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©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 1
Pass Ultrasound Physics Exam
Study Guide Notes Volume I & Volume II
Second Edition
By
Mansoor Khan
MBBS, RDMS, RDCS
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 2
©Copyright 2014 Blue Cube Venture, LLC All rights reserved.
The Pass Ultrasound Physics Exam Study Guide Notes is protected by copyright. No part of this study
guide review may be reproduced in any form without written permission from the copyright owner.
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 3
Introduction
The second edition of the bestselling Pass Ultrasound Physics Exam Study Guide Notes is divided into two volumes, Volume I and Volume II. The volume I covers the topics such as Pulse Echo Instrumentation, Ultrasound transducers, Sound beam, Bioeffects, Intensity, Resolution and Quality assurance. The volume II covers the topics such as Doppler physical principles, Doppler spectral analysis, Hemodynamics, propagation of ultrasound wave through tissues, Artifacts, Ultrasound physics elementary principles, and Real time imaging. This book is devoted to the ARDMS SPI exam and the material is based on the ARDMS physics exam outline. It explains the concepts in very simple and easy to understand way. It also contains Important to Remember notes related to the topic which are SPI exam questions. If you are preparing to take ARDMS SPI exam and looking for an ultrasound book which can help you, the Pass Ultrasound Physics Exam Study Guide Notes is for you. You can increase your chances to pass ARDMS Ultrasound Physics and Instrumentation exam by memorizing the Important to Remember notes.
Thank you for giving me the opportunity to help you. I will be improving this study guide notes and adding more material in future. I am in this field for more than ten years and my goal is to provide help to students and sonographers to become successful in their field.
Mansoor Khan
MBBS, RDMS, RDCS
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 4
TableofContentsChapter 1 – Pulse Echo Instrumentation .................................................................................................... 15
Ultrasound System .................................................................................................................................. 15
Master Synchronizer ........................................................................................................................... 15
Transducer .......................................................................................................................................... 15
Output Power ...................................................................................................................................... 16
Pulsar................................................................................................................................................... 17
Beam Former ...................................................................................................................................... 19
Noise ................................................................................................................................................... 20
Electronic Noise .................................................................................................................................. 21
Electrical Interference ......................................................................................................................... 21
Receiver ................................................................................................................................................... 21
Amplification ....................................................................................................................................... 22
Pre Amplification ................................................................................................................................. 24
Compensation ..................................................................................................................................... 24
TGC Curve ............................................................................................................................................ 26
Compression ....................................................................................................................................... 27
Dynamic Range ................................................................................................................................... 29
Demodulation ..................................................................................................................................... 30
Reject .................................................................................................................................................. 32
Display ................................................................................................................................................. 34
LCD Display .......................................................................................................................................... 36
Display Modes ......................................................................................................................................... 38
A‐Mode (Amplitude Mode) ................................................................................................................. 38
B‐Mode (Brightness Mode) ................................................................................................................. 38
M‐Mode (Motion Mode) .................................................................................................................... 39
Storage .................................................................................................................................................... 40
Chapter 2 ‐ Scan Converter ......................................................................................................................... 41
Analog to Digital Scan Converter ........................................................................................................ 42
Digital to Analog Scan Converter ........................................................................................................ 43
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Preprocessing ...................................................................................................................................... 44
Post Processing ................................................................................................................................... 45
Read Magnification ................................................................................................................................. 45
Write Magnification ................................................................................................................................ 46
Pixel ..................................................................................................................................................... 46
Bit ........................................................................................................................................................ 46
Shades of gray ..................................................................................................................................... 48
Spatial Resolution ............................................................................................................................... 49
Harmonic Imaging ................................................................................................................................... 49
Chapter 3 ‐ Ultrasound Transducers ........................................................................................................... 50
Piezoelectric principle ............................................................................................................................. 51
Piezoelectric Material ............................................................................................................................. 51
Piezoelectric Effect .................................................................................................................................. 52
Reverse Piezoelectric Effect .................................................................................................................... 52
Polarization ............................................................................................................................................. 52
Depolarization ..................................................................................................................................... 52
Curie point .............................................................................................................................................. 53
Curie temperature .............................................................................................................................. 53
Sterilization ......................................................................................................................................... 53
Disinfection ......................................................................................................................................... 53
Chapter 4 ‐ Components of Ultrasound Transducer ................................................................................... 54
Active Element ........................................................................................................................................ 54
Acoustic Insulator ............................................................................................................................... 57
Wire ..................................................................................................................................................... 57
Matching Layer ....................................................................................................................................... 57
Backing Material ..................................................................................................................................... 59
Bandwidth ............................................................................................................................................... 61
Quality Factor .......................................................................................................................................... 63
Characteristics of Imaging Transducers .............................................................................................. 64
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Characteristics of Non Imaging Transducers ...................................................................................... 65
Pulsed Wave Transducers ................................................................................................................... 65
Continuous Wave Transducers ........................................................................................................... 65
Chapter 5 ‐ Types of Transducers ............................................................................................................... 66
Mechanical Transducer ........................................................................................................................... 66
Oscillatory or Rotary Type ................................................................................................................... 67
Oscillatory Mirror ................................................................................................................................ 67
Array Transducers ................................................................................................................................... 68
Linear Sequential Array (Linear Switched Array, Linear Array)........................................................... 68
Curved Sequential Array Transducer .................................................................................................. 69
Phased Array Transducers .................................................................................................................. 71
Linear Phased Array ............................................................................................................................ 73
Convex Phased Array Transducer ....................................................................................................... 74
Annular Phased Array Transducer ...................................................................................................... 74
Mechanically steered transducers ...................................................................................................... 76
Mechanical Array Transducers ............................................................................................................ 77
Focusing Techniques ............................................................................................................................... 78
Mechanical Focusing ........................................................................................................................... 78
Electronic Focusing ............................................................................................................................. 79
Transmit Focusing ............................................................................................................................... 79
Receive Focusing ................................................................................................................................. 79
Electronic Beam Steering ........................................................................................................................ 80
Coded Excitation ................................................................................................................................. 81
Frequency compounding .................................................................................................................... 81
1D Transducers ................................................................................................................................... 81
1.5 D Transducers ............................................................................................................................... 82
2D Transducers ................................................................................................................................... 82
Chapter 6 ‐ Sound Beam ............................................................................................................................. 82
Constructive Interference ....................................................................................................................... 82
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Destructive Interference ......................................................................................................................... 83
Sound Beam ............................................................................................................................................ 83
Near Zone (Fresnel Zone) .................................................................................................................... 84
Focal Depth (Focal Length) ..................................................................................................................... 85
Focal Point (Focus) .................................................................................................................................. 85
Far Zone (Fraunhofer Zone) .................................................................................................................... 86
Focal Zone ........................................................................................................................................... 86
Sound Beam Divergence ..................................................................................................................... 87
Diffraction ........................................................................................................................................... 88
Huygens Principle .................................................................................................................................... 89
Chapter 7 – Bioeffects ................................................................................................................................. 90
Bioeffects ................................................................................................................................................ 91
Thermal Effects ................................................................................................................................... 91
Thermal Index ..................................................................................................................................... 93
Cavitation ................................................................................................................................................ 93
Stable Cavitation ................................................................................................................................. 94
Transient Cavitation ............................................................................................................................ 95
Mechanical Index .................................................................................................................................... 95
Bioeffects Study Techniques ................................................................................................................... 97
Mechanistic Approach ........................................................................................................................ 97
Empirical Approach ............................................................................................................................. 97
Ultrasound contrast agents .................................................................................................................... 98
Chapter 8 – Intensity ................................................................................................................................... 99
Intensity .................................................................................................................................................. 99
Spatial .................................................................................................................................................. 99
Peak ................................................................................................................................................... 100
Average ............................................................................................................................................. 100
Temporal ........................................................................................................................................... 100
Pulsed ................................................................................................................................................ 100
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Spatial peak intensity (Isp) ................................................................................................................ 100
Spatial Average Intensity (Isa) ........................................................................................................... 100
Temporal Peak Intensity (Itp)............................................................................................................ 100
Imax or Im ......................................................................................................................................... 100
Pulse Average Intensity (Ipa) ............................................................................................................ 101
Temporal Average Intensity (Ita) ...................................................................................................... 101
SPTP ................................................................................................................................................... 102
SPTA .................................................................................................................................................. 102
SATA .................................................................................................................................................. 102
SPPA .................................................................................................................................................. 102
Beam Uniformity Coefficient ................................................................................................................ 104
Chapter 9 – Resolution .............................................................................................................................. 105
Spatial Resolution ................................................................................................................................. 105
Lateral Resolution ............................................................................................................................. 105
Axial Resolution................................................................................................................................. 108
Temporal Resolution ............................................................................................................................. 112
Factors Which Can Improve Temporal Resolution ........................................................................... 115
Factors Which Can Reduce Temporal Resolution ............................................................................. 115
Slice Thickness Resolution (Elevational Resolution) ............................................................................. 116
Contrast Resolution .............................................................................................................................. 116
Chapter 10 – Quality Assurance ................................................................................................................ 117
AIUM Test Object .................................................................................................................................. 117
Tissue Equivalent Phantom ................................................................................................................... 118
Doppler Phantom .................................................................................................................................. 118
Sensitivity .............................................................................................................................................. 120
Minimum system sensitivity ............................................................................................................. 120
Hydrophone ...................................................................................................................................... 120
Radiation Force Balance .................................................................................................................... 120
Sterilization ....................................................................................................................................... 121
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Disinfection ....................................................................................................................................... 121
Chapter 11 ‐ Doppler Physical Principles .................................................................................................. 122
Doppler Effect ....................................................................................................................................... 122
Doppler Shift ......................................................................................................................................... 122
Positive Doppler Shift ........................................................................................................................ 123
Negative Doppler Shift ...................................................................................................................... 123
No Doppler Shift ................................................................................................................................ 123
Doppler Equation .................................................................................................................................. 128
Pulsed Wave Doppler ............................................................................................................................ 129
Continuous Wave Doppler .................................................................................................................... 130
Dedicated Continuous Wave (Pedoff) .............................................................................................. 132
Color Flow Doppler ............................................................................................................................... 132
Ensemble Length ................................................................................................................................... 135
Aliasing .................................................................................................................................................. 135
Nyquist Limit ......................................................................................................................................... 136
Wall Filter .......................................................................................................................................... 138
Color Power Doppler (Energy Mode, Color Angio) ............................................................................... 138
Non Directional Doppler ....................................................................................................................... 139
Bi Directional Doppler ........................................................................................................................... 139
Chapter 12 ‐ Doppler Spectral Analysis..................................................................................................... 140
Spectral Analysis ................................................................................................................................... 140
Spectral Waveform ............................................................................................................................... 140
Fast Fourier Transform (FFT) ................................................................................................................. 140
Autocorrelation ..................................................................................................................................... 141
Phase Quadrate Detection .................................................................................................................... 142
Chapter 13 ‐ Hemodynamics ..................................................................................................................... 143
Energy ................................................................................................................................................... 143
Energy Gradient .................................................................................................................................... 143
Types of Energy ..................................................................................................................................... 143
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Kinetic Energy ................................................................................................................................... 144
Potential Energy ................................................................................................................................ 144
Gravitational Energy ......................................................................................................................... 144
Law of Conservation of Energy ............................................................................................................. 144
Velocity ................................................................................................................................................. 145
Flow ....................................................................................................................................................... 145
Types of Flow ........................................................................................................................................ 146
Steady Flow ....................................................................................................................................... 146
Pulsatile Flow .................................................................................................................................... 146
Phasic Flow ........................................................................................................................................ 147
Laminar Flow ..................................................................................................................................... 147
Turbulent Flow .................................................................................................................................. 148
Energy loss during blood flow and its causes ....................................................................................... 150
Friction Loss ...................................................................................................................................... 150
Inertial Loss ....................................................................................................................................... 150
Viscous Loss ...................................................................................................................................... 151
Viscosity ............................................................................................................................................ 151
Pressure ................................................................................................................................................ 152
Pressure Gradient ............................................................................................................................. 152
Hydrostatic Pressure ......................................................................................................................... 153
Venous Circulation ................................................................................................................................ 154
Veins .................................................................................................................................................. 155
Effect of Respiration on Venous Circulation ......................................................................................... 156
Venous return from upper extremities ............................................................................................. 157
Venous return from lower extremities ............................................................................................. 157
Hydrostatic Pressure ............................................................................................................................. 158
Continuity Equation .............................................................................................................................. 159
Continuity Rule ...................................................................................................................................... 160
Bernoulli’s Principle .............................................................................................................................. 160
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Bernoulli Effect ...................................................................................................................................... 160
Poiseuille’s Law ..................................................................................................................................... 161
Effects of stenosis on arterial blood flow: ............................................................................................ 162
Pulsatility Index (PI)............................................................................................................................... 163
Advantages of Pulsatility Index: ........................................................................................................ 163
Disadvantages of Pulsatility Index: ................................................................................................... 164
Resistive Index (RI) ................................................................................................................................ 164
Advantages of Resistive Index: ......................................................................................................... 164
Disadvantages of Resistive Index: ..................................................................................................... 165
Chapter 14 ‐ Propagation of Ultrasound Wave through Tissues .............................................................. 165
Propagation Speed ................................................................................................................................ 165
Density .................................................................................................................................................. 166
Stiffness ................................................................................................................................................. 166
Attenuation ........................................................................................................................................... 168
Decibel .................................................................................................................................................. 170
Factors That Contribute To Attenuation ............................................................................................... 171
Absorption ........................................................................................................................................ 171
Reflection .......................................................................................................................................... 171
Types of Reflections .............................................................................................................................. 172
Specular Reflection ........................................................................................................................... 172
Diffuse Reflection .............................................................................................................................. 173
Scattering .......................................................................................................................................... 173
Attenuation Coefficient ........................................................................................................................ 175
Half Value Layer Thickness .................................................................................................................... 176
Impedance ............................................................................................................................................ 178
Incidence ............................................................................................................................................... 179
Normal Incidence .............................................................................................................................. 179
Oblique Incidence ............................................................................................................................. 180
Incident Intensity .............................................................................................................................. 181
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Reflected Intensity ............................................................................................................................ 181
Transmitted Intensity ........................................................................................................................ 182
Intensity Reflection Coefficient (IRC) .................................................................................................... 182
Intensity Transmission Coefficient (ITC) ............................................................................................... 183
The Law of Conservation Of Energy ...................................................................................................... 184
Snell’s Law ............................................................................................................................................. 186
Range Equation ..................................................................................................................................... 187
13 Microsecond Rule ............................................................................................................................ 188
Chapter 15 – Artifacts ............................................................................................................................... 189
Types of Artifacts ...................................................................................................................................... 190
Resolution Artifacts ............................................................................................................................... 190
Axial Resolution Artifact.................................................................................................................... 190
Lateral Resolution Artifact ................................................................................................................ 191
Spatial Resolution Artifact ................................................................................................................ 191
Temporal Resolution Artifact ............................................................................................................ 192
Attenuation Artifacts ............................................................................................................................ 192
Shadowing Artifact ............................................................................................................................ 192
Enhancement Artifact ....................................................................................................................... 193
Refraction Artifact ................................................................................................................................. 194
Edge Shadowing ................................................................................................................................ 194
Reverberation ................................................................................................................................... 194
Comet Tail ......................................................................................................................................... 195
Ring Down ......................................................................................................................................... 195
Multipath Artifact ................................................................................................................................. 196
Mirror Image Artifact ........................................................................................................................ 196
Miscellaneous Artifacts ......................................................................................................................... 198
Side Lobe Artifact .............................................................................................................................. 198
Grating Lobe Artifact ......................................................................................................................... 198
Propagation Speed Error Artifact .......................................................................................................... 199
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Range ambiguity artifact, Range error artifact ................................................................................. 200
Vertical Misregistration .................................................................................................................... 200
Slice Thickness Artifact (Elevational Resolution Artifact) ................................................................. 201
Speckle .............................................................................................................................................. 201
Cross Talk .......................................................................................................................................... 203
Chapter 16 – Ultrasound physics Elementary Principles .......................................................................... 203
Sound .................................................................................................................................................... 203
Mechanical wave .............................................................................................................................. 205
Longitudinal Wave ............................................................................................................................ 205
Transverse Wave ............................................................................................................................... 205
Interference ...................................................................................................................................... 206
Acoustic Variables ................................................................................................................................. 206
Pressure ............................................................................................................................................ 207
Density .............................................................................................................................................. 207
Distance............................................................................................................................................. 207
Acoustic Parameters ............................................................................................................................. 208
Frequency ......................................................................................................................................... 208
Period ................................................................................................................................................ 211
Wavelength ....................................................................................................................................... 212
Propagation Speed ............................................................................................................................ 214
Amplitude .......................................................................................................................................... 216
Power ................................................................................................................................................ 218
Intensity ............................................................................................................................................ 218
Chapter 17 ‐ Real Time Imaging ................................................................................................................ 220
Frame .................................................................................................................................................... 220
Frame Rate ........................................................................................................................................ 220
Factors which affect the frame rate are: .............................................................................................. 222
Field of View (FOV) ............................................................................................................................ 222
Line density ....................................................................................................................................... 223
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Multi Focus ........................................................................................................................................ 223
Number of pulses per frame ............................................................................................................. 224
Imaging Depth ................................................................................................................................... 224
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Chapter1–PulseEchoInstrumentation
UltrasoundSystem
An ultrasound machine is a system which produces ultrasound images by using ultrasound waves.
The ultrasound sound machine consists of many components which are connected to each other and perform different functions to form an ultrasound image.
The components of a pulse-echo ultrasound system are:
1. Master Synchronizer
2. Transducer
3. Pulsar
4. Receiver
5. Display
6. Storage
MasterSynchronizerMaster Synchronizer is a component of ultrasound machine which maintains and organizes the proper timing and interaction of all components of the ultrasound machine so that ultrasound system can operate as a single integrated system.
TransducerThe ultrasound transducer is a part of ultrasound machine. It contains a piezoelectric material which converts electrical energy into acoustic energy during the transmission phase, and converts acoustic energy into electrical energy when the echoes return to the transducer after traveling in the body during reception phase.
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OutputPower
Output Power is the amount of voltage applied to the piezoelectric element to produce an ultrasound pulse. The output power of the electrical signal produced by the pulsar determines the strength or intensity of the ultrasound wave produced. The greater the output power of the electrical signal produced by the pulsar to excite the piezoelectric crystal, the greater the intensity of the ultrasound wave produced by the transducer.
Increasing the power output will increase the sound intensity.
The advantages of increasing output power is that, it produces sound waves with higher amplitude. The higher amplitude sound waves have improved depth penetration and improved signal to noise ratio.
The disadvantage of increasing output power is that it increases the patient’s exposure to potential bioeffects. Therefore the lowest output power without affecting the image quality and shortest scanning time should be used to reduce the potential risks of bioeffects.
To avoid the exposure to potential bioeffects, the sonographer should follow the ALARA principle which means As Low as Reasonably Achievable.
The other names used for output power are Output Gain, Power, Gain, Acoustic Power, Transmit Output, and power.
The sonographer can change the output power.
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Pulsar Pulsar is a component of ultrasound machine which determines frequency, amplitude, and pulse repetition period of ultrasound waves. The pulsar generates electrical signals that are applied to the piezoelectric element. When electrical signal is applied, the piezoelectric element contracts and expands and produces the ultrasound pulse. The strength of the ultrasound wave produced is directly proportional to the strength of the electrical signal applied to the PZT crystal. The greater the strength of the electrical signal the greater will be the amplitude of the ultrasound wave produced. Each electric pulse generates an ultrasound pulse.
The Pulsar also sends signals to the receiver and scan converter that the transducer has been activated.
In mechanical transducer, a single electrical signal excites the piezoelectric crystal to produce an ultrasound pulse.
In phased array transducers, a large number of crystals are excited with miniscule time delays to generate the ultrasound pulse. This technique is used to focus and steer the ultrasound beam, therefore, phased array systems have more complex pulsars.
The strength of pulsar output voltage that excites the piezoelectric crystals ranges from 1-300 volts and lasts less than 1 microsecond.
Important To Remember
The transducer output power ranges from 0 to 500 volts. Changing the Output Power changes the brightness on the
displayed image. Increasing the Output power increases the signal to noise
ratio which improves image quality. Thermal Index and Mechanical Index are related to output
power. Both are mathematical numbers that are used to help determine bioeffects of ultrasound on human body.
Bioeffects are related to output power, not changes in amplification.
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The pulsar creates the firing pattern for phased array systems.
The pulsar suppresses grating lobe formation.
Coded Excitation is a technique in which a series of encoded pulses sent to form a single scan line instead of sending one pulse per line. This technique allows for multiple focal zones, speckle reduction, improved penetration and better contrast resolution.
The following are the functions performed by the Pulsar:
generates the electrical signals which are applied to the piezoelectric elements
controls the timing of electrical signals controls the strength and amplitude of the electrical signal determines the Pulse Repetition Period determines the Pulse Repetition Frequency
Important To Remember
The timing of the voltage applied to the PZT is determined by the pulsar. The duration of the voltage applied determines the pulse length. The longer duration voltages produce longer pulses and shorter duration pulses produce shorter pulses.
The pulsar is responsible for the delay and variations in pulse amplitude which is needed for electronic control for beam scanning, beam steering, and beam shaping.
The pulsar determines the pulse amplitude, pulse repetition frequency, and pulse repetition period.
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BeamFormer
Beam Former is a component of ultrasound machine. It creates the phase delays and pulse sequencing to create the transmit beams and receive beams.
Beam former determines the firing delay patterns in phased array transducers for steering and focusing of the ultrasound beam.
Important To Remember
Beam Former is considered part of the transmitter.
Beam Former controls the timing to excite the PZT crystals which shapes the beam for focusing and steering.
Beam Former creates the phase delays and pulse sequencing to create the transmit beam and the receive beam.
Beam Former also adjusts the electrical signal voltages in the transducer to reduce the side lobes and grating lobes which is called Apodization.
During reception, the beam former creates time delays for dynamic receive focusing. It also varies the number of crystals used and controls dynamic aperture.
Digital beam former produces these signals in digital format.
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Noise
Noise
Noise is low level signals that degrade the image.
A ratio between original signal and the degraded signal is called signal to noise ratio.
A high signal to noise ratio means that there is much better signal than noise which improves the image quality.
A low signal to noise ratio means that there is more noise or degraded signals which degrades the image quality.
Increasing the output power increases the signal to noise ratio which improves the image quality.
Increasing the output power also increases the patient’s exposure to potential bioeffects. Therefore while scanning the sonographer should use the lowest output power without affecting the image quality.
Changing the receiver gain does not improve the signal to noise ratio. By increasing the gain the signal and noise are amplified by the same amount. It gives the appearance of improved signal to noise ratio also called apparent SNR.
A strong signal does not guarantee a good signal to noise ratio.
A weak signal does not guarantee a bad signal to noise ratio.
Noise Floor is the amplitude level below which no signals are visible because of the presence of the noise.
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ElectronicNoise
Electronic noise is random excitation of electrons within the electronics.
In Doppler spectrum electronic noise looks like random white speckle.
In color Doppler electronic noise looks like random color pixels where there is no flow.
ElectricalInterference
Electrical Interference occurs when the transducer receives signals from other electrical devices or electromagnetic waves such as radio transmission. Electrical interference can be carried through the air or from the power supplying the system.
The electrical interference looks like a bright flashlight down the middle of an image or a barber pole flashing.
The electrical interference on spectral Doppler looks like bright white horizontal or zigzag lines in the spectrum called Doppler tones.
Receiver
Receiver is the electronic component of ultrasound machine which processes the electrical signals received from the transducer during the reception phase. The electronic signals produced by returning sound waves are weak. The receiver increases the strength of these weak electrical signals, processes them and transforms them into a suitable form for display as an ultrasound image.
Receiver is also known as signal processor.
The signals produced by the transducer upon reception and sent to the receiver of the ultrasound system are extremely weak and are in the range of millivolt to microvolt.
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The receiver performs the following functions on the received electrical signal:
1. Amplification
2. Compensation
3. Compression
4. Demodulation
5. Rejection
AmplificationAmplification is the first function of the receiver. The returning echo signals are very weak and produce very weak electrical signals. Amplification increases the strength of these electrical signals received in the transducer to a level suitable for further processing. All electrical signals are made larger equally in the amplification process. The received signals stay the same; they become either brighter or darker depending on the adjustments made. Amplification or overall gain increases the brightness of the entire image.
Amplification is also called overall gain or receiver gain.
Amplification is controlled by the operator or sonographer. Sonographer can increase or decrease the strength of all of the returning echoes equally.
Amplification or overall gain is measured in decibels. Decibel is a relative value. The final signal strength leaving the receiver is compared to the initial signal strength entering the receiver.
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Important To Remember
Incorrect levels of amplification affect the entire image rather than a portion of an image.
If the sonographer uses too little amplification or overall gain, some of the processed echoes will remain too weak and will not appear on the display.
When too much amplification is used, all the echoes in the image appear too bright and the ability to distinguish different tissues or structures is lost.
Amplification does not affect the signal to noise ratio because both signal and noise are amplified equally.
Amplification does not affect the amount of patient exposure to ultrasound energy because only returned signals are amplified.
The amplification of the weak electrical signals that first reach the receiver ranges from 50 to 100 decibels.
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PreAmplification
CompensationCompensation is the second function of the receiver. Compensation corrects for attenuation and creates an image which is uniformly bright from top to bottom.
The ultrasound waves become weaker as they travel deep in the body, therefore, the returning sound waves from the deeper regions of body are weak and have low intensity. The weakening of sound waves while they travel in the body is called attenuation. The process of adjusting for attenuation is called compensation. The amplitude of received weak echo signals is increased in the receiver which makes these signals suitable for further processing in ultrasound machine to create an image.
The other names used for compensation are swept gain, depth gain compensation (DGC), and time gain compensation (TGC).
TGC is adjusted by the sonographer to improve image quality.
If TGC is set incorrectly, only a portion of the image will be affected, rather than the entire image.
The unit for compensation is Decibels (dB).
Pre Amplification
It is the process which helps to improve the quality of the signal before it gets amplified.
Pre Amplification prevents electronic noise from contaminating the small signals received by the transducer.
Preamplification improves the signal strength quality before it is amplified and is designed to prevent electronic noise in the system from degrading the receive signal.
Pre Amplification is performed as close to the crystal within the transducer as possible during reception.
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Important to Remember
TGC compensates for attenuation
The purpose of compensation is to produce images of uniform brightness from top to bottom or from near field to far field.
Compensation treats echoes differently, depending upon the depth from which they return.
Compensation is different from amplification. Amplification increases the strength of small electrical voltages received in the transducer. In amplification all signals are treated equally and it increases overall brightness of the image. In compensation the weaker signals due to lower amplitude echoes from greater depths are amplified.
Adjustments to TGC are related to the transducer frequency
By using a higher frequency transducer, the ultrasound beam undergoes more attenuation. Therefore, more TGC will be used to compensate for attenuation. On the diagram, the TGC curve will be shifted upward and to the right.
By using a lower frequency transducer, the ultrasound beam undergoes less attenuation. Therefore, less TGC will be needed to compensate for the attenuation. On the diagram, the TGC curve will be shifted downward and to the left.
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TGCCurve
Use of TGC compensates for attenuation and improves the image quality which helps the ultrasound system to produce images with detailed information.
On a TGC curve, the X axis represents amount of compensation and the Y axis represents the depth of the reflector.
NearGain – superficial depths at which constant small amounts of compensation is used
Delay – depth at which compensation begins
Slope – compensation corrects attenuation effects over path length
Knee – depth at which maximum compensation is used
FarGain – maximum amount of compensation that the receiver can provide
The region of minimum amplification on a TGC curve is called delay and is associated with area close to the transducer.
The low frequency transducers are more likely to have longer delay in the TGC curve. A long delay in TGC curve is consistent with less compensation in the area close to the transducer.
The far gain setting of a TGC curve represents the maximum compensation that a reflected ultrasound wave undergoes during the compensation process.
The use of TGC is most effective in focal zone. Use of TGC compensates for attenuation in focal zone and improves the image quality which helps the ultrasound system to produce images with detailed information.
With the use of low frequency ultrasound waves, the delay of the TGC curve is deeper. There is less attenuation and do not require much compensation. Therefore, the TGC curve is less steep with the use of low frequency ultrasound.
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CompressionCompression is the third function of the receiver.
Compression decreases the dynamic range of the electrical signals by decreasing the difference between the smallest and largest electrical voltages passing through the system. Compression keeps electrical signals within the operating range of the ultrasound system electronics. Compression is done without altering the relationships between the voltages. The largest electrical voltages stay largest, and smaller electrical voltages remain smallest. Electrical signals are treated differently based on strength.
Compression also changes the gray scale mapping and keeps gray scales within the range of what we can see. Compression decreases the dynamic range of the signals and increases the image contrast. The ultrasound systems treat higher voltages and lower voltages differently. The higher voltages are assigned image brightness and lower voltages are assigned different shades of gray.
Compression is also called log compression and dynamic range.
The unit of Compression is decibels (dB). Compression is measured in decibels which is a relative unit comparing one signal to another.
The compression of electrical signals done in receiver is determined by the ultrasound system electronics and cannot be adjusted by the sonographer.
Compression for gray scale can be adjusted by the sonographer and sonographer can change the gray scale mapping by changing the compression.
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Important To Remember
Compression is a process that maps larger dynamic range into a smaller dynamic range.
Compression maps the range of returning signals into a smaller dynamic range that our eyes can distinguish.
Human eye can distinguish 20 shades of gray. Compression allows us to visualize different tissues within the 20 visible shades.
The sonographer can change the compression. There is also internal compression that cannot be changed.
Receiver function of compression is a preprocessing function of compression. It is set by the manufacturer and cannot be changed by the sonographer.
Video compression occurs later in the signal processing. It compresses the displayed gray scale appearance.
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 29
DynamicRangeDynamic Range is the ratio of the largest to the smallest signal strength or amplitude of a component such as transducer, receiver, scan converter, or display. Dynamic range is the ratio of maximums to minimums of any quantity.
The dynamic range of a signal decreases the more it is processed.
The unit of dynamic range is decibel.
The different types of dynamic range are:
Input Dynamic Range Output Dynamic Range Display Dynamic Range Gain Dynamic Range
InputDynamicRangeThe input dynamic range is the ratio of the maximum input signal to the minimum input signal. It is the range of the signal amplitudes a system can receive and process without causing image distortion. The input dynamic range is the default dynamic range.
OutputDynamicRangeThe output dynamic range is the ratio of the maximum output signal to the minimum output signal.
DisplayDynamicRangeThe dynamic range of the display indicates the number of shades of gray.
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 30
DemodulationDemodulation is the fourth function of the receiver. Demodulation is the process which modifies the electrical signals within the receiver. It changes the shape of the electrical signal from one form to another which is more suitable for display.
As a wave propagates through a medium the interactions cause changes or modulations in the wave. Demodulation is the process by which modulations are removed or detected.
The main purpose for demodulation of a signal in an ultrasound system is to prepare it for display on the monitor.
The process of demodulation is fixed by the manufacturer in the ultrasound system and cannot be changed by the sonographer.
Important To Remember
Within different components of the ultrasound system, the amplifier has the greatest dynamic range.
Within different components of the ultrasound system, the display has the lowest dynamic range.
Signal dynamic range is generally much greater than the display dynamic range.
Range of returning signals and monitor or display ranges are usually outside our visible dynamic range.
Display dynamic range will exceed our visual dynamic range.
©Copyright 2014 Pass Ultrasound Physics Exam Study Guide Notes 31
Demodulation is done in two steps. These steps are:
RectificationRectification changes negative components of a signal into positive.
SmoothingorEnvelopingSmoothing wraps an envelope around the bumps to even out the rough edges. It traces the signal peaks and valleys and applies averaging or smoothing to them.
Important to Remember
Demodulation is the process of extracting the low frequency from the higher transducer frequency.
Demodulation does not have a visible effect on the image. It simply modifies the electrical signal so it becomes more suitable to be displayed on the monitor.
Demodulation is the function of the receiver.
Demodulation is also known as signal detection.
Smoothing is also known as envelope detection.
Envelope detection is done after rectification.
Demodulation is a process which transforms one form of signal into another form of signal such as changing analog signal into video signal.