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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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