3.1.

Speed

The image acquisition time with respect to the a-scan rate is a function of the variation speed of the reference mirror in the time domain technique, while in frequency domain OCT, it depends on the camera speed in the detection spectrometer or on the rate of sweep of the laser. According to the design of scanning and focusing, the field of view of OCT reaches a lateral image dimension of up to 10 mm and a depth of about 1 to 2 mm into the skin, depending on the light source and detector system. This way OCT allows the detection of architectural details within the tissue, but lacks the resolution of single cells in skin.

3.2.

OCT Systems

4.1.

Normal Human Skin

OCT demonstrates clearly the layered structure of healthy skin (Fig. 1). The stratum corneum manifests as a split entrance signal visible as two thin lines. On the palms and soles, where the stratum corneum is much thicker, it presents as the first wavy, signal intense layer. The epidermis is slightly less signal-intense. The dermo-epidermal junction and the stratum papillare of the upper dermis are again more signal intense, while the reticular dermis offers a less intense signal.

Fig. 1

OCT image of healthy skin of the forearm imaged by the VivoSight device ($6\times 2{\mathrm{mm}}^2$). The entrance signal of the Stratum corneum presents as two thin lines (center arrow), the epidermis (E) below offers a little less intense signal. The right arrow shows the level of the dermo-epidermal junction (DEJ), and further in the depths the dermis (D) gives a more intense signal again. Also vessels in the papillary dermis (V) and hair follicles (HF) are visible.

The nail unit can also be investigated by OCT. The healthy nail plate appears as well demarcated structure, presenting mostly with signal rich parallel layers, in some cases with a granular pattern.¹⁴

Differences in scattering caused by different structures and components within the tissue, and differences in thicknesses of layers and scattering can be used to quantify acanthosis, atrophy, or oedema.^15–19

4.2.

Skin Tumors

4.2.1.

Nonmelanoma skin cancer

Epithelial skin tumors such as basal cell carcinoma (BCC) and actinic keratoses (AK) are the prime examples of indications for OCT. BCC show characteristic tumor conglomerates with a signal-intensity similar to the one of the epidermis. Superficial tumor nests derive from the epidermis, while deeper parts of the BCC are sharply demarcated from the dermis by a dark line, correlating to the surrounding fibrous stroma (Figs. 2Fig. 3Fig. 4 to 5).²⁰ Typically the tumor is accompanied by enlarged blood vessels.^21–25

Fig. 2

OCT image of a BCC with superficial and deeper tumor islands (arrow) captured with the Callisto device ($4\times 1.9{\mathrm{mm}}^2$).

Fig. 3

OCT image of the same BCC (arrow) as in Fig. 2 captured with the VivoSight device ($6\times 2{\mathrm{mm}}^2$).

Fig. 4

OCT image of the same BCC imaged by the SKINTELL system, vertical section ($1.8\times 0.6{\mathrm{mm}}^2$).

Fig. 5

OCT image of the same BCC (arrow) imaged by the SKINTELL system, in the en face section mode (horizontal section, $1.8\times 1.5{\mathrm{mm}}^2$).

Actinic keratoses are characterized by a thickening and a stronger scattering of the stratum corneum due to parakeratosis (Fig. 6).²⁶

Fig. 6

OCT image of an actinic keratosis with few hyperkeratosis (VivoSight, $6\times 2{\mathrm{mm}}^2$).

The entire epidermis appears thicker, but the demarcation to the dermis is still detectable.²⁷ In contrast, in invasive squamous cell carcinoma (SCC), the infiltration of tumor cells into the dermis leads to a loss of the dark line normally representing the dermo-epidermal junction. In cases of severe hyperkeratosis in AK and SCC, a signal shadow is produced that can disguise the tumor below.

4.2.2.

Melanoma and Benign Pigmented Tumors

Due to the melanin within, pigmented lesions show an irregular scattering in OCT. The dermo-epidermal junction is visible in junctional nevi, together with an acantopapillomatosis, but can disappear in compound nevi. The dermo-epidermal junction is also not detectable in malignant melanoma, because of the infiltrative tumor growth (Fig. 7). As currently there are no firm specific features or characteristics to differentiate between benign pigmented nevi and malignant melanoma, OCT is not a suitable technique for a clear-cut diagnosis of pigmented lesions at this time.²⁸ Yet in thin melanomas, OCT can be used to measure the tumor thickness, as the scattering within the tumors differs from the surrounding collagen fibers, and therefore the lower border in thin tumors is detectable.^29,30

Fig. 7

OCT image of a malignant melanoma on the back. The arrows mark the lower tumor border (TB) and also dense tumor nests (TN) with an intense signal are seen (VivoSight, $6\times 2{\mathrm{mm}}^2$).

5.1.

Inflammatory Diseases

Psoriasis as well as eczema show a thickening of the stratum corneum and the epidermis. Inflammatory infiltration and oedema in the dermis lead to a lower scattering and blood vessels are dilated.^31,32

In Darier’s disease, OCT can detect the acantholytic papules. In bullous autoimmune disease, such as pemphigus or bullous pemphigoid, the location of the depth of blistering—intra- or subepidermal—can be differentiated by OCT.³³ In porokeratosis, the cornoid lamellae show typical features in OCT images.

Also the changes in inflammatory skin diseases during treatment can be appreciated by OCT by measuring the thickness of layers and the signal attenuation coefficient.

5.2.

Parasite Infestation

Parasites such as scabies mites, larva migrans, and hookworms can be detected by OCT.³⁴ Scabies mites impress as signal strong structures below the stratum corneum with a burrow behind (Figs. 8 and 9).

Fig. 8

OCT image of a skabies mite (M), vertical section (VivoSight, $2\times 2{\mathrm{mm}}^2$).

Fig. 9

OCT image of a skabies mite (M) and its burrow (B), horizontal section (VivoSight, $2\times 2{\mathrm{mm}}^2$).

5.3.

Nails

In the investigation of diseased nails by OCT, leukonychia and onychomycosis lead to signal rich artifacts. Although the exhibition of several white lines and longish structures in onychomycosis shows a high sensitivity, specificity of this method is too low to replace other diagnostic methods.^14,35

5.4.

Quantification of Changes and Therapy Monitoring

Additionally, OCT offers a valuable tool for monitoring of therapeutical effects and quantification of skin changes, as shown in several studies of UV radiation therapy, steroid atrophy, wound healing, or nail hydration over time.^36–38 Especially, when using nonsurgical treatment options such as imiquimod or photodynamic therapy, OCT can help to keep the chosen noninvasive approach, to avoid repetitive biopsies and to control the therapeutic efficacy.³⁹

5.5.

Comparison with Other Imaging Methods

As with all imaging techniques, the principle of “the longer wave length offers deeper penetration, but always correlates with a lower resolution” still holds true. OCT is able to fill the gap between the other established methods, especially the gap between dermoscopy and ultrasound on one side and confocal laser scanning microscopy on the other end. Dermoscopy offers a 2-D image of the skin surface with a tenfold magnification, where diagnosis is based on specific patterns, depending on colors, differential structures, asymmetry of the lesion etc. It is well established for the differentiation of melanocytic tumors, but offers also specific patterns for most other skin tumors, which is useful for fast screening for melanoma and other skin cancers in everyday practice. High-frequency ultrasound—usually used around 20 MHz but available up to 100 MHz—is mainly used for the estimation of tumor thickness in melanoma to plan one-step excisions with guideline conform safety margins and helps to decide whether sentinel node excision should be performed at the same time. It can also be used for the evaluation of scleroderma and other indications. Its penetration depth lies around 8 mm for the 20 MHz sonography but of course resolution is much lower compared with OCT. OCT’s penetration depth lies around 1 to 2 mm depending on the device, but with the higher resolution as stated above, offering the wide range of indications described in this review. The confocal laser scanning microscopy offers the highest resolution comparable with histopathology (almost 1 μm), but at the expense of a limited penetration depth of only about 250 μm, allowing the evaluation of structures within the epidermis and down to the papillary dermis in great detail. Confocal laser scanning microscopy is especially helpful in the differentiation between benign melanocytic nevus or malignant melanoma, but helps also in nonmelanoma skin cancers and their precursor lesions and in several inflammatory skin diseases. Due to their different technical parameters and the consequently differing range of ideal indications, that of course do overlap, there is no wonder technique best for all indications. Rather the experience of the doctor to choose the ideal imaging device for each specific clinical question will help to profit by the complementation of the different techniques available.

For comparison and to show how well the different methods complement each other, the following images (epiluminescence microscopy, confocal laser microscopy, and OCT) show the case of a 78-year-old man with a BCC on the neck (Figs. 10Fig. 11 to 12).

Fig. 10

Dermatoscopic image of a BCC, showing a typical vascular pattern for BCC with arborizing vessels and teleangiectasias (VivaCam, $10\times 10{\mathrm{mm}}^2$).

Fig. 11

OCT image of the same BCC (arrow) as in Fig. 10. Typical tumor nest formation of the BCC can be seen (arrow) in whole and their location in correlation to other skin structures can be appreciated allowing more like an overview of the tumor (VivoSight, $6\times 2{\mathrm{mm}}^2$).

Fig. 12

CLSM image of the same BCC (arrows) as in Figs. 10 and 11 giving details on the tumor formations and offering the discrimination of tumor cells and surrounding fibrous stroma around the tumor nests (VivaScope, Mavig, $500\times 500{\mu \mathrm{m}}^2$).

Therefore a combination of OCT with other imaging modalities—such as dermoscopy, confocal laser microscopy, or in the future perhaps multiphoton tomography or Raman spectroscopy—will lead to greater knowledge and better understanding of pathological and maybe even pathogenetic principles, will offer new diagnostic approaches, and will sustain the trend toward nonsurgical diagnostic methods and therapies.^40–42