Basics of echo & principles of doppler echocardiography

BASICS OF ECHOCARDIOGRAPHYAND PRINCIPLES OF DOPPLER ECHOAbraha HailuAugust 29, 2010

1. BASICSultrasound (1923) vibrations of the same physical nature as sound but with frequencies above the range of human hearing, namely frequencies greater than 20,000 cycles per second.the diagnostic or therapeutic use of ultrasound and esp. a noninvasive technique involving the formation of a two-dimensional image used for the examination and measurement of internal body structures and the detection of bodily abnormalitiesa diagnostic examination using ultrasound

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 wavelengthHigh 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 resolutionHowever, 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 penetrationThe 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 or the chest wall, such as the apex of the left ventricle with transthoracic imaging.INTERACTION OF ULTRASOUND WAVES WITH TISSUESWhen 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 transducerRefraction: Attenuation:Signal strength is progressively reduced due to absorption of the US energy by conversion to heat (frequency and, wavelength dependent)

12 cm for 2.5 MHz transducer, and

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 imagingUltrasound waves sent from chest wall

ULTRASOUND TRANSDUCERS 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 signalThe amplitude is proportional to the incident angle and acoustic impedance, and timing is proportional to the distance from the transducer

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 frequencyproduces more harmonics the further it travels through tissue

uses broadband transducers that receive double the transmitted frequencyWhen 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 obtainharmonic imaging improves interphase definition

2. IMAGING MODALITIESTwo 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.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

OPTIMIZATION OF 2-D IMAGESTechnical Factors ITRANSDUCER:High frequency increases backscatter and resolution but lacks depth penetration

Low-frequency transducers permit good penetration but reduced image resolutionDEPTH: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 employedFOCUS:Indicates the region of the image in which the ultrasound beam is narrowest

Resolution is greatest in this regionGAIN:This function adjusts the displayed amplitude of all received signalsTRANSTHORACIC ACOUSTIC WINDOWS

IMAGING PLANESThe long-axis plane is the plane perpendicular to the posterior and anterior surfaces of the body and parallel to the long axis of the heart

Sweeping begins at the base of the heart which appears on the rt of the screenThe left atrium, the mitral valve and the right ventricular outflow tract are seen.

Basics of echo & principles of doppler echocardiography

Parasternal WindowRight Long-Axis ViewSweeping begins at the right atrium which is on the right of the screen. In this view we can see the right atrium and the right ventricle, with the tricuspid valve in between

The short-axis plane is the plane that runs parallel to the posterior and anterior surfaces of the body and transects the heart from its apex to its base encompassing all four cardiac chambers

Parasternal Short-Axis ViewsObtained from the parasternal window by rotating the transducer clockwise by 90ºHence, the image index marker is pointed toward the patient´s right supraclavicularfossa. A series of sweeps transect the heart from the base to the apex by changes in transducer position and angulation. There are three characteristic levels: 1) great vessels, 2) mitral valve, 3) ventricles.

Parasternal Short-Axis ViewGreat Arteries LevelSweeping begins at the left edge of the atrium which appears on the right of the screen

At aortic valve level it demonstrates all three aortic valve leaflets. The pulmonary valve, the right ventricular outflow tract, the right atrium and the left atrium are seen in this view.

Parasternal Short-Axis ViewGreat Arteries LevelIn this view we can see the pulmonary valve (PV) and pulmonary artery (PA) with right (RPA) and left branches (LPA).

Parasternal Short-Axis ViewMitral Valve LevelThe anterior and posterior mitral leaflets are seen as they open in diastole and close in systole

Parasternal Short-Axis ViewMitral Valve Level

The four-chamber plane is the plane that runs parallel to the posterior and anterior surfaces of the body and transects the heart from its apex to its base encompassing all four cardiac chambers

Apical WindowSweeping begins at the left edge of the heart which appears on the right of the screen.

4-Chamber ViewIn this view we can see the left ventricle, the right ventricle, the left atrium and the right atrium. The tricuspid annulus lies slightly (up to 10 mm) closer to the apex than the mitral annulus. The septal and posterior tricuspid leaflets are seen in this view.

Apical Window5-Chamber ViewBy tilting the transducer anteriorly, the aortic root is seen in an oblique long view.

Apical Window2-Chamber ViewSweeping begins at the anterior face of the left ventricle which appears on the right of the screen. From the four-chamber view, the transducer is rotated anti-clockwise by 60º to obtain the two-chamber view of the left ventricle, the mitral valve and the left atrium.

Apical Window2-Chamber ViewThis view shows the LV, the LA and the MV. The LV shows the inferior wall on the left and the anterior wall on the right.

Subcostal Window4-Chamber ViewSweeping starts at the apex which appears on the right of the screen. A view of all four chambers shows the right ventricular free wall, the midsection of the interventricular septum and the posterolateral left ventricular wall. In this view, the interatrial septum is perpendicular to the direction of the ultrasound beam

Subcostal WindowThe inferior vena cava as it enters the right atrium and the central hepatic vein are seen.

Suprasternal Notch WindowSweeping begins at the descending aorta which appears on the right of the screen. The long-axis view shows the ascending aorta, the arch, the proximal descending aorta and the origins of the right brachiocephalic and left common carotid and subclavian arteries.

B. M-MODE ECHOCARDIOGRAPHYa single line of sight is included, the repetition frequency of the pulse transmission is very high and sampling rates of around 1800 cycles/sec are usedContinuously-moving structures may be identified more accurately when motion versus time, as well as depth, is displayed clearly on the M-mode recording

AORTIC VALVE AND LEFT ATRIUMThe aortic root is moving anteriorly in systole and posteriorly in diastole. The left atrium is posterior to the aortic root. The aortic leaflet coaptation point is seen as a thin line in diastole. AO is measured in end-diastole and LA in end-systole.

In early diastole, the leaflets separate widely, with the maximum early diastolic motion of the anterior leaflet termed the E point. The leaflets move together in mid-diastole and then separate again with atrial systole, the A point. Closure at the end of diastole is termed C point.

M-mode recording perpendicular to the long axis of and through the centre of the left ventricle at papillary muscle level provides standard measurements of systolic and diastolic thickness and chamber dimensions:

Normal ValuesLeft ventricular end-diastole: 37 - 57 mm (23 -31 mm/m²)Left ventricular end-systole: 21 - 40 mm (14 -21 mm/m²)Interventricular septum: 7 - 11 mmPosterior wall: 7 - 11 mm

3. DOPPLER ECHOCARDIOGRAPHYBASIC 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 frequencyThis 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 targetA 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 [(Vx cos ø)] ÷ CThis 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 calculationFor 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 hypertensionIt is generally assumed that ø is 0º and cos ø is therefore 1Ideally, 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 equationSPECTRAL 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).

Right Ventricular InflowCan be recorded from an apical approach. The pattern is similar to mitral flow.

Left Ventricular OutflowAn apical window is used with a pulsed Doppler sample volume positioned on the left ventricular side of the aortic valve. Note the narrow band of the systolic velocities.

Right Ventricular OutflowFrom a parasternal short-axis view, the sample volume is located in the right ventricular outflow tract. The Doppler shape is similar to the left ventricular outflow curve

Suprahepatic Vein FlowFrom the longitudinal subcostal view, the sample volume is located at the main suprahepatic vein. There are two positive waves, (A) and (V), and two negative ones, (X) and (Y).

DOPPLER MODALITIES Doppler methods used for cardiac evaluation :continuous wave dopplerPulsed wave dopplercolor flow doppler

CONTINUOUS WAVE DOPPLERemploys two dedicated ultrasound crystals, one for continuous transmission and a second for continuous receptionThis permits measurement of very high frequency Doppler shifts or velocitiesLimitations 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)

An ideal Doppler profile is one with a smooth "outer" contour, well-defined edge and maximum velocity, and abrupt onset and termination

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 beamis always performed with 2D guidance to determine the sample volume positionParticularly useful for assessing the relatively low velocity flows associated with:transmitral or transtricuspid blood flow, pulmonary venous flow, left atrial appendage flow, or for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation

From a four-chamber view, a pulse wave Doppler signal is placed in the superior portion of the left atrium at the entry site of the right upper pulmonary veinNote that the outline of pulmonary flow signal is seen S: systolic wave; D: diastolic wave.

Because PWD repeatedly samples the returning signal, there is a maximum limit to the frequency shift that can be measured unambiguouslyThe maximum detectable frequency shift (the Nyquist limit) is one-half the PRFIf velocity exceeds the Nyquist limit, signal aliasing is seen with the signal cut off at the edge of the display and the top of the waveform appearing in the reverse partHigh-PRF increases the number of sample volumes

COLOR FLOW IMAGINGWith 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 blueColour DopplerThe option of displaying “variance” allows an additional colour (usually green) to be added to indicate that a mean velocity has excessive variability (turbulent flow).

apical four chamber view with color flow Doppler during diastole This color signal is used to position a pulsed wave Doppler sample volume so that quantitatable signals of flow can be obtained from the pulmonary veins and from the mitral leaflet tips

Basics of echo & principles of doppler echocardiography

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Basics of echo & principles of doppler echocardiography