Ut testing instrumentation

Pulse-Echo Instrumentation

The Circuitry:

Voltage activation of the PE crystal Ultrasound formation Propagation Reflection Charge formation of crystal Processing Display

Transmitter

TGC

ReceiverAmplifier Detector Scan

Converter

Display

TRX


TGC – Time Gain Compensation Circuit

Pulse-Echo InstrumentationPulser Components

1. HV pulse generator

2. The clock generator

3. The transducer

TIME

TIME

+ +

-

P

Generated WaveApplied Voltage

V

-


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.


PULSER

ATTENUATOR

TRX


CLOCK GENERATOR

Controls the actual number of pulses which activate the crystal.Responsible for sending timing signal to the

1. Pulse generator2. TGC circuitry3. Memory


CLOCKGENERATOR

HVGENERATOR

TGC UNIT

MEMORY

CRT DISPLAY

TRS

TRX


Sensitivity refers to the weakest echo signal that the instrument is

capable of detecting and displaying.

Factors that determine sensitivity are

1. Transducer frequency2. Overall and TGC receiver gain3. Reject control4. Variable focal zone on array real-time instruments.


Increasing the voltage causes

1. Greater amplitude – greater penetration

2. Longer pulses – degrades axial resolution

3. Increase exposure


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.


The Receiver Unit consist of

1. Radiofrequency Amplifier

2. Time gain compensation TGC unit

3. Demodulation Circuit

4. Detector Circuit

5. Video Amplifier


PULSER TGC UNITMEMORY

CRT DISPLAY

TRSTRXRF

RECEIVER

DEMODULATOR

DETECTOR

VIDEOAMPLIFIER

Radio-Frequency Amplifier

• Amplify weak voltage signals.

• This is called GAIN


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 OutVoltage In


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


Saturation Level

Distance

Am

plitu

de

Normal Gain


Saturation Level

Distance

Am

plitu

de

Excess Gain


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

TGC - electronic process of adjusting the overall system gain as a function of the transmit time.


TGC Controls

• Near Gain

• Slope Delay

• Slope

• Knee

• Far Gain

• Body Wall


GaindB

Depth cmDELAY

SLOPE

KNEE MAX GAIN

NEAR GAIN


GaindB

Depth cm

SLOPE

KNEE MAX GAIN

NEAR GAIN

Body wall


GaindB

Depth cmDELAY

SLOPE

KNEE

CUT-OFF


The slide potentiometer allows adjustment of receiver gain for small discrete depth increments.


GaindB

Depth (Time)

Slide Potentiometer


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.


Types of Amplifiers

• Wide-Band• Narrow-Band


Frequency MHz

Gain Gain

Frequency MHz

Wide-band amplifier Narrow-band amplifier


Receiver B

Receiver A

Receiver C

Receiver D

TRXOutputTo System

Frequency SelectorSwitch

Receiver Unit


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.


Large range in signal amplitudes is due to:

1. Normal variation in the reflection amplitude.2. Frequency dependent tissue attenuation.


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.


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


Input signal

GainA

B

Linear Amplification

Logarithmic Amplification


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 transducerhousing reduces system noise.


REJECTIONRejection 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.


NoiseLevel

DynamicRange

Saturation Level

Rejection Level

Zero Signal Level


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.


DEMODULATION

Rectification

Detection


RECTIFICATION

• Rectification results in the elimination of the negative portion of the RF signals

• Half Wave Rectification

• Full wave Rectification


Half-WaveRectification


Full-WaveRectification


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.


Smoothing


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


DIGITAL SCAN CONVERTER

The device that stores the echo signal is called a Scan converter.


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


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

1x1y

3x3y

5x5y

8x7y

10x10y

X, Y ADDRESS

In the Scan converter the echoes are processed on a first-come first-in basis.


XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

5050

5050

5050

5050

5050

5050

5050

5050

5050

5050

5050

5050

Raster Process

DIGITAL SCAN CONVERTER

• Convert echo voltage signal into a numerical value.

• Each numerical value corresponds to a shade of gray.


The number of shades of gray is determined by the BIT CAPACITY.

# of shades of gray = 2


EchoesdB


214283

16432564612872568

Shades of GrayBit


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.


100%

50%

0%

% AvailableShade of gray

Echo Strength

1

2

34


POST PROCESSING

Assignment of specific display brightness to numerical echo amplitudes read out ofthe digital memory.


9887

7898

8879

8888

8888

8888

SMOOTHING


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


Zoom Magnification

• Read Zoom

• Write Zoom


Resolution at the DSC

1. Find Matrix size

2. Determine FOV ( width/length)

3. Calculate pixels/cm

4. Find linear distance/pixel = resolution


DataReformatting

DataPost-Processing

DataCollection&Formatting

ADC

DataPre-Processing

Display

RAM

EchoSignal

PositionalData


1. ROM

2. PROM

3. RAM


Engineering

Ut testing instrumentation