Ut testing add01b-equipment calibrations

Addendum-01bEquipment CalibrationMy ASNT Level III UT Study Notes2014-June.

Pulse-Echo Instrumentation

The Circuitry:

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

Transmitter

ReceiverAmplifier Detector Scan

Converter

Display

TGC – Time Gain Compensation Circuit

Pulse-Echo InstrumentationPulser Components

1. HV pulse generator

2. The clock generator

3. The transducer

Generated WaveApplied Voltage

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

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

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

Normal Gain

Saturation Level

Distance

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

KNEE MAX GAIN

NEAR GAIN

GaindB

Depth cm

KNEE MAX GAIN

NEAR GAIN

Body wall

GaindB

Depth cmDELAY

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

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

10x10y

X, Y ADDRESS

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

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.

% AvailableShade of gray

Echo Strength

POST PROCESSING

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

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

DataPre-Processing

Display

EchoSignal

PositionalData

1. ROM

2. PROM

3. RAM

65. In Figure 3, transducer A is being used to establish:

A. Verification of wedge angleB. Sensitivity calibrationC. ResolutionD. An index point

66. In Figure 3, transducer C is being used to check:

A. Distance calibrationB. ResolutionC. Sensitivity calibrationD. Verification of wedge angle

67. In Figure 3, transducer D is being used to check:

A. Sensitivity calibrationB. Distance calibrationC. ResolutionD. Verification of wedge angle

68. When the incident angle is chosen to be between the first and second critical angles, the ultrasonic wave generated within the part will be:

A. LongitudinalB. ShearC. SurfaceD. Lamb

69. In Figure 4, transducer B is being used to check:

A. The verification of wedge angleB. ResolutionC. Sensitivity calibrationD. Distance calibration

Q: In a UT test system where signal amplitudes are displayed on a CRT, an advantage of a frequency-independent attenuator over a continuously variable gain control is that:

A. the pulse shape distortion is lessB. the signal amplitude measured using the attenuator is independent

of frequencyC. the dynamic range of the system is decreasedD. the effect of amplification threshold is avoided

Q: An amplifier in which received echo pulses must exceed a certain threshold voltage before they can be indicated might be used to:

A. suppress amplifier noise, unimportant scatter echoes, or small flaw echoes which are of no consequence

B. provide a screen display with nearly ideal vertical linearity characteristicsC. compensate for the unavoidable effects of material attenuation lossD. provide distance amplitude correction automatically

Q: The output voltage from a saturated amplifier is:

A) 180 degrees out of phase from the input voltageB) lower than the input voltageC) nonlinear with respect to the input voltageD) below saturation

Q: The transmitted pulse at the output of the pulser usually has a voltage of 100 to 1000V, whereas the voltages of the echo at the input of the amplifier are on the order of:

A) 10 VoltsB) 50 VoltsC) .001 to 1 VoltsD) 1 to 5 Volts

Q: The intended purpose of the adjustable calibrated attenuator of a UT instrument is to:

A) control transducer dampeningB) increase the dynamic range of the instrumentC) broaden the frequency rangeD) attenuate the voltage applied to the transducer

Ut testing add01b-equipment calibrations

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