Pulse-Echo Instrumentation
The Circuitry:
Voltage activation of the PE crystal Ultrasound formation Propagation Reflection Charge formation of crystal Processing Display
Pulse-Echo Instrumentation Transmitter
TRX
Receiver Amplifier
Detector
Scan Converter Display
TGC
TGC – Time Gain Compensation Circuit
Pulse-Echo Instrumentation Pulser Components 1. HV pulse generator 2. The clock generator 3. The transducer
Pulse-Echo Instrumentation Generated Wave
Applied Voltage
+
+
V
P TIME
TIME
-
-
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation Switch that controls the output power of the HV generator is the attenuator.
Pulse-Echo Instrumentation
TRX
PULSER
ATTENUATOR
Pulse-Echo Instrumentation CLOCK GENERATOR Controls the actual number of pulses which activate the crystal. Responsible for sending timing signal to the 1. Pulse generator 2. TGC circuitry 3. Memory
Pulse-Echo Instrumentation CLOCK GENERATOR
TGC UNIT
HV GENERATOR
MEMORY
TRS
TRX
CRT DISPLAY
Pulse-Echo Instrumentation Sensitivity refers to the weakest echo signal that the instrument is capable of detecting and displaying. Factors that determine sensitivity are 1. 2. 3. 4.
Transducer frequency Overall and TGC receiver gain Reject control Variable focal zone on array real-time instruments.
Pulse-Echo Instrumentation Increasing the voltage causes 1. Greater amplitude – greater penetration 2. Longer pulses – degrades axial resolution 3. Increase exposure
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation The Receiver Unit consist of 1. Radiofrequency Amplifier 2. Time gain compensation TGC unit 3. Demodulation Circuit 4. Detector Circuit 5. Video Amplifier
MEMORY PULSER
TRX
TGC UNIT
TRS
RF RECEIVER CRT DISPLAY DEMODULATOR
DETECTOR
VIDEO AMPLIFIER
Pulse-Echo Instrumentation Radio-Frequency Amplifier • Amplify weak voltage signals. • This is called GAIN
Pulse-Echo Instrumentation 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 Voltage In
Pulse-Echo Instrumentation The Imaging effect of adjusting gain are: 1. Increasing the gain - increased sensitivity, better penetration 2. Decreasing the gain – decreased sensitivity, less penetration 3. Too high a gain – overloads the display, loss or spatial resolution
Amplitude
Pulse-Echo Instrumentation
Saturation Level
Normal Gain
Distance
Pulse-Echo Instrumentation Excess Gain Amplitude
Saturation Level
Distance
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation Time Gain Compensation TGC TGC - electronic process of adjusting the overall system gain as a function of the transmit time.
Pulse-Echo Instrumentation TGC Controls • Near Gain • Slope Delay • Slope • Knee • Far Gain • Body Wall
Pulse-Echo Instrumentation KNEE
Gain dB
NEAR GAIN
DELAY
SLOPE
Depth cm
MAX GAIN
Pulse-Echo Instrumentation KNEE
Gain dB
NEAR GAIN SLOPE
Depth cm Body wall
MAX GAIN
Pulse-Echo Instrumentation KNEE
Gain dB
SLOPE CUT-OFF
DELAY
Depth cm
Pulse-Echo Instrumentation The slide potentiometer allows adjustment of receiver gain for small discrete depth increments.
Pulse-Echo Instrumentation Slide Potentiometer
Gain dB
Depth (Time)
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation Types of Amplifiers • Wide-Band • Narrow-Band
Pulse-Echo Instrumentation Wide-band amplifier
Narrow-band amplifier
Gain
Gain
Frequency MHz
Frequency MHz
Pulse-Echo Instrumentation Receiver Unit
Receiver A
Receiver B Output To System
TRX Receiver C
Frequency Selector Switch
Receiver D
Pulse-Echo Instrumentation DYNAMIC RANGE 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.
Pulse-Echo Instrumentation Large range in signal amplitudes is due to: 1. Normal variation in the reflection amplitude. 2. Frequency dependent tissue attenuation.
Pulse-Echo Instrumentation RF amplifier can handle a wide range of signal amplitude at its input – but cannot accommodate the corresponding output using linear amplification.
Pulse-Echo Instrumentation Linear amplification - all voltages amplitudes, regardless of size at the point of input are amplified with the same gain factor.
Pulse-Echo Instrumentation LOGARITHMIC AMPLIFICATION 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
Pulse-Echo Instrumentation
A
Linear Amplification
Gain
B Logarithmic Amplification
Input signal
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation Pre-amplification is a technique to reduce system noise. Positioning of part of the amplifier circuitry in the transducer housing reduces system noise.
Pulse-Echo Instrumentation REJECTION 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.
Pulse-Echo Instrumentation Saturation Level
Rejection Level Dynamic Range Noise Level Zero Signal Level
Pulse-Echo Instrumentation SIGNAL PROCESSING 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.
Pulse-Echo Instrumentation DEMODULATION Rectification Detection
Pulse-Echo Instrumentation RECTIFICATION • Rectification results in the elimination of the negative portion of the RF signals •
Half Wave Rectification
•
Full wave Rectification
Pulse-Echo Instrumentation Half-Wave Rectification
Pulse-Echo Instrumentation Full-Wave Rectification
Pulse-Echo Instrumentation DETECTION 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.
Pulse-Echo Instrumentation Smoothing
Pulse-Echo Instrumentation The video signal is then further amplified by the VIDEO AMPLIFIER. The output from the video amplifier is forwarded to 1. CRT or 2. Scan converter
Pulse-Echo Instrumentation DIGITAL SCAN CONVERTER The device that stores the echo signal is called a Scan converter.
Pulse-Echo Instrumentation All Scan Converters are designed to 1. Store echoes in appropriate location 2. Encode echoes in shade of gray 3. Read out echoes in a horizontal raster format
Pulse-Echo Instrumentation 4. Digital Memory is divided into small squares = Pixel. 5. The Pixels form the Image Matrix 6. Total # of storage location = rows x columns 7. x and y location = ADDRESS
Matrix
Rows x, coordinates
Matrix
Columns, y coordinates
Matrix Pixel
10x 10y
X, Y ADDRESS 8x 7y
5x 5y
3x 3y
1x 1y
Pulse-Echo Instrumentation In the Scan converter the echoes are processed on a firstcome first-in basis.
X X X
X
X X
X X X
X X X
X X X
X
X X
X X X
X X X
50
50
50
50
50
50
50
50 50
50 50
50
Raster Process
50
50
50
50
50
50
50
50 50
50 50
50
Pulse-Echo Instrumentation DIGITAL SCAN CONVERTER • Convert echo voltage signal into a numerical value. •
Each numerical value corresponds to a shade of gray.
Pulse-Echo Instrumentation The number of shades of gray is determined by the BIT CAPACITY. # of shades of gray = 2
Pulse-Echo Instrumentation Echoes dB
Pulse-Echo Instrumentation Bit
Shades of Gray
1
2
2
4
3
8
4
16
5
32
6
64
7
128
8
256
Pulse-Echo Instrumentation Gray Scale Resolution = dynamic range (dB) # of gray shades
Pulse-Echo Instrumentation 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.
Pulse-Echo Instrumentation % Available Shade of gray
100% 1 2 50% 3
4
0% Echo Strength
Pulse-Echo Instrumentation POST PROCESSING Assignment of specific display brightness to numerical echo amplitudes read out of the digital memory.
Pulse-Echo Instrumentation 9
7
8
8
8
8
8
8
8
9
8
7
8
8
8
8
7
8
8
9
8
8
8
8
SMOOTHING
Pulse-Echo Instrumentation The DSC is not necessary for image display, but is needed for the following post-processing functions. • Video Invert • Display Invert • Display Subdivision • Zoom Magnification
Pulse-Echo Instrumentation Zoom Magnification • Read Zoom • Write Zoom
Pulse-Echo Instrumentation Resolution at the DSC 1. Find Matrix size 2. Determine FOV ( width/length) 3. Calculate pixels/cm 4. Find linear distance/pixel = resolution
Pulse-Echo Instrumentation Data PreProcessing
ADC
Echo Signal
Data Collection & Formatting
M A R
Data PostProcessing
Data Reformatting
Positional Data
Display
Pulse-Echo Instrumentation 1. ROM 2. PROM 3. RAM