Image Processing Reference
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3. Comparison of six kinds of Doppler audio processing
The digitization of Doppler ultrasound system had progressed in recent years, and the
digital signal processing using DSP etc. can realize complex processing easily from the
conventional analog-circuit. We made the target performances of the direction separating
process of digital Doppler audio, and evaluated six kinds of digital-signal processing ideas
that were pre-existing or were newly devised.
3.1 Design of a target specification
For the digitization, the target performance is investigated and taken up to Table 1.
item
target
1. time-delay
bellow 20ms (PRF 4kHz)
2. direction separation
above 30dB
fs/128 to 63*fs/128 (both direction)
flat as possible
4. frequency resolution fs/100
5. calculation volume light as possible
Table 1. Requirement specification of Doppler audio direction separation
3. frequency characterization
Time-delay:
A user usually sets up the Doppler range gate on a tomogram, moves it, and performs blood
flow diagnosis with the Doppler ultrasound system. In searching for a small blood vessel,
the Doppler audio is effective, because its response is faster than that of the spectrum image.
This is because a tomogram set with the Doppler audio delays the outputs of about 20 ms,
compared with the spectrum image that has a typical delay of about 40 ms. The time delay
of tomogram processing is a few cycle of one frame (13.3 - 16.7 ms). In the Doppler signal
processing system, it has a total processing delay of 10 ms by quadrature-detection and HPF
processing, except for the Doppler processing part. Therefore, to make the tomogram and
audio agree, a time delay of 3.3 - 6.7 ms is required at the Doppler signal processing part.
However, because the direction separation process, which is the main factor of the Doppler
signal processing part delay, requires a number of series samplings for processing, a target
time delay is theoretically difficult to achieve. Therefore, the target time delay was set to be
20 ms or smaller, so that the target delay time required for the direction separation process
to store the Doppler audio is about one frame cycle at maximum in a tomogram.
Direction separation:
It has been reported that human's direction distinction requires a right-and-left signal
difference of 15 to 20 dB or lager. In an actual Doppler ultrasound system, considering that
the Doppler signal has a broad band, that the angle between the right-and-left speakers is
small, and that blood flow velocity changes with time, a larger signal difference is required.
The target performance of direction separation was set to be 30 dB or higher at observation
frequency.
Frequency characteristic:
A signal processing frequency range is the range fs from negative-side Nyquist frequency to
positive-side Nyquist frequency, where fs is input IQ-signal sampling frequency. We made
the frequency characteristic flat in the region of
fs
/ 128
to
 
63
fs
/ 128
range.
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