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