The Heart Deformation Coordinate System

The movement of any object in space is always described in three dimensions, and, therefore, a three-dimensional coordinate system must be defined:

Tissue Doppler Imaging

The Doppler principle/effect has long been used in echocardiography to track and display the velocities of moving objects. This technique has been used extensively to track the movement of red blood cells within the cardiac chambers. Generally the Doppler filters are set to detect high-velocity and low-amplitude signals (ranging from 10 cm/sec in the venous circulation to 150 cm/sec in the arterial circulation). These signals can be used to produce both pulse and continuous wave spectra plotted over time or as two-dimensional color flow patterns. These techniques can produce an array of hemodynamic and flow-related information. These same principles also can be applied to the myocardium. However, the Doppler filters must be set to detect lower-velocity and higher-amplitude signals (myocardial velocities range from 1–20 cm/sec, and the amplitude is approximately 40 dB higher than that seen in blood). By changing the Doppler filter settings, one can display the differences between blood and myocardial movement.

One of the major limitations of any Doppler technique is the dependency of velocity measurements on the angle of imaging relative to the object’s direction of motion. The velocity will be underestimated by approximately 6% if the angle of interrogation is 20 degrees, 13% at 30 degrees, and 29% at 40 degrees.¹ The second major limitation to TDI is its inability to distinguish between myocardial movement that is due to active contraction and myocardial movement that is due to passive tethering. These limitations have led to the development of alternate imaging techniques such as Doppler-based strain and strain rate imaging and speckle tracking.

Two major TDI techniques have been used in the assessment of ventricular function. Pulsed wave TDI (similar to routine pulsed Doppler) involves placing a defined sample volume over the area of interest. This technique has the advantage of displaying myocardial velocities in the defined small area of interest (usually <1 cm) with high temporal resolution. Alternatively, a color-coded template can be superimposed over a two-dimensional 2D or M-mode image (color TDI). This has the advantage of analyzing multiple segments and larger sample areas simultaneously. When tissue moves toward the transducer, it is color-coded red and when it moves away from the transducer it is color-coded blue (Fig. 18-2).

TDI has been found to be useful in many different clinical applications, including the following:

TDI can be used to evaluate both global and regional LV function. Mitral annular velocity has been used to estimate global LV systolic function. In a patient with normal LV function, the systolic velocity (S′ or Sa) of the mitral valve annulus is generally >6 cm/sec.² A prior study that investigated the average mitral annular descent velocity (color-coded TDI M-mode) found that a velocity of >5.4 cm/sec predicted a LV ejection fraction of 50% or greater with a sensitivity and specificity of 88% and 97%, respectively.³ To evaluate regional systolic function, the TDI sample volume can be placed in the area of interest. Currently, color TDI is used most often to look for areas with low TDI velocities. Tissue velocities are decreased in areas where there is myocardial dysfunction or ischemia.

Tissue Doppler imaging has become a standard component of the assessment of diastolic function and the estimation of left ventricular filling pressures. The most frequently assessed regions are the lateral and medial mitral valve annulus. The lateral E′ is usually higher than the medial E′. In a normal individual, the lateral E′ is >15 cm/sec and the medial E′ is >10 cm/sec.² As an individual ages or develops diastolic dysfunction, the E′ velocity decreases. It has been demonstrated that the ratio of E/E′ (E = mitral inflow early diastolic velocity/E′ = TDI E′) correlates well with increased pulmonary capillary wedge pressures (PCWP). An E/E′ >10 (lateral annulus) and a E/E′ >15 (medial annulus) have been shown to correlate with a PCWP greater than 20 mm Hg.^4,5

Tissue Doppler imaging is a useful tool for the accurate timing of various myocardial events. This usefulness has led to its application in patients with congestive heart failure who are being evaluated for and treated with cardiac resynchronization therapy. The most commonly measured interval is the time from the onset of the QRS to the peak systolic velocity in the ejection phase (Ts). This measurement can be calculated in multiple walls and segments to assess for dyssynchrony. This topic is further discussed in Chapter 17.

TDI has also been found to be useful in the differentiation of constrictive pericarditis from a restrictive cardiomyopathy. A restrictive cardiomyopathy causes impaired myocardial function. This pathologic process leads to a decrease in the E′ velocity. In the setting of pericardial constriction, the E′ velocity is preserved as myocardial function is preserved. In a study looking at these patients, a E′ >8 cm/sec had a high sensitivity and specificity for the diagnosis of constrictive pericarditis (95% sensitivity, 96% specificity).⁶

Finally, TDI has been shown in several studies to be helpful in separating the athletic heart from other conditions such as hypertrophic cardiomyopathy. In patients with hypertrophic cardiomyopathy, the S′ and E′ velocities are reduced compared to normal individuals.⁷ Patients with the athletic heart have preserved myocardial tissue Doppler imaging velocities.

Strain and Strain Rate Imaging

Following electrical stimulation, the myocardium deforms in a three-dimensional fashion due to sarcomeric shortening. There is longitudinal shortening, radial thickening, and circumferential shortening. Strain is a dimensionless measure of this deformation and is generally expressed as a percentage. Strain (ε) is defined as the deformation (change in length) of an object normalized to its original length. Lengthening is represented by positive values and shortening by negative values (Fig. 18-3).

Strain rate imaging is defined as the speed at which deformation (change in length) occurs and is expressed in seconds^–1. Within a region of interest, the strain rate is calculated as the instantaneous difference in velocity between two points relative to the distance between the points (ν1 distal and ν2 proximal). A difference between two point velocities indicates that there is movement relative to one another, either compression or expansion (Fig. 18-4).

Speckle Tracking Echocardiography

The myocardial fibers are organized and aligned differently throughout the various regions of the ventricle. This variability leads to unique speckle patterns of reflection, interference, and scattering. These patterns (20–40 pixels) are often referred to as a “kernel,” and their movements (displacement and velocity) can be tracked. This forms the basis of speckle tracking echocardiography or two-dimensional strain determination.

Since the “kernel” or speckle pattern can be tracked in any direction, it has the advantage of not being limited to the angle of the ultrasound beam. As this technique is angle independent, circumferential and radial strains can be estimated. As with all techniques, there are technical factors that limit its use. Image quality is very important to the accurate application of this technique. Artifact, dropout, and poor-quality images make the identification and tracking of speckle patterns difficult. Through-plane motion with a temporary loss of the speckle pattern also can make accurate tracking difficult. This can be particularly troublesome at the base, where there is significant apical displacement during systole. Speckle tracking is disadvantaged by its much lower temporal resolution when compared to tissue Doppler imaging. This makes it less able to distinguish events separated by short time intervals (milliseconds).

The potential future clinical applications of speckle tracking imaging are yet to be defined and currently are an area of active research. It has been shown that the rate of early diastolic untwist correlates with the degree of diastolic dysfunction.¹³ This correlation has been demonstrated in medical conditions where diastolic dysfunction is expected. There is also ongoing research defining its utility in diagnosing and quantifying cardiac dyssynchrony. It may prove helpful in managing patients who are candidates for cardiac resynchronization therapy. Initial research has found the assessment of radial strain in determining the septal-to-posterior wall thickening delay to be a predictor of cardiac resynchronization therapy response.¹⁴ Speckle tracking combined with longitudinal TDI measurements predicts a better response to CRT than either parameter alone.¹⁵

Figure 18-1 Heart deformation coordinate system.

Figure 18-2 Example of a normal pulsed tissue Doppler imaging study of the medial mitral valve annulus. There are three major velocity tracings: the S′ (Sa) occurs during left ventricular systolic contraction; E′ (Ea) occurs during the early filling phase of diastole; and A′ (Aa) occurs during atrial contraction. The isovolemic relaxation time and isovolemic contraction also can be measured.

Figure 18-3 Strain: Deformation or change in length (L₁ – L₀) of an object normalized to its original length (L₀)

Figure 18-4 Strain rate imaging data. The d represents the instantaneous distance between the proximal velocity (ν_b) and the distal velocity (ν_a).

Figure 18-5 Data on timing of global events is implanted into normal regional velocity, strain rate, and strain rate curves to subdivide the cardiac cycle into mechanical components. ICT, isovolemic contraction time; IVRT, isovolumic relaxation time.

(From Sutherland GR, Di Salvo G, Claus P, et al. Sutherland strain and strain rate imaging: a new clinical approach to quantifying regional myocardial function. J Am Soc Echocardiogr. 2004;17:788–802, fig. 2. Used with permission.)

Figure 18-6 Example of a myocardial speckle pattern and the tracking of this pattern.

(From Pavlopoulos H, Nihoyannopoulos P. Strain and strain rate deformation parameters: from tissue Doppler to 2D speckle tracking. Int J Cardiovasc Imaging. 2008;24:479–491. Used with permission.)