Abstract
Pulsed Doppler systems require a high signal-to-noise ratio (SNR) to differentiate the low amplitude of the blood flow echoes from the floor noise. Conventional quadrature demodulation (QDM) assumes that the central frequency of the emitted pulse is the same as the received echoes and uses this frequency as demodulation frequency. Nevertheless, the central frequency of the received echoes is actually downshifted due to the frequency-dependent attenuation (FRDA). This downshifting produces loss of SNR due to discarding energy of the down-mixed signal. In this study, by estimating the spectrum of a Gaussian modulated pulse, the downshift caused by the FRDA was obtained to create a new compensated pulse with the aim of forcing the echo central frequency to match with the demodulation frequency. The method was evaluated by using Field II simulation. Considering an attenuation of 0.5 dB/MHz/cm, a 5 MHz transducer and a relative bandwidth of approximately 50%, at a depth of 16 cm, compensated pulses reduced the frequency shift from 1.11 to 0.32 MHz and the SNR degradation from 9.25 to 0.41 dB. [This study was supported by the MEXT-Support Program for the Strategic Research Foundation at Private Universities, 2013–2017].
Published Version
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