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ECHO BASICS

PHYSICS AND INSTRUMENTATION

- DR. NAIR ANISHKUMAR P.K.V

Mechanical vibration transmitted through an

elastic medium.

Spectrum of sound

Sound

• Ultrasound can be directed as a beam and

focused

• As ultra sound passes through a medium it

obeys laws of reflection and refraction

• Targets of relatively small size reflect

ultrasound thus can be detected and

characterised.

Advantages for Diagnostic utility

• Ultrasound is poorly transmitted through

a gaseous medium

• Attenuation occurs rapidly, Especially at

higher frequency.

Disadvantages

Particles of the medium vibrate parallel to the

line of propagation producing longitudinal waves.

Areas of compression alternates with areas of

rarefaction.

Amount of reflection , refraction and attenuation

depends on acoustic properties of medium

Denser medium reflect higher percentage of

sound energy

Mechanics :

Mechanics :

The loss of ultrasound as it propagates through a

medium is referred to as attenuation

It is the rate at which the intensity of the ultrasound

beam diminishes as it penetrates the tissue.

Attenuation has three components: absorption,

scattering, and reflection

INTERACTION BETWEEN ULTRASOUND AND TISSUE

Always increases with depth

It is affected by the frequency of the transmitted beam

and the type of tissue through which the ultrasound

passes

The higher the frequency is, the more rapidly it will

attenuate

Attenuation increases with increase in density of

medium.

Attenuation

Expressed as the half-power distance, which is a �measure of the distance that ultrasound travels before

its amplitude is attenuated to one half its original

value.

As a rule of thumb, the attenuation of ultrasound in

tissue is between 0.5 and 1.0 dB/cm/MHz.

Attenuation

The velocity and direction of the ultrasound beam as

it passes through a medium are a function of the

acoustic impedance of that medium

Acoustic impedance (Z, measured in rayls) is the

product of velocity (in meters per second) and

physical density (in kilograms per cubic meter).

Acoustic impedance

Acoustic impedance

The phenomena of reflection and refraction obey the

laws of optics and depend on the angle of incidence

between the transmitted beam and the acoustic

interface as well as the acoustic mismatch, i.e., the

magnitude of the difference in acoustic impedance’

Use of a acoustic coupling gel during transthoracic

imaging

Acoustic impedance Importance :

The interaction between an ultrasound beam and a

reflector depends on the relative size of the targets

and the wavelength of the beam

As the size of the target decreases, the wavelength of

the ultrasound must decrease proportionately to

produce a reflection and permit the object to be

recorded.

Specular echoes and scattered echoes

Specular echoes are produced by reflectors that are

large relative to ultrasound wavelength

The spatial orientation and the shape of the reflector

determine the angles of specular echoes.

Examples of specular reflectors include endocardial

and epicardial surfaces, valves, and pericardium

Specular echoes

Specular echoes

Targets that are small relative to the wavelength of

the transmitted ultrasound produce scattering

Such objects are referred to as Rayleigh scatterers.

The resultant echoes are diffracted or bent and

scattered in all directions.

Scattered echoes

Scattered echoes contribute to the visualization

of surfaces that are parallel to the ultrasonic

beam and also provide the substrate for

visualizing the texture of grey-scale images

The term speckle is used to describe the tissue-

ultrasound interactions that result from a large

number of small reflectors within a resolution

cell.

Scattered echoes

Without the ability to record scattered echoes, the

left ventricular wall, for example, would appear as

two bright linear structures, the endocardial and

the epicardial surfaces, with nothing in between .

High-frequency ultrasound though has good

resolution , is reflected by many small interfaces

within tissue, resulting in scattering, much of the

ultrasonic energy becomes attenuated and less

energy is available to penetrate deeper into the

body..

Importance :

THE TRANSDUCER

Piezoelectricity

A period of quiescence during which the transducer

listens for some of the transmitted ultrasound energy to �be reflected back is known as dead time.�

The amount of acoustic energy that returns to the

transducer is a measure of the strength and depth of

the reflector.

The time required for the ultrasound pulse to make the

round-trip from transducer to target and back again

allows calculation of the distance between the

transducer and reflector

Piezoelectricity

Piezoelectric ceramics : ferroelectrics, barium

titanate, and lead zirconate titanate

Piezoelectric elements are interconnected

electronically

The frequency of the transducer is determined by the

thickness of these elements.

Each element is coupled to electrodes, which transmit

current to the crystals, and then record the voltage

generated by the returning signals.

Piezoelectricity

The dampening material shortens the ringing

response of the piezoelectric substance after the

brief excitation pulse.

An excessive ringing response (or ringdown) �lengthens the ultrasonic pulse and decreases range

resolution.

Thus, the dampening material both shortens the

ringdown and provides absorption of backward and

laterally transmitted acoustic energy

Backing material

At the surface of the transducer, matching layers

are applied to provide acoustic impedance

matching between the piezoelectric elements and

the body.

This increases the efficiency of transmitted

energy by minimizing the reflection of the

ultrasonic wave as it exits the transducer surface.

Matching layers

An ultrasound beam as it leaves the transducer is

parallel and cylindrically shaped beam. Eventually,

however, the beam diverges and becomes cone

shaped .

The proximal or cylindrical portion of the beam is

referred to as the near field or Fresnel zone.

When it begins to diverge, it is called the far field or

Fraunhofer zone.

Wave motion

Imaging is optimal within the near field

The length of the near field (l) is described by the

formula:

where r is the radius of the transducer and λ is the

wavelength of the emitted ultrasound.

Near field

From the above formula optimal ultrasound imaging :

large-diameter & high-frequency transducer maximize

the length of the near field.

Near field

Factors preventing this approach from being

practical.

1) The transducer size is predominantly limited by

the size of the intercostal spaces.

2) Although higher frequency does lengthen the near

field, it also results in greater attenuation and lower

penetration of the ultrasound energy

Near field

the ultrasound beam is both focused and steered

electronically

it is primarily achieved through the use of

phased-array transducers, which consist of a

series of small piezoelectric elements

interconnected electronically

MANIPULATING THE ULTRASOUND BEAM

By adjusting the timing of excitation, the beam

can be steered

Dynamic transmit focusing

Near field Focusing

An undesirable effect of focusing is its effect on

beam divergence in the far field. Because focusing

results in a beam with a smaller radius, the angle

of divergence in the far field is increased.

Divergence also contributes to the formation of

important imaging artefacts such as side lobes

Resolution is the ability to distinguish

between two objects in close proximity.

two components:

spatial

temporal.

Resolution

It is defined as the smallest distance that two targets

can be separated for the system to distinguish

between them.

Two components:

Axial resolution

lateral resolution

Spatial resolution

Ability to differentiate two structures lying along the

axis of the ultrasound beam

The primary determinants are the frequency of the

transmitted wave and its effect on pulse length.

Axial resolution

the ability to distinguish two reflectors that lie

side by side relative to the beam

affected by the width or thickness of the

interrogating beam, at a given depth

lateral resolution diminishes as beam width

(and depth) increases.

Lateral resolution

The distribution of intensity across the beam profile

will also affect lateral resolution

both strong and weak reflectors can be resolved within

the central portion of the beam, where intensity is

greatest.

Lateral resolution

Gain is the amplitude, or the degree of

amplification, of the received signal.

Gain

Contrast resolution refers to the ability to distinguish

and to display different shades of grey

For accurate identification of borders display texture or detail

within the tissues.

Useful to differentiate tissue signals from background noise.

Dependent on target size.

A higher degree of contrast is needed to detect small

structures

Contrast resolution

Ability of the system to accurately track moving

targets over time.

It is dependent on speed of ultrasound and the depth

of the image as well as the number of lines of

information within the image.

Greater the number of frames per unit of time, the

smoother and more aesthetically pleasing the real-

time image.

Temporal resolution

CREATING THE IMAGE

The pulse, which is a collection of cycles traveling

together, is emitted at fixed intervals

TRANSMITTING ULTRASOUND ENERGY

How one can use ultrasound to obtain an image of an object.

Modes :

SIGNAL PROCESSING

Dynamic range is the extent of useful ultrasonic

signals that can be processed to reduce the range of

the voltage signals to a more manageable number

It is defined as the ratio of the largest to smallest

signals measured at the point of input to the display

It is expressed in decibels

Concept of dynamic range

The range of voltages generated during data

acquisition, by post-processing, is transformed to 30

shades of grey which the human eye is able to

distinguish

Grey scale :

The new frequencies generated due to nonlinear

interactions with the tissue ,which are integer

multiples of the original frequency, are referred to

as harmonics.

The returning signal contains both fundamental

and harmonic frequencies. By suppressing or

eliminating the fundamental component, an image

is created primarily from the harmonic energy

Tissue harmonic imaging

After destructive interference the remaining

harmonic energy can then be selectively

amplified, producing a relatively pure harmonic

frequency spectrum.

The strong fundamental signals produce intense

harmonics and weak fundamental signals

produce almost no harmonic energy thus

reducing artefacts.

The net result is that harmonic imaging reduces

near field clutter ,the signal-to-noise ratio is

improved and image quality is enhanced.

Tissue harmonic imaging

Side lobes occur because a portion of the energy

concentrate off to the side of the central beam and

propagate radially, a phenomenon known as edge

effect

A side lobe may form where the propagation

distance of waves generated from opposite sides of

a crystal differs by exactly one wavelength.

Side lobes are three-dimensional artefacts, and their

intensity diminishes with increasing angle.

ARTIFACTS : Side lobes

The artefact created by side lobes occurs because

all returning signals are interpreted as if they

originated from the main beam.

A prerequisite for a dominant side lobe artefact is

that the source of the artefact must be a fairly

strong reflecting target like The atrioventricular

groove and the fibrous skeleton of the heart

ARTIFACTS : Side lobes

Result from the beam reflecting from the

transducer or from other strong echo-producing

structures within the heart or chest .

Typically, a reverberation artefact that originates

from a fixed reflector will not move with the

motion of the heart.

It appears as one or more echo targets directly

behind the reflector, often at distances that

represent multiples of the true distance

ARTIFACTS : reverberations

Shadowing occurs beyond a region of unusually high

attenuation, such as a strong reflector.

It results in the absence of echoes directly behind

the target .

Eg: prosthetic valves & heavily calcified Native

structures.

ARTIFACTS : shadowing

ARTIFACTS : shadowing

Ring down artefact arises from high-amplitude �oscillations of the piezoelectric elements.

This only involves the near field

Eg : right ventricular free wall or left ventricular

apex

ARTIFACTS : near field clutter

ARTIFACTS : near field clutter

Doppler imaging is concerned with the direction,

velocity, and then pattern of blood flow through

the heart and great vessels.

The primary target is the red blood cells

It focuses on physiology and hemodynamics

The Doppler equations rely on a more parallel

alignment between the beam and the flow of

blood.

DOPPLER ECHOCARDIOGRAPHY

The increase or decrease in frequency due to

relative motion between the transducer and the

target is referred to as the Doppler shift.

It is the mathematical relationship between the

magnitude of the frequency shift and the velocity

of the target relative to the source

Doppler shift

the Doppler shift (∆f) depends on the transmitted

frequency (f₀ ) of the ultrasound, the speed of sound

(c ), the intercept angle between the interrogating

beam and the flow ( ө ), and, finally, the velocity of the

target (v ).

Doppler shift

Because the velocity of sound and the transmitted

frequency are known, the Doppler shift depends on

the velocity of blood and the angle of incidence, ( ө )

Doppler shift

Transducer or carrier frequency is the primary

determinant of the maximal blood flow velocity

that can be resolved

A lower frequency is advantageous because it

allows high flow velocity to be recorded.

Doppler shift

Five basic types

1 ) continuous wave Doppler

2 ) pulsed wave Doppler

3) color flow imaging

4 ) tissue Doppler

5 ) duplex scanning

Doppler Formats

It is similar to echocardiography. Short, intermittent

bursts of ultrasound are transmitted into the body

and listens at a fixed and very brief time interval �after transmission of the pulse.

This permits returning signals from one specific

distance from the transducer to be selectively

received and analysed, a process called range

resolution

Pulsed wave Doppler

The number of pulses transmitted from a Doppler

transducer each second is called the PRF.

To accurately represent a given frequency, it must

be sampled at least twice, that is

This formula establishes the limit (Nyquist limit)

below which the sampling rate is insufficient to

characterize the Doppler frequency.

Aliasing

This Imaging simultaneously transmits and receives

ultrasound signals continuously.

2 types

1) Transducer employs two distinct elements: one to

transmit and the other to receive

2) With phased-array technology, one crystal within

the array is dedicated to transmitting while another

is simultaneously receiving.

Continuous wave Doppler

A major advantage of continuous wave Doppler

imaging is that aliasing does not occur and very high

velocities can be accurately resolved.

A form of pulsed wave Doppler imaging that uses

multiple sample volumes to record the Doppler

shift

By overlaying this information on a two-

dimensional or M-mode template, the colour flow

image is created.

Based on the strength of the returning echo ,

flow velocity, direction, and a measure of

variance are then integrated and displayed as a

colour value

Colour Flow Imaging

The primary determinant of jet size is jet momentum,

which depends on both flow rate and velocity. Thus,

factors that affect velocity, including blood pressure, will

also affect jet size.

If colour Doppler imaging is performed when blood

pressure is either very high or very low, this clinical

information should be noted and taken into account

when the study is interpreted.

Technical Limitations of Color Doppler Imaging

The eccentric jets that become entrained along a

wall, making them appear smaller than they actually

are (Chamber constraint ).

For similar reasons, chamber size can also influence

the apparent area of a colour flow jet

Technical Limitations

By adjusting the colour scale, PRF is altered, and jet

size can change dramatically.

By lowering the scale (or Nyquist limit), the lower

velocity blood at the periphery of the jet becomes

encoded and displayed, making the jet appear larger.

Increasing the wall filter will reduce the jet size by

excluding velocities at the periphery.

Technical Limitations : Instrument settings

Power and instrument gain will also alter jet size.

Increasing these settings will increase jet area.

Transducer frequency has a complex effect on

colour jet area.

The jet size will tend to increase with high carrier

frequency because of the relationship between

velocity and the Doppler shift. On the other hand,

greater attenuation at higher frequency will make

jets appear smaller.

Technical Limitations : Instrument settings

Doppler imaging records velocity, not flow. It cannot

distinguish whether the moving left atrial blood

originated in the ventricle (the filled triangles) or

atrium (the filled circles), simply

that it has sufficient

velocity to be detected.

(billiard ball effect)

Related directly to the Doppler principle. For

example, aliasing occurs when pulsed wave

Doppler techniques are applied to flow velocities

that exceed the Nyquist limit

Mirror imaging / crosstalk :the appearance of a

symmetric spectral image on the opposite side of

the baseline from the true signal

Doppler Artifacts

Shadowing may mask colour flow information

beyond strong reflectors

Ghosting is a phenomenon in which brief swathes

of colour are painted over large regions of the

image

It is produced by the motion of strong reflectors

such as prosthetic valves..

Doppler Artifacts

Too much gain can create a mosaic distribution of

color signals throughout the image.

Too little gain eliminates all but the strongest

Doppler signals and may lead to significant

underestimation of jet area.

Doppler Artifacts

By adjusting gain and reject settings, the

Doppler technique can be used to record the

motion of the myocardium rather than the

blood within it

1) adjusting the machine to record a much lower

range of velocities

2) additional adjustments to avoid oversaturation

because the tissue is a much stronger reflector of the

Doppler signal compared with blood.

Tissue Doppler Imaging

One obvious limitation is that the incident angle between

the beam and the direction of target motion varies from

region to region.

This limits the ability of the technique to provide absolute

velocity information, although direction and relative

changes in tissue velocity are displayed.

The biologic effects of ultrasound energy are

related primarily to the production of heat

the amount of heat produced depends on the

intensity of the ultrasound, the time of exposure,

and the specific absorption characteristics of the

tissue.

BIOLOGIC EFFECTS OF ULTRASOUND

The perfusion of tissue have a dampening

effect on heat generation and physically allow

heat to be carried away from the point of

energy transfer.

Limited imaging time, occasional repositioning

of the probe, and constant monitoring of the

probe temperature help to ensure an

impeccable safety record

BIOLOGIC EFFECTS OF ULTRASOUND

Cavitation : Formation and behaviour of gas

bubbles produced when ultrasound penetrates

into tissue

Because of the relatively high viscosity of blood

and soft tissue, significant cavitation is unlikely.

BIOLOGIC EFFECTS OF ULTRASOUND

Few reports have suggested that some changes

might occur at the chromosomal level that would be

relevant to the developing foetus .

No evidence that any of physical phenomena

(oscillatory, sheer, radiation, pressure, and micro-

streaming ) has a significant biologic effect on

patients.

BIOLOGIC EFFECTS OF ULTRASOUND

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