Image Processing Reference
In-Depth Information
frequency range of -fs/2 to +fs/2 on the baseline (0Hz) shown in Fig. 12(a). At the time (A) in
Fig. 12, the frequency of the spectrum exceeds +fs/2 and aliasing is induced. The Doppler
ultrasound system has an anti-aliasing display function (BLS: baseline-shift) that shifts a
baseline to a negative side, as shown in Fig. 12(b), and expands a positive velocity range
seemingly. Thus we can measure the peak velocity of blood flow easily. The power
spectrum at the zero baseline-shift is shown in Fig. 13(a). The spectrum image at the -0.25*fs
baseline-shift and the power spectrum corresponding to the time (A) in Fig. 12 are shown in
Fig. 13(b). In the spectrum image, a baseline-shift is easily realized by changing the
frequency read-out operation of the spectrum after FFT processing. However, since there is
no baseline-shift function in the Doppler audio, a baseline-shift is not realized in spectrum
imaging and Doppler audio processing. For example, although a negative-component is lost
in the spectrum image shown in Fig. 13(b), since Doppler sound is still in the state shown in
Fig. 13(a), it displays a negative-output and does not correspond to the Doppler image.
 5
0
fs
 75
0
fs
(A)
 5
0
fs
Baseline
Baseline
(A)
Freq
Freq
Time
Time
 5
0
fs
 25
0
fs
(b) baseline-shift=-0.25・ fs
(a) baseline-shift=0
Fig. 12. Spectrum Doppler image
Power
+NF :
Nyqui s t freq of f o rward
-NF :
Ny q uist freq o f reverse
Freq
 5
1
fs
 0
1
fs
 5
0
fs
0
 5
0
fs
 0
1
fs
 5
1
fs
reverse
forward
(a) baseline-shift=0
(a) Display area
reverse
forward
(b) baseline-shift=-0.25・ fs
(b) Display area
Fig. 13. Spectrum display area and baseline shift.
4.2 Anti-aliasing processing of Doppler audio and its target performance
To solve the problem of the spectrum image and Doppler audio not working together, we
examined the signal processing system of the Doppler audio to determine the possible type
of baseline-shift. On the other hand, since IQ-signals after quadrature-detection had little
merit at a small operation load in narrow-band processing, we examined a realization
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