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Chapter 4: image instruments
Beam Former
音束形成器
Analog-to-digital
converters
BOX
4-1 Function
of the Beam Former
Generate voltages that drive the transducer.
Determine pulse repetition frequency, coding, frequency, and intensity.
Scan, focus, and apodize the transmitted beam.
Amplify the returning echo voltage.
Compensate for attenuation.
Digitize the echo voltage stream.
Direct, focus,增幅器
and apodize the reception beam.
An ultrasound pulse is shown as B.
The ultrasound PRF is equal to the voltage PRF.
The PRF range is from 4 to 15 kHz.
Image depth = pen x PRF <= 77 (cm/ms).
The pulser generates the voltages that drive the transducer.
T = (2*pen)/1.54
Example: Frequency = 5 MHz
Image depth = 15 cm
The round-trip travel time (13 us/cm x 15 cm = 195 us)
Image using 3 MHz frequency
Image using 5 MHz frequency
Coded Excitation
Code excitation uses a series of pulses and gaps rather than a single driving pulse.
A shorter and stronger pulse yielding good resolution and sensitivity.
Coded Excitation
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Coded excitation has been applied in radar.
Pulse compression in the conversion, using a matched filter, of a relative
long coded pulse to one of short time duration, excellent resolution, and
equivalent high intensity and sensitivity.
A match filter maximizes the signal-to-noise ratio (SNR) of the returning
signal.
The longer the coded pulse ,the higher the SNR in matched-filter
implementations will be.
It is also called “Barker codes”.
– Reference:
• http://mathworld.wolfram.com/BarkerCode.html
• http://en.wikipedia.org/wiki/Barker_code
•
An better match scheme is called “Golay codes”.
– Using pairs of transmitted pulses with the second being a bipolar sequence in
which the latter portion of the pulse is the inverse of the first.
– Reference:
• http://en.wikipedia.org/wiki/Binary_Golay_code
• http://mathworld.wolfram.com/GolayCode.html
• http://www-math.mit.edu/phase2/UJM/vol1/MKANEM~1.PDF
•
Bipolar: http://en.wikipedia.org/wiki/Bipolar
A channel is an independent signal path consisting of
a transducer element, delay, and possibly other electronic
components
An increased number of channels allows
more precise control of beam characteristics.
Transmit/receive (T/R) switch
A. Amplification (gain) increases voltage amplitude and electric power.
B. A gain of 3 dB corresponds to an output power equivalent
to input power x 2; 10 dB corresponds to an input power x 10.
See details in Table 4-1.
E. Gain is too low.
F. Proper gain
G. Gain is too high.
Gain is set subjectively so that echoes
appear with appropriate brightness.
I. Abdominal (腹部) scan with low gain.
H. Abdominal (腹部) scan with proper gain.
Time Gain Compensation (TGC)
• Compensation equalizes difference in
received echo amplitudes cause by
different reflector depths.
• TGC compensates for the effect of
attenuation on image.
• TGC controls are adjusted to yield onaverage uniform brightness over image.
Figure 4-14
High gain, electronic noise can be seen on image.
Depth Gain Compensation (DGC)
Figure 4-13
Lateral Gain Control
Figure 4-14
Analog-to-Digital Converters (ADCs)
(Digitizer)
1. The higher sampling rates of the ACDS
,the better the temporal detail of the voltage is preserved.
2. ADC convert the analog voltage representing echoes to
numbers for digital signal processing and storage.
Echo Delays and Summer
• The echo voltage pass through digital
delay lines to accomplish reception
dynamic focus and steering functions.
• Summer is added signals together to
produce resulting scan line along with all
the others.
Signal Processor (訊號處理)
• Filtering
– Bandpass filtering
• Detection
– Amplitude detection (radio frequency to video)
• Compression
– Dynamic range reduction
Filtering (Bandpass filter)
• Reducing electronic noise.
• A tuned amplifier is simply an amplifier
with an electronic filter called bandpass
filter.
Fig. 17
Harmonic Image (諧波影像)
(See details in page 22 of Chapter 2, and page 3 in refrence book)
• Demo in harmonic.xls