Physics Of Ultrasound And Echocardiography

History of Ultrasound Imaging

▫ 1760 - Abbe Lazzaro Spallanzani – Father of ultrasound▫ 1912 - First practical application for rather unsuccessful search

for Titanic▫ 1942 - First used as diagnostic tool for localizing brain tumors

by Karl Dussik▫ 1953 - First reflected Ultrasound to examine the heart, the

beginning of clinical echocardiography – Dr.Helmut Hertz , a Swedish Engineer and Dr. Inge Edler a cardiologist

▫ 1970s - Origin of TEE ,Lee Frazin, a cardiologist from Chicago mounted M-mode probe on a Transoesophageal probe.

Topic outline

1. Echo

basics

•Tips on Ultrasound waves / interaction with tissues

•Ultrasound transducers /probes

•Image Resolution

2. Imaging modes

•2-D Imaging & Imaging planes (normal 2D Echo)

•M-Mode (Normal M-Mode Echo)

3. Doppler Echo

•Basic principles•Doppler Imaging

Modalities•CW Doppler •Pulsed

Doppler•CF Doppler

•Relationship between Doppler velocity and pressure gradient

Sound

Mechanical vibration transmitted through an elastic medium

Pressure waves when propagate thro’ air at appropriate frequency produce sensation of hearing

Vibration Propagation

Surface Vibration Pressure Wave Ear

As sound propagates through a medium the particles of the medium vibrate

Air at equilibrium, in the absence of a sound wave

Compressions and rarefactions that constitute a sound wave

“Sine wave”

Amplitude - maximal compression of particles above the baseline

Wavelength - distance between the two nearest points of equal pressure and density

One Compression and rarefaction constitute one sound wave . It can be represented as “Sine wave”.

Velocity = frequency x Wavelength

Frequency – No. of wavelengths per unit time. 1 cycle/ sec = 1 Hz

So, Frequency is inversely related to wavelength Velocity – Speed at which waves propagate

through a medium– Dependent on physical properties of the medium

through which it travels – Directly proportional to stiffness of the material– Inversely proportional to density within a

physiological limit

Sound velocity in different materials

Material Velocity ( m/s)

Air 330

Water 1497

Metal 3000 - 6000

Fat 1440

Blood 1570

Soft tissue 1540

ULTRASOUND

Ultrasound is sound with a frequency over 20,000 Hz, which is the upper limit of human hearing.The basic principles and properties are same as that of audible sound

Frequencies used for diagnostic ultrasound are between 1 to 20 MHz

Medical ultrasound imaging typically uses sound waves at frequencies of 1,000,000 to 20,000,000 Hz (1.0 to 20 MHz). In contrast, the human auditory spectrum (between 20 and 20,000 Hz)

Frequency and wavelength are mathematically related to the velocity of the ultrasound beam within the tissue:

Velocity =  Wavelength (mm)  x  frequency (Hz) The speed with which an acoustic wave moves through a medium is

dependent upon the density and resistance of the medium. Media that are dense will transmit a mechanical wave with greater

speed than those that are less dense. The resolution of a recording, ie, the ability to distinguish two objects

that are spatially close together, varies directly with the frequency and inversely with the wavelength

High frequency, short wavelength ultrasound can separate objects that are less than 1 mm apart.

Imaging with higher frequency (and lower wavelength) transducers permits enhanced spatial resolution

However, because of attenuation, the depth of tissue penetration or the ability to transmit sufficient ultrasonic energy into the chest is directly related to wavelength and therefore inversely related to transducer frequency

As a result, the trade-off for use of higher frequency transducers is reduced tissue penetration

The trade-off between tissue resolution and penetration guides the choice of transducer frequency for clinical imaging.

As an example, higher frequency transducers can be used in echocardiography for imaging of structures close to the transducer.

Interaction of ultrasound wave with tissues

1. Attenuation2. Reflection 3. Scattering 4. Absorption

Attenuation

Loss of intensity and amplitude of ultrasound wave as it travels through the tissues

Due to reflection, scattering and absorption Proportional to Frequency and the distance the wave

front travels – Higher frequency , more attenuation Longer the distance (Depth), more the attenuation

And also on the type of tissue through which the beam has to pass

Expressed as “Half – power distance” For most of soft tissues it is 0.5 – 1.0 dB/cm/MHz

Reflection

Basis of all ultrasound imaging

From relatively large, regularly shaped objects with smooth surfaces and lateral dimensions greater than one wavelength – Specular Echoes

These echoes are relatively intense and angle dependent.

From endocardial and epicardial surfaces, valves and pericardium

Amount of ultrasound beam that is reflected depends on the difference in Acoustic impedance between the mediums

The resistance that a material offers to the passage of sound wave

Velocity of propagation “v” varies between different tissues

Tissues also have differing densities “ρ” Acoustic impedance “Z = ρv” Soft tissue / bone and soft tissue / air

interfaces have large “Acoustic Impedance mismatch”

Acoustic Impedance

Scattering

Type of reflection that occurs when ultrasound wave strikes smaller(less than one wavelength) , irregularly shaped objects - Rayleigh Scatterers ( e.g.. RBCs)

Are less angle dependant and less intense.

Weaker than Specular echoes

Result in “Speckle” that produces the texture within the tissues

Interaction Of Ultrasound Waves With Tissues

When an ultrasonic wave travels through a homogeneous medium, its path is a straight line. However, when the medium is not homogeneous or when the wave travels through a medium with two or more interfaces, its path is altered; either of the ff:

 Scattering: Small structures, eg, less than 1 wavelength in lateral dimension, result in scattering of

the ultrasound signal Unlike a reflected beam, scattering results in the US beam being radiated in all

directions, with minimal signal returning to the transducer Refraction: Attenuation:

Signal strength is progressively reduced due to absorption of the US energy by conversion to heat (frequency and, wavelength dependent)

The depth of penetration: 30 cm for a 1 MHz transducer, 12 cm for 2.5 MHz transducer, and 6 cm for a 5 MHz transducer

Air has a very high acoustic impedance, resulting in significant signal attenuation when imaging through lung tissue, especially emphysematous lung, or pathologic conditions such as pneumomediastinum or subcutaneous emphysema

In contrast, filling of the pleural space with fluid, generally enhances ultrasound imaging

How is ultrasound imaging done?

“From sound to image”

Pierre Curie (1859-1906),Nobel Prize in Physics, 1903

Jacques Curie (1856-1941)

PIEZOELECTRIC EFFECT

Piezoelectric effect

Crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt have the ability to generate an electric charge in response to applied mechanical stress

“Piezoelectricity" after the Greek word Piezein, which means to squeeze or press.

“Converse” of this effect is also true

Construction of a Transducer

Backing Material

Electrodes

Piezoelectric crystal

US transducers use a piezoelectric crystal to generate and receive ultrasound wavesImage formation: is related to the distance of a structure from the transducer, based upon the time interval between ultrasound transmission and arrival of the reflected signal The amplitude is proportional to the incident angle and acoustic impedance, and timing is proportional to the distance from the transducer

Ultrasound Transducers

Production of ultrasound

1. Piezoelectric crystal2. High frequency electrical signal with continuously changing

polarity 3. Crystal resonates with high frequency 4. Producing ULTRASOUND 5. Directed towards the area to be imaged 6. Crystal “listens” for the returning echoes for a given period

of time 7. Reflected waves converted to electric signals by the crystal8. processed and displayed

Schematic representation of the recording and display of the 2-D image

Electronic Phased Array which uses the principle of Electronic Delay

Phased Array Transducers

Electronic Focusing

Electronic beam steering

Characteristics of ULTRASOUND BEAM

Length of near field = ( radius)2 / wavelength of emitted ultrasound

TEE workstation

Resolution

Ability to distinguish two points in space

Two components – Spatial – Smallest distance that two targets can be separated for the system to distinguish between them.

Two components – Axial and Lateral

Temporal

• Axial Resolution ▫ The minimum separation

between structures the ultrasound beam can distinguish parallel to its path.

▫ Determinants:▫ Wavelength – smaller the

better▫ Pulse length – shorter the

train of cycles greater the resolution

• Lateral Resolution▫ Minimum separation

between structures the ultrasound beam can distinguish in a plane perpendicular to its path.

▫ Determinants: ▫ Depends on beam width –

smaller the better ▫ Depth ▫ Gain

Temporal resolution

Ability of system to accurately track moving targets over time

Anything that requires more time will decrease temporal resolution

Determinants:Depth

Sweep angle

Line density

PRF

The Trade off ..

To visualize smaller objects shorter wavelengths should be used which can be obtained by increasing frequency of U/S wave.Drawbacks of high frequency –

More scatter by insignificant inhomogeneityMore attenuation Limited depth of penetration

For visualising deeper objects lower frequency is useful, but will be at the cost of poor resolution

So..

The reflected signal can be displayed in four modes..

A- mode

B- mode

M- mode

2-Dimensional

A. Two dimensional (2-D) imaging :– A 2D image is generated from data obtained mechanically (mechanical

transducer) or electronically (phased-array transducer)– The signal received undergoes a complex manipulation to form the final

image displayed on the monitor including signal amplification, time-gain compensation, filtering, compression and rectification.

B. M-mode: Motion or "M"-mode echocardiography is among the earliest forms of

cardiac ultrasound The very high temporal resolution by M-mode imaging permits:

– identification of subtle abnormalities such as fluttering of the anterior mitral leaflet due to aortic insufficiency or movement of a vegetation.

– dimensional measurements or changes, such as chamber size and endocardial thickening, can be readily appreciated

2-D & M Mode

A –mode

shows the

Amplitude of

reflected

energy at

certain depth

B- Brightness mode shows the energy as the brightness

of the point

M- Motion mode the reflector is moving so if the

depth is shown in a time plot, the

motion will be seen as a curve

A

B

C

M- mode

• Timed Motion display ; B – Mode with time reference

• A diagram that shows how the positions of the structures along the path of the beam change during the course of the cardiac cycle

• Strength of the returning echoes vertically and temporal variation horizontally

M – Mode uses..

Great temporal resolution- Updated 1000/sec. Useful for precise timing of events with in a cardiac cycle

Along with color flow Doppler – for the timing of abnormal flows

Quantitative measurements of size , distance & velocity possible with out sophisticated analyzing stations

2 – D MODE

Provides more structural and functional information

Rapid repetitive scanning along many different radii with in an area in the shape of a fan

2-D image is built up by firing a beam , waiting for the return echoes, maintaining the information and then firing a new line from a neighboring transducer along a neighboring line in a sequence of  B-mode lines.

2-D imaging by steering the transducer over an area that needs to be imaged

Mechanical Steering of the Transducer

Electronic Phased Array Transducers for 2-D imaging

Linear Array Curvilinear Array

A single ‘FRAME’ being formed from one full sweep of beams

A ‘CINE LOOP’ from multiple FRAMES

Resembles an anatomic section – easy to interpret2-D imaging provides information about the spatial relationships of different parts of the heart to each other.Updated 30- 60 times/sec ; lesser temporal resolution compared to M-mode

OPTIMIZATION OF 2-D IMAGESTechnical Factors I

TRANSDUCER: High frequency increases backscatter and resolution but lacks depth

penetration Low-frequency transducers permit good penetration but reduced image

resolution

DEPTH: The deeper the field of the image, the slower the frame rate The smallest depth that permits display of the region of interest should be

employed

FOCUS: Indicates the region of the image in which the ultrasound beam is narrowest

Resolution is greatest in this region

GAIN: This function adjusts the displayed amplitude of all received signals

Study of blood flow dynamics

Detects the direction and velocity of moving blood within the heart.

Doppler Study

Comparison between 2-D and Doppler

2-D Doppler

Ultrasound target

Tissue Blood

Goal of diagnosis

Anatomy Physiology

Type of information

Structural Functional

So, both are complementary to each other

Christian Andreas Doppler (1803 – 1853)

DOPPLER EFFECT

DOPPLER EFFECT-

Certain properties of light emitted from stars depend upon the relative motion of the observer and the wave source.

Colored appearance of some stars as due to their motion relative to the earth, the blue ones moving toward earth and the red ones moving away.

OBSERVER 2 Long wavelength Low frequency

OBSERVER 1Small wavelengthHigh frequency

Doppler Frequency Shift - Higher returned frequency if RBCs are moving towards the and lower if the cells are moving away

Doppler principle as applied in Echo..

The Doppler equation

Velocity is given by Doppler equation..

V = c fd / 2 fo cos V – target velocity C – speed of sound in tissue fd –frequency shift fo –frequency of emitted U/S - angle between U/S beam & direction of target velocity( received beam , not the emitted)

Doppler Equation

Doppler blood flow velocities are

displayed as waveforms

When flow is perpendicular to U/S beam angle of incidence will be 900/2700 ; cosine of which is 0 – no blood flow detected

Flow velocity measured most accurately when beam is either parallel or anti parallel to blood flow.

Diversion up to 200 can be tolerated( error of < or = to 6%)

Important consideration !

“Twin Paradoxes of Doppler”

Best Doppler measurements are made when the Doppler probe is aligned parallel to the blood flow

High quality Doppler signals require low Doppler frequencies( < 2MHz)

Importance of being parallel to flow when detecting flow through the aortic valve

Velocity is directly proportional to frequency shift and for clinical use it is usual to discuss velocity rather than frequency shift ( although either is correct)

V a fd / cos V = c fd / 2 fo cos V a fd

BASIC PRINCIPLES:

utilizes ultrasound to record blood flow within the cardiovascular system (While M-mode and 2D echo create ultrasonic images of the heart)

is based upon the changes in frequency of the backscatter signal from small moving structures, ie, red blood cells, intercepted by the ultrasound beam

A moving target will backscatter an ultrasound beam to the transducer so that the frequency observed when the target is moving toward the transducer is higher and the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency This Doppler phenomenon is familiar to us as the sound of a train

whistle as it moves toward (higher frequency) or away (lower frequency) from the observer

This difference in frequency between the transmitted frequency (F[t]) and received frequency (F[r]) is the Doppler shift:

  Doppler shift (F[d]) = F[r] - F[t]

Doppler effect(Pairs of transmitting (T) and receiving (R) transducers):• With a stationary target (panel A): the carrier frequency [f(t)] from the transmitting transducer strikes the target and is reflected back to the receiving transducer at the reflected frequency [f(r)], which is unaltered• with a target moving toward the transducer (panel B): An increase in f(r) is seen

• with a target moving away from the transducer (panel C): f(r) is reduced

•In all cases, the extent to which f(t) is increased or reduced is proportional to the velocity of the target

A flow moving toward the transducer has a higher observed frequency than a flow moving away from the transducer.

Blood flow velocity (V) is related to the Doppler shift by the speed of sound in blood (C) and ø (the intercept angle between the ultrasound beam and the direction of blood flow) A factor of 2 is used to correct for the "round-trip" transit time to and from the

transducer.

  F[d] = 2 x F[t] x [(V x cos ø)] ÷ C

This equation can be solved for V, by substituting (F[r] - F[t]) for F[d]:

  V = [(F[r] -F[t]) x C] ÷ (2 x F[t] x cos ø)

the angle of the ultrasound beam and the direction of blood flow are critically important in the calculation For ø of 0º and 180º (parallel with blood flow), cosine ø = 1 For ø of 90º (perpendicular to blood flow), cosine ø = 0 and the Doppler shift is 0 For ø up to 20º, cos ø results in a minimal (<10 percent) change in the Doppler shift For ø of 60º, cosine ø = 0.50

The value of ø is particularly important for accurate assessment of high velocity jets, which occur in aortic stenosis or pulmonary artery hypertension

It is generally assumed that ø is 0º and cos ø is therefore 1

•Ideally, the beam should be placed parallel to blood flow

When the beam does not lie parallel, it is possible to introduce a correction into the calculation of flow velocity by measuring the cosine of the angle of interrogation and introducing this value into the Doppler equation

SPECTRAL ANALYSIS  When the backscattered signal is

received by the transducer, the difference between the transmitted and backscattered signal is determined by comparing the two waveforms with the frequency

content analyzed by: fast Fourier transform (FFT)

The display generated by this frequency analysis is termed spectral analysis

By convention, time is displayed on the x axis and frequency shift on the y axis

Shifts toward the transducer are represented as "positive" deflections from the "zero" baseline, and shifts away from the transducer are displayed as "negative" deflections

• Spectral information can be displayed in real time (Doppler figure)

The Doppler signal portrays the entire period of flow, ie: acceleration (a), peak flow (pf), and deceleration (d).

Applications of Doppler - Different modes to measure blood velocities

Continuous wave

Pulsed wave

Colour Flow Mapping

Modern echo scanners combine Doppler

capabilities with 2D imaging capabilities

Imaging mode is switched off (sometimes with the image held in memory) while the Doppler modes are in operation

CONTINUOUS WAVE DOPPLER

Continuous generation of ultrasound waves coupled with continuous ultrasound reception using a two crystal transducer

CWD at LVOT in Deep TG Aortic Long axis view

Can measure high velocity flows ( in excess of 7m/sec)Lack of selectivity or depth discrimination -Region where flow dynamics are being measured cannot be precisely localizedMost common use – Quantification of pressure drop across a stenosis by applying Bernoulli equation

1/2 PV2 Pressure

Kinetic Energy

Potential Energy

P = 4V2

Bernoulli EquationBalancing Kinetic and Potential

energy

This goes down..As this goes up..

Doppler Velocity And Pressure Gradient

 Doppler echo can estimate the pressure difference across a stenotic valve or between two chambers

This r/n ship is defined by the Bernoulli equation and is dependent on : velocity proximal to a stenosis (V1) velocity in the stenotic jet (V2) density of blood (p), acceleration of blood through the orifice

(dv/dt), and viscous losses (R[v]): The pressure gradient (Δ P) can be calculated from:

  Δ P = [0.5 x p x (V2 x V2 - V1 x V1)] + [p x (dv/dt)] + R[v]

(If one assumes that the last two terms (acceleration and viscous losses) are small, and then enter the constants, the formula is simplified to):

  Δ P (mmHg) = 4 x (V2 x V2 - V1 x V1) Thus, the Bernoulli formula may be further simplified:

  Δ P (mmHg) = 4V2

PULSED WAVE DOPPLER

Doppler interrogation at a particular depth rather than across entire line of U/S beam.Ultrasound pulses at specific frequency - Pulse Repetition Frequency (PRF) or Sampling rateRANGE GATED - The instrument only listens for a very brief and fixed time after the transmission of ultrasound pulseDepth of sampling by varied by varying the time delay for sampling

Transducer alternately transmits and receives the ultrasound data to a sample volume. Also known as Range-gated Doppler.

PWD at LVOT in Deep TG aortic long axis view

PRF for a given transducer of a given frequency at a particular depth is fixed; But to measure higher velocities higher PRFs are necessary

Drawback – ambiguous information obtained when flow velocity is high velocities (above 1.5 to 2 m/sec)

This effect is called Aliasing

ALIASING

Aliasing will occur if low pulse repetition frequencies or velocity scales are used and high velocities are encountered

Abnormal velocity of sample volume exceeds the rate at which the pulsed wave system can record it properly.

Blood velocities appear in the direction opposite to the conventional one

Full spectral display of a high velocity profile fully recorded by CW Doppler

PW display is aliased, or cut off, and the top is placed at the bottom

Aliasing occurs if the frequency of the sample volume is more than the Nyquist limit

Nyquist limit = PRF/2

To avoid Aliasing - PRF = 2 ( Doppler shift frequency or Maximum velocity of Sample volume)

Can be achieved by – Decreasing the frequency of transducer, decrease the depth of interrogation by changing the view ( this increases the PRF)

Color Flow Doppler

Displays flow data on 2-D Echocardiographic imageImparts more spatial information to Doppler data Displays real-time blood flow with in the heart as colors while showing 2D images in gray scaleAllows estimation of velocity, direction and pattern of blood flow

Multigated, PW Doppler in which blood flow velocities are sampled at many locations along many lines covering the entire imaging sector

Echo data is processed through two channels that ultimately combine the image with the color flow data in the final display.

Color Flow Doppler..Flow toward transducer – red Flow away from transducer – blue Faster the velocity – more intense is the colour Flow velocity that changes by more than a preset value within a brief time interval (flow variance) – green / flame

CFM v/s Angiography

CFM Angiography

Records velocity not flow; So in MR, CFM jet area consists of both atrial and ventricular blood – Billiard Ball Effect

Records flow

Larger regurgitant orifice area there will be smaller jet area

Larger regurgitant orifice area there will be larger jet area

Instrumentation factors in Color Doppler Imaging

Eccentric jets appear smaller than equivalently sized central jets – Coanda Effect

High pressure jet will appear larger than a low-pressure jet for the same amount of flow

As gain increases, jet appears larger

As ultrasound output power increases, jet area increases

Lowering PRF makes the jet larger

Increasing the transducer frequency makes the jet appear larger

Advantages & disadvantages Doppler methods used for cardiac evaluation :

A. continuous wave doppler

B. Pulsed wave doppler

C. color flow doppler

CONTINUOUS WAVE DOPPLER employs two dedicated ultrasound crystals, one for

continuous transmission and a second for continuous reception This permits measurement of very high frequency Doppler

shifts or velocities

Limitations of this technique: It receives a continuous signal along the entire length of

the US beam Thus, there may be overlap in certain settings, such as:

stenoses in series (eg, left ventricular outflow tract gradient and aortic stenosis) or

flows that are in close proximity/alignment (eg, AS and MR)

PULSED DOPPLER 

permits sampling of blood flow velocities from a specific region In contrast to continuous wave Doppler which records signal along

the entire length of the ultrasound beam is always performed with 2D guidance to determine the sample

volume position

Particularly useful for assessing the relatively low velocity flows associated with:

1) transmitral or transtricuspid blood flow,

2) pulmonary venous flow,

3) left atrial appendage flow, or

4) for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation

COLOR FLOW IMAGING

• With CF imaging, velocities are displayed using a color scale:with flow toward the transducer displayed in orange/red flow away from the transducer displayed as blue

SECOND HARMONIC IMAGING(Improving Resolution) An ultrasound wave traveling through tissue becomes distorted,

which generates additional sound frequencies that are harmonics of the original or fundamental frequency

produces more harmonics the further it travels through tissue uses broadband transducers that receive double the transmitted

frequency

When compared to conventional imaging, it reduces variations in ultrasound intensity along endocardial and myocardial surfaces, enhancing these structures

of particular benefit for patients in whom optimal echocardiographic images are technically difficult to obtain

harmonic imaging improves interphase definition

Thank You..