Download echocardiography

ECHOCARDIOGRAPHY
Lupu Silvia, Șuș Ioana
The purpose of this laboratory practice is:
- to understand the basic physical principles of ultrasound-based imaging
- to comprehend the basics of major imaging modes
- to recognize echocardiography views and anatomical structures
- to understand the Doppler principle and its medical imaging applications
1. Definition
Echocardiography (cardiac ultrasound) is a non-invasive ultrasound-based imaging
technique which allows the examination of the heart and great vessels.
2. Physical principle
Ultrasound waves are high-frequency (over 20,000 Hz) mechanical vibrations of an elastic
medium, which are represented graphically as sinusoid waves (Figure 1) and are
characterized by: frequency – cycles per second or hertz (Hz), wavelength – meters (m),
velocity of propagation (m/s) and amplitude – decibels (dB).
Figure 1. Sinusoidal ultrasound waveform.
Frequency is the number of ultrasound waves per second and is measured in cycles per
second or hertz (Hz; 1 Hz = 1 cycle/s).
Propagation velocity is the speed of ultrasound passing through the body and is different
for each type of tissue; in soft tissues such as the myocardium, valves and blood vessels, the
propagation velocity is estimated at around 1,540 m/s.
Wavelength represents the peak-to-peak distance between two consecutive ultrasound
waves and is measured in meters (m). Wavelength may be calculated by dividing the
frequency of the ultrasound to the propagation velocity. Therefore, assuming that the
propagation velocity remains constant, higher frequency ultrasound waves have shorter
wavelength, while lower frequency waves have longer wavelength. Shorter wavelengths
(higher frequencies) provide a better image resolution, while increased wavelengths (lower
frequencies) allow deeper penetration in the tissues. Conventional diagnostic medical
ultrasound devices use transducers with a frequency ranging between 1 and 20 MHz.
Echocardiography transducers emit low frequency waves (2-4 MHz), thus providing the
necessary penetration (up to 30 cm), at the expense of lower resolution.
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Ultrasound amplitude indicates the energy of the ultrasound signal and is measured in
decibels (dB).
While passing through the body, ultrasound waves encounter different types of tissue and
may undergo two major phenomena: reflection and refraction (Figure 2). When the
ultrasound beam emitted by the transducer reaches the limit between two media with
different elastic properties, some of the waves return to the source (reflection) and will
provide the echocardiography image of superficial structures, while others pass into the
next medium with a change in direction (refraction), thus penetrating further into the
tissues, and, when reflected, will provide images of deeper structures.
Figure 2. Ultrasound propagation at the interface of two media with different elastic
properties.
3. The echocardiograph
The echocardiograph has three main components: transducer (probe), amplifying system
and display system.
3.1. The transducer
The transducer (probe) is one of the main components of the echocardiograph, containing
piezoelectric crystals which are able to both generate and receive ultrasound waves based
on the piezoelectric effect. Piezoelectricity is the ability of certain polarized materials (such
as titanate ceramic or quartz) to generate an electric field when submitted to mechanical
pressure and vice versa. In medical ultrasound imaging devices, an alternative current is
applied to the piezoelectric crystals of the transducer, resulting in mechanical deformation
of the crystals. The crystals contract and expand generating ultrasound waves (Figure 3).
Conversely, reflected ultrasound waves from the tissues exert oscillating pressure on the
crystals, which generate an alternative electric current that is received by the
echocardiograph and used for constructing the image.
Figure 3. The piezoelectric effect.
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Over time, different types of transducers have been developed:
Monocrystal transducers: contained a single piezoelectric crystal (element) and only
provided mono-dimensional images;
Mechanical transducers:
- Single crystal transducers: contained only one element which was steered in an arc of
60-90⁰
- Multi-crystal transducers: contained 4 elements on a rotating system which
generated an ultrasound beam when they reached the slit of the transducer.
Linear transducers: contain up to 30 elements arranged in a linear fashion; the crystals are
activated simultaneously and the final image is created by combining the signals received
by each element. Today, this type of transducer is mainly used for peripheral vessels
imaging and may contain up to 512 elements.
Phased array transducers: may contain 16 to 256 elements; piezoelectric crystals are
usually arranged in a square, and are activated sequentially.
Each transducer has a marker on its surface which allows the examiner to orient the
exploring ultrasound beam along a certain axis plane in order to get a specific view of the
heart.
4. Imaging modes
4.1. B (Brightness) mode (2D) provides bi-dimensional images of studied structures.
Images are displayed in different shades of grey, black and white, depending on the density
of the tissues. Low density structures (e.g. blood, other fluids) appear black, while higher
density structures (e.g. myocardium, valvular tissue) appear grey/white. Because images
are acquired and displayed with a high frequency (called frame rate, which can be as high as
≥30 frames/sec), real-time recordings of the moving heart are obtained.
To acquire a good quality B-mode image, certain echocardiographic windows are used.
Echocardiographic windows
Echocardiographic windows are areas of the thorax or abdomen on which the transducer can
be placed to provide the best view of the heart, avoiding bones (the sternum or ribs) or airfilled pulmonary parenchyma. When ultrasounds pass through the bone or air-filled
pulmonary parenchyma, they are absorbed (converted into heat), leading to attenuation of
the signal, which may impair image quality. To minimize attenuation at the transducer-skin
interface, a special water-soluble gel is applied to the skin.
The most commonly used echocardiography windows are the parasternal, apical,
suprasternal, and subcostal windows (Figure 4). To acquire the suprasternal and subcostal
windows, the patient must lie in supine position; for the other windows, the left lateral
decubitus position is preferred.
Figure 4. Echocardiography windows: 1 – suprasternal window; 2 – parasternal window; 3 –
apical window; 4 – subcostal window.
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From each of these windows, several views can be acquired by orienting the transducer
differently, to achieve certain imaging planes; the most commonly used imaging planes are
the short-axis, long-axis, and four-chamber planes.
Echocardiography views
Echocardiography views are images of the heart that can be obtained by placing the
transducer in certain windows and by directing the ultrasound wave plane along certain
planes. Most commonly used views are:
- the parasternal long-axis view (Figure 5)
- the parasternal short axis views:
o at the level of the great vessels (Figure 6)
o at mitral valve level (Figure 7)
o at papillary muscles level (Figure 8)
- the apical 4-chamber (Figure 9, A), 5-chamber (Figure 9, B) and 2-chamber views
- the subcostal view
- the suprasternal view.
Figure 5. Parasternal long-axis view. Ao = aorta; LA = left atrium; LV = left ventricle; RV =
right ventricle; IVS = interventricular septum; LVPW = left ventricular posterior wall. The
parasternal long-axis view can be used for measuring LV, RV, LA and Ao diameters, IVS and
LVPW thickness, as well as the aortic valve annulus.
Figure 6. Parasternal short-axis view at the level of the great vessels. Ao = aortic valve
(opened); PA = pulmonary artery; RV = right ventricle; RA = right atrium; LA = left atrium.
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Figure 7. Parasternal short-axis view at mitral valve level. ALMV = anterior leaflet of the
mitral valve; PLMV = posterior leaflet of the mitral valve; RV = right ventricle.
Figure 8. Parasternal short-axis view at papillary muscles level. LV = left ventricle; RV =
right ventricle.
Figure 9. Apical (A) 4-chamber and (B) 5-chamber view. LA = left atrium; RA = right atrium;
LV = left ventricle; RV = right ventricle; Ao = aorta.
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4.2. M (Motion) mode imaging provides unidirectional images of the heart, displaying the
movement of structures along a single axis. To get an M-mode image, a B-mode view must
be acquired first. Most commonly, the parasternal long axis view is used; a cursor is
superimposed on the B-mode image along a chosen single axis; an exploring M-mode
ultrasound beam passes through the heart along that particular axis, with a high pulse
repetition frequency, providing a mono-dimensional image of the intersected structures at
different moments in time. Therefore, M-mode can be very useful for studying the
movements of cardiac structures during each cardiac cycle, yielding the advantage of a
high temporal resolution.
Using the B-mode parasternal long axis view as a support, several M-mode recordings can
be acquired: the aortic valve recording (Figure 10), the mitral valve recording (Figure 11), and
the mid-ventricular recording (Figure 12).
Figure 10. M-mode aortic valve recording. In this view, aortic valve opening (1), the
diameter of the aortic root (3) and the left atrium (2) can be measured, as described in the
image above. Two of the three aortic leaflets are seen in the parasternal long-axis view;
when the valve is closed, the leaflets come into contact with each other, generating a single
echo; when the valve is opened, the leaflets move away from each other, describing a
rectangle shape. Ao = aorta; LA = left atrium; RV = right ventricle.
All measurements that can be performed from this M-mode view may also be measured in
B-mode from the parasternal long-axis view. These parameters and their normal values are
listed in Table 1.
Table 1. Normal ranges of parameters measured in parasternal long-axis view.
Parameter
Normal values
1
Aortic annulus
21 – 29 mm
Ascending aorta diameter
26 – 34 mm
Aortic valve opening
17 – 26 mm
Left atrial diameter
27 – 38 mm
1
- the transition point between the left ventricle and the aortic root, also referred to as
aortic ring.
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Figure 11. M-mode mitral valve recording. In this view, the movements of the mitral valve
during the different phases of the cardiac cycle can be recorded. During ventricular systole,
the mitral valve is closed and therefore, the mitral leaflets generate a single echo; during
ventricular diastole, the valve opens and the anterior mitral valve leaflet (AMVL) moves
towards the interventricular septum (IVS), describing an “M” shape in M-mode images,
while the posterior mitral valve leaflet (PMVL) moves towards the posterior wall of the left
ventricle, describing a “W” shape. RV = right ventricle; D = mitral valve opening; E =
maximum early diastolic motion of the mitral valve; F = mid-diastolic transient closure of
the mitral valve; A = late diastolic motion of the mitral valve due to atrial contraction; C =
mitral valve closure.
Figure 12. M-mode recording of the left ventricle. In this view, interventricular septum (IVS)
and left ventricular posterior wall (LVPW) thickness, as well as left ventricular end-systolic
diameter (ESD) and end-diastolic diameter (EDD) can be measured during systole and
diastole. During ventricular systole, left ventricular diameter decreases, during diastole, the
diameter increases. End-systolic and end-diastolic diameters, as measured in M-mode, may
be used for quantifying systolic function by determining the left ventricular performance
indices. RV = right ventricle. Normal values for these parameters are listed in Table 2.
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Table 2. M-mode mid-ventricular view parameters.
Parameter
Abbreviation
Interventricular septum thickness
IVS
Left ventricular posterior wall LVPW
thickness
End-systolic left ventricular diameter ESD
End-diastolic left ventricular diameter EDD
Normal range
6 – 10 mm
6 – 10 mm
25 – 40 mm
39 – 59 mm
Echocardiography is an important tool for evaluating ventricular systolic function, which
can be assessed by calculating the left ventricular performance indices listed in Table 3.
Fractional shortening is defined as the difference between the end-diastolic and endsystolic left ventricular diameters.
Left ventricular ejection fraction is an indicator of the amount of blood that is ejected by
the left ventricle during systole (left ventricular stroke volume, calculated as the difference
between the end-diastolic and end-systolic left ventricular volumes) divided by the
maximum volume of the left ventricle (the end-diastolic volume) and expressed in
percentages.
Table 3. Left ventricular performance indices.
Parameter
Abbreviation Formula
Left ventricular ejection fraction
LVEF
(SV/EDV) x 100
Stroke volume
SV
EDV – ESV
Cardiac output
CO
SV x HR
EDV = end-diastolic volume; ESV = end-systolic volume; HR = heart rate.
Normal range
≥55%
70 – 80 ml
5 – 6 l/min
End-diastolic and end-systolic volumes can be calculated by several methods using
geometrical assumptions regarding the shape of the left ventricle.
In most echocardiographs, left ventricular volumes are automatically calculated using Mmode measured end-systolic and end-diastolic diameters based on the Teicholz formula.
The Teicholz formula relies on the assumption that the left ventricle resembles a sphere,
which is grossly inaccurate. The echocardiograph simply introduces the measured values
into the formula to calculate ventricular volumes. Historically, the cube method was also
used.
Left ventricular volumes may also be calculated based on measurements that are acquired
from B-mode images. For that purpose, the apical 4-chamber view is usually preferred. In
different models, the left ventricle is presumed to resemble 1) an ellipsoid, having two
equal short axes and a long axis which is the double of the short one or 2) a bullet. For each
of these geometrical assumptions, a formula has been derived and used for volume
calculation.
Another, more accurate method that relies on the apical 4-chamber view is the Simpson’s
monoplane modified method (Figure 13) in which the entire surface of the left ventricular
chamber, usually traced manually by the examiner, is subdivided automatically into 20
separate disks, having the same height, but different diameters; the echocardiograph
automatically measures each diameter and calculates the volume of each disk, then adding
the twenty values to provide a reasonably accurate measurement of ventricular volumes in
diastole and then in systole.
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All these methods can provide measurements of left ventricular volumes and the derived
left ventricular performance indices. The corresponding formulas are detailed in table 4.
Table 4. Formulas for left ventricular volumes and performance indices
Method
Geometric assumption
Formula for LV volume
Cube method
Cube
D3
Teicholz formula
Sphere
7D2 / (2.4 + D)
Monoplane ellipsoid method Ellipsoid with two equal 8A2/3πL
short axes and a long axis
double the short axis
Bullet model
Cylinder (base of the
5/6 x AmL
ventricle) + ellipsoid (apex)
Simpson’s modified
None; left ventricular cavity Σ [Ai x L/20]
monoplane method
is divided into 20 disks of
equal height and different
diameters
D = diameter; A = area; Ai = area of the disk; L = long-axis length of the left ventricle; AmL =
short-axis left ventricular area at the mid-ventricular level.
Figure 13. Left ventricular ejection fraction calculated using Simpson’s modified rule, from
the apical 4-chamber view.
5. Doppler echocardiography
5.1. Physical principle. The Doppler effect.
The Doppler effect was first described by the Austrian physicist Christian Doppler in 1842
and consists in a change in the frequency of sound waves when the source of the sound and
the target are moving relatively to each other. In echocardiography, the transducer serves
as the ultrasound source, generating waves with a certain frequency; when these
ultrasound waves reach moving targets – the red blood cells – they are backscattered and
undergo a change in frequency; if the blood stream is directed away from the transducer,
the frequency of the ultrasound waves is reduced (Figure 14b), while if the blood stream
approaches the transducer, the frequency is increased (Figure 14a).
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Figure 14. The Doppler effect. In figure 14a the targets (red blood cells) approach the
transducer, therefore the frequency of the backscattered signal increases by comparison to
the initial Doppler ultrasound wave. In figure 14b the targets move away from the
transducer, therefore the frequency of the backscattered signal decreases.
Blood flows are recorded by superimposing a cursor on a B-mode image in the area of
interest. The exploring ultrasound beam will pass through the body on that particular axis.
The echocardiograph records the Doppler shift or Doppler signal (the difference between
the initial frequency and the frequency of the reflected waves) and uses it to calculate blood
flow velocity based on the Doppler equation:
∆f= (2 x f0 x v x cos α)/c, where
∆f = Doppler shift;
f0 = initial frequency of the ultrasound wave;
v = moving target (blood flow) velocity;
cos α = cosine of the angle between the direction of the blood flow and the direction
of the ultrasound beam;
c = velocity of ultrasound propagation in the body (1,540 m/s).
In medical ultrasound imaging, most commonly used types of Doppler imaging include
Continuous Wave Doppler (CWD), Pulsed Wave Doppler (PWD), and Color Doppler.
Continuous Wave Doppler (CWD) uses two piezoelectric crystals, one of which
continuously emits while the other continuously receives ultrasound waves. The continuous
emission and reception of ultrasound beams provides the advantage that even very high
frequency shifts (and, implicitly, velocities) can be recorded accurately. However, the
sampling is made along the entire length of the ultrasound beam, and, as a consequence,
signals from different targets along the same plane can overlap, which makes it challenging
to accurately locate the origin of the signal.
Pulsed Wave Doppler (PWD) uses a single piezoelectric crystal that emits an ultrasound
pulse and then waits for the backscattered signal to return to the transducer. With PWD,
signals from a small volume of blood located at a certain depth (called sample volume) are
acquired, providing an accurate localization of the origin of the signal. The main
disadvantage of PWD is the fact that very high blood flow velocities can be
underestimated.
In Color Doppler, the Doppler shift of the pulsed wave mode is color-coded. Accordingly,
blood flows moving away from the transducer are coded in blue, while blood flows
approaching the transducer are coded in red; if there is turbulence in the blood flow,
yellow-green signals may be recorded. Color Doppler also allows a semi-quantitative
assessment of blood flow velocity: if velocity is increased, the blue or red color becomes
brighter; if velocities are lower, the colors turn darker.
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