New echocardiographic technique - Tissue Doppler imaging

Abstract

Tissue Doppler imaging (TDI) is a relatively new echocardiographic technique that uses Doppler principles to measure the velocity of myocardial motion. Doppler echocardiography relies on detection of the shift in frequency of ultrasound signals reflected from moving objects. With this principle, conventional Doppler techniques assess the velocity of blood flow by measuring high-frequency, low-amplitude signals from small, fast-moving blood cells. In TDI, the same Doppler principles are used to quantify the higher-amplitude, lower-velocity signals of myocardial tissue motion. Tissue Doppler has been validated extensively and examined in a variety of cardiac pathologies. Although initial work reported tissue velocity from the septal or posterior wall in the parasternal projections, recent work almost exclusively interrogates tissue velocities in the longitudinal direction (apical projections). In the longitudinal direction, myocardial motion is such that the apex is generally immobile, whereas the base moves toward the apex in systole and away from the apex in diastole. This differential motion between base and apex results in a velocity gradient along the myocardial wall, with the highest velocities at the base and low or zero velocity at the apex. Because TDI interrogates motion at a single point in the myocardium with reference to a point outside the chest (the transducer), it is influenced by translational motion and tethering (normal apical segments pull an abnormal basal segment toward the apex). Moreover, single-point interrogation (depicting tissue displacement) does not fully capture true myocardial mechanics. With color TDI, a color-coded representation of myocardial velocities is superimposed on gray-scale 2-dimensional or M-mode images to indicate the direction and velocity of myocardial motion. Color TDI mode has the advantage of increased spatial resolution and the ability to evaluate multiple structures and segments in a single view