Physics basic

PROF.DR SYED AMIR GILANIPROF.DR SYED AMIR GILANI

Basic UltrasoundBasic UltrasoundPhysicsPhysics

InstrumentationInstrumentation

Prof.Dr.Syed Amir GilaniProf.Dr.Syed Amir Gilani

UltrasoundUltrasound


The use of high frequency (inaudible) sound togenerate images


The use of high frequency (inaudible) sound togenerate images

Ultrasound images are Tomographic, Real-time and in Grey scale

Tomographic Images in sections or slices,

as in CT, MR, SPECT etc



Real-time Physiological movements

within body are depicted

as such



Real-time Physiological movements

within body are depicted

as such

Grey Scale Number of grey shades

between absolute white and

absolute black

Mechanical energy producing alternating compression and rarefaction

Sound waves are Longitudinal waves unlike electromagnetic waves

SoundSound

The audible frequency ranges from 20 Hz– 20 kHz. Ultrasound frequency is in the MHz range.

Mechanical energy producing alternating compression and rarefaction

Sound waves are Longitudinal waves unlike electromagnetic waves

SoundSound

T

Time

Pre

ssur

eThe Sound WaveThe Sound Wave

SOUND BEAMS & DISPLAY MODES

Beam widthBeam width o As sound travels, the width of the beam changes:

o Starts out at exactly the same size as the transducer diameter,

o Gets progressively narrower until it reaches its smallest diameter, and then

o It diverges

FocusFocus

o The location where the sound beam reaches its minimum diameter.

Focal DepthFocal Depth

o The distance from the transducer face to the focus. Also called focal length or near zone length.

Near ZoneNear Zone

o The region or zone in between the transducer and the focus.

(Fresnel Zone)(Fresnel Zone)

o The beam converges in the near zone

Far ZoneFar Zone

o The region or zone beyond the near field. The beam

o (Fraunhofer Zone) diverges in the far zone.

Near zone - short name - Fresnel

Far zone - long name - Fraunhofer

Focal ZoneFocal Zone

o The region surrounding the focus where the beam is "sort of narrow" and the picture is relatively good.

Note Note

o For an unfocused continuous wave disc transducer:

o At the end of the near zone, the beam diameter is ½ the transducer diameter.

o At two near zone lengths, the beam diameter is equal to the transducer diameter.

Focal DepthFocal Depth

o Definition

o Distance from transducer to the narrowest part of the beam (the focus)

Determined by two factorsDetermined by two factors

o 1. Transducer diameter and

o 2. Frequency of the ultrasound.

Sound Beam DivergenceSound Beam Divergence

o Definition

o Describes the spread of the sound beam in the deep far zone. Determined by two factors:

o 1. The transducer diameter and

o 2. The frequency of the ultrasound,

o Larger diameter crystals producing higher frequency sound produce beams that diverge less in the far field.

o Smaller diameter crystals producing lower frequency sound produce beams that diverge substantially in the far field.

DiffractionDiffractiono Diffraction Pattern o a V-shaped wave, also called a Huygen's wavelet.

When produced by a tiny source, with a size near the wavelength of the sound, waves will diverge in this shape as they propagate. If sound waves produced by imaging transducers acted in this manner, the wave would be spread broadly as it travels.

Huygen's PrincipleHuygen's Principle

•Definition

•Sound waves produced by imaging transducers are shaped like an hourglass and do not diffract because they obey Huygen's principle. Each tiny part of the surface of the large transducer face may be considered an individual sound source. The hourglass shape of a sound beam is the result of the constructive and destructive interference of the many sound wavelets emitted from these numerous sound sources.

Lateral Resolution Lateral Resolution

o It has been found that when all of these multiple wavelets are combined according to Huygen's Principle, they produce an hourglass-shaped main sound beam. This principle explains the shape of an imaging transducer's sound beam.

DefinitionDefinition

o The minimum distance that two structures are separated by side-to-side or perpendicular to the sound beam that produces two distinct echoes.

SynonymsSynonyms

Lateral Angular Transverse Azimuthal

o LATA resolution.

UnitsUnits

o mm, all units of length

o smaller number, more accurate image

High Frequency Pulsed UltrasoundHigh Frequency Pulsed Ultrasound

o Special notes for pulses made of high frequencies:

o Beam Shape & Divergence—pulses of sound at higher frequencies tend to have less divergence in the far field. Higher frequency pulsed US has narrower beams compared to lower frequencies.

FocusingFocusing

o Results in:

o 1. A narrower "waist" in the US beam.

o 2. A decrease in focal depth (the focus is shallower).

o 3. A reduction in the size of the focal zone. Effective mainly in the near field and the focal zone.

Three Modes of FocusingThree Modes of Focusing

o 1. Lens - external focusing (fixed, mechanical or conventional)

2. Curved Piezoelectric2. Curved Piezoelectric

o Crystal-internal focusing, (fixed, mechanical or conventional)

o A lens or curvature of the crystal can focus a wave produced by a single PZT crystal. This is conventional,

o fixed, or mechanical focusing. The focal depth using either

o of these two methods cannot be changed.

3. Electronic Focusing3. Electronic Focusing

o Phased array technology provides dynamic, variable (adjustable) focusing or multi-focusing.

Single crystal transducers are always Single crystal transducers are always fixed focus.fixed focus.

o Phased array transducers generally have "better" lateral resolution because the focus is adjustable by the sonographer.

o With phased arrays, the sonographer can locate the focus within the anatomic region of clinical importance.

o The greater the extent of focusing, the smaller the focal zone.

Summary—Sound Beam AnatomySummary—Sound Beam Anatomy

CharacteristicCharacteristic DeterminantsDeterminantsFrequency (CW)Frequency (CW) Electronic frequency :Electronic frequency :

Frequency (Pulsed)Frequency (Pulsed) Thickness of ceramic & Speed of Thickness of ceramic & Speed of sound in ceramicsound in ceramic

Focal lengthFocal length Diameter of ceramic & Frequency Diameter of ceramic & Frequency of sound Diameter of sound Diameter

DivergenceDivergence Diameter of ceramic & Frequency Diameter of ceramic & Frequency of soundof sound

Lateral resolution Lateral resolution Beam width Beam width

Graph AxesGraph Axes

Display ModesDisplay Modeso A-Mode o Amplitude o Mode—When the US pulse is emitted, a dot

moves across the screen of an US machine at a constant speed. When the echo returns, an upward deflection proportional to the amplitude of the returning echo is observed. A-mode provides very precise measurements of distances between the probe and the reflector.

o Looks like a big-city skyline.

o X-axis - reflector depth,

o measured by pulse's time-of-flighty

o Y-axis - amplitude of echo,

o measured by echo strength

B-ModeB-Mode

o Brightness Mode—Returning echoes are presented as spots on the line of travel of the emitted US pulse. The stronger the returning echo, the brighter the spot. The brightness of the dot is proportional to the strength of the returning echo.

o X-axis - depth of reflector

o measured by pulse's time-of-flight

o Z-axis - amplitude of echoes

o measured by echo strength

M-ModeM-Mode

o Motion Mode or T-M mode Dragging a photosensitive across a B-mode produces squiggly lines. These lines represent the motion of the reflecting body surfaces as they occur in time. Not related to echo amplitude, related to location. displays the changing position of Mime reflectors with respect to time.

••X-axis -X-axis - time time•Y-axis -•Y-axis - depth, measured by pulse's time-of- depth, measured by pulse's time-of-flightflight

PULSED ECHO PULSED ECHO INSTRUMENTATION INSTRUMENTATION

Information Information

o Time of flight

o Strength

Ultrasound SystemUltrasound System

o The entire device that produces US beams, retrieves the echoes and produces visual images and audio signals.

o System Components

o Six interconnected components - information is transferred to and from each.

Master SynchronizerMaster Synchronizer

o Communicates with all of the individual components of the ultrasound system.

o Organizes and times their functions.

o Prepares them to operate as a single integrated system.

TransducerTransducer

o Converts electrical into acoustic energy during transmission

o Converts returning acoustic into electrical energy during reception.

PulserPulser

o The component that controls the electrical signals sent to the transducer for sound pulse generation.

o Determines the PRF, pulse amplitude, and pulse repetition period.

o Creates the firing pattern for phased array systems. This is called the 'beam former.

ReceiverReceiver

o The electronics associated with processing the electronic signal produced by the transducer during reception and producing a picture on an appropriate display.

DisplayDisplay

o The device associated with the presentation of processed data for interpretation.

o CRT (television), audio speakers, a paper record.

StorageStorage

o Any number of devices and "media" that are used to permanently archive the US data

o Video tape, paper, film, transparent film, computer discs.

PulserPulser

o Function

o Receives timing signal from synchronizer

o Creates an electrical signal that will excite the PZT crystal.

o Produces electrical voltage, 10 - 500 volts, that excites piezoelectric crystal during transmission.

o Greater electrical voltage increases sound intensity created by the transducer and sent into the patient.

o Pulser signals depend upon system and transducer.

o Do not" use a transducer with a crack in the housing because of the potential for electrical shock to the patient.

Pulser ModesPulser Modeso Continuous Wave:o Constant electrical signal in the form of a sine wave,

electrical frequency = US frequency.o Pulsed Wave,o Short duration electrical "spike", one electrical spike per.o Single Crystal: ultrasound pulse.o Pulsed Wave, Many elements fired for each ultrasound

pulse.

ArraysArrays

o Thus, for each sound pulse, many short duration electrical "spikes" are required—one electrical spike per fired element. ^ For phased array systems, the pulser is also called the 'beam former."

Transducer OutputTransducer Output

o Synonyms: output gain, acoustic power, pulser power, energy output transmitter output.

o Changes in transducer output affect the brightness of the entire image.

o Determined by the excitation voltage from the pulser.

o Piezoelectric crystal vibrates with a magnitude related to pulser voltage.

Adjusted by sonographerAdjusted by sonographer

oYes

Effect upon imageEffect upon image

o When transducer output changes, every pulse transmitted to the body changes.

o All reflections from structures in the body also change. The brightness of the entire image changes

Note:Note:

o Increasing transducer output improves improves signal – to- nose ratio.

o The meaningful signal strength is altered, while the noise level remains constant.

Receiver and Its FunctionsReceiver and Its Functions

o Overall Function:

o The signals returning from the transducer are extremely weak. The receiver boosts the strength of these signals, processes them and prepares them for display.

OrderOrder

o Amplification, compensation, compression, demodulation, rejection (hint: alphabetical order).

AmplificationAmplification

o Purpose:

o Increasing the strength of all electrical signals in the receiver prior to further processing.

o Synonyms:

o receiver gain

PreamplifierPreamplifier

o Preamplifier may condition signal before it is amplified, often performed in the probe.

CompensationCompensation

o Purpose:

o Used to create image of uniform brightness from top to bottom,

o Since attenuation is strongly related to path length, echoes returning from great depths have lower amplitudes than those returning from shallow depths.


o Compensation makes all echoes from similar reflectors appear identical regardless of their depth.

SynonymsSynonyms

o Time gain compensation (TGC), depth compensation (DGC), swept gain.

IMPROPER TGC IMPROPER TGC

NoteNote

o Compensation makes an image equally bright at all depths. Ask the question "Is the image of uniform brightness from the top to the bottom?"

TGC & FrequencyTGC & Frequency

o Adjustments to TGC are related to transducer frequency:

o • With a higher frequency transducer the beam undergoes more attenuation. Therefore, more TGC must be used. On the diagram, the TGC curve is shifted upward & to the right.

TGC & FrequencyTGC & Frequency

o • With a lower frequency transducer, the beam undergoes less attenuation. Therefore, less TGC is needed. On the diagram, the TGC curve is shifted downward and to the left.

TQC CURVETQC CURVE ADJUSTING TGCADJUSTING TGC

CompressionCompression

o Reducing the total range, the smallest to the largest signal.

PurposePurpose

o Keeps signals within the operating range of the system's electronics and the gray scale within the range of what the human eye can see.

o • Done without altering the relative relationships between voltages; largest stays largest, smallest remains smallest.

o • Decreases the dynamic range of the signals,


o Changes the gray scale mapping.

DemodulationDemodulation

o Purpose

o Changes the signal's form to one more suitable for TV display.

Adjusted by sonographerAdjusted by sonographer

o No. Fixed by the manufacturer. Has two steps:

1. Rectification1. Rectification

o Turning all of the negative voltages into positive ones. Corrects for or eliminates negative voltages.

2. Smoothing (or Enveloping)2. Smoothing (or Enveloping)

o Putting an envelope around the "bumps" to even them out.

RejectionRejection

o Purpose:

o Displays low level echoes only when clinically meaningful.

SynonymsSynonyms

o Suppression, threshold

o Very low level echoes may or may not be important. Reject determines whether they appear on the image.

o Effect upon image Affects all low level signals everywhere on the image,

o but does not affect bright echoes.

Order of Receiver OperationsOrder of Receiver Operations

o These five operations must be performed in the appropriate order for proper system function. Alphabetical order:

o 1. Amplification

o 2. Compensation

o 3. Compression

o 4. Demodulation

o 5. Reject

Summary—Receiver FunctionsSummary—Receiver Functions

FunctionFunction Adjustable?Adjustable? Signals ProcessedSignals Processed

AmplificationAmplification Yes Yes All signals treated the same All signals treated the same

CompensationCompensation Yes Yes Signals treated differently Signals treated differently based on reflector depth based on reflector depth

Compression Compression Yes Yes Decreases dynamic range, Decreases dynamic range, changes gray scale map changes gray scale map

DemodulationDemodulation NoNo Changes form of signals Changes form of signals

RejectionRejection Yes Yes All weak signals manipulated All weak signals manipulated Strong signals not affected. Strong signals not affected.

Dynamic Frequency TuningDynamic Frequency Tuning

o Pulses contain a wide range of frequencies (wide bandwidth).

o On some US systems, higher frequencies create shallow parts of the image and lower frequencies create deeper parts.

o Eechoes arising from superficial structures are filtered to process only higher frequencies, since higher frequencies make better images.

o Lower frequency signals are used to image deeper structures, since the higher frequencies have attenuated and are no longer present.

Harmonic ImagingHarmonic Imaging

DefinitionDefinition

o Transmitting sound at a particular frequency (called the fundamental frequency), but creating an image from sound reflected at twice the fundamental frequency (called the harmonic or second harmonic).

Fundamental frequencyFundamental frequency

o The frequency of the transmitted sound wave

Harmonic frequencyHarmonic frequency

o Twice the transmitted frequency. Also called the second harmonic.

ExampleExample

o A transducer transmits a sound pulse with a fundamental frequency of 2 MHz. In the harmonic mode, an image created from 4MHz sound reflections will be displayed.

o As a sound wave travels in the body, a miniscule amount of energy is converted from the fundamental frequency to the harmonic frequency due to non-linear behavior.

Contrast AgentsContrast Agents

o Also called "micro-bubbles" of gas entrapped in a shell.

o Contrast agents have a different acoustic fingerprint than blood or tissue.

Contrast AgentsContrast Agents

o Injected into the circulation (usually intravenously). These agents create strong reflections that actually "light up" blood chambers or vessels.

o Currently indicated only for determining the borders of the left ventricle of the heart.

Requirements Requirements

o • Safe

o • Long persistence

o • Metabolically inert

o • So small as to pass through capillaries

o • Strong reflector of ultrasound

without contrast with contrast

Output Power vs. Receiver GainOutput Power vs. Receiver Gain

o Adjustments to either output power or receiver gain change the brightness of the entire image.

Output PowerOutput Power

o Affects brightness by adjusting the strength of the sound pulse sent to the body by the transducer.

o When the pulse is more powerful, all of the returning echoes from the body are stronger, and the image is brighter.

o When the image is too bright due to high output power, the lateral and longitudinal resolution degrade.

Receiver GainReceiver Gain

o Affects the brightness by changing the amplification of the electronic signals after returning to the receiver.

o When amplification is increased, the electronic signals in the receiver are boosted, and the image will be brighter.

Which one?Which one?

o To determine whether a control affects output power or receiver gain, look at the its description. When the term suggests "outgoing" the function is probably output power. When the word indicates "reception or incoming" the function is most likely receiver gain.

ALARAALARA

o When an entire image is either too bright or too dark, changes in output power or receiver gain may correct the problem. As a first option, always choose the option that will minimize patient exposure.

Use the ALARA Principle – As Low As Use the ALARA Principle – As Low As Reasonable Achievable Reasonable Achievable

Image too dark—first, image too bright—first, reduce increase receiver gain

INTERACTION OF INTERACTION OF SOUND AND MEDIA SOUND AND MEDIA

AttenuationAttenuationo Definition

o The decrease in intensity, power and amplitude of a sound wave as it travels.

o The farther US travels, the more attenuation occurs.

UnitsUnits

o dB, decibels (must be negative, since the attenuation causes intensity to decrease)

In soft tissueIn soft tissue

o Attenuation of sound in soft tissue depends upon the wave's 1) frequency and the 2) distance the wave travels.

o In soft tissue, higher frequency results in greater attenuation. Thus we can image deeper with lower frequency sound.

NoteNote

o Attenuation is unrelated to propagation speed.

Three ComponentsThree Components

o 1. Absorption (sound energy converted into heat energy)

o 2. Scattering

o 3. Reflection

Media Media

o Air—much, much more attenuation than in soft tissue vo Bone—more than soft tissue, absorption & reflection y|/o Lung—more than soft tissue, due to scattering vo Water—much, much less than soft tissue yo Air >> Bone & Lung > Soft Tissue » Water

Reflection and ScatteringReflection and Scattering

ReflectionReflection

o Occurs when propagating sound energy strikes a boundary between two media and some returns to the transducer.

Specular ReflectionSpecular Reflection

o Reflections from a very smooth reflector (mirror) are Specular

o Specular reflections also occur when the wavelength is much smaller than the irregularities in the boundary.

ScatteringScattering

o If the boundary between two media has irregularities (with a size similar to or a bit smaller than the pulse's wavelength), then the wave may be chaotically redirected in all directions.

Rayleigh ScatteringRayleigh Scattering

o If a reflector is much smaller than the wavelength of sound, the sound is uniformly diverted in all directions. Higher frequency sound undergoes more Rayleigh scattering. A red blood cell is a Rayleigh scatterer.

o Rayleigh scattering is related to frequency

Attenuation & ImagingAttenuation & Imaging DepthDepth

o Attenuation ultimately limits the maximum depth from which images are obtained. The goal in diagnostic imaging is to use the highest frequency that still allows us to image to the depth of the structures of clinical interest. That is why we use 2-l0Mhz sound waves.

OrganizedOrganized

systematicsystematic

DisorganizedDisorganized

chaoticchaotic


(back to Transducer)(back to Transducer)

SpecularSpecular DiffuseDiffuse

ScatteringScattering

(in all directions)(in all directions)

RayleighRayleigh ScatterScatter

Attenuation CoefficientAttenuation Coefficient

o Definition The amount of attenuation per centimeter.

UnitsUnits

o dB/cm, decibels per centimeter.

o In soft tissue

o • With higher frequency, the attenuation coefficient increases.

o • This is why lower frequencies are used to image to greater depths.

o As the frequency of a wave increases, attenuation coefficient increases.


o In soft tissue, attenuation coefficient (dB/cm) is approx. half of the frequency (MHz), or 0.5dB/cm/MHz.

o Hint Attenuation coefficient is related to frequency.

EquationEquation

o total attenuation (dB) = path length (cm) x attenuation coefficient (dB/cm)

ImpedanceImpedance

o Characteristic of the medium only.

o Impedance is not measured, it is calculated,

UnitsUnits

o Rayls, often represented by the letter "Z"

Typical ValuesTypical Values

o Between 1,250,000 and l,750,000rayls (1.25 - 1.75Mrayls)

o Reflection of an ultrasound wave depends upon a difference In the acoustic impedances at the boundary between the two media.

Oblique IncidenceOblique Incidence

o Anything other than 90 degrees; not at right angles,

Reflection and TransmissionReflection and Transmission

o Incident Intensity

o The intensity of the sound wave at the instant prior to striking a boundary.

Reflected IntensityReflected Intensity

o The portion of the incident intensity that, after striking a

o boundary, changes direction and returns back from where it came.

Transmitted IntensityTransmitted Intensity

o The portion of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling.

At the boundary between two mediaAt the boundary between two media

o 1. When IRC and ITC are added, the result is l00%g

o 2. When reflected and transmitted intensities are added, the result is the incident intensity.

o There is "conservation of energy" at a boundary.


Typically, only 1% or less of the incident US energy is reflected at a soft-tissue boundary between different biologic media (such as blood and muscle).

99% is reflected at an air-tissue interface \y 50% at a bone-tissue interface^.

Thus, there is a great deal of attenuation at an air-tissue interface or a bone-tissue interface.

Transmission With Normal IncidenceTransmission With Normal Incidence

o With NORMAL incidence:

o These are simply reflection questions, whatever remains after transmission, must be reflected!

o Incident Intensity = Reflected Intensity + Transmitted Intensity

o Transmitted Intensity = Incident Intensity x Intensity Transmission Coefficient

Reflection & Transmission With Reflection & Transmission With Oblique IncidenceOblique Incidence

o Extremely complex physics regarding transmission & reflection with obliquity.

RememberRemembero With oblique incidence, we are uncertain as to

whether reflection will occur. Simply say "I don't know)

Specular reflections arise when the interface is smooth.

RefractionRefraction

o Definition Refraction is a change in direction of wave propagation when traveling from one medium to another. A process associated with transmission (not reflection!)

o Refraction is transmission with a bend.

Occurs when two conditions are metOccurs when two conditions are met

o 1. oblique incidence and

o 2. different propagation speeds

o Cannot occur with normal incidence or with identical propagation speeds.

Snell’s LawSnell’s Law

o The physics of refraction are described by Snell's Law

TermsTerms

Frequency is the number of complete cycles/sec

Hertz is the frequency of sound, 1 Hz = 1cycle/sec

Audible sound 20 Hz – 20 kHz

Medical ultrasound frequency in MHz

PiezoelectricityPiezoelectricity

1

2

3

4

EchoesEchoesEchoes, arising from acoustic interfaces

form the basis of ultrasound images.

Interfaces are boundaries between materials

with different acoustic properties....

if the medium is totally homogenous, and has

no interfaces, no echoes will form and the

structure will appear anechoic.

Echoes Echoes (cont’d)(cont’d)

The intensity of echoes depends

upon the difference in the acoustic

impedance of the material at the interface.


The intensity of echoes depends

upon the difference in the acoustic

impedance of the material at the interface.

Acoustic impedance (Z) is the product

of the density of the material () and the

propagation velocity of sound (c) so:

Z = cGreater the mismatch, greater the echoes


The echoes are not only related to the densityof the substance, so harder is not necessarilybrighter....it is the impedance mismatchwhich is important....so air:tissue interfaceis very echogenic as is air:bone interfacewhile muscle:fat interface is not very echogenic

Velocity of SoundVelocity of Sound

0

1000

2000

3000

4000

5000

Air

Blo

o d

Liv

e r

Wa t

e r

Kid

n ey

Mu s

cle

Fat

So f

t Tis

sue

Bo n

e

4080m/sec

1540m/sec

330m/sec


Types of reflectors:

Specular.....large and smooth

will reflect like a mirror

(diaphragm).

Diffuse...the reflectors are

small; individually smaller

than the wavelength of

ultrasound, tissue substanceRememberSpecular reflection isangle dependent

Ultrasound images are made up of small dots on the screen, the brightness of the individual dotcorresponds to the intensity of the echo receivedso the greater the echo, the brighter the dot...Theecho is translated into the Brightness in formingimages and this is called “B” mode ultrasound.

Ultrasound images are made up of small dots on the screen, the brightness of the individual dotcorresponds to the intensity of the echo receivedso the greater the echo, the brighter the dot...Theecho is translated into the Brightness in formingimages and this is called “B” mode ultrasound.The position is calculated by accurately measuringthe time of return of the echo at the transducerafter an ultrasound pulse.

Ultrasound ranging

depends on the

assumption that

sound speed is

constant and a

calibrated velocity

of 1540m/sec

As sound passes through a tissue, the pressure wave diminishes in amplitude as energy is lost through absorption reflection and scattering...This is called Attenuation


Attenuation determines the depth at which structurescan be imaged and depends both on the tissue being examined as well as the transducer frequency...


Attenuation determines the depth at which structurescan be imaged and depends both on the tissue being examined as well as the transducer frequency...The higher the frequency, the higher the attenuation and lower the penetration

• The higher the probe frequency the shallower

the tissue depth that can be imaged.

• The higher the probe frequency, the better

the resolution.

Use the highest frequency possible to image any structure

TRANSDUCERSTRANSDUCERS

DEFINITIONSDEFINITIONS

o Transducer

o Any device that converts one form of energy into another:

o Electric motor (electric to kinetic)

o Light bulb (electric to heat & light)

o Muscle (chemical to kinetic)

o Loudspeaker (electric to acoustic)

Common Transducer TypesCommon Transducer Types

o Linear

o Convex

o Endocavitary

o Sector

LinearLinear

ConvexConvex

EndocavitaryEndocavitary

Piezoelectric EffectPiezoelectric Effect

o A property of certain materials to create a voltage when they are mechanically deformed.

o Also, these materials deform or change shape when a voltage is applied to them (the reverse piezoelectric effect).

o PZT in ultrasound transducers is also called the ceramic, active element, or crystal.

Curie TemperatureCurie Temperature

o If PZT is heated above this temperature (approximately 360C° or 680F°), it loses its piezoelectricity—i.e., the PZT is depolarized. Thus, we should not heat sterilize or autoclave transducers. Also called the Curie point.

SterilizationSterilization

o The complete destruction of all living microorganisms by means of exposure to heat, chemical agents, or radiation,

DisinfectionDisinfection

o Refers to the application of a chemical agent to reduce or eliminate infectious organisms on an object, such as a transducer.


o With regard to infection control, the most critical instruments are those that are intended to penetrate skin or mucous membranes. These require sterilization.


o Less critical instruments that simply come into contact with mucous membranes (such as fiber-optic endoscopes) require a lower level of disinfection than sterilization.


o As a rule, transducers should be disinfected using Cidex™ or other cold germicides. Ultrasound transducers should never be sterilized using either dry or moist heat, or chemicals because this could likely damage the transducer

Transducer ArchitectureTransducer Architecture

Active Element The piezoelectric crystal.

CaseCase

o Protects the internal components from damage and insulates the patient from electrical shock.

NoteNote

o Do not use a transducer with a cracked housing.

WireWire

o Each active element in a transducer requires electrical contact so that the voltage from the US system can excite the crystal to vibrate thereby producing an ultrasonic wave. Similarly, during reception the sound wave deforms the crystal, producing a voltage. The voltage must be sent back to the ultrasound system for processing into an image.

NoteNote

o Do not use a transducer with a frayed wire.

Matching LayerMatching Layer

o Recall that impedance differences result in reflection at boundaries.


o The matching layer has an impedance between those of the skin and the active element to increase the percentage of transmitted US between the active element and the skin. Gel's impedance is in between those of the matching layer and the skin.


o The matching layer is one-quarter wavelength thick. Impedances: PZT > matching layer > gel > skin

Damping ElementDamping Element

o A material that is bonded to the active element that limits the "ringing" of the PZT.

o Commonly made of epoxy resin impregnated with tungsten. Also called backing material.

Damping material - advantagesDamping material - advantages

o Shortens spatial pulse length, pulse duration decreases numerical value of LARRD resolution.

o Increases picture accuracy by improving LARRD resolution

Damping material - also causesDamping material - also causes

o Decreased transducer's sensitivity

o Increased bandwidth (range of frequencies) in the pulse - also called wide bandwidth

o Decreased "Q" factor. Imaging probes are low-Q

Bandwidth and Quality Factor Bandwidth and Quality Factor

BandwidthBandwidth

o It is uncommon for a transducer to emit a sound beam with only a single pure frequency. Rather, the pulse is more like a sound 'click' and contains a range of frequencies below and above the main frequency.

o The bandwidth is the range of frequencies^ between the highest and the lowest frequency emitted from the transducer.

Quality FactorQuality Factor

o A unitless number representing the degree of damping. Imaging transducers are low-Q transducers when compared to therapeutic transducers because imaging transducers use backing material.

Quality FactorQuality Factor

o The Q-factor of typical imaging transducers can be approximated by the number of cycles in the pulse produced by the transducer (approximately 2 - 4).

When Q-factor is lowWhen Q-factor is low

o 1. damping is effective

o 2. pulse length & duration are short.

o 3. bandwidth is wide

o 4. axial resolution is improved

Transducer FrequenciesTransducer Frequencies

o What determines the resonant frequency of a transducer?

Continuous Wave TransducersContinuous Wave Transducers

o Sound wave's frequency equals the frequency of the voltage applied to the PZT by the machine's electronics.

Pulsed TransducersPulsed Transducers

o The pulse repetition frequency (PRF) is determined by the number of electrical pulses the US machine delivers to the active element.

The frequency of the US for a pulsed txr The frequency of the US for a pulsed txr is determined by 2 factorsis determined by 2 factors

o 1. The thickness and

o 2- The propagation speed of the piezoelectric material.

o propagation speed for PZT is approx. 4-6 mm.

For Pulsed TransducersFor Pulsed Transducers

o The thinner the active element, the higher the transducer's resonant or natural frequency (think of a crystal glass.)

o The higher the active material's propagation speed, the higher the transducer's frequency.

TWO-DIMENSIONALTWO-DIMENSIONAL IMAGING IMAGING

o We desire "images" and slices of anatomy

o Ultrasound only travels in a straight line

o Narrow beams provide high quality imaging and good lateral resolution

Solution NoteSolution Note

o construct a two dimensional image from multiple ultrasound pulses transmitted into the body in different directions.

o 2-D images may be referred to as "B-scans" or "B-modes" since they are gray scale.

Mechanical ScanningMechanical Scanning

o Crystals Scanhead contains one active element

o Element Shape The crystal is circular and disc-shaped (like a coin).

o Steering The active element is moved by a motor oscillating crystal or mirror through a pathway, automatically creating a scan plane.

o Focusing Conventional or Fixed: curvature (internal) of the PZT or an acoustic lens (external) focuses the beam at a specific depths

Transducer ArraysTransducer Arrays

o Array A collection of active elements in a single transducer.

o Element A single slab of PZT cut into a collection of separate pieces called elements,

o Channel The electronic circuitry connected to each element.

Linear ArrayLinear Arrayo A collection of elements in a line. There are 2 types:

o 1. linear switched (or sequential) array

o 2. linear phased array

Annular ArrayAnnular Array

o A group of ringed elements with a common center.

Convex, curved orConvex, curved or curvilinear curvilinear ArrayArray

o Elements arranged in an arc. There are 2 types:

o 1. convex switched (or sequential) array

o 2. convex phased array

Linear Switched or Sequential Linear Switched or Sequential Arrays CrystalsArrays Crystals

o Large transducers with multiple elements arranged in a line. Image is no wider than the transducer.

Element ShapeElement Shape

o Each element in a linear-switched array is rectangular.

o A few elements (S-10), but not all, are fired at exactly the same time.

o The sound wavelets from the multiple crystals interfere with each other to create a single sound beam that travels straight ahead into the body. Then, the next group is fired.

Switched arraysSwitched arrays are are also called also called sequential.sequential.o Steering No steering, pulses sent down parallel

lines.

o Focusing Fixed-focusing without steering.

o Image Shape Rectangular image shape, no wider than transducer.

o Defective Crystal Dropout extending from superficial to deep.

Vertical focusing achieved conventionally with lens or Vertical focusing achieved conventionally with lens or curvature of active elements.curvature of active elements.

Phased ArraysPhased Arrays

o Meaning Adjustable focus or multi-focus; achieved electronically,

Crystals, Steering &Crystals, Steering & FocusingFocusing

o A collection of electric pulses is delivered to all of the transducer's elements in various patterns.

o The patterns ' focus & steer the US beam during transmission. Thus, focusing and steering are electronic.

Image ShapeImage Shape

o The image is fan or sector-shaped.

o Electronic signals excite all of the elements and create only one sound pulse.

o There are miniscule time delays (10 ns) between electronic pulses delivered to the individual array elements. The elements are fired nearly simultaneously.

o If we imagine the delays to represent the surface of a reflecting mirror, the direction and the shape of the wave become apparent.

Defective CrystalDefective Crystal

o If one element malfunctions, the steering and focusing becomes erratic.

o Electrical Patterns no steering no transmit focusing steered upward no transmit focusing

Beam FormerBeam Former

o The component of an US system that creates these electronic patterns is called the beam former.

Dynamic ReceiveDynamic Receive

o Similarly, time delays during reception are also applied to the Focusing electrical signals from the transducer to the US system. This reception zone focusing relates to the depth of the returning echoes and optimizes image quality

Annular Phased Arrays Annular Phased Arrays

o Crystals Concentric rings (donut shaped) cut from the same circular slab of PZT.

o Element Shape Ring (like a donut)

o • Small diameter rings have a shallow focus but diverge rapidly.

o • Large diameter rings have a deep focal length,

StrategyStrategy

o Selected focal zones, use inner crystals for shallow regions and outer crystals for deep regions,

ImageImage

o The image is fan or sector-shaped. Shape

FocusingFocusing

o Phasing provides electronic focusing in all planes at all

o Depths a core sample, This provides optimal lateral resolution at all depths.

SteeringSteering

o Steering is performed mechanically. HINT: This is unlike other phased array transducers.

Defective CrystalDefective Crystal

o Defective crystal causes a horizontal (side-to-side) band of dropout.

Convex or Curved ArraysConvex or Curved Arrays

o Piezoelectric crystals arranged in a curve to provide a natural sector image.

o May be sequential or phased array (just like a linear array). Image shape blunted-sector, fan-shaped image.

Large transducerLarge transducer

o Convex sequential array that places a large acoustic footprint

o on the patient.

Acoustic footprintAcoustic footprint

o Describes the area of contact between the transducer and the skin.

Small transducerSmall transducer

o Typically a convex phased array.

Dynamic ApertureDynamic Aperture

o A form of receive, electronic focusing,

o As the returning sound beam strikes the transducer, the size of the transducer surface listening for echoes is varied. This is accomplished by varying the number of elements used to receive the reflected signal,

o Echoes arising early (from superficial structures) are received using only a few crystals from the array.

o As the echoes return from deeper structures, the aperture is increased. More and more elements in the array are used to listen.

o This allows the beam to be as narrow as possible at all depths, and optimizes lateral resolution at all depths.

Multidimensional ArraysMultidimensional Arrays

o Two-dimensional arrays provide real-time acquisition of data in three planes. Creates 3-D ultrasound images.

o Same number of elements in the up & down and side-to-side directions.

o 1 ½ - dimensional arrays allow focusing in the plane of the beam width. Improves elevational resolution — makes a thinner slice, see "Slice Thickness" on page 149\|/

o More elements side-to-side than up & down.

Vector ArraysVector Arrays

o Vector arrays combine linear sequential and linear phased array technologies. Phasing is applied to a linear sequential array.

Image ShapeImage Shape

o Trapezoidal, basically a sector with a flat top that does not come to a point.

Summary—TransducersSummary—Transducers

Transducer Transducer TypeType

Image Image ShapeShape

Steering Steering TechniqueTechnique

Focusing Focusing TechniqueTechnique

Crystal Crystal DefectDefect

MechanicalMechanical SectorSector MechanicalMechanical fixedfixed Image tossImage toss

linear switchedlinear switched rectangulrectangularar

nonenone fixedfixed Vert line Vert line dropoutdropout

Linear phased Linear phased

array array

SectorSector Electronic Electronic Electronic Electronic Poor Poor steering & steering & focusing focusing

annular annular phasedphased

SectorSector MechanicaMechanical l

electronicelectronic horiz line horiz line dropoutdropout

Summary—TransducersSummary—Transducers

Transducer Transducer TypeType

Image Image ShapeShape

Steering Steering TechniqueTechnique

Focusing Focusing TechniqueTechnique

Crystal Crystal DefectDefect

convex convex sequentialsequential

blunted blunted sectorsector

NoneNone FixedFixed vert line vert line dropout dropout

Convex Convex phasedphased

blunted blunted sectorsector

electronicelectronic electronicelectronic poor poor steering & steering & focusingfocusing

vectorvector flat top flat top sectorsector

electronicelectronic electronicelectronic poor poor steering & steering & focusing focusing

AXIAL AXIAL RESOLUTIONRESOLUTION

Resolution Resolution

o The ability to image accurately (accuracy, not merely quality)

Axial ResolutionAxial Resolution

o The ability to distinguish two structures that are close to each other front to back, parallel to, or along the beam's main axis.

UnitsUnits

o mm, cm — all units of distance

o The shorter the pulse, the smaller the number, the more accurate is the image

o The shorter the pulse, the better the LARRD resolution.

LARRD resolution improves withLARRD resolution improves with

o Less ringing, fewer cycles in pulse (fewer cars in the train)

o Higher frequency sound (each car in the train is shorter), shorter wavelength (each car is shorter).

o changing either of these factors requires a new transducer

Note Note

o As frequency increases, the numerical value of the LARRD resolution decreases. This means that we have improved LARRD resolution and higher quality images with high frequency transducers.

BIOEFFECTS BIOEFFECTS

Measuring the Output of Measuring the Output of Ultrasound MachinesUltrasound Machines

oHydrophone:o A small needle with a piezoelectric crystal

at its end. The needle is placed in the ultrasound beam.

HydrophoneHydrophoneo It attaches to an oscilloscope and displays

acoustic signals received by the crystal. Can quantitate amplitude, period, pulse duration and pulse repetition period.

Radiation ForceRadiation Forceo An incident sound wave can exert a small but

measurable force on an object. If the object is a balance or a float, we can measure the SATA intensity. If a small suspended ball is used, we can measure the SPTA intensity.

Acoustic-OpticsAcoustic-Opticso Based on the interaction of two

types of waves, sound and light. It quantifies amplitude, period, pulse duration & pulse repetition period.

o A shadowing system, called a Schlieren, uses this principle to measure beam profiles.

CalorimeterCalorimeter

o A transducer which turns acoustic energy into heat. When the total heat gain is measured along with the time that it took to obtain the heat, the total power of the US beam can be calculated.

ThermocoupleThermocouple

o A small device embedded in an absorbing material. The US is absorbed, turned into heat, and the thermocouple measures the change in temperature. The US intensity at specific points in space are measured by a thermocouple.

CrystalsCrystals

o Cholesteric liquid crystals or starch/iodine blue, when struck by different US intensities, turn different colors. The color of the crystals give us insight into the shape and strength of the beam.

Biologic Effects & SafetyBiologic Effects & Safety

o In-vivo & In-vitro Bioeffects have been studied in experiments with:

o Living (in vivo) as well as in

o Nonliving settings {in vitro).

DosimetryDosimetry

o The science of identifying and measuring those characteristics of an US field which are especially relevant to its potential for producing biological effects.

Thermal MechanismThermal Mechanism

o Temperature elevation via absorption resulting from interaction of biologic tissue and US. A second mode of thermal injury may result from localized scattering of acoustic energy, especially at in homogencities within the medium (Rayleigh scattering.)

Tissues BoneTissues Bone

o Tissue-bone interface is an absorber. Therefore temperature elevation at a tissue-bone interface is more likely.

Fetal tissuesFetal tissues

o Temperature elevation in fetal soft tissue is considered of potentially greater harm than in adults. Thus, fetal soft tissues adjacent to bone is of great concern.

Thermal IndexThermal Index

o The Thermal Index is a number proposed in the most recent AIUM guidelines that relates to tissue heating. Thermal index is a theoretical calculation related to the possible temperature elevation, measured in degrees centigrade, that could be produced by the sound beam.

o TIS—thermal index calculated assuming that the sound beam travels in soft tissue.

o TIB—thermal index calculated assuming that bone is at the beam's focus, usually a higher number than the TIS

o TIC—thermal index calculated assuming that cranial bone is at or near the skin's surface.

Focused vs. Unfocused Thermal Focused vs. Unfocused Thermal EffectsEffectso Focused beams are less likely to cause

temperature elevation in tissues.

o Unfocused beams are more likely to cause temperature elevation in tissues.

o This phenomenon occurs because a narrow beam heats only a small region of tissue and the heat is rapidly transferred to and dissipated by adjacent tissues that were not heated by the US beam.

Intensity LimitsIntensity Limits

o Therefore, the AIUM maximum intensity limit is lower for unfocused sound (lOOmW/cm2) than for focused sound (l,000)mW/cm2 or 1 watt/cm2.)

Cavitation MechanismCavitation Mechanism

o Gaseous nuclei

o Microbubbles (gaseous nuclei) may be excited by US. This takes the form of shrinking and expanding of the bubble.

o Potential of near total energy absorption where the nuclei exist may lead to thermal injury.

o So far, minimal evidence that cavitation really occurs in diagnostic US. Current data indicate cavitation can occur in mammals at SPTP intensities exceeding 3,500W/cm2.

Mechanical IndexMechanical Index

o The Mechanical Index (MI) is a number proposed in the AIUM guidelines that relates to cavitation.

o Mechanical index is higher (more likely to produce cavitation) with:

o 1) Higher pressure beams

o 2) Lower frequency sound

STABLE CAVITATIONSTABLE CAVITATION

o Bubbles tend to oscillate when exposed to acoustic waves of small amplitude. Bubbles do not burst. Bubbles that are a few micrometers in diameter double in size.

o Bubbles intercept, reradiate and absorb acoustic energy.

TRANSIENT CAVITATIONTRANSIENT CAVITATION

o Bubbles expand and collapse violently. Bubbles burst.

o Depends upon the pressure of ultrasound pulses (MPa).

o Synonyms also called normal or inertial cavitation

ThresholdThreshold

o Threshold for transient cavitation is only 10% greater than the pressure for stable cavitation.

EffectsEffects

o Highly localized violent effects:

o Enormous pressures— shock wave— mechanical stress.

o Colossal temperatures—thousands of degrees.

Bioeffects—General ConceptsBioeffects—General Concepts

o • In Vitro (out of the body) Studies—results are real; although important scientifically, "reports of in vitro studies which claim direct clinical significance should be viewed with caution" (AIUM statement.)

o Very high US intensities—genetic alteration or damage which may be lethal to cells.

o Under controlled conditions, bioeffects are considered beneficial!

o Low level US intensities—no known effects.

Epidemiology and StatisticsEpidemiology and Statistics

o Epidemiological Studies (population studies)

o Require large numbers of patients when the occurrence rate of the measure effect is small. This is the case in diagnostic imaging.

o Randomized clinical studies require large data sets on each patient to account for other factors which may contribute to the finding.

o Using this method, diagnostic ultrasound has shown no adverse effects on fetal outcome or birth weight or any other measurement.

Best study isBest study is

o 1. Prospective and

o 2. Randomized

Limitations of Epidemiologic StudiesLimitations of Epidemiologic Studies

o 1. often retrospective (looking at charts and information acquired in the past); it is best to collect data prospectively so that the record is complete

o 2. ambiguities —justification for exam, gestational age, number of exams, exposure, and mode

o 3. other risk factors —maternal age, nutrition, smoking, alcohol, drugs

Statistical ConsiderationsStatistical Considerations

o The smaller the effect, the harder it is to detect

o Requires large number of patients

AIUM Statement on Clinical SafetyAIUM Statement on Clinical Safety

o Approved: March 1988 o Diagnostic ultrasound has been in use since the

late 1950s.o Given its known benefits and recognized efficacy

for medical diagnosis, including use during human pregnancy, the American Institute of Ultrasound in Medicine herein addresses the clinical safety of such use:

o No confirmed biological effects on patients or instrument operators caused by exposure at intensities typical of present diagnostic ultrasound instruments have ever been reported.

o Although the possibility exists that such biological effects may be identified in the future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the risks, if any, that may by present.

Benefits should outweigh the risks. Benefits should outweigh the risks.

o The AIUM suggests:o Do not perform studies without reason

o Do not prolong studies without reason

o Minimize exposure time (but perform a complete diagnostic exam).

o Use minimum output power and highest receiver gain required to produce optimal images

o ALARA Principle .

Electrical and Mechanical HazardsElectrical and Mechanical Hazards

o Several different instruments as well as the individual components of the US system may be linked to a patient at any given time. Precautions such as proper electrical grounding should always be taken to avoid electrical hazard. Instruments should be routinely checked for proper condition.

Shadowing Shadowing And

Enhancement

Acoustic Shadowing

Acoustic Shadowing

Acoustic Enhancement

Controls

Contrast and BrightnessContrast and Brightness

o Monitor specific

o Does not need to be adjusted very frequently

GainGain

o This is the most important control for image optimization

Attenuation causes amplitude lowering in deeper tissues....amplitude is translated into brightness

Similar lesionsat different depths should look the same!!

Time Gain CompensationTime Gain Compensation

The shape of the basic gain filtercan be altered and machines providevarious controls for altering the gain atspecific depths.

Gain Gain

o Use high gain to get detail in dark areas

o Low gain for detail in bright areas

Gain ControlsGain Controls

Gain Gain

FocusFocus

Ensure that the structure of interestis in the focal zone

TransducersTransducers

o Piezoelectric

o Both the transmitter and receiver of ultrasound

o Central or preferential frequency, the range of frequency is the “Bandwidth”

Classification of TransducersClassification of Transducers

o Method of beam steering, mechanical or electronico Element arrangement and shape of field...linear,

sector (phased), convex or annularo Special probes, encocavitary, intravascular probes

Transducer SelectionTransducer Selection

o The higher the frequency, the better the resolutiono The higher the frequency, the lower the penetrationo THI combines the advantages of better resolution and

higher penetration

Select the highest frequency transducer

permitting penetration to the required depth.....

for superficial structures like breast, thyroid,

scrotum and for appendix use 7.5 – 10 MHz,

for abdomen 3.5 – 6 MHz and for heart 2.5-5 MHZ

Transducer selectionTransducer selection

oProbe frequency

oFreeze

oDynamic Range

oPost processing

Transducer FrequencyTransducer Frequency

Transducer selection Transducer selection (Cont’d)(Cont’d)

o Select the foot-print appropriate for the structure size and depth and intervening obstacles

o For heart, a micro-convex or sector probe that will allow scanning between ribs; for small parts a linear face and for abdomen a linear or convex face

Dynamic RangeDynamic Range

o Use the highest possible dynamic range without making the picture too soft

Dynamic RangeDynamic Range

DR 90 65 30

Image QualityImage Quality

o Spatial resolution

o Contrast resolution

o Temporal resolution

Contrast

Contrast

N=16

EEndnd

Physics basic

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