Spectral Doppler technique is widely used in today's ultrasonic diagnostic system since it is especially suitable for the noninvasive detection and measurement of human body. For example, in a blood flow Doppler system, ultrasonic signals are transmitted into human body, then the echo signals scattered by the body tissue or blood flow are detected and analyzed so as to obtain a series of parameters, such as the velocity of blood, all of which are valuable for clinic diagnosis.
FIG. 1 is a block diagram of a typical ultrasonic Doppler system. As shown in FIG. 1, the received echo signals, as being weak, are generally amplified by a low noise amplifier, especially in a blood flow Doppler system. Then, the amplified echo signals are beam-formed and quadrature demodulated to obtain the quadrature Doppler signals. As for a blood flow Doppler system, the amplitude of the echo signals from the tissue or blood vessel wall is normally much higher than that from blood flow. For this reason, a high-pass filter (or known as wall filter) is needed to process the obtained quadrature Doppler signals after gap filling. In this way, most of the echo signals from the tissue and blood vessel wall, characterized in high amplitude and extremely low frequency, can be successfully cancelled.
As shown in FIG. 1, after high-pass filtering, the quadrature Doppler signals, in one path, are fed into a spectral analysis unit to calculate the spectrogram. Then, a parameter calculating unit extracts the mean frequency waveform, maximum frequency waveform and etc. based on the spectrogram, thereby producing some valuable parameters for clinic use. The spectrogram and the parameters, such as the maximum frequency waveform and etc., are then converted by a DSC (Digital Scan Converter) and sent to a monitor for real time display. In the other path, the filtered quadrature Doppler signals are fed into a direction separating unit, and then are separated into forward and backward blood flow Doppler signals. At last, the separated Doppler signals for forward and backward directions are converted by a DAC (digital-analog converter) and output to the right and left stereo speakers respectively. By using such a Doppler system, doctors can make more accurate diagnosis under the help of the spectrogram displayed in the monitor and the voice from the speaker.
In a practical system of FIG. 1, a large amount of noise is normally introduced into the Doppler signals by the amplifier when the echo signals are amplified. This kind of noise is generally regarded as white noise within the band of Doppler signals. As the detection depth increases, the amplitude of the scattered echo signals will be reduced, and thereby the amount of noise in the amplified echo signals will become obvious. As a result, the image quality of the spectrogram and the voice quality of the audio Doppler signals will be degraded significantly. However, it is difficult to remove the background noise introduced by the amplifier or other devices in the Doppler system, only through a simple high-pass or low-pass filter. This is because the spectral distribution of the obtained Doppler signals may be various in each scan. For voice signals, this will be more obvious as it is much more difficult to remove the background noise from the voice signals only through a simple high-pass or low-pass filter in the time domain.
To solve the above problem, Mo and etc. propose a method and apparatus for real-time noise reduction for Doppler audio output in U.S. Pat. No. 6,251,007. More specifically, the method comprises the following steps: performing FFT (Fast Fourier Transform) on the obtained Doppler signals to obtain a power spectrum; extracting the maximum frequency from the obtained power spectrum; setting a frequency as the cut-off frequency of a low-pass filter according to the maximum frequency; filtering the Doppler signals in frequency or time domain so as to reduce the noise. However, in this solution, the effect of noise reduction is highly dependent on the precision of the maximum frequency estimation. That is, when there is a wrong estimation of the maximum frequency, the voice output from the speaker will be distorted severely.
In addition, some other methods of noise reduction in Doppler system, are described in “Doppler ultrasound signal denoising based on wavelet frames”, IEEE, Trans Ultrason Ferroelectr Freq Contr, Vol. 3 P709-716, 2001; “Denoising quadrature Doppler signals using the wavelet frame” see Ibid, Vol. 5 P561-564, 2003, and “Doppler ultrasound spectral enhancement using the Gabor transform based spectral subtraction”, see Ibid, Vol. 10 P1861-1868, 2005. These methods are directed to reducing the background noise in time domain by using wavelet transform without subsampling, or by utilizing Gabor transform and Gabor expansion. But, these methods are not suitable to the practical applications for their high cost.
Therefore, there is still a need to provide a new method and apparatus capable of reducing the background noise in a Doppler system with high reliability and low cost.