Showing papers on "Photoacoustic Doppler effect published in 1988"

Theory of ultrasound Doppler-spectra velocimetry for arbitrary beam and flow configurations

Abstract: In conventional ultrasound Doppler systems, the velocity component along the beam axis is derived from the observed frequency shift. Recently, it was verified that by using a pulsed-Doppler system with the beam transversely oriented with respect to the flow, the velocity component transverse to the beam can be derived from the edges of the spectrum. These results are generalized to take into account arbitrary angles of incidence, effects of velocity gradients, arbitrary apertures, and arbitrary source pulses. For uniform apertures and transverse flow, it has been previously shown that the Doppler output spectrum is symmetrical about zero frequency, with its width depending on the Doppler effect due to the transverse velocity and the geometry of the problem. For a beam direction oblique to the velocity, it is shown that the spectrum is now shifted, and is centered about the classical Doppler frequency. For velocity gradients and transverse flows the spectrum remains symmetrical, with the edges corresponding to the maximal velocity; however, the profile becomes peaked at the center. For oblique incidence, an asymmetrical spectrum is obtained and its edges are related to the maximal and minimal velocities within the sampling volume. >

A time-shared ultrasound Doppler measurement and 2-D imaging system

Abstract: Methods for obtaining image-guided Doppler blood velocity measurements are briefly reviewed Preference is given to a time-sharing scheme in which the Doppler measurement is turned off during the data-acquisition period for a 2-D image frame (typically 20 ms) The signal dropout that occurs during the image-updating period is removed from the Doppler audio by inserting a synthetic signal segment The synthetic signal is generated by passing white noise through a discrete-time FIR (finite-impulse response) filter with filter coefficients that are a windowed version of the Doppler signal measured immediately prior to the imaging interrupt It is shown that the artificial signal has spectral properties (and thus audible sound) similar to those of the real Doppler signal segment on which it is based The time-sharing method is analyzed and evaluated experimentally, using dedicated hardware The proposed algorithm allows for the design of time-shared Doppler/imaging systems that carry out pulsed or continuous Doppler measurements with essentially real-time imaging guidance >