Continuous Wave Versus Pulsed Wave Doppler Handheld Ultrasound Comparison

Continuous wave and pulsed wave are the two spectral Doppler modes on a handheld heart scanner. Both read the speed of moving blood and draw it as a trace on the screen. Both run on the same probe, a tap apart on the screen. They part ways on a single trade between speed and place. A clinician picks the mode that fits the flow in front of them and reads its speed off the trace.

What continuous wave does

Continuous wave reads the speed of blood along a whole line through the heart. A clinician aims the beam down a jet. The mode reports the fastest blood found anywhere on that line. It reads that speed at any value. A jet at six metres a second reads as cleanly as one at two. The mode runs two crystals at once, one always sending, the other always listening. The steady stream of sound leaves no gap, so the mode meets no ceiling on speed. The reading could come from any point along that line. This is range ambiguity, the price of listening to the whole beam. A clinician settles where the speed sits with the two-dimensional image, which shows the jet on the screen. On a handheld the same imaging probe carries the mode, with no separate pencil transducer to swap in. A lower transmit frequency reaches a deeper jet through the chest.

The Doppler effect drives the reading. Blood moving toward the beam raises the pitch of the returning sound, and the faster it moves, the further that pitch shifts. The mode turns that shift in pitch into a speed on the screen. It also plays the flow as sound through the speaker. A clinician hears a swish, loudest at the fastest moment of the jet, and uses it to guide the aim by ear. The screen shows the speed as a filled envelope. The outer edge marks the peak. The fill below holds every slower speed the beam crosses along its line. A clinician reads the number off the outer edge. A clinician sets the gain to fill the trace and keep the noise down. The trace sweeps left to right with time, a fresh curve for each beat. A clinician sets the sweep speed to spread each beat wide enough to read the top of its curve.

Continuous wave does its best work on the fast jets of the heart. A narrowed valve and a leaking valve both drive blood at speed. The mode reads the peak of such a jet with no ceiling to cap it. A tight aortic valve sends its jet through at high speed. The peak speed gives the pressure the heart works against to clear the valve. A clinician turns that peak into a pressure gradient and grades the lesion from the number. The peak gives the instantaneous gradient, the highest pressure across the valve in the beat. The mode reads the same way wherever the jet runs. The aortic jet reads from the apex, the mitral from the same window, the tricuspid from the lower left sternal edge. A clinician finds each jet in turn and marks its peak. The tricuspid jet carries the pressure in the lungs, read from its peak and added to the pressure in the right atrium. The mode reads a shunt across a hole in the wall by the same fast jet it throws.

The reading hangs on the aim. The beam must run along the jet to read its full speed. A clinician swings the probe and watches the trace climb to its tallest. The highest clean peak is the truest reading. An off-axis beam falls short of the real speed, so a clinician keeps adjusting until the peak stops rising. A small turn of the wrist can lift a peak into the right range. The cardiac read assumes the beam sits near zero degrees to the flow, with no angle correction dialled in. A rough angle is enough to spoil a peak, so a clinician builds the alignment by hand at the probe. The loudest audio and the tallest trace agree when the aim is good. The colour image helps lay the beam down the jet before the trace is read. A clinician reads a few beats and keeps the cleanest for the record.

Continuous wave finds a fast jet at the far edge of what Doppler can catch. A tight regurgitant leak can run narrow, with a signal that is easy to miss. The continuous beam listens the whole time, so it picks that signal up in full. The look of the envelope carries a clue of its own. The brighter and fuller the trace, the stronger the jet behind it. A faint wisp of a trace points to a small leak. A clinician reads the brightness of the envelope alongside the peak. The shape of the envelope adds to the read, with its rise and fall timed to the lesion behind it. A clinician notes the timing of the peak within the beat to place the lesion in the cardiac cycle. On a handheld, the mode shares the probe with the live image. A clinician reads the trace as it forms. A clinician reads the density of the trace as a rough guide to how much blood the leak carries.

What pulsed wave does

Pulsed wave reads the speed at one chosen depth. A clinician drops a sample gate on the line of the beam. The mode reads only the flow that crosses that gate. It ties the speed to the exact spot of the gate. This is range resolution. A clinician knows where every number on the trace comes from. The gate shows as a small box on the screen. A clinician slides it along the line to the depth they want and sets its width to match the flow, from a tight spot to a longer stretch. The gate length sets how much of the line the mode averages into one reading. A clinician sizes the gate to the job in hand. A short gate pins the reading to a sharp point on the line. The reading belongs to whatever sits inside the box at that instant.

The mode works by sampling in pulses. One crystal sends a short pulse of sound, then waits to hear the echo return from the gate. The round trip fixes the depth of the reading. The mode reads that one gate, pulse after pulse, beat after beat. A clinician watches the flow at a single spot hold steady over time. The waiting sets a ceiling on speed. Pulses can go out only so fast. That rate caps the highest speed the mode can read. A deeper gate needs a longer wait for its echo, which pulls the ceiling down further. The everyday filling and ejection of the heart run a metre or two a second. The mode reads those flows with room to spare under its rate. A clinician reads the same depth on every beat, so a slow drift in the flow stands out across the beats.

Pulsed wave belongs on the flows that sit at a known depth. A clinician puts the gate at the tips of the mitral leaflets and reads the filling of the ventricle. A clinician moves the gate below the aortic valve to read the ejection. The gate slides into a pulmonary vein to read the flow there. Each spot gives a clean speed at a depth a clinician can name. The mode reads the velocity-time tracing for a stroke volume, the inflow pattern for filling, the vein flow for pressures. These flows run slow enough to stay under the ceiling. The mode reads them cleanly, with the gate holding the reading to its place. A clinician reads the right-sided inflows and outflows the same way, gate by gate. A clinician builds a picture of the heart’s filling and emptying one gate at a time.

The strength of the mode is in knowing the place. A speed without a place can mislead a reading. A clinician reads the mitral inflow and knows the number belongs to the flow right at the gate. A clinician moves the gate a centimetre and reads a different flow. The mode follows the gate wherever it lands. A clinician sets the gate by eye on the live image, drops it on the spot the picture shows, and reads the flow at that depth. The wall filter clears the slow thud of moving tissue from the trace, so the blood flow stands alone. A clinician keeps the gate small to read one stream on its own. A clinician reads an inflow or an outflow at the chair, with the place of every number known.

The trade each makes

Each mode gives up one thing to gain another. No single mode reads every speed at every depth at the same time. The trade runs along one axis, speed against place. A clinician who grasps that axis can drop any flow onto it and choose the mode that fits. The pick is rarely a close call once the flow has a name. The table below sets the two modes out point by point. A clinician runs an eye down the rows and lands on the column the flow calls for.

The table reads as a set of questions about one flow. Each row asks one thing of the mode. How fast can it read? Where on the line does the speed sit? What happens to a flow past the limit? What kind of flow does it suit? What does a clinician usually read with it? A clinician answers each question for the flow in hand and follows the answers to a column. The rows sit in plain order, the one that matters near the top. A clinician short of time reads the first row and has the gist of the choice. The table holds the whole trade in a frame the eye takes in at a glance. A clinician reads it once and carries its shape in mind for the next scan.

In practice the pick is quick. A clinician reads the flow in front of them and names the mode at once. The hand learns the trade and moves ahead of the thought. A clinician keeps both modes a tap apart on the one probe and switches when the flow changes. The trade sits behind every Doppler reading on the heart. A clinician who respects it reads each flow with the mode that fits. The table waits for the moment the pick is not obvious and settles it on the spot. A clinician runs the chosen mode, reads the speed off the trace, and turns to the next flow. A clinician runs the whole spectral study this way, flow by flow, with the trade as the guide.

Aliasing on the pulsed mode

Pulsed wave carries one limit. Aliasing is the sign of it. The mode sends pulses at a set rate. Half that rate is the highest speed it can read straight. This ceiling is the Nyquist limit. A flow that climbs past it folds over on the trace. The peak runs off the top of the scale and reappears at the bottom, pointing the wrong way. A clinician sees the signal wrap around the baseline. The wrap is the mode’s way of saying the speed has outrun it. The further past the limit the flow runs, the more tangled the wrap looks. Colour Doppler shows the same wrap as a patch of the wrong colour where the flow speeds past the limit. A clinician treats the wrap as a flag, a sign to fix the read before trusting a number. A clinician meets aliasing first on the brisk flows, the mitral inflow in a fast heart or a stenotic jet caught by the gate.

A clinician has a few moves against a wrap. Raising the pulse rate lifts the ceiling. Shifting the baseline hands the trace more room on the side it needs. Dropping to a lower transmit frequency reads a higher speed before the wrap sets in. Each move buys a little headroom. A high-pulse-rate mode drops extra gates along the line to reach a faster flow. The depth of the gate caps how high the rate can climb, since a deeper gate waits longer for its echo. A clinician reads the depth and the rate together, since the two set the ceiling between them. A clinician works through the easy fixes first. A jet can run so far past the ceiling that no setting brings it back. A clinician then switches to the continuous mode to read the speed whole.

A wrap can fool a reading if a clinician takes it at face value. The folded peak shows a speed lower than the truth. A clinician who misses it can read a fast jet as a slow one and undercall the lesion. The look of the signal is the safeguard. A wrap breaks apart at the edges of the scale and spills past the baseline. A clinician checks for it before trusting any peak from the pulsed mode. A review of Doppler echocardiography in Mayo Clinic Proceedings sets out the Nyquist limit and the role of each mode in a cardiac study. A clinician learns the look of a wrap early and reads around the limit without trouble after that. A clinician confirms a fast jet on the continuous mode whenever a pulsed trace wraps.

Grading a tight valve with continuous wave

A tight aortic valve is the classic job for continuous wave. The narrowed opening drives blood through at speed. A clinician finds the jet from the apex and lays the beam down its length. The jet runs away from the probe, so the trace fills in below the baseline. The peak sits at the outer edge of the envelope. A clinician marks it and reads the speed. A tight valve can push the jet past four metres a second. The peak alone shows how hard the heart works to clear the opening. A clinician steps through the apical, right parasternal, and suprasternal windows to chase the highest jet. A clinician reads the jet at the end of a quiet held breath to steady the trace. The window that lines up best with the jet gives the truest number. A complete envelope marks a peak a clinician can trust.

The peak speed turns into a pressure gradient. A clinician squares the peak velocity and multiplies by four. A jet at four metres a second gives a gradient near sixty-four millimetres of mercury. The gradient is the pressure the heart builds to drive blood past the valve. A higher peak means a higher gradient and a tighter valve. A clinician reads the mean gradient too, traced across the whole envelope, for a fuller grade of the lesion. A clinician grades the stenosis from the numbers and tracks them across visits. On a narrowed mitral valve the mode reads how slowly the gradient falls, a pressure half-time that grades the stenosis on its own. A clinician sets the gradient beside the valve area for a grade that holds even when the heart pumps weakly. The continuous mode carries this read on a handheld at the bedside. A clinician finds the jet, marks the peak, and reads the gradient in under a minute. The same method grades a leaking valve or a shunt, each on the strength of the fast jet it drives.

Timing the filling with pulsed wave

Pulsed wave times the filling of the left ventricle. A clinician sets the gate at the tips of the mitral leaflets from the apex. The trace shows two waves with each beat: an early filling wave as the ventricle relaxes, then a late wave as the atrium contracts. A clinician reads the height of each wave and the time across the pair. The shape of the two waves shows how stiff the ventricle has grown. The flow runs slow enough to sit well under the Nyquist limit, so the trace stays clean. The slope of the early wave back to the baseline gives a deceleration time, another mark of a stiff ventricle. A clinician switches the gate to tissue Doppler at the mitral ring to read the muscle’s own speed, and pairs it with the inflow for an estimate of the filling pressure. The same gate moves below the aortic valve to time the ejection, and into a vein to read its flow. The mode builds a record of the filling one gate at a time.

Both modes on one handheld

A handheld scanner carries both Doppler modes on the one probe. A clinician switches from one to the other with a tap on the screen. The probe is a pocket device. The screen is a phone or a tablet in the hand. A clinician brings the modes to the patient at the chair or the bedside. The colour image guides the beam for either mode. A clinician finds the flow on the picture, drops the line or the gate, and reads the trace. The whole study runs without a cart or a referral. Some handhelds trace the envelope and report the peak on their own, with a clinician checking the mark. A clinician saves the trace and the clip to the record from the same screen, and reads a heart’s flows in a few minutes where the patient sits.

A clinician moves through a study by the flow in hand. Each flow calls for the mode that fits it. A tap brings that mode up in a second. A clinician grades a tight valve, then times a filling, in one sitting. The handheld holds the speed read and the depth read in the one device. A clinician needs no second machine for the pair. The modes meet the patient where they are, at the chair, the ward, or the clinic. A clinician reads the heart’s flows with the right mode each time. A clinician works through a focused cardiac study, valve by valve and chamber by chamber, on the one device. The pocket probe carries both.

Common questions about CW and PW Doppler

Can a handheld scanner run both continuous and pulsed wave Doppler?

Yes. A modern handheld carries both spectral Doppler modes on the one probe, alongside the colour image. A clinician taps from one mode to the other on the screen. The device runs the read at the bedside, with no cart and no second machine to wheel in.

Why does continuous wave have no speed limit?

Continuous wave runs two crystals at once, one sending and one listening without pause. The steady stream leaves no gap to sample, so the mode meets no ceiling on speed. It reads a jet at any velocity along the line of the beam. A clinician fixes where that speed sits with the two-dimensional image.

What is aliasing in pulsed wave Doppler?

Aliasing is what a clinician sees when a flow runs faster than the pulsed mode can read. The mode samples at a set pulse rate, and half that rate, the Nyquist limit, is the top speed it can show straight. A flow past that limit folds over on the trace and points the wrong way. A clinician raises the pulse rate, shifts the baseline, or switches to continuous wave to read the speed whole.

When does a clinician choose pulsed wave to read a flow?

A clinician chooses pulsed wave to read a flow at a known depth. The sample gate ties the speed to one spot, so the reading belongs to that exact place. It suits the slower flows of the heart, like the mitral inflow or the outflow below the aortic valve. These run under the Nyquist limit and read clean.

Does continuous wave Doppler measure the pressure across a valve?

Continuous wave reads the peak speed of the jet through the valve. A clinician squares that speed and multiplies by four to get the pressure gradient in millimetres of mercury. A peak of four metres a second works out to a gradient near sixty-four. The gradient is the pressure the heart builds to drive blood past the valve, and it grades how tight the valve has become.

About the Author

Julien Mercier

Senior R&D Engineer · Medical Ultrasound Transducer Development

Senior R&D Engineer with an M.S. in Applied Physics and over 15 years of experience in medical ultrasound transducer development, specializing in the design verification and performance testing of high-frequency imaging transducers. Currently leading the development and verification of the company’s next-generation high-frequency linear-array transducer, responsible for imaging performance evaluation and reliability analysis in preclinical testing. Brings extensive hands-on experience in piezoelectric element tuning, beamforming parameter optimization, and system-level performance testing.