IMAGING OF THE MECHANICAL PROPERTIES OF SKIN USING HIGH RESOLUTION ULTRASOUND ELASTOGRAPHY

Purpose:

The measurement of skin elasticity has the potential to play an important role in the clinical assessment of a range of skin conditions, including acne scars, wounds, skin cancer, radiation fibrosis and lymphoedema but many of these applications would require measurement of highly local values of elastic properties. Existing methods of assessing skin elasticity, such as measuring overall strain while applying suction, torsion or a uniaxial load to the skin, provide only an overall value of skin stiffness on a scale no smaller than many centimeters.

Our aim was to evaluate the ability to use ultrasound to produce high resolution in vivo images of the stiffness distribution within the skin and investigate the effects of surface extensive loading, of varying direction, on the strain generated within normal skin and the underlying tissue layers.

Methods and Materials:

Pseudo-static linear extensive strains of about 20%, large enough for collagen fibre recruitment, were applied to the skin surface whilst measuring the load and acquiring a sequence of ultrasound radio-frequency images.

Results:

The small strain (1-2%) between images in such a sequence allowed incremental correlation-based displacement tracking for imaging the total displacement. Least squares strain estimation then formed lateral strain images with a resolution of about 0.5mm for a field of about 15mm deep by 40mm wide.

Conclusions:

In general uniform strains were seen within both the skin and subcutis, despite the use of surface extensive loads, with a pronounced anisotropy observed as a modulation factor of about two and a periodicity of 90 degrees for the variation of relative stiffness (measured as the ratio of load to strain) with direction of loading. On the other hand, little or no strain was generated in underlying muscle.