On completion of this chapter, you should be able to:
Explain the transducer selection and patient position for a cardiac examination
Describe the imaging planes used in echocardiography
Define suprasternal, subcostal, apical, and parasternal
Describe a normal cardiac examination using two-dimensional, color flow, Doppler, and M-mode imaging modes
List and discuss the applications of color flow Doppler in the echocardiographic examination
Analyze comparative anatomy of cardiac structures
The widespread clinical acceptance of two-dimensional (2D) imaging has tremendously aided the diagnostic results of a typical echocardiographic examination. Improved transducer design, resolution capabilities, focus parameters, gray-scale differentiation, gain control factors, cine loop functions, and other software capabilities have aided the cardiac sonographer in the attempt to record consistent, high-quality images from the multiple scan planes necessary to obtain a dynamic composite image of the cardiac structures. Color flow Doppler allows the cardiac sonographer to obtain additional velocity information in detecting intracardiac shunt flow, mapping regurgitant jets, and determining obstructive flow pathways.
The evaluation of cardiac structures by echocardiography is regarded as an essential diagnostic tool in clinical cardiology. The reason for its widespread use in the evaluation of cardiac disease is its noninvasive, reproducible, and accurate assessment of cardiac structures. Echocardiography exploded when 2D echocardiography was developed, which allowed the cardiac structures to be dynamically visualized in real-time. Thus the echocardiographer can assess the four chambers of the heart, the cardiac valves, the intracardiac anatomy, and the intracardiac lesions; observe contractility; determine valvular function; and assess hemodynamics. The combination of 2D, Doppler, tissue Doppler imaging, strain, and color flow mapping provides an extremely accurate means to evaluate wall or valve thickness, valvular orifice and chamber size, and contractility of the cardiac structures. The introduction of transesophageal echocardiography (TEE) and three-dimensional (3D) echocardiography has enabled exquisite visualization of the heart while the transducer is guided through the mouth and into the esophagus to image the cardiac anatomy.
Contrast injected through an intravenous line into the bloodstream has provided an additional pathway to enhance cardiac endocardial borders and demonstrate cardiac thrombus and mass lesions. Saline bubble injections have been a clinical aid to determine the presence and direction of interatrial septal shunt flow.
Exercise stress echocardiography, supine bike stress echocardiography, and dobutamine stress echocardiography have provided additional information about the contractility, hemodynamics, and performance of the left ventricle in a simulated stress situation.
To perform an echocardiographic examination of good diagnostic quality, the sonographer must understand the anatomic, hemodynamic, and pathophysiologic parameters of the heart and be able to incorporate the physical principles of sonography into the routine examination. This chapter introduces the reader to the basic technical components of echocardiography through 2D, M-mode, Doppler, and color Doppler imaging. Emphasis will be on common findings the general sonographer may encounter “above the diaphragm” in the Clinical Echocardiography chapters.
The echocardiographic patient is examined in the left lateral decubitus position. This position allows the heart to move away from the sternum and closer to the chest wall, thus allowing a better cardiac window. The “cardiac window” may be considered that area on the anterior chest where the heart is just beneath the skin surface, free of lung interference, and is usually found between the third and fifth intercostal spaces, slightly to the left of the sternal border.
The cardiac sonographer must keep in mind that different body shapes require variations in transducer position. An obese patient may have a horizontal transverse heart, and thus a slight lateral movement from the sternal border may be needed to record cardiac structures. A thin patient may have a long and slender heart, requiring a lower, more medial transducer position. Barrel-chested patients may have echocardiographic difficulties because of the lung absorption interference, and it may be necessary to turn these patients completely on their left side. Sometimes the upright or slightly forward-bent position is useful in forcing the heart closer to the anterior chest wall.
The following techniques are guidelines for the average patient. In the initial echocardiographic study, moving the transducer freely along the mid–left sternal border until all the cardiac structures are easily identified is a better practice than restricting the transducer to one interspace. This procedure saves time and gives the examiner a better understanding of cardiac relationships. If the heart is actually medial, the best study is performed with the patient completely on his or her left side. Observing the patient’s respiratory pattern may help the sonographer identify if the lungs overshadow the cardiac structures. Controlled breathing will help to obtain good quality images. If the lung interference clouds the cardiac structures, the patient should breathe in and then exhale for as long as possible to move the lungs away from the field of view. This usually gives the examiner adequate time to record valid information.
Several types of transducers are available for echocardiographic techniques. Most adult cardiac sonographers use multifocal transducers that range from 1.5 to 4.5 MHz with either a manually controlled or an automatic focus. A pediatric patient generally requires a multifocal higher-frequency transducer for improved resolution and near-field definition.
Transducer location and imaging planes.
The Committee on Nomenclature and Standards in Two-Dimensional Echocardiography of the American Society of Echocardiography recommends the following nomenclature and image orientation standards for transducer locations ( Figure 32-1 ):
Suprasternal. Patient is supine. Transducer placed in the suprasternal notch.
Subcostal. Patient is supine. Transducer located near the body midline and beneath the costal margin.
Apical. Patient in left lateral decubitus position. Transducer located over the cardiac apex (at the point of maximal impulse).
Parasternal. Patient in left lateral decubitus position. Transducer placed over the area bounded superiorly by the left clavicle, medially by the sternum, and inferiorly by the apical region.
The imaging planes are described by the manner in which the 2D transducer transects the heart ( Figure 32-2 ):
Long axis. Transects the heart perpendicular to the dorsal and ventral surfaces of the body and parallel with the long axis of the heart.
Short axis. Transects the heart perpendicular to the dorsal and ventral surfaces of the body and perpendicular to the long axis of the heart.
Four chamber. Transects the heart approximately parallel with the dorsal and ventral surfaces of the body.
Cardiac color flow examination
The color flow mapping (CFM) examination is generally performed along with the conventional 2D examination. The advantage of CFM is its ability to rapidly investigate flow direction and movement within the cardiac chambers ( Box 32-1 ). Flow toward the transducer is recorded in red, and flow away from the transducer is blue ( Figure 32-3 ). This is denoted on the color bar on the right upper side of the image. As the velocities increase, the flow pattern in the variance mode turns from red to various shades of red, orange, and yellow before it aliases. Likewise flow away from the transducer is recorded in blue; this color turns to various shades of blue, turquoise, and green before it aliases. Depending on the location of the transducer, the flow signals from various structures within the heart appear as different colors. An understanding of cardiac hemodynamics helps the examiner understand the flow patterns.
The color flow mapping examination is generally performed in the same planes used for conventional Doppler examination.
Parasternal long-axis view: MV, TV, AO
Parasternal short-axis view: AO, PA, RVOT, IAS, TV
Parasternal short-axis view: MV, TV, AO, PV
Apical four-chamber plane: MV, TV
Apical five-chamber plane: LVOT, AV
Apical long-axis, two-chamber view: LV, MV, LA
Subcostal four-chamber view: IAS, IVS, RV, LV, RA, LA
Subcostal view: IVC, hepatic veins
Subcostal 5 chamber view: AO, LVOT
Subcostal short-axis view: AO, PA, RVOT
Suprasternal view (long axis): ascending and descending aorta, SVC
Suprasternal view (short axis): arch, RPA, LA, SVC, pulmonary veins
AO, Aorta; AV, aortic valve; IAS, interatrial septum; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; IVS, interventricular septum; LA, left atrium; LV, left ventricle; LVOT, left ventricular outflow tract; MV, mitral valve; PA, pulmonary artery; PV, pulmonary valve; RA, right atrium; RPA, right pulmonary artery; RV, right ventricle; RVOT, right ventricular outflow tract; SVC, superior vena cava; TV, tricuspid valve.
Although normal cardiac flows are difficult to accurately time during the CFM examination because of its slow frame rate, the use of color M-mode (with a faster frame rate) allows one to precisely determine specific cardiac events in correlation with the ECG. The color M-mode is made in the same manner as a conventional M-mode study ( Figure 32-4 ). The cursor is placed through the area of interest, and the flow is evaluated using an autocorrelation technique.
Doppler applications and technique
Doppler echocardiography is established as a valuable noninvasive tool in clinical cardiology to provide hemodynamic information about the function of the cardiac valves and chambers of the heart. When combined with conventional 2D echo, Doppler techniques may be focused to provide specific information on the velocity flow patterns of a particular area within the heart. The ability to provide qualitative and quantitative information in evaluating valvular function, intracardiac shunts, dysfunction of a native or prosthetic valve, or the obstruction of a surgically inserted shunt has contributed to the clinical care of the cardiac patient. Understanding of cardiac physiology and hemodynamics is critical to the interpretation of the Doppler information. In addition, the sonographer must clearly understand Doppler principles, artifacts, and pitfalls in order to produce a quality study.
Normal cardiac doppler flow patterns
It is important to understand the relationship between the 2D echo and the Doppler flow study. The 2D echo allows assessment of cardiac anatomy and function; Doppler velocity flow analysis allows examination of blood flow rather than cardiac anatomy. The Doppler principle on which this technique is based involves the backscatter of transmitted ultrasonic waves from circulating red blood cells. The difference in frequency between transmitted and backscattered sound waves (Doppler shift) is used to quantify forward or backward blood flow velocity.
The Doppler signals should be obtained with the sample volume parallel to the direction of flow. Recall that the flow of blood occurs in three-dimensional space, whereas the real-time image is only in two dimensions. Therefore the two-dimensional image serves as a guide to the operator as small adjustments of the transducer and sample volume are made in the valve orifice to record the optimal Doppler signals. The key is to produce a spectral signal to show a well-defined velocity envelope along with a clearly defined audio tone. The clarity of the audio tone cannot be emphasized enough. Frequently the clarity of tone is used to guide the Doppler cursor into the correct plane to record the maximum velocity.
Blood flow toward the transducer is displayed by a time velocity waveform above the baseline at point zero, or a positive deflection ( Figure 32-5 ). Flow away from the Doppler signal is displayed below the baseline or as a negative deflection. A simultaneous ECG should be displayed to help time the cardiac cycle.
Pulsed wave doppler
A pulsed wave transducer is constructed with a single crystal that sends bursts of ultrasound at a rate called the pulse repetition frequency (PRF). The transducer receives sound waves backscattered from moving red blood cells during a limited time between transmitted pulses. A time gating device is then used to select the precise depth from which the returning signal has originated because the signals return from the heart at different times.
The particular area of interest undergoing Doppler evaluation is referred to as the sample volume. The sample volume and directional line placement of the beam are moved by use of the trackball. The exact size and location of the sample volume can be adjusted at the area of interest. Some instruments have a fixed sample volume size. Others allow the operator to select the size appropriate for the particular study.
Velocities under 2 m/sec are recorded without an alias pattern ( Figure 32-6 ). However, pulsed Doppler is limited in its ability to record high-velocity patterns. The maximum frequency shift that can be measured by a pulsed Doppler system is called the Nyquist limit and is one half the PRF. Velocities that exceed this limit are known to produce an aliasing pattern. Normal cardiac structures do not exceed the Nyquist limit and are easily measured with the pulsed Doppler system.
Continuous wave doppler
The continuous wave probe differs from the pulsed wave probe in that it is able to both send and receive sound ( Figure 32-7 ). One crystal continuously emits sound; the other receives sound as it is backscattered to the transducer. This probe may be part of a phased or annular array imaging probe or may be a stand-alone independent probe. If it is part of a 2D imaging transducer, the sample direction can be steered by use of the trackball.
The Pedoff continuous wave (nonimaging) probe is smaller than the 2D imaging probe and thus has advantages in obtaining a good Doppler study ( Figure 32-8 ). This small diameter allows greater flexibility to angle between small rib interspaces or obtain signals from the suprasternal notch or right parasternal border. The audio portion of the Doppler examination becomes a critical factor in this study because there is not 2D available to guide the transducer location.
Often both 2D–continuous wave and Pedoff probes are used within the echocardiographic study. Once the proper transducer position is found with the imaging transducer, the angulation and window are marked for proper placement of the Pedoff probe. The audio sound and spectral wave pattern are then used to guide the correction angulation of the beam for maximum-velocity recordings. Because there is not a particular sample volume site within the continuous wave beam, velocities are recorded from several points along the linear beam. This technique has the ability to record maximum velocities without alias patterns and is especially useful for very-high-velocity patterns as seen in regurgitant lesions or stenotic valves.
Audio signals and spectral display of doppler signals
The best Doppler signals are obtained when the ultrasound beam is parallel or nearly parallel to the flow of blood. Therefore the best windows used to record the 2D images may not be the best windows to record Doppler flow patterns. This section discusses the technique for recording quality Doppler signals from the inflow and outflow tracts through the cardiac valves.
The signal from the arterial flow is very different from that of the venous flow; likewise, mitral and tricuspid patterns differ from the aortic and pulmonary valve patterns. The blood flow velocity determines the pitch or frequency of the audio signal. As the velocity becomes higher, the pitch becomes higher; as the velocity decreases, so does the pitch.
Normal blood flow across the cardiac valves demonstrates a narrow range of velocity with a smooth and even Doppler audio signal. When the flow becomes disturbed, as occurs distal to an obstruction or regurgitation of the valve, the tone becomes harsh. The high-velocity flows produce a very-high-frequency signal with a sharp whistling-hissing tone. This signal may be found in obstruction, shunts, and regurgitant lesions.
Other movements within the cardiac chambers produce audio signals, but these signals are not as well defined. The valve opening and closure can be heard as a discrete click when the Doppler window is located too close to the valve. The normal cardiac function causes the valve to move in and out of the Doppler beam, producing a lower-frequency signal. Therefore careful angulation along with the audio signal helps the sonographer observe the dynamics of the cardiac cycle for correct beam placement to obtain the best-quality Doppler signal.
The spectral analysis waveform allows the sonographer to store a graphic display of what the audio signal is recording because it provides a representation of blood flow velocities over time. The velocity on the vertical axis is measured in centimeters per second or meters per second, and time is shown on the horizontal axis ( Figure 32-9 ). Therefore the direction and velocity of flow may be measured very accurately when the beam is parallel to the flow.
A normal spectral display pattern has a typical appearance. In normal blood flow, the cells generally have a uniform direction with similar velocities. The spectral tracing appears as a smooth mitral velocity pattern bordered by a narrow band of velocities. As the velocity increases, so does the turbulence within the border of the narrow-band velocities, producing a filling of the velocity curve. As the cardiac structure moves in and out of the beam, the Doppler frequency shift is recorded as tall artifact spikes.
Quantitation of the Doppler signal to obtain hemodynamic information is derived from the measurement of blood flow velocity ( Table 32-1 ). As explained previously, it is critical that the angle of the Doppler signal be as parallel to flow as possible. The Doppler equation is based on the principle that the velocity of blood flow is directly proportional to the Doppler frequency shift and the speed of sound in tissue, and it is inversely related to twice the frequency of transmitted ultrasound and the cosine of the angle of incidence between the ultrasound beam and the direction of blood flow. Therefore the relationship between the angle and its cosine becomes significant and can be a source of error if ignored. If the angle is less than 20 degrees, the cosine is close to 1 and can be ignored. If the angle increases beyond 20 degrees, the cosine becomes less than 1 and may produce an underestimation of velocity.
|Children (cm/sec)||Adults (cm/sec)|
The Doppler examination is performed along with the 2D study of the cardiac structures. During this conventional study, the sonographer notes structures that may need special attention during the Doppler examination (e.g., a redundant mitral valve leaflet may indicate the need to search for mitral regurgitation). Throughout the Doppler study, various patient positions and transducer rotations are necessary to place the sample volume parallel to blood flow ( Box 32-2 ). There are basically five transducer positions used to record quality Doppler flow patterns: the apical four chamber, the left parasternal, subcostal, suprasternal, and the right parasternal. The patient should be forewarned about the audio sounds produced by the Doppler signal because some find the sound alarming if the volume is set too high.
Mitral valve, tricuspid valve, left ventricular outflow tract, aortic valve, pulmonary vein inflow, superior vena cava inflow, interventricular septum, interatrial septum
Parasternal short-axis window
Pulmonary valve, main pulmonary artery, right and left branches pulmonary artery (patent ductus arteriosus flow), tricuspid valve
Suprasternal notch window
Ascending aorta, descending aorta, patent ductus arteriosus flow, right pulmonary artery
Interatrial septum, interventricular septum, inferior vena cava flow, superior vena cava flow
Parasternal long-axis window
Mitral regurgitation, tricuspid regurgitation, aortic regurgitation
Right parasternal window
The echocardiographic examination
It is the responsibility of the cardiac sonographer to acquire the blood pressure, height, and weight (to calibrate the body surface area) of each patient. Ideally, the sonographer should digitally acquire the cine loop of one or more cardiac cycles as needed for quantification and analysis. If an irregular rhythm is present (i.e., atrial fibrillation, flutter, or frequent ectopy), the sonographer should acquire 3 to 5 consecutive cardiac cycles for 2D; Doppler will need 4 to 10 consecutive beats averaged. If two or more myocardial segments of the left ventricle are not well visualized, the use of an approved injectable contrast agent, such as Definity, should be considered if no contraindications are present. The sonographer should always compare the echocardiographic images to the previous study when preparing the preliminary report.
The protocol for the evaluation cardiac structures begins with the parasternal long- and short-axis views, followed by the apical four-chamber, long-axis, and two-chamber views ( Figure 32-10 ). The subcostal and suprasternal views complete the study. The sonographer should acquire the respective cine loop(s) for 2D and color flow Doppler and acquire the representative still frames for M-modes and pulsed wave/continuous wave Doppler. The color Doppler sector should be long, spanning the entire cardiac image from top to bottom. This sector should be narrow enough to obtain the frame rate greater than or equal to 17 Hz ( Box 32-3 ).
Five-chamber view (including aorta)
Inferior vena cava, hepatic veins
RV and LV inflow
Suprasternal notch window
Right pulmonary artery
Right parasternal window
The following protocol is a minimum standard to be performed for all complete 2D/M-mode, color flow Doppler, and spectral Doppler examinations. Additional views are often required and are based on presence of disease and clinical indications. The order of acquisition is important and should always follow this sequence: (1) 2D image, then (2) full-screen color flow Doppler of same image, followed by (3) spectral Doppler of the same view. The intent is to show anatomy first (zoom as needed), then color flow of that anatomy (zoom as needed), then spectral. Labeling views or structures is strongly recommended whenever there is an interruption in the 2D/color flow Doppler/spectral format or if nonstandard images are used. Either mode may not increase frame rate depending on harmonic frequency selection. Digitally acquire the following views in the order listed below.
Parasternal long-axis two-dimensional view.
The parasternal long-axis view (PLA) is the initial image in the complete echographic examination. An attempt should be made to record as many of the cardiac structures as possible, from the base of the heart to the apex. Generally, this is accomplished by placing the long axis of the transducer slightly to the left of the sternum in about the fourth intercostal space. When the bright echo reflection of the pericardium is noted, the transducer is gradually rotated until a long-axis view of the heart is obtained. This view will demonstrate the right ventricle, aorta/ascending aorta, left atrium, mitral leaflets, interventricular septum, and left ventricle ( Figure 32-11 ). If it is not possible to record the entire long axis on a single scan, the transducer should be gently rocked cephalad to caudad in an “ice pick” fashion to record all the information from the base to the apex of the heart. See Boxes 32-4 and 32-5 for protocols.
Record deep PS LAX (regardless of presence of effusion); typical depths are 20 to 24 cm; far field should not be dark unless there is effusion; TGC gain accordingly
PS LAX full screen (not zoomed)
Color Doppler full screen of aortic, mitral valve, LVOT, and include RVOT in color Doppler
PS LAX 2D zoom as needed to show anatomy/pathology better and then zoom of same image with color
PS LAX color IVS to rule out shunt
M-mode cursor through the minor axis of the aorta and left atrium
M-mode cursor through mitral valve leaflet tips
M-mode cursor through minor axis of LV just superior to papillary muscle
High PS LAX window to assess ascending aorta; color flow Doppler as needed to differentiate between artifact and a dissection; also high PS SAX 2D and with color flow Doppler if needed to help differentiate between an artifact and a dissection
RA/RV, full screen
Color Doppler of RA/RV
Whether or not tricuspid regurgitation is seen, attempt continuous wave Doppler for peak tricuspid regurgitation velocity and record
Medical angulation of the probe in the PLA will demonstrate the tricuspid valve, right ventricle, and right atrium ( Figure 32-12 ).
The cardiac sonographer should observe the following structures and functions in the parasternal long-axis view:
Composite size of the cardiac chambers
Contractility of the right and left ventricles
Thickness of the right ventricular wall
Continuity of the interventricular septum with the anterior wall of the aorta
Pliability of the atrioventricular and semilunar valves
Coaptation of the atrioventricular valves
Presence of increased echoes on the atrioventricular and semilunar valves (increased echoes may represent calcification, fibroelastoma, vegetations, or other abnormality)
Systolic clearance of the aortic cusps (aortic leaflets should open fully in systole)
Presence of abnormal echo collections in the chambers or attached to the valve orifice (thrombus may occur if there are wall motion abnormalities)
Presence and movement of chordal-papillary muscle structure
Thickness of the septum and posterior wall of the left ventricle
Uniform texture of the endocardium and myocardium
Size of the aortic root and left atrium