Combined Two-Dimensional Tissue/Flow Imaging
A functional sectorscan system has been built which provides two-dimensional (2D) images based on a coherent echo/Doppler processor.
The phase and magnitude of the demodulated signal (complex video) are analyzed by means of a fast digital processor. An image field of 256 X 256 pixels is stored in memory. A digital scan convertor and color coding scheme formats the image for standard raster display. The tissue and flow features are distinguished by contrasting color or grey scale assignments. A microprocessor-based operating system provides flexible control of sector format, pulse repetition frequency, sample clock, encoding scheme, etc.
The significant features of the echo/Doppler processor are: The output of the receiver is log compressed and demodulated by a quadrature pair of double-balanced mixers to preserve the phase and magnitude information within a 1 MHz bandwidth. A flash 8 bit A/D convertor is multiplexed between the “real” and “imaginary” components at a 7 MHz rate. A linear approximation (hypotenuse function) is used to calculate the magnitude term. The phase is computed by means of a look-up table. To obtain the Doppler shift for any given point, the derivative dΘ/dt is substituted by the change of phase (ΔΘ/Δt) between consecutive transmit/receive cycles.
Initial in vivo trials show that the system is capable of producing a longitudinal view of the carotid artery. Both tissue and flow features are clearly distinguishable.
KeywordsDigital Controller Demodulate Signal Doppler Angle Quadrature Pair Complex Video
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