Development of a high frequency ultrasound skin imaging system: optimization utilizing time domain reflectometry and network analysis

High frequency ultrasound (HFUS) can advantageously be applied for high resolution imaging of tissues over a limited penetration depth in applications like skin and eye imaging. Broadband and sensitive electronics is essential for a good resolution in these system, whereby HFUS specific problems have to be taken into account to preserve bandwidth and resolution. In this paper the design, the development and the optimization of a HFUS system for applications in dermatology are presented. In our setup, a nonlinear expander/limiter network is used to protect the receiver against the large pulsed transmit signal and to keep noise signals off. Furthermore, the transducer is connected through a cable, whose length is not negligible compared to the wavelengths at the applied frequencies. Large and small signal reflectances and transmittances were independently measured based on a time domain reflectometry (TDR) concept and network analysis (NWA), which allowed a subsequent optimization of the imaging properties. A dedicated calibration technique was developed and time/frequency domain equivalency was exploited for the measurements. Results of measurements on technical objects (wires, glass plate) for the assessment of the imaging properties as well as in vivo skin images are presented.