrate the total thickness of the dermis; hence, dermal thickness cannot be assessed. On the other hand, the epidermal extinction coefficient cannot be obtained with the current specifications of the OCT system, so the dermal extinction coefficient was measured instead. A detailed discussion on the determination of extinction coefficients is previously published by Galvez et al. (2021) [23].

3. Results and Discussion

OCT attenuation has been increasingly used for tissue analysis and characterization. For heterogeneous samples like the skin, the A-scan may contain several prominent peaks representing boundaries of various surfaces [23,25]. Figure 2 is an A-scan of porcine skin at the belly. The use of multiple A-scans presents a possible use of OCT to characterize porcine skin at different sites [27].

Aside from the surface peak of the epidermis, another prominent peak is the dermis. Histologically, a very thin dermo-epidermal junction separates these two skin layers.

Table 2. Epidermal thickness and extinction coefficient of porcine skin from various sites.

Epidermal Thickness (μm)	Belly	Leg	Buttocks	Cheek	Ear
Mean	92.38	95.22	64.64	77.72	60.13
Median	85.13	93.01	61.48	78.83	59.91
St Dev	27.30	16.64	13.18	12.03	4.70
Extinction Coefficient (1/mm)	Belly	Leg	Buttocks	Cheek	Ear
Mean	6.42	2.65	4.67	4.36	2.31
Median	6.33	2.45	3.80	3.82	2.48
St Dev	2.87	0.77	1.87	1.94	1.97

shows the epidermal thickness of porcine skin at various sites. It can be shown that the epidermal thickness in the belly produces higher standard deviations compared to other parts. Higher errors were also observed in the belly’s extinction coefficient. Larger errors are partly due to having less particles at these regions which leads to low values and slow variation of backscatter intensity. It also causes significant scattering intensity fluctuations [28,29].

The ear epidermis was observed to be the thinnest, followed by the buttocks and cheek. These findings were consistent with other works [13,30], except for the belly, which in this research is one of the thickest compared to others. Since the epidermal layer is fragile, the data points were insufficient to compute the extinction coefficient. The high standard deviations of the values may be due to the heterogeneous nature of the skin. Another study limitation is that the skin samples were frozen for about a week before scanning.

4. Conclusions

Using OCT, epidermal thickness and dermal extinction coefficients were obtained at different pig body parts. The thinnest epidermis was found on the ear, consistent with the present literature. Even without using B-scans, which require more processing time than a single A-scan. This study showed preliminary measurements and the importance of an A-scan in epidermal thickness. Thus, we can use OCT as a rapid, non-invasive tool as an aid for the diagnosis of skin conditions, not just in humans but in other animals as well.