Image Processing Reference
In-Depth Information
flow. Also inside of blood vessel, a blood vessel wall and a transmit-wavelength influence
the blood-flow signal. Then, in order to prevent the saturation of the frequency analysis or
the Doppler audio processing, a wall-filter is arranged in pre-processing of them. The wall-
filter is HPF with high order cut-off property. The details of Spectrum Doppler signal
processing are shown in Fig. 2(c). Range gate processing is the integration of L(t) in the
range direction in the range gate. Wall-filter processing removes a clatter component. The
complex IQ-signal x(t) after these processing is inputted into the spectrum Doppler display
processing and the Doppler audio processing. The former displays the spectrum Doppler as
a time-change image of a flow velocity. The latter separates the direction of Doppler signal,
and outputs them as stereo sounds from a right-and-left speaker.
Probe
T&R Proc
Tx waveform
Envelope detection
t
B mode
Image Proc
Tx Fr e q
fp
1/ fs
Rx waveform
t
Phase detection
t
R1
M(t)
R(t)
L(t)
Spectrum
Doppler Proc
R1
R2
R3
LPF
R2
R3
fp
(a) The outline of signal-processing of ultrasound system
R3
R1
R2
Power
Power
Power
Doppler
Clutter
- s /2
0
- s /2
0
+ s /2
- s /2
0
+ s /2
+ s /2
(b) Spectra od baseband IQ-signal
Display
Output
L(t)
X(t)
Wall Filter
(HPF)
FFT/Power
Calc
Spectrum
Image Proc
Range Gate
Audio
Output
Direction
Separation
Audio Proc
Spectrum Doppler Processing
(c) Spectrum Doppler processing
Fig. 2. Doppler ultrasound signal-processing.
2.2 Conventional analog signal-processing
An analog phase-shift processing system that consists of all-pass filters has been used in the
direction separation processing. The outline of it is shown in Fig. 3. This is a processing
system that shifts the phase between the IQ-signals of 90 degree, and adds them or subtracts
them. Since an all-pass filter has the characteristic that the phase reverses on cut-off
frequency, this system shifts the phase in a target frequency range combining all-pass filter
arrays. If it assumes that the input IQ-signal x(t) has a frequency component of
 .
xt
() exp(
j
t
)
(4)
d
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