Direct sampling and baseband conversion in Doppler systems for high-frequency ultrasound blood flow measurements

The design of a 50 MHz range ultrasound pulsed wave Doppler system for high resolution flow measurements in small blood vessels is proposed, in which radio frequency echo signals inside a single range gate are directly sampled and digitised. Unidirectional Doppler spectrograms are calculated by means of the fast Fourier transform of digitised data. Furthermore, the Doppler signal is made audible. In addition to making use of a high-frequency sample-and-hold circuit for a direct baseband conversion, the concept presented utilises an analogue wall-filter implementation quite advantageously. In addition, only a low-frequency analogue-to-digital converter is required for digitisation, which enables an inexpensive system implementation with a high dynamic range and real-time processing capabilities.