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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