FIGURE 1-1-1 “Tools” available in a modern echocardiography laboratory.
M-MODE ECHOCARDIOGRAPHY
M-mode echocardiography, first displayed on strip chart recordings in the early 1970s, provides a one-dimensional, “ice-pick” view of the heart (Figure 1-1-2). While initially obtained with a “blind” dedicated M-mode transducer, M-mode echocardiograms are now obtained with 2-dimensional guidance, thus reducing the errors generated by oblique, off-axis images. Since the heart is a 3-dimensional structure, a single linear assessment of the heart has obvious limitations. However, M-mode echo still has a role to play in current day echocardiography.15
FIGURE 1-1-2 Normal 2-D guided M-mode of the aortic valve and left atrium.
M-mode echocardiography provides excellent spatial and temporal resolution (far exceeding the temporal resolution of 2-dimensional imaging), which allows for more precise timing of events and an assessment of the motion of rapidly moving cardiac structures such as valves. There remain many clinically useful roles. For example, M-mode assessment of the free wall of the right atrium or right ventricle may help to determine the presence of diastolic chamber collapse, a sign of cardiac tamponade (Figure 1-1-3). The assessment of right ventricular function using M-mode to assess TAPSE (tricuspid annular plane systolic excursion) is a useful marker of right ventricular function.16
FIGURE 1-1-3 M-mode echocardiogram demonstrating right ventricular diastolic collapse in a patient with cardiac tamponade.
While 2-dimensional (2-D) and 3-dimensional (3-D) imaging clearly provide a more comprehensive assessment of cardiac chambers, linear dimensions of cardiac structures remain an important quantitative measure. For example, current guidelines for the management of valvular heart disease17 still employ linear dimensions in the decision-making algorithms for the management of patients with aortic and mitral regurgitation.
TWO-DIMENSIONAL ECHOCARDIOGRAPHY
Two-dimensional echocardiography (2DE), which provides real-time dynamic images of cardiac structure and function, is the mainstay of current clinical echocardiography. Initially introduced in the 1970s, image quality has dramatically improved over the years due to improvements in transducer technology, image processing algorithms, and advances in computer technology that allow for more rapid processing.
Current 2DE systems provide a robust platform for the evaluation of cardiac structure and function, including the assessment of left and right ventricular structure and function, cardiac chamber dimensions, the assessment of valvular heart disease, and the evaluation of the great vessels and the pericardium. Two-dimensional imaging forms the back-bone of the modern day echocardiography laboratory (Figure 1-1-4).
FIGURE 1-1-4 Two-dimensional image of the left ventricle demonstrating a normal left ventricle in the parasternal long axis view (L) and short axis (R) views.
DOPPLER ECHOCARDIOGRAPHY
Based on the Doppler principal of frequency shifts of the ultrasound signal generated by moving targets in relationship to the transducer, Doppler echocardiography utilizes red blood cells as the moving ultrasound reflector to provide an evaluation of blood flow, allowing an assessment of valvular function (regurgitation and stenosis) and the assessment of intracardiac shunts. Recent advances in the use of the Doppler technique now also allow for the assessment of tissue velocity data (see Tissue Doppler Imaging).
Both spectral (pulsed wave and continuous-wave) and color flow Doppler techniques are utilized to assess valvular function. For example, continuous-wave Doppler can be used to define aortic valve gradients (Figure 1-1-5) in order to evaluate the presence and severity of aortic stenosis. Color flow Doppler can be employed to assess valvular regurgitation (Figure 1-1-6) and intracardiac shunts (Figure 1-1-7).
FIGURE 1-1-5 Continuous-wave Doppler demonstrating an elevated aortic valve gradient in a young patient with severe aortic stenosis due to a bicuspid aortic valve.
FIGURE 1-1-6 Color Doppler demonstrating mitral and aortic regurgitation in a patient with rheumatic valve disease.
FIGURE 1-1-7 Subcostal view demonstrating a secundum atrial septal defect.
COLOR DOPPLER M-MODE
By combining the benefits of M-mode and color Doppler imaging, this technique results in an image with high temporal and spatial resolution. Color M-mode has been employed in the assessment of left ventricular filling to assess diastolic function and can be used to better define the width of the aortic regurgitation (AR) signal to aid in quantification of the severity of AR (Figure 1-1-8).
FIGURE 1-1-8 Color M-mode demonstrating aortic regurgitation (AR).
TISSUE DOPPLER IMAGING (TDI)
By changing various system settings and filters to better detect tissue velocity information as opposed to blood flow, the Doppler technique has been used to assess tissue (myocardial) velocities (Figure 1-1-9).18 In recent years, tissue Doppler imaging (TDI) has emerged as an important adjunctive imaging modality. By extracting the tissue velocity information, computation of myocardial strain and strain rate is possible. Utilizing this technique has further advanced our understanding of systolic and diastolic function, played a role in evaluating the timing of cardiac motion in the evaluation of patients with left ventricle (LV) dyssynchrony, and provides insight into myocardial mechanics.19
FIGURE 1-1-9 Tissue Doppler imaging of the lateral mitral valve annulus.
SPECKLE TRACKING ECHOCARDIOGRAPHY (STE)
Utilizing speckles that are present in grayscale B-mode images, speckle tracking echocardiography (STE) is a new technique employed to assess myocardial function.20 Unlike the TDI technique, STE is angle-independent. By processing data from 2-D images offline, this technique allows for the derivation of myocardial velocity, strain, and strain rate. This information can be used to assess systolic and diastolic function, evaluate patients with LV dyssynchrony, and provide further insight into myocardial mechanics. While it is currently used largely a research tool, STE is finding its way into clinical echocardiography.
THREE-DIMENSIONAL ECHOCARDIOGRAPHY (3DE)
Initial 3-D echocardiograms were generated by reconstructing a series of 2-D echocardiographic data sets. Significant technological advances over the past decade now allow for real-time image acquisition and image display in current 3DE systems (for both transthoracic and transesophageal probes) (Figure 1-1-10). This 3-D capability has further expanded the use of echocardiography in a variety of settings. While clearly not as commonly employed as standard 2-D imaging, 3-D imaging has been demonstrated to provide more accurate assessment of chamber volumes and mass, and regional LV function and dyssynchrony. It also provides more comprehensive views of valvular structure and quantitation of valvular regurgitation, as well as playing a role in stress imaging.21 As clinicians become more familiar with this new imaging capability, the role of 3-D imaging will continue to expand.
FIGURE 1-1-10 Three-dimensional TEE image of the mitral valve demonstrating a torn (arrow) chord and flail P2 segment.
STRESS ECHOCARDIOGRAPHY
Stress echocardiography is predicated on the principal that myocardial ischemia results in regional left ventricular dysfunction. By imaging the heart prior to and during or after stress, both resting regional wall motion abnormalities (due to prior myocardial infarction or hibernating myocardium) and transient stress provoked wall motion abnormalities (due to myocardial ischemia) can be detected. Stress echo improves diagnostic sensitivity and specificity compared to a nonimaging stress test. Other than standard contraindications and risks for stress testing, there are no contraindications to the procedure.
Stress echocardiography was initially reported in 1979 using M-mode imaging to detect transient regional dysfunction.22 Subsequently, 2-D imaging was employed, which allows for a comprehensive assessment of all segments of the LV.23 The development of digital echocardiographic image acquisition, which enabled side-by-side comparison of rest and stress images, furthered the clinical application of stress echo using both exercise (treadmill and supine bicycle) and pharmacologic stress agents.
Stress echocardiography now plays a significant role in the evaluation and management of patients with known or suspected coronary artery disease, offering an alternative to nuclear perfusion imaging. Stress echocardiography has many advantages over nuclear imaging (lack of radiation, lower cost, faster turnaround time, immediate results, greater specificity, the ability to assess valves and the pericardium), but it also has several limitations (more subjective, issues with image quality, operator dependence, lower sensitivity). Overall accuracy is comparable to stress perfusion imaging.24 In addition, stress echocardiography can be utilized in the assessment of patients with valvular heart disease and pulmonary hypertension to assess exercise capacity and the hemodynamic response to exercise.
The clinical application of stress echocardiography will be highlighted in the sections on coronary artery disease and valvular heart disease.
TRANSESOPHAGEAL ECHOCARDIOGRAPHY (TEE)
First described by Side and Gossling in 197125 who used a continuous-wave Doppler transducer to measure blood flow in the aorta, transesophageal echocardiography (TEE) (Figure 1-1-11) rapidly evolved from an M-mode only imaging modality to current multi-plane transducers with the full spectrum of imaging and Doppler modalities.
FIGURE 1-1-11 Transesophageal echocardiogram demonstrating a left atrial thrombus in a patient prior to planned cardioversion.
The clinical use of TEE has developed in both the intraoperative setting as a monitoring tool and as a diagnostic imaging modality in the awake/sedated patient. As TEE probe capabilities have evolved, its clinical applications have expanded. There are currently many clinical scenarios in which TEE is useful either as an adjunct to a transthoracic (TTE) study or as the primary echocardiographic study (Table 1-1-1). TEE does provide several advantages over TTE, including excellent image quality in virtually all patients and the ability to assess posterior cardiac structures. It is invaluable in the assessment of mitral valve disease and in the assessment of the left atrium, as well as having an important role in the assessment of prosthetic valves. Intraoperative TEE remains an important monitoring tool in a variety of settings.
TABLE 1-1-1 Indications for TEE
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