Skin imaging involves a non-invasive digital technique which produces better images of the skin. These high quality images make assessment of the skin condition easier. Additionally, digital imaging makes storage and retrieval of data much easier. It also enables objective computer analysis and quicker evaluation of images.

Non-invasive skin imaging is now widely used by researchers, dermatologists and the cosmetics industry. The objective study of skin properties is of great interest to the cosmetic industry.

Skin imaging is used for four purposes.

• For studying the normal skin
• For examining the diseased skin
• For noninvasive monitoring of the skin during treatment
• For use as a diagnostic tool in the dermatology clinic.

Skin imaging devices

Various imaging devices examine various aspects of the skin surface, sub surface and skin sections, but almost all depend on the optical and sonographic properties of the skin.

Surface examination

A magnifying glass is the simplest device where a magnified image is examined by eye. Great reliance is still placed on such visual examination since the superiority of computer analysis is yet to be established.

Surface photography is the most widely used device for detecting melanoma skin cancer, often making biopsy unnecessary. It is also used for evaluating many other kinds of skin lesion. Polaroid and ultra violet filters are used to magnify the surface image to reveal wrinkles, erythema and pigmentation for example.

Profilometry is the assessment of the topography or the natural and unnatural features of the skin surface. Optical profilometry is a rapid procedure and exploits the different ways oblique light is reflected from various surface features. A computer image of the surface topography is then generated detailing parameters such as skin roughness and depth of skin defects. Laser profilometry is excellent for assessing skin wrinkle line disorders.

Optical profilometry is most widely used for evaluating hydrating or anti-wrinkle creams. It can also be used to understand how skin ages and the consequences of photoaging due to UV light exposure.

Subsurface examination

Dermatoscopy: Reflected light from the skin surface interferes with images of the deeper skin structures. Dermatoscopy is a technique where this problem is partially resolved by reducing the surface reflection thereby increasing light penetration. Fluid immersion dermatoscopy does this by inserting a liquid-glass interface between the light source and the epidermis. Some dermatoscopes also use a cross-polarized lens to absorb the surface light reflection.

With the advent of image digitization, mole scanners are being increasingly used in dermatoscopy for clinical observation, monitoring, automatic lesion identification and even automatic diagnosis, tasks traditionally performed by dermatologists. However, superiority of mole scanners over dermatologists is not yet established. Dermatoscopy is currently the most popular skin imaging technique.

Spectroscopy: Chromophores are skin components that variably absorb light. After some time period of light absorption, some chromophores, called fluorophores, emit radiation of different wavelengths, which a spectroscope can separate into its component radiations. Each component is then separately analysed. Spectroscopy is currently used mostly in research.

Diuse reflectance spectroscopy or DRS examines the absorption characteristics of the 320nm to 1100nm range of wave lengths. The major chromophores here are hemaglobins, melanins, keratins and water. Differences in absorption can distinguish a melanoma from benign nevi.

In fluorescence spectroscopy, a specific emitted wavelength is sought after scanning with a particular range of wavelengths. The method requires knowledge of fluorophores. It is useful in detecting basal cell carcinoma.

Raman spectroscopy uses the phenomenon of Raman scattering where monochromatic light from the infrared region falling on the skin is reflected back as light of different wavelengths. The scattering is caused by molecular structures, which have their own particular spectral signatures. Healthy and diseased skin have different molecular structures and this may make diagnostic raman imaging possible in the future.

Optical coherence tomography or OCT: All methods described so far have the common limitation that different skin layers cannot be separated. Therefore descriptive morphology is not possible.

OCT produces interference patterns that correspond to different skin layers. OCT was first used for retinal imaging in 1991 and the image resembled an unstained histology section. While currently only the upper layers can be studied, in future the technique may approach the cellular level making it a potential diagnostic tool.

High-resolution ultra sonography or HRU: HRU relies on different tissues having different impedance. As with OCT it is possible to use HRU to obtain images of a histology section. HRU can be used to monitor and assess inflammatory conditions.

Conclusions

Currently dermatoscopy with mole scanners is the best technique in skin imaging although some doubt the usefulness of automated computer mole scanners. Generally, the diagnostic capabilities of all skin imaging devices need to be accurately studied and compared to that of clinicians before making any firm conclusions about their usefulness.