Image Processing Reference
In-Depth Information
3.
All the systems fill the frequency characteristic. However, the frequency characteristics
near the DC and near the Nyquist region are dependent on the filter characteristics of
each processing system.
4. Signal processing for Doppler audio anti-aliasing
The direction separation system of the foregoing section is developed further, and the
Doppler audio technology exceeding the Nyquist frequency is examined. Some direction-
separation systems for a Doppler audio that is interlocked with the baseline-shift of a
spectrum image are investigated. First, section 4.1 explains a problem peculiar to the
Doppler audio corresponding to the Doppler display processing. In section 4.2 we defined
the target performance of anti-aliasing Doppler audio processing selected three kinds of
signal-processing systems. In section 4.3 the various systems of the
modulation/demodulation system, the FFT/IFFT system and the complex IIR Filter system
are explained. Next, in section 4.4 the signal-processing algorithms are compared with the
target performances. It was confirmed that the complex IIR band-pass filter system has an
excellent response and a low calculation load. Finally, in section 4.5 using the blood-flow
data collected from Doppler phantom, we performed functional and performance analyses
by simulation shown in Fig. 22.
4.1 Anti-aliasing display and conventional problem
The Doppler ultrasound system extracts the blood flow component used in the quadrature-
detection of the Doppler signal from the blood (mainly an erythrocyte), which moves inside
a blood vessel, and removes a reflective signal from tissue, such as a blood vessel wall with
a high-pass filter, and transforms the Doppler component into an image and sound. The
Doppler ultrasound system is shown in Fig. 11. The signal obtained after HPF processing is
divided into two lines. Spectrum image processing generates a Doppler signal as a spectrum
time change image corresponding to blood velocity, and Doppler audio processing outputs
direction separation signals as stereo sound from the right-and-left speakers.
Tx/Rx
Processing
B-Mode Image Processing
Display
Probe
Spectrum
Image Proc
Quadrature
detection
Left Speaker
Right Speaker
Doppler
Audio Proc
HPF
Fig. 11. Doppler ultrasound system.
Because the Doppler signal contains phase information, the signal includes both positive-
side (forward) and negative-side (reverse) frequency components. If sampling frequency is
set to be fs , the detection of a Doppler frequency component corresponding to the frequency
range of -fs/2 to +fs/2 is possible. A spectrum image is shown in Fig. 12. The horizontal axis
corresponds to time. The vertical axis corresponds to the velocity derived from Doppler shift
frequency, and luminosity corresponds to the spectrum intensity of each time. Since a
spectrum image is a power spectrum generated by complex FFT processing, it has the
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