Pulse-Echo Imaging Instrument. Pulse-Echo Instrumentation. Voltage activation of the PE crystal Ultrasound formation Propagation Reflection Charge formation of crystal Processing Display. Pulse-Echo Instrumentation. Transmitter. Display. Receiver Amplifier. Detector. Scan Converter.
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The Pulser rate is known as the pulse repetition frequency (PRF).
Typical PRF 3,000 – 5,000.
PRF automatically adjusted as a function of imaging depth.
Switch that controls the output power of the HV generator is the attenuator.
Transducer has dual roles; transmitting and receiving signals.
The transducer is capable of handling a wide range of voltage amplitude.
The Receiver is capable of handling only smaller signals
Therefore it is desirable to isolate the pulser circuit from the receiver circuit.
The Transmit Receive Switch
TRS – positioned at the input of the receiver and is designed to pass only voltages signals originating at the transducer by the returning echoes.
Electric signals generated by the transducer
are weak and needs amplification.
The gain is the ratio of the output to input
Voltage or Power.
Gain = Voltage Out
Primary objective of grayscale pulse-echo imaging is to make all like reflectors appear the same in the Image regardless where they are located in the sound beam.
Time Gain Compensation
TGC - electronic process of adjusting the overall system gain as a function of the transmit time.
The slide potentiometer allows adjustment of receiver gain for
small discrete depth increments.
Frequency Tuning of the Receiver
The frequency band width of the receiver refers to the range of ultrasound signal frequencies that the receiver can amplify with a maximum gain.
The dynamic range is a measure of the range of echo signal amplitudes.
The dynamic range can be measured at any point.
The dynamic range decreases from transducer, to receiver to scan converter and finally to display.
RF amplifier can handle a wide range of signal amplitude at its input – but cannot accommodate the corresponding output using linear amplification.
Linear amplification - all voltages amplitudes, regardless of size at the point of input are amplified with the same gain factor.
In Logarithmic amplification weak echoes amplitudes are amplified more than strong echoes.
This can reduced the dynamic range by as much as 50%.
The process of reducing the signal DR by electronic means is called COMPRESSION
R-F amplifier can also set the electronic level in the machine.
S-N level – compares real echo signals the system can handle versus the non-echo signals presents (Noise).
The Higher the SN ratio – better the operation of the system.
Pre-amplification is a technique to reduce system noise.
Positioning of part of the amplifier circuitry in the transducer housing reduces system noise.
Rejection is the receiver function that enables the operator to systematically increase or decrease the minimum echo signal amplitude which can be displayed.
Alternate names = Threshold, Suppression.
Zero Signal Level
RF waveform – oscillating type of voltage signal (AC)
First Step in processing the signal is Demodulation.
Demodulation is the process of converting the electric signal from one form to another.
The main effect of detecting the rectified RF signal is to round out or smooth the signal as to have a single broad peak.
The rectified RF signal following detection is referred to as a Video Signal.
DIGITAL SCAN CONVERTER
The device that stores the echo
signal is called a Scan converter.
Rows x, coordinates
Columns, y coordinates
X, Y ADDRESS
In the Scan converter the echoes are processed on a first-come first-in basis.
The number of shades of gray is determined by the BIT CAPACITY.
# of shades of gray = 2
Gray Scale Resolution = dynamic range (dB)
# of gray shades
Operator can select different A/D conversion scheme (Preprocessing).
Each preprocessing curve is called an algorithm and assigns a specific percentage amount of shades of gray to regions of the echo amplitude.
Shade of gray
Assignment of specific display brightness
to numerical echo amplitudes read out of
the digital memory.