Performance Issues in Doppler Ultrasound 1

2 Fundamental Tradeoffs In pulsed modes (PW and color), maximum velocity without aliasing is In pulsed modes, maximum depth of the Doppler gate is Combining the above two equations, we have In Doppler, the lowest acceptable frequency is usually used.

Performance Issues in Doppler Ultrasound 3 Fundamental Tradeoffs The velocity (frequency) resolution is determined by the inverse of the smaller of transit time and observation time. It may be preferable to increase the sample volume (i.e., degrade spatial resolution) in order to reduce spectral broadening (i.e., increase velocity resolution). Sample volume Scatterer motion Sample volume Scatterer motion

Performance Issues in Doppler Ultrasound 4 Fundamental Tradeoffs Longer pulses (Doppler gates) are often used for increased SNR. Thus, axial resolution is degraded. Higher frame rates require larger beam spacing. Thus, lateral resolution is not optimal.

Performance Issues in Doppler Ultrasound 5 Matched Filtering A matched filter is a time-delayed version of the time reversed input signal. A matched filter on the receiver maximizes the SNR given a transmit waveform. By maximizing the SNR, both the frequency and the time errors can be reduced. Gaussian signal gives the poorest estimation from this point of view.

Performance Issues in Doppler Ultrasound 6 Doppler Ambiguity Function The Doppler ambiguity function is designed to evaluate the amount of ambiguity in both time and frequency given a transmit waveform. Matched filtering is typically assumed at the receiver end. The total potential ambiguity is the same for all signals that possess the same energy. Therefore, the goal of choosing an “optimal”waveform is to distribute the ambiguity in an optimal way based on specific imaging requirements.

Performance Issues in Doppler Ultrasound 7 Doppler Ambiguity Function Typical examples: –CW. –Single pulse. –PW waveform. –Color Doppler waveform. Matched filtering may not be implemented in practical systems.

Performance Issues in Doppler Ultrasound 8 Design Problems Adaptive wall filter: –Design a wall filter that adaptively change the cut-off frequency based on characteristics of the Doppler signal. –Particularly effective for reducing “flash” artifact.

Performance Issues in Doppler Ultrasound 9 Design Problems High PRF (measuring high velocity at a large depth). –Strong signals from secondary gates at shallower depths. –The receiver needs to have very large dynamic range. –Digital system implementation.

Performance Issues in Doppler Ultrasound 10 Design Problems Simultaneous B and Doppler imaging: –Duplex. –Triplex. –B and Doppler interleave. –Recovery of missing samples. –Effects on spectral and audio quality.