Ultrasonic assessment of skin and surgical wounds utilizing backscatter acoustic techniques to estimate attenuation

Backscatter acoustic techniques at high ultrasonic frequencies (10-40 MHz) were utilized to investigate the ultrasonic attenuation coefficient of normal skin and the relationship between attenuation and the healing of surgical wounds ranging in age from 9 to 49 days. The attenuation coefficient was calculated with measurements from depths of 0.5 and 1.0 mm, primarily in the reticular dermis. The values for control skin ranged from 6.0 Np cm-1 at 10 mHz to 19.6 Np cm-1 at 40 MHz and a corresponding slope of 0.45 Np cm-1 MHz-1. Wound attenuation initially increased with wound age from approximately 15% of control at day 9 to 30% of control at day 34 but did not continue to increase through day 49. During the same period, however, total collagen (as a percentage of wet weight) increased at a constant rate from approximately 45% of control at day 9 to 70% of control at day 49. Thus the attenuation coefficient in healing wounds over the time period studied may be sensitive to more than total collagen content in the tissue. It may be affected by other competing factors as wounds mature such as intermolecular cross-linking, collagen fiber bundle size, and structural arrangement of fiber bundles or other tissue constituents such as proteoglycans and elastin. The observations that wound integrated attenuation using backscatter techniques over the range of 10-40 MHz substantially increased in early wounds, did not increase beyond day 34, and remained significantly lower than control values over the entire healing period studied are in agreement with the results of an independent study of the same tissue using transmission techniques at 100 MHz.