Echocardiography is a unique noninvasive method for imaging the living heart. It is based on detection of echoes produced by a beam of ultrasound (very high frequency sound) pulses transmitted into the heart.
History
Spallanzani 1700's is referred as father of ultrasound. He demonstrated that bats were blind and navigated by means of echo reflection using inaudible sound. In 1842, Christian Doppler introduced pitch of sound. Currie in 1880 discovered that first ultrasound waves were created using piezoelectrode. In 1929 Sokolov detected metal flaw, Karl Dussic in 1941, first used it in medicine. In 1950, Keidel, first used it for heart. Hertz+ and Elder in 1853, used echo for heart. In 1963, Joyner used first echo for Mitral stenosis (MS). Then in 1963, Feigenbaum placed ultrasound probe on his chest and saw moving heart images. Later in 1965, detected Feigenbaum pericardial effusion. Doppler was introduced in 1960's. Again in 1970 Color Flow was discovered. Transesophageal echo in 1982 and intracardiac Echo in 1990's were started. Hence evolved the technique of echocardiography. 2It started with Motion Mode, 2-D echocrdiography and color Doppler, followed by transesophageal and intracardiac. More recently, neonatologists have become interested in the echocardiographic assessment of hemodynamic instability in infants. The terms functional echocardiography and point-of-care echocardiography have been introduced to describe the use of echocardiography as an adjunct in the clinical assessment of the hemodynamic status in neonates.1–4 The increasing availability of echocardiography, with miniaturization of the technology, has resulted in more widespread use of echocardiography in NICUs around the world.5 Perhaps the most significant challenge for the application of so-called functional studies is that newborns in the NICU with hemodynamic instability are at a much higher risk for having underlying congenital heart disease (CHD).
How are Echo Images Produced?
Echo images are produced by piezoelectric effect. In this electric energy is converted to crystal that makes it vibrate resulting in sound waves. The transducer frequency has characteristics of the piezocrystal. Transducer frequency is measured in MEGAHERTZ (MHz). Higher the MHz, closer is the vision.
From its introduction in 1954 to the mid 1970's, most echocardiographic studies employed a technique called M-mode, in which the ultrasound beam is aimed manually at selected cardiac structures to give a graphic recording of their positions and movements. M-mode recordings permit measurement of cardiac dimensions and detailed analysis of complex motion patterns depending on transducer angulation. They also facilitate analysis of time relationships with other physiological 3variables such as ECG, heart sounds, and pulse tracings, which can be recorded simultaneously.
Echocardiography animation, utilizing both M-mode and two-dimensional recordings, now 3-D and 4-D images therefore provides a great deal of information about cardiac anatomy and physiology, the clinical value of which has established echocardiography as a major diagnostic tool.
In order to understand the basic principles of echocardiography it is extremely important to understand the physical principles behind the process and also the definition of the commonly used terms.
Ultrasound is sound having frequency of > 20,000 cycles/sec. It can be directed in a beam and it obeys the laws of reflection and refraction. It is reflected by objects of small size. Ultrasound beam should be extremely narrow so that it can obtain an icepick view or slice of the heart.
Resolution is the ability to distinguish or identify two objects that are close together.
Two-dimensional (2-D) Echocardiography (Fig. 1.1) is the study of cardiac structures in a two-dimensional view.
Three-dimensional (3-D) echo. The clinician's ability to image the heart by echocardiography had been limited to two-dimensional techniques.6 Improving transducer technology, beam-forming and miniaturization have led to significant improvements in spatial and temporal resolution using 2-DE. However, 2-DE has fundamental limitations. The very nature of a 2-DE slice, which has no thickness, necessitates the use of multiple orthogonal ‘sweeps’. The only three-dimensional image of the heart is the ‘virtual image’ that exists in the echocardiographer's mind, and is then translated into words. Since myocardial motion occurs in three-dimensions, 2-DE techniques inherently do not lend themselves to accurate quantitation. Early reconstructive approaches were based on 2-DE image acquisitions that were subsequently stacked and aligned based on phases of the cardiac cycle, in order to recreate a 3-DE dataset.7–10 In 1990, von Ramm and Smith published their early results with a matrix array transducer that provided real-time images of the heart in three-dimensions.11 Over the past five years, dramatic technological advances have facilitated the ability to perform live 3-DE scanning, including the ability to steer the beam in three-dimensions and to render the image in real time.12 The usefulness of 3-D echocardiography has been demonstrated in (1) the evaluation of cardiac chamber volumes and mass, which avoids geometric assumptions; (2) the assessment of regional left ventricular (LV) wall motion and quantification of systolic dysynchrony; (3) presentation of realistic views of heart valves, (4) volumetric evaluation of regurgitant lesions and shunts with 3-DE color Doppler imaging, and (5) 3-DE stress imaging.
Doppler echo is a study of cardiac structures and blood flow profiles using an ultrasound beam Doppler echocardiography is a method for detecting the direction and velocity of moving blood within the heart. Doppler examination is based on the observation that the frequency of sound increases when a source of sound is moving towards the listener and vice versa. The same applies when a reflecting object is moving towards a transducer. It is used to determine the direction and velocity of RBC with respect to ultrasound beam. The common system used is to encode flow towards the transducer as red and away from transducer as blue [BART (Blue Away; Red Toward)]. The best Doppler information is obtained when the ultrasonic beam is parallel to the moving target (opposite of that for imaging with M-mode or 2-D echo). Also better information is obtained with higher frequency transducer compared with a lower frequency transducer.
Continuous wave Doppler is used to assess valvular stenosis and regurgitation and velocity of flow inshunts.
Pulse Doppler information can be used sending a short burst of ultrasound, the frequency of that burst is distorted if the target from which it is reflected is moving. It helps to assess the normal valve functions, LV diastolic function, stroke volume and cardiac output.
The major disadvantage of the pulse Doppler system is that the velocity one can measure is limited. Pulse Doppler system has inherently pulse repetition frequency (PRF).
The PRF determines the ability of the Doppler to detect high frequency Doppler shifts. The inability of the Doppler system to detect high frequency Doppler shifts is known as aliasing. The upper limit of frequency that 6this limit can detect is known as "Nyquist limit". This limit is one-half of PRF. If the flow exceeds this limit it is detected as flow in the opposite direction. ‘Alaising’ and ‘wrap around’ may thus add to the confusion. The Nyquist limit of color flow Doppler imaging is usually lower than ordinary pulsed spectral Doppler.
Color flow imaging is pulsed Doppler and therefore is limited in its abilities to measure high velocities. Basically two flow patterns can be detected with spectral Doppler. First is laminar, i.e. reflecting red blood cells are travelling in same direction with little difference in velocities. The Doppler signal from such a flow indicates a narrow velocity spectrum (Figs 1.2 and 1.3). Second is turbulent, this happens when blood is flowing through a narrow orifice. Multiple velocities in turbulent flows shown in Figure 1.4. As velocity increases aliasing occurs (Fig. 1.5). One of the major functions of color Doppler system is to identify abnormal flow patterns such as valvular regurgitation and shunts revealed by multiple color bands.
Note:
- Ultrasonic beam is perpendicular to blood flow no flow or color is recorded.
- Ultrasonic beam is parallel to blood flow, e.g. apical views, high velocity, color is brighter.
- If the velocity is too low flow is not visualized.
Transducers
Transthoracic
There are 4 transthoracic transducers, two phased array and two mechanical. The phased array transducers have a flat surface and no visible moving parts. The phased array transducers vary with the frequency and number of elements in the transducers. Transducers used have varying frequencies. Children frequencies vary 3.5–7.0 MHz. Higher frequency has better resolution. Adults lower frequency probe is used.
The higher frequency transducers are usually smaller. The number of elements varies from 32–128. Transducers with more elements are usually larger. The low frequency transducers have better penetration and produce better Doppler recordings. High frequency transducers give better resolution and finer image.
Transesophageal
Echo needs separate probe, adult and pediatric. Transesophageal transducers are placed in endoscopic 9instruments. Both mechanical and phased array devices can be used.
Intravascular
It can be placed in intravascular catheter of almost any size.
Digital Echocardiography
Digital echocardiography is the recording and display of echocardiographic data in digital form. The echocardio-graphic recording can be viewed and manipulated by computers.
Stress Echocardiography
Stress testing helps to identify latent or known cardiac abnormalities, only become manifest when provoked with some form of stress. Patients with valvular heart disease can show significant hemodynamic changes with stress. Doppler recordings are important under these circumstances.
Contrast Echocardiography
When liquid was injected within cardiovascular system, tiny suspended bubbles produce a cloud of echoes.
This technique is used for various purposes. The most common is right to left shunting. This technique is a sensitive means of finding small shunts.
REFERENCES
- Kluckow AM, Seri AI, Evans AN. Functional echocardiography: An emerging clinical tool for the neonatologist. J Pediatr 2007; 150:125–30.
- Sehgal AA, McNamara APJ. Does point-of-care functional echocardiography enhance cardiovascular care in the NICU? J Perinatol 2008; 28:729–35.
- Sehgal AA, McNamara APJ. Does echocardiography facilitate determination of hemodynamic significance attributable to the ductus arteriosus? Eur J Pediatr 2009; 168:907–14.
- Evans AN, Gournay AV, Cabanas AF, Kluckow AM, Leone AT, Groves AA, et al. Point-of-care ultrasound in the neonatal intensive care unit: International perspectives. Seminars in Fetal and Neonatal Medicine 2011; 16:61–6.
- Tajik AJ, Seward JB, Hagler DJ, Mair DD, Lie JT. Two-dimensional real-time ultrasonic imaging of the heart and great vessels: Technique, image orientation, structure identification and validation. Mayo Clin Proc 1978; 53:271–303.
- Ariet M, Geiser EA, Lupkiewicz SM, Conetta DA, Conti CR. Evaluation of a three-dimensional reconstruction to compute left ventricular volume and mass. Am J Cardiol 1984;54:415–20.
- Dekker DL, Piziali RL, Dong E Jr. A system for ultrasonically imaging the human heart in three-dimensions. Comput Biomed Res 1974; 7: 544–53.
- Linker DT, Moritz WE, Pearlman AS. A new three-dimensional echocardiographic method of right ventricular volume measurement: In vitro validation. J Am Coll Cardiol 1986;8:101–6.
- Matsumoto M, Matsuo H, Kitabatake A, Inoue M, Hamanaka Y, Tamura S, et al. Three-dimensional echocardiograms and two-dimensional echocardiographic images at desired planes by a computerized system. Ultrasound Med Biol 1977; 3: 163–78.
- Von Ramm OT, Smith SW. Real time volumetric ultrasound imaging system. J Digital Imaging 1990; 3:261–6.
- Salgo IS. Three-dimensional echocardiographic technology. Cardiol Clin 2007;25:231–9.