Though Doppler seems relatively simple to implement in ultrasound, analog performance in the latest systems has much higher, more-difficult-to-achieve specifications than standard black-and-white systems..
We have all heard the change in pitch when an ambulance or police car drives by with its siren blaring. The pitch is higher as the vehicle comes toward you, and shifts to a lower pitch as it goes away from you. This is called the Doppler effect, named after Christian Doppler, who came up with the idea in 1842.
To hear this effect best, you must be directly in front or back of the moving car. The more at a right angle (and farther away) you are from the source of the sound, the less well you will hear the effect.
The Doppler effect can be used to measure blood flow via ultrasound in a human or animal. An ultrasound transducer produces a beam that goes through a blood vessel. As with the automobile/sound example above, the beam needs to be as close to parallel with the blood flow as possible. As the beam moves more toward a right angle with the flow, less of the signal will be sensed.
To achieve parallelism, a beam is periodically pointed in exactly the same direction and location in the blood vessel. This period is called the pulse-rate frequency (PRF). The PRF must be fast enough so that it doesn’t alias with the speed of the blood flow, also known as the Nyquist rate.
| Download this article in .PDF format |
This file type includes high resolution graphics and schematics when applicable.
Subsequently, each beam is compared to the previous beam. Things that are not moving show no change in the beam, while moving objects are sensed in the differences from beam to beam. Observing these differences is known as measuring the Doppler effect.
Several useful measurements can be performed when looking at these differences, otherwise known as Doppler modes, in an ultrasound machine. One involves measuring the power of the difference, which is called Power-Doppler Imaging (PDI) in ultrasound literature. It’s also possible to measure the direction of flow. Using a slightly different Doppler algorithm, you can find this direction and display it with different colors—with a Doppler mode often called Color-Flow Mapping (CFM)—depending on whether the direction is toward or away from the probe.
A third Doppler mode, called Pulsed-Wave Doppler (PWD), takes the spectra of the Doppler information. Many other Doppler modes use the fundamental Doppler signal to determine various things about the flow.
These modes make Doppler sound relatively easy to implement. Just observe a periodic beam, take the difference, and do a little more processing. How hard can that be? Well, as many companies have found out, it actually isn’t so simplistic. Why? Because the analog performance of the system has much higher and harder-to-attain specifications than a standard black-and-white ultrasound-imaging system.
Linearity is an important factor in an analog front-end system. IMD3 or intermodulation distortion is critical and comes from the third-order nonlinearity of the system. Simply put, it has to be very good. Another key parameter is the overall system-noise performance—no power-supply noise can be found in the receive chain. It’s a difficult problem, though, because the transmit power supply is typically of the 200-V p-p variable variety (that can go down to 6 V p-p). The noise from this power supply can easily leak into the receive path of the ultrasound machine. These and many more considerations create tough challenges for designers looking to exploit Doppler’s advantages.
We’ll cover more ground about solving these issues in future articles.
Richard Tobias, CEO and president of Cephasonics, has over 30 years of experience in corporate management, engineering management, business development and engineering for the medical imaging, electronics, embedded systems, DSP, and CAD/EDA industries. He holds a BSEE from the University of Minnesota—Institute of Technology and an MS in mathematics from Stanford University.