Tissue Doppler Imaging
As we saw in Chapter 4, the principles of Doppler imaging can be used not only to assess blood flow but also to assess the movement of the myocardium. This is known as tissue Doppler imaging (TDI). The Doppler signals returning from myocardium are distinct from signals from blood (myocardial motion generates a stronger but lower-velocity signal) and so can be selected with appropriate filtering. The resulting signals can be displayed as colour Doppler images to show myocardial motion or as spectral pulsed-wave Doppler traces to assess motion in specific myocardial regions.
TDI provides valuable information on myocardial mechanics. It does however have certain limitations. Like all Doppler techniques, TDI is heavily angle dependent in that it can only measure motion parallel to the direction of the ultrasound beam. Furthermore, TDI is unable to distinguish between active motion (i.e. myocardial contraction/relaxation) and passive motion (where a region of myocardium is ‘pulled’ by an adjacent segment). The alternative technique of speckle tracking overcomes these limitations and is discussed in detail in Chapter 11.
Pulsed-wave TDI permits the measurement of myocardial velocity in a specific region. To perform pulsed-wave TDI, the sonographer places the sample volume in the region of interest. In the assessment of left ventricular (LV) longitudinal contraction, the sample volume is placed in the myocardium within 1 cm of the mitral valve annulus, either medially (septally) or laterally, in the apical 4-chamber view (Fig. 10.1). In each location a pulsed-wave TDI recording is made using a low gain setting and an aliasing velocity 15-20 cm/s.
The resultant pulsed-wave TDI recording (Fig. 10.2) shows a mitral annular velocity towards the transducer which corresponds to the annular myocardium moving towards the apex as the LV contracts longitudinally during systole (Sm or S′). This is followed by an early myocardial velocity (Em or E′) which corresponds to early diastolic relaxation, the myocardium moving away from the transducer, and finally by a further movement away from the transducer, corresponding to atrial contraction (Am or A′).