Laser tomography gets under your skin

09 Oct 2006

German scientists have developed an optical technique that for the first time allows the effects of skin aging to be monitored non-invasively and in real time.

The experimental technique, known as multiphoton laser-scanning tomography, measures the level of damage to the skin caused by sun exposure and aging, without the need to analyse tissue samples in the lab. What's more, initial results indicate that women's skin ages faster than men's (Optics Letters 31 2879).

The technique is being pioneered by a research collaboration that involves Friedrich Schiller University, Jena; the Fraunhofer Institute of Biomedical Technology, St Ingbert; and JenLab, a Jena-based laser technology company.

Put simply, near-infrared femtosecond laser pulses are used to scan the fabric of the deeper layers of the skin (the dermis). This optical interrogation tracks changes in the content of the proteins collagen and elastin, degeneration of which causes the appearance of wrinkles and the progressive loss of skin smoothness.

To quantify these histological changes, the researchers measure two-photon excited autofluorescence (AF) and second-harmonic generation (SHG). From these measurements it is possible to calculate the so-called SHG-to-AF aging index (SAAID), which exhibits a direct correlation to the ratio of dermal collagen and elastin (the collagen/elastin factor).

According to the Optics Letters paper: "As collagen is able to generate second harmonics and elastin has an excitation mechanism in the blue spectral range, both components can ideally be excited at 820 nm by two-photon processes. A filter system can be used for the separate measurement of elastin and collagen because emission of second harmonics is at 410 nm, whereas elastin AF has a maximum in the green spectral range."

The initial clinical evaluation involved tests on the forearms of 18 patients, and measured the collagen/elastin factor in each instance. The team was also able to obtain images of tiny swathes (0.2 mm wide) of the proteins' fibrous matrices, showing the physical appearance of the dermis -- the white lower-layer of skin that gets exposed in deep abrasions.

Large variations appeared from patient to patient, and even from one part of a patient's forearm to another. "In a healthy 35-year-old, some areas can appear like the skin of a 25-year-old, and others like that of someone who's 50," said Martin Johannes Koehler, a dermatologist at Friedrich Schiller University and a co-author of the Optics Letters paper.

On average, though, both the collagen/elastin factor and the physical appearance of the network showed a clear dependence on the patient's age. The dependence appeared to be sex-dependent, with women's skin losing collagen at faster rates than men's.

Currently, dermatologists who want to check out the collagen network of a patient's dermis need to remove a sample of tissue and analyse it in the laboratory. This means it is impossible to monitor variations in the same location as aging progresses.

"You would like to measure changes in collagen content over time," Koehler said. "Moreover, current techniques provide a qualitative assessment of the state of the matrix, but no precise measure of the collagen or of the elastin content, which is what the new technique does."

Although the research is still at the experimental stage, the authors hope that one day it could become useful in studying skin diseases that affect the collagen structure. These include scleroderma, a poorly understood disease characterized by excessive deposits of collagen in the skin, and some chronic complications of graft-versus-host disease, which occur when the tissues of bone-marrow transplant patients are attacked by immune cells from the donor.

Testing the effectiveness of anti-aging cosmetic products could also become easier. "Some cosmetics are thought to change the content of collagen in the skin but, until now, to measure that you had to cut out a piece of skin," added Koehler.