High frequency ultrasonic characterization of human skin In vivo

Abstract

High frequency (>20 MHz) ultrasound has numerous potential applications in dermatology because of its ability to penetrate several millimeters into the skin and provide information at a spatial resolution of tens of microns. However, conventional B-scan images of skin tissues often lack the capability to characterize and differentiate various skin tissues. In this work, quantitative ultrasonic methods using the attenuation coefficient, backscatter coefficient, and echo envelope statistics were studied for their potential to characterize human skin tissues in vivo. A high frequency ultrasound system was developed using polymer transducers, a pulser/receiver, high-speed digitizer, 3-axis scanning system, and a PC. Data collected using three different transducers with center frequencies of 28, 30 and 44 MHz were processed to determine the characteristics of normal human dermis and subcutaneous fat. Attenuation coefficients were obtained by computing spectral slopes vs. depth, with the transducers axially translated to minimize diffraction effects. Backscatter coefficients were obtained by compensating recorded backscatter spectra for system-dependent effects, and additionally for one transducer, using the reference phantom technique. Good agreement was seen between the results from the different transducers/methods. The attenuation coefficients were well described by a linear frequency dependence whose slope showed significant differences between the forearm and fingertip dermis, but not between the forearm dermis and fat. The backscatter coefficient of the dermis showed an increasing trend with frequency and was significantly higher than that of fat.

(cont.) A maximum likelihood fit of six probability distributions (Rayleigh, Rician, K, Nakagami, Weibull, and Generalized Gamma) to fluctuations in echo envelope data showed that the Generalized Gamma distribution modeled the envelope better than the other distributions. Fat was seen to exhibit significantly more pre-Rayleigh behavior than the dermis. Data were also obtained from the skin of patients patch-tested for contact dermatitis. A significant increase in skin thickness, decrease in mean backscatter of the upper dermis, and decrease in attenuation coefficient slope was found at the affected sites compared to normal skin. However, no differences in terms of echo statistics were found in the mid-dermis. These results indicate that a combination of ultrasonic parameters have the potential to non-invasively characterize skin tissues.