physics of ultrasound and echocardiography
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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..
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