What’s the Deal with Tissue Doppler?

When an ultrasound probe sends out sound waves at a certain frequency, they bounce off various structures and return to the probe.  If the structures that the sound waves bounce off of are moving relative to the probe, there will be a shift in the frequency returning to the probe known as a “Doppler shift”.  From measuring this shift in frequency, and using some very simple math, the velocity of the object that encountered the sound wave can be determined.  

This sounds straightforward, however in reality, the ultrasound probe can encounter many different Doppler shifts at once.  For example, if we are sampling blood flow in a blood vessel or in the heart, some red blood cells will be traveling fast, and some will be traveling slow, and some may be traveling in directions at angles not quite straight back at the probe and this will further affect the magnitude of the Doppler shifts (due to reasons of basic trigonometry).  As you can probably guess, something like blood will travel at very different speeds compared to something like contracting heart tissue, and  they will have different Doppler shifts.

In addition, when sound bounces off tissues, the strength of the sound waves returning back to the probe will have different magnitudes.  For example, the strength of the sound waves returning after bouncing off a red blood cell will be weaker than the strength of the sound waves that bounce back after encountering tissue.  In most ultrasound modes, this difference in the strength of returning ultrasound waves is “coded” as different shades of gray on 2-D ultrasound.  This is why blood, urine, and water are dark on a 2-D ultrasound image, while tissue is displayed in various shades of gray.  

Now, since the ultrasound probe is encountering a multitude of different Doppler shifts and amplitudes of returning sound waves at the same time, in order to make sense of it all, the machine has a set of “filters” that ignore returning ultrasound waves that are likely to confuse rather than enlighten the sonographer as to what is going on.  In other words, sound that is “signal” is kept, and sound that is “noise” is discarded.

Pulsed wave Doppler uses a single crystal to transmit a burst of soundwaves (with a given frequency) out to the tissues.  After sending the sound waves out, the same crystal stops doing anything except to wait a certain amount of time for the sound waves to bounce back and return to the probe, then starts listening to what came back for a very short time.  The machine then calculates the speed of the tissue measured in the gate by using the Doppler shifts.  The timing of the listening and length of time spent listening is based on where the sonographer sets the placement and size of the “gate” of the pulsed wave Doppler beam.  Because sound travels about the same speed through most tissues, velocities at the point of interest (where we placed the gate) can be detected by waiting the right amount of time to start listening again.  Thus, the sound waves bouncing back from things nearer or further away from the gate can be ignored because they arrive back to the probe at a time the machine is not listening, and the machine can focus on movement exclusively within the gate.  This, however, is different from the “filters” I just mentioned above.  The filters are where tissue Doppler imaging or “TDI” come in.

Sound detected by the probe that interacts with blood is usually very low amplitude and travels at much faster speeds than moving tissue.  For example, in echocardiography, we are usually measuring velocities faster than 0.2 m/sec and up to around 5 or 6 m/sec, with most things we look at traveling around 0.5 to 1.5 m/sec.  Heart tissue, however, is usually of higher amplitude and travels at speeds a bit less than 20 cm/sec.  Tissue Doppler differs from traditional pulsed wave Doppler by setting filters that discard the low amplitude/higher Doppler shift signals and pays attention to higher amplitude/lower Doppler shift signals (figure 1).