Ultrasonic Characterization of the Layered Structure of Human Scalp

Interest in transcranial ultrasound has motivated numerous ultrasonic studies of the skull and brain; however, the ultrasonic properties of the scalp are relatively unknown. The goal of this study was to ultrasonically characterize the transmural tissue structure of the scalp. Sixty-four formalin-fixed specimens were prepared from 4 human donors and scanned in a water tank with a 25 MHz transducer to create parametric images of the speed of sound (SOS), frequency slope of attenuation (FSA), integrated attenuation coefficient (IAC), integrated backscatter coefficient (IBC), and apparent integrated backscatter (AIB). Parametric images revealed a layered structure of which the 3 thickest (>1 mm) layers were analyzed: dermal, subcutaneous and subgaleal. Measured values for these 3 layers, respectively, were SOS = 1560 ± 29 m/s, 1510 ± 19 m/s, and 1520 ± 21 m/s; FSA = 2.75 ± 0.77 dB/cm/MHz, 2.51 ± 0.60 dB/cm/MHz, and 0.83 ± 0.47 dB/cm/MHz; IAC = 56 ± 11 dB/cm, 29.7 ± 6.1 dB/cm, and 20.3 ± 7.1 dB/cm; IBC = 0.14 ± 0.13 cm −1 sr −1 , 0.072 ± 0.056 cm −1 sr −1 and 0.07 ± 0.15 cm −1 sr −1 ; AIB = −45.1 ± 3.1 dB, −46.9 ± 3.2 dB and −50.2 ± 5.0 dB. All 5 measured parameters were significantly different between the dermal and subcutaneous layers and between the dermal and subgaleal layers. AIB, IAC, and FSA were significantly different between the subcutaneous and subgaleal tissue layers. These findings demonstrate that the ultrasonic properties of the scalp vary by layer.