Using The Pedoff Probe
©Walter Rasmussen, R.D.C.S.
Above: CW Doppler of the aortic valve.
The Pedoff probe is designed for the acquisition of Continuous Wave Doppler in areas where there is restricted access to the area of interest and when a probe with the highest possible sensitivity is needed.
The Pedoff probe was the original Doppler probe and is of simple design, having only 2 large crystals, one for transmit and one for receive, with static focus. When compared with today’s sophisticated multi-focus, multi-frequency probes containing hundreds of crystals and a larger footprint, the Pedoff probe has the advantage of a simple design and small size.
The Pedoff probe is perhaps the most difficult to use because it provides no visual reference for positioning of the ultrasound beam. A considerable amount of practice is therefore required in order to develop sufficient skill and confidence in its use. The sonographer must also be knowledgeable of the general position of cardiac structures within the thorax from specific points on the body that will provide the best windows.
Above: Principal acoustic windows used for the Pedoff Probe.
The Doppler audio sound is a key component for obtaining the most accurate spectral Doppler images because spectral Doppler sound provides higher signal resolution than the spectral Doppler display itself (the display is a compressed image of the sound). This is why a sonographer will often hear a higher velocity in the background, while barely seeing it on the display. With the guidance of Doppler audio, minor adjustments in angle, position and pressure should increase the visualization of the target blood flow.
Doppler audio should never be disabled during an echocardiography exam for the same reason that automobile drivers are safer when they can hear all the sounds around them. If one removes the sounds of engines, wind, sirens and horns, while not impossible; driving becomes a much more difficult and dangerous endeavor.
Aortic stenosis using the Pedof probe positioned at the right supra-clavicular fossa.
It is required that the Pedoff probe be used as part of a thorough evaluation of aortic stenosis or an aortic valve prosthesis. See the related chapter on the subject: http://echocardiographyskills.com/pedoff-transducer-aortic-stenosis
From the apical window, the best placement of the Pedoff probe is most easily found by first locating the area of interest with the two-dimensional probe, then noting the transducer position and angle, and carefully exchanging it for the Pedoff probe.
Guiding the Pedoff transducer requires mental visualization of where the cardiac anatomy is in relationship to where the probe is placed on the chest wall.
While observing the CW spectral display and listening to the blood flow, locate one of the typical patterns and sound of either the mitral, aortic or tricuspid valves. The relationship between the valves as viewed from the apical window must also be understood in order to move from one valve to the other.
Once one of the spectral Doppler patterns is located, adjust the angle of the transducer to find the velocity profile pattern of the valve being sought. Attempt to make the audio signal become stronger, the spectral display brighter and the velocity higher by tilting in various directions within the blood flow and/or rotating the transducer minutely clockwise and counter-clockwise. At some point, maximizing the velocity of the blood flow on the spectral display will become the main focus and the audio, somewhat less important as it is localized.
The strength of the signal is conveyed through the loudness of the sound and confirmed by the brightness of the spectral display, helping to position the CW cursor within the center of the aortic jet. Experience really counts in obtaining quality spectral Doppler recordings because it is easy to obtain a blunted peak velocity, which may result in underestimation of peak and mean pressure gradients.
It is advisable that the novice Sonographer practice the use of the Pedoff probe as much as possible. It is not too much more effort to spend a little extra time even on normal patients, in order to gain competency or stay in practice with the Pedoff probe.