Eight human skin samples were excised postmortem from the upper and lower back, chest and abdomen from two cadavers. The acoustical speed, attenuation and backscatter were measured as a function of frequency (20 to 30 MHz) at 100 positions on a uniform grid over a cross-sectional slice through each sample with the sound incident in a direction parallel to the skin surface. Measurements were made at 24 +/- 0.5 degrees C. Samples were then frozen, cut and stained for histological examination and quantification of fibrous proteins and fat content. The mean attenuation coefficients obtained for whole skin agreed well with previously published results. Employing the model alpha = alpha 1f" where alpha is the attenuation coefficient in decibels per centimeter, alpha 1 is the value of the attenuation coefficient at 1 MHz and f is frequency raised to the power n, mean values (+/- 1 standard deviation) for epidermis were alpha 1 = 0.44 +/- 0.26 and n = 1.55 +/- 0.12, and for dermis alpha 1 = 0.264 +/- 0.17 dB cm-1 and n = 1.69 +/- 0.084. Using a similar model the mean backscatter coefficient was defined by mu 1 = (5.01 +/- 25.76) x 10(-8) Sr-1 cm-1, n = 3.77 +/- 1.5 for the epidermis, and mu 1 = (1.79 +/- 19.5) x 10(-6) and n = 2.76 +/- 1.4 for the dermis. The speed of sound values fell within the range to be found in the literature with a mean value in the epidermis of 1645 m s-1 and in the dermis of 1595 m s-1. Significant, strong correlation existed between the spatially averaged fibrous protein content in the epidermis and dermis and the spatially averaged integrated attenuation measurements. Likewise, strong correlation existed between integrated backscatter and fibrous protein content in the epidermis but not in the dermis. Further research is required to confirm these preliminary findings and to evaluate the role of collagen fibre orientation as a source of variation in the backscattering coefficient of dermis.

Ultrasonic propagation properties of excised human skin.

This chapter discusses the properties of tissue at ultrasonic frequencies, although there is also a brief mention of transmission properties at audio frequencies. The mechanical response of tissues to the ultrasonic range of frequencies, that is, those over 20 kHz, with emphasis on the frequency range 1–10 MHz, can be studied to add to available knowledge because of its wide use in clinical medicine. Shear waves propagate in hard tissues at velocities around 30–50% lower than the equivalent longitudinal wave velocity. Thereafter, the scattering of sound from tissue is anisotropic. The total attenuation in tissue results from the combined losses as a result of absorption and scattering and can be expressed by the equation: α = α a + α s , where α a is the amplitude absorption coefficient and α s is the amplitude scattering coefficient. The intensity attenuation coefficient, μ, is 2α. Subsequently, the intensity coefficients μ a and μ s are related by μ = μa + μs = 2α, where μ a , and μ s are also called absorption and scattering cross section per unit volume respectively. The measurement of attenuation has commonly been achieved using a substitution method, recording the reduction in signal strength at a receiver following the introduction of a tissue sample of known thickness into an acoustic path. Factors affecting attenuation are essential, the same as those affecting acoustic velocity, such as temperature, frequency, anisotropy, tissue composition, and tissue fixation among others.

Acoustic Properties of Tissue at Ultrasonic Frequencies

In vivo attenuation and backscatter coefficients of normal human forearm dermis and subcutaneous fat were determined in the ranges 14 to 50 MHz and 14 to 34 MHz, respectively. Data were collected using three different transducers to ensure that results were independent of the measurement system. Attenuation coefficient was obtained by computing spectral slopes vs. depth, with the transducers axially translated to minimize diffraction effects. Backscatter coefficient was 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 computed attenuation and backscatter results from the different transducers/methods. The attenuation coefficient of the forearm dermis was well described by a linear dependence with a slope that ranged between 0.08 to 0.39 (median = 0.21) dB mm(-1) MHz(-1). The backscatter coefficient of the dermis was generally in the range 10(-3) to 10(-1) Sr(-1) mm(-1) and showed an increasing trend with frequency. No significant differences in attenuation coefficient slope between the forearm dermis and fat were noted. Within the range of 14 to 34 MHz, the ratio of integrated (average) backscatter of dermis to that of fat ranged from 1.03 to 87.1 (median = 6.45), indicating significantly higher backscatter for dermis than for fat. Data were also recorded at the fingertip where the attenuation coefficient slope of the dermis was seen to be higher than that at the forearm.

High-frequency ultrasonic attenuation and backscatter coefficients of in vivo normal human dermis and subcutaneous fat

The digital imaging revolution that swept other medical specialties in the late 1970s and early 1980s is beginning to find new and important roles in dermatology. This technology has a wide range of educational, clinical, and research applications. Dermatologists should understand certain basic concepts about images and imaging techniques to take advantage of progress in this field and eventually apply it to their own research and/or clinical practice.

What dermatologists should know about digital imaging.

The incidence and mortality rate of cutaneous melanoma continue to increase throughout the world, making the study of melanoma biology an important area of current research. While recent break-throughs in transgenic mouse technology have led to promising mouse skin models of melanoma, there is presently no technique available for quantitatively studying subsurface melanoma progression, in vivo. We demonstrate the first application of an imaging method called ultrasound backscatter microscopy (UBM) for imaging early murine melanomas with spatial resolution of 30 μm axial and 60 μm lateral. Murine B16 F10 melanomas have been imaged from their earliest detection, over several days, until they are 2 to 5 mm in diameter. Melanoma dimensions measured by UBM were found to be in excellent agreement with those determined histopathologically on the excised tumours. The relative rms errors in UBM-determined melanoma height and width were found to be 8.7% and 4.2%, respectively. The mean rate of increase in tumour height of early murine melanoma was found to be 0.37±0.06 mm/day. Computer-generated volumetric renderings of melanomas have been produced from three-dimensional image data, allowing quantitative comparisons of tumour volumes to be made. Using a priori assumptions of ellipsoid tumour shape, the relative error in UBM-determined volume was shown to be less than 17%. These results should be of considerable interest to investigators studying melanoma biology using mouse skin models, and have implications in the use of high frequency ultrasound imaging for the clinical assessment of cutaneous melanoma.

Ultrasound backscatter microscope analysis of mouse melanoma progression.

The propagation velocity (c) and attenuation coefficient (α) of ultrasound were measured at room temperature in skin tissues excised from twenty aged (27 months) and eight young (2 months) Bar Harbor strain female mice. Frequencies used were 2.25, 5.0, 7.5 and 10.0 MHz. Both velocity and attenuation data were obtained using a pulse-echo technique. Transit times were measured with a universal counter timer to 10 nsec resolution, and corrected for the “dead time” due to the transducer matching layer. Aged skin had significantly lower values of c and α at all frequencies than did the corresponding young tissues. This preliminary study suggests that the tissue changes which collectively constitute aging may, in some instances, be predictably associated with measurable changes in the acoustic properties of those tissues.

Wasson Kerrick

Papers

Ultrasonic characterization of aging in skin tissue