TIME COMPRESSION ANALYSER

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Frequency analysis of ultrasonic doppler blood flow signals can be achieved using a bank of electronic filters, but a more modern method is to use a computer to perform fast Fourier transform (FFT) analysis. Doppler blood flow signals may contain frequency components from (say) 200 Hz to 15 kHz. This wide range of frequencies cannot be analysed quickly enough in a sequential mode.

This problem is solved by dividing the signal into short periods (e.g. 10 ms) and reading these into a computer memory. These are then read out at a much faster rate (e.g. 250 times faster) and rapid analysis applied to the time compressed signal. The flow pattern is then displayed on a strip chart usually produced on a fibre-optic recorder with the frequency components (velocity components) displayed across the strip. By using the amplitude of these components as well as the frequency, the rate of blood flow may also be calculated.

Such devices may be used by vascular surgeons in the investigation of flow patterns in diseased arteries.