Monitoring skin photodamage using quantitative optical coherence tomography
In clinical medicine, 8-methoxypsoralen (8-MOP) combined with ultraviolet A (UVA; 320–400nm) (PUVA) therapy is an effective and widely used treatment for several types of dermatose, including psoriasis, mycosis fungoides, and vitiligo. However, many studies have shown that PUVA has both potent mutagenic and carcinogenic effects and also immunosuppressive consequences. The latter can allow tumor development or changes to the immune function in both immuno-suppressed and healthy individuals.1,2 Long-term PUVA therapy significantly increases the risk of skin-cancer development.3,4
The effects of UV-induced photodamage on the morphology of skin tissue have been studied extensively ex vivo based on histological analysis.5,6 However, skin biopsy alters the original skin morphology (e.g., by introducing artifacts caused by dehydration, fixation, and staining of the tissue), cannot be done repeatedly at the same site, and always requires iatrogenic trauma. Innovative skin-imaging techniques, such as optical coherence tomography (OCT), might have advantages for in vivo skin studies because of the inherently high resolution, which enables visualization of micromorphological structures.7,8
In a recent study, we applied OCT for quantitative analysis of PUVA-induced photodamage of BALB/c mice dorsal skin. We used an OCT system9–11 (see Figure 1) with a central wavelength and bandwidth of 1310 and 50nm (full width at half maximum), respectively, at 10–15μm axial resolution. The system's transverse resolution is approximately 25μm, which we determined from the focal-spot size produced by the probe beam.
We divided our mice randomly into PUVA- and UVA-treated groups, as well as a control group. We monitored changes to the same region of interest on mice dorsal skin after treatment and stored the OCT images on a PC for further qualitative and quantitative processing. Figure 2 shows an OCT image of normal mice dorsal-skin structure. Three separate layers are clearly visible, i.e., an upper layer (the cutaneous cover, including stratum corneum, epidermis, and dermis), a muscle-tissue layer at the bottom, and a layer of stratum mucosum (subcutaneous tissue). A darker, thinner region is located between these latter two layers. These tree layers have also been described in previous studies based on histological analysis. However, after PUVA or UVA treatment, the hierarchical structure became vague.
To further illustrate the differences, we took an average of more than 20 adjacent amplitude-modulation (‘A’) scans from a horizontal surface of skin tissue in each group. We averaged the 2D images into a single curve to obtain the distribution of the mean signal (or mean grey level) as a function of depth (see Figure 3). OCT imaging is based on intensity differences of backscattered light. Its depth-related signal simply follows the Lambert-Beer law, which states that light attenuation in tissues follows an exponential dependence. Therefore, we adopted an exponential equation to fit the OCT intensity profiles. Figure 4 shows that the coefficient correlations were 0.974, 0.981, and 0.982 in the control, UVA, and PUVA groups, respectively. The attenuation coefficients of the three groups were 2.691±0.080, 2.089±0.102, and 1.702±0.061mm−1, respectively. Comparing the three groups, the attenuation coefficients of the UVA and PUVA groups were much lower than those of the control group (P<0.01). We conclude that the low attenuation coefficient of UVA-induced skin tissue is caused by an inflammation response in mice dorsal skin. Application of 8-MOP to mice skin obviously accelerates this.
In summary, we successfully applied OCT for qualitative and quantitative evaluation of PUVA-induced skin photodamage. Our result implies that PUVA phototherapy carries a high risk. Dermatologists must carefully choose the appropriate dosage once PUVA phototherapy has been adopted for treatment. Moreover, application of in vivo real-time imaging technology is necessary for clinical assessment of long-term therapy. We will continue our characterization of the effects of UV radiation on mice dorsal skin.
We acknowledge financial support from the National Natural Science Foundation of China (grant 60778047), the Key Science and Technology Project of Guangdong province (grants 2005B50101015 and 2008B090500125), and the Key Science and Technology Project of Guangzhou City (grant 2008Z1-D391).
South China Normal University
Zhouyi Guo is laboratory director. His research area is biomedical optics and photonic Chinese medicine.
