Ut testing instrumentation

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