The Difference Between Colour Doppler Velocity Imaging and ...

1 Eur J Echocardiography(2002)3,240 244 , available online at onTECHNICAL NOTEThe Difference Between Colour Doppler VelocityImaging and Power Doppler ImagingW. N. McDicken and T. AndersonMedical Physics, University of Edinburgh, Edinburgh, Doppler effect, which occurs on the reflection ofultrasound from moving blood or tissue, is observed as ashift in frequency of the reflected ultrasound from thatof the incident ultrasound. This Doppler shift has beenwidely used in clinical practice as a means of measuringblood Velocity [1]. Two types of Doppler Imaging areoften discussed as facilities on ultrasonic scanners,namely Colour Doppler Velocity Imaging and PowerDoppler Imaging . In this article the Difference betweenthe two will be described and it will be noted that,although both are used in vascular disease, PowerDoppler Imaging is not extensively used at present incardiology.

2 A number of other names are used for theseDoppler techniques, for example Normal Doppler forColour Doppler Velocity Imaging and Energy Doppler for Power Doppler . The echocardiographer is obliged toascertain what exactly each name stands Doppler devices that provide Velocity infor-mation when their beam is directed at a blood vesselhave been available for many years. Indeed images canbe produced by these devices if a narrow ultrasoundbeam is slowly moved across a blood vessel and theblood Velocity at each position displayed on a real-time Doppler Imaging only became avail-able when a breakthrough was made in fast signalprocessing[2]. By real-time Imaging we mean the pro-duction of several images per second of the scannedregion. The fast signal processing rapidly produces val-ues of the mean blood Velocity at neighbouring pointsalong the ultrasound beam and allows the beam to beswept quickly to generate a cross-sectional image ofregions of blood flow.

3 Static or slow moving tissue doesnot produce significant Doppler shifts in the reflectedultrasound and is therefore not presented in a Dopplerimage. Of course a grey shade B-mode image of thesetissues is usually combined with the Doppler early instruments, 15 Doppler images per secondwere typical but now the rate can be in excess of 50 of Measurement from EchoSignalsTo understand the two Doppler techniques let us revisithow echoes are produced from tissues within the bodyand the information these echoes carry. When a shortpulse of transmitted ultrasound is scattered from a smallsample volume of moving blood and an echo signal fromthat volume is detected by a transducer (Fig. 1), twopieces of information are obtained (Fig. 2).1. A very accurate measurement of the time taken forthe echo to return to the The amplitude (size) of the echo signal the size ofthe pressure fluctuations in the all the blood cells in the sample volume to bemoving at the same speed with respect to the transducerand that a series of ultrasound pulses are transmitted atequal time intervals.

4 Due to the changing position of theblood cells, small changes in the time taken for theechoes to return to the transducer can be detected andelectronically processed to produce a signal contain-ing Doppler effect information. The frequency of thissignal is the Doppler shift in the ultrasound frequencyand from it the Velocity of blood is calculated by themachine. The direction of flow, toward or away fromthe transducer, can also be power of the echoes (derived from the amplitudeof the echoes) determines the power of the Dopplersignal, both of which are related to the number of bloodcells in the sample volume. Power is therefore a readilyobtained measure of the number of moving cells in thesample the above we can see that we have the means toobtain the Velocity and a measure of the number ofcells moving with that Velocity .

5 The latter is not aparticularly accurate measurement since the power ofthe echo signals is greatly influenced by effects such asattenuation of the ultrasound in tissue. However, it canbe used successfully to indicate that flow is occurring at1525-2167/02/030240 + 05 $ 2002 The European Society of Cardiology. Published by Elsevier Science Ltd. All rights location of a sample volume. Also the effects on theechoes are likely to be similar for neighbouring samplevolumes so their power signals can be a sample volume is small it is usual to assumethat the velocities of the blood cells in it are all thesame, whereas for a large sample volume a number ofvelocities may be present and hence there is a spectrumof velocities. Since the number of blood cells is the samein sample volumes of equal size, the power of the signalfrom each sample volume is also the same.

6 Two factorscan alter this situation, inclusion in the sample volumeof some static tissue such as vessel wall reduces thepower of the Doppler signal and turbulence increases it,as seen in jet ImagingIn Doppler Imaging , echoes are collected from an areaof tissue through which the narrow ultrasound beam isswept. For each beam direction a pulse is transmittedand a train of echoes is collected from consecutivesample volumes located along the beam axis. With atypical commercial machine, there will be 128 volumesalong the beam. The process is repeated about 10 timesto obtain 10 echoes from each sample volume. Note theprocess of collecting 10 echoes from each sample volumecan be completed in perhaps 2 ms so the process is veryfast. The very small differences in the return times of theechoes from each sample volume are used to derive theDoppler shift signal for each sample volume.

7 In suchimaging the number of transmitted pulses in each beamdirection is limited, since it is necessary for the beam tobe swept across the scan plane as quickly as signal processing of the Doppler signal from eachsample volume rapidly calculates the mean Velocity ofthe blood cells and the power of the signal. It is thereforepossible to produce two images namely the ColourDoppler Velocity Image and the Power Doppler Image(Fig. 3). The direction of flow information extracted bythe electronics from the sequence of returning echoes isincluded in the Velocity images, normally shades of reddenote flow toward the transducer and blue away fromthe Velocity Doppler images, it is worth noting that thevelocity depicted is usually not the actual Velocity , butthe component of that Velocity along the beam direction(Fig.)

8 4). The values of the Velocity components, pre-sented as different colours in image pixels, are thereforedependent on the angle Between the true Velocity direc-tion and the beam direction at each sample volume. Thisangle is often not known, so care has to be taken ininterpreting images since a considerable alteration incolour can result from a change in angle rather than achange in Velocity . Another common artefact is knownFigure production of an echo signal when an ultrasonic pulse is transmitted toward a sample volume W. N. McDicken and T. AndersonEur J Echocardiography, Vol. 3, issue 3, September 2002 Figure amplitude and echo return time information obtained from an ultrasonic instrument for one beamdirection toward the images produced by an ATL 5000 scanner, (a) a Colour Velocity Doppler image of blood flowin the left ventricle, (b) a Power Doppler image of blood flow in the left Power Doppler Imaging 242 Eur J Echocardiography, Vol.

9 3, issue 3, September 2002as aliasing . The measurement of high Doppler shiftfrequencies due to high blood velocities requires echosignals to be collected at a high rate. However there is alimit to the rate at which echoes can be returned sincetime must be allowed Between each pulse transmissionfor the related echoes to be collected. There is thereforean upper limit on the Doppler shift which can bemeasured and hence a corresponding upper limit onvelocity. Velocities above the upper limit are calculatedwith the wrong value and direction of flow. The aliasingartefact appears as regions of wrongly coloured pixels ina Colour Doppler Velocity Doppler images are relatively straightforwardto interpret: the power level at each pixel is presented asa level of brightness. Since the power at each pixel isfairly similar, as noted above, the images have a uniformbrightness except perhaps at vessel walls or in turbulentareas.

10 The main attraction of Power Doppler Imaging isthat it is a sensitive technique which is good for depict-ing flow in small vessels, it therefore gives more completeimages of vascularity than Doppler Velocity Doppler Imaging is also not prone to aliasingartifacts but this is not too surprising since it onlyindicates the presence of flow and does not attemptto measure Velocity . Power Doppler images are notextensively used in cardiology. They can give someimprovement to the definition of myocardial boundariesand hence be of value in stress tests. This improvementarises since averaging over a few consecutive imagescan be used to build up the power signal relative tonoise and therefore enhance the depiction of blood atthe myocardial boundary. Averaging does not improvethe low Velocity Colour images at the boundary to thesame extent since small Velocity signals have differentFigure component of Velocity along the beam W.