Tissue Doppler Imaging
Tissue Doppler Imaging (TDI) is a novel use of ultrasound to image the motion of tissue with Doppler echocardiography. Doppler echocardiography records and displays the velocities of the moving targets. When Doppler echocardiography is used to measure blood flow velocity, erythrocytes are the targets. Their normal velocities range from 10 cm/s in the venous circulation to 150 cm/s in the arterial circulation. However, the velocities of myocardial tissue are much lower (1-20 cm/s), but their amplitudes are greater than those produced by blood. 
Therefore, Doppler ultrasounds instruments have been modified to record the low velocities of myocardial tissue and to reject the high velocities generated by blood flow. TDI requires a high frame rate. A special function key needs to be selected to activate TDI. After TDI has been selected, the subsequent operation is identical to that used to perform regular pulsed wave Doppler echocardiography, except that the TDI gain needs to be lowered from the regular gain setting used for blood flow Doppler recordings and the velocity scale needs to be adjusted to a lower aliasing velocity (about 20-30 cm/s or even lower) to optimize TDI signals.
TDI can be displayed in the color mode, just as in colour imaging of blood flow. Tissue velocities are colour coded by autocorrelation: red for tissue moving toward the tranducer and blue for tissue moving away from the tranducer. Movement and velocities of cardiac structures are regulated by the underlying systolic function and diastolic function of the heart.
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