Addendum-01b Equipment Calibration

Addendum-01b 

Equipment Calibration 

My ASNT Level III UT Study Notes 

2014-June.

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


Pulser Components 

1. HV pulse generator 

2. The clock generator 

3. The transducer


Applied Voltage 

Generated Wave 

V 

+ + 

P 

TIME 

- 

TIME 

-


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 

TRX 

ATTENUATOR


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


CLOCK 

GENERATOR 

TGC UNIT 

HV 

GENERATOR 

MEMORY 

TRS 

TRX 

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 frequency 

2. Overall and TGC receiver gain 

3. Reject control 

4. 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 UNIT 

MEMORY 

TRX 

TRS 

RF 

RECEIVER 

CRT 

DISPLAY 

DEMODULATOR 

DETECTOR 

VIDEO 

AMPLIFIER


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 Out 

Voltage 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


Amplitude 

Saturation Level 

Normal Gain 

Distance


Excess Gain 


Amplitude 

Distance


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


KNEE 

MAX GAIN 

Gain 

dB 

NEAR GAIN 

SLOPE 

DELAY 

Depth cm


KNEE 

MAX GAIN 

Gain 

dB 

NEAR GAIN 

SLOPE 

Body wall 

Depth cm


KNEE 

Gain 

dB 

SLOPE 

CUT-OFF 

DELAY 

Depth cm


The slide potentiometer allows adjustment of receiver gain for 

small discrete depth increments.


Slide Potentiometer 

Gain 

dB 

Depth (Time)


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


Wide-band amplifier 

Narrow-band amplifier 

Gain 

Gain 

Frequency MHz 

Frequency MHz


Receiver Unit 

Receiver A 

TRX 

Receiver B 

Receiver C 

Output 

To 

System 

Frequency 

Selector 

Switch 

Receiver D


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


Gain 

A 

Linear Amplification 

B 

Logarithmic Amplification 

Input signal


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 

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.



Rejection Level 

Dynamic 

Range 

Zero Signal Level 

Noise 

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-Wave 

Rectification


Full-Wave 

Rectification


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

10x 

10y 

X, Y ADDRESS 

8x 

7y 

5x 

5y 

3x 

3y 

1x 

1y


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


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


Echoes 

dB


Bit 

1 

2 

3 

4 

5 

6 

7 

8 

Shades of Gray 

2 

4 

8 

16 

32 

64 

128 

256


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.


% Available 

Shade of gray 

100% 

1 

2 

50% 

3 

4 

0% 

Echo Strength


POST PROCESSING 

Assignment of specific display brightness 

to numerical echo amplitudes read out of 

the digital memory.


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


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


Data 

Pre- 

Processing 

RAM 

Data 

Post- 

Processing 

ADC 

Data 

Collection 

& 

Formatting 

Data 

Reformatting 

Echo 

Signal 

Positional 

Data 

Display


1. ROM 

2. PROM 

3. RAM

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

A. Verification of wedge angle 

B. Sensitivity calibration 

C. Resolution 

D. An index point

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

A. Distance calibration 

B. Resolution 

C. Sensitivity calibration 

D. Verification of wedge angle 

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

A. Sensitivity calibration 

B. Distance calibration 

C. Resolution 

D. 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. Longitudinal 

B. Shear 

C. Surface 

D. Lamb

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

A. The verification of wedge angle 

B. Resolution 

C. Sensitivity calibration 

D. 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 less 

B. the signal amplitude measured using the attenuator is independent 

of frequency 

C. the dynamic range of the system is decreased 

D. 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 characteristics 

C. compensate for the unavoidable effects of material attenuation loss 

D. provide distance amplitude correction automatically

Q: The output voltage from a saturated amplifier is: 

A) 180 degrees out of phase from the input voltage 

B) lower than the input voltage 

C) nonlinear with respect to the input voltage 

D) 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 Volts 

B) 50 Volts 

C) .001 to 1 Volts 

D) 1 to 5 Volts

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

instrument is to: 

A) control transducer dampening 

B) increase the dynamic range of the instrument 

C) broaden the frequency range 

D) attenuate the voltage applied to the transducer

Addendum-01b Equipment Calibration

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