The velocity pattern in the ascending aorta of 15 healthy adults was measured quasisimultaneously from the Doppler-shifts produced in 16 gates distributed equally within the cross-section along a narrow ultrasound beam which centrally traversed the vessel upstream of the brachiocephalic trunk. A comparison between the time integrals of the velocities in gates 9 (centre line), 4 and 13 (off centre) and the time integral of the weighted mean of the velocities of all gates correlated with r=0.90, SEE=1.05 (gate 9), r=0.90, SEE 0.88 (gate 4) and r=0.92, SEE 0.94 (gate 13). A better correlation (r=0.96, SEE=0.60) was found between the linear mean of all gates and the weighted mean. These results show that Doppler measurements in single small gates are not appropriate to determine the average cross-sectional blood flow velocity in healthy adult

ANLIKER, M.

JENNI, R.

KRAYENBUEHL, H.-P

RUFFMANN, K.

VIELI, A.

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European Heart Journal (1984) 5, 948-953A comparison between single gate and multigateultrasonic Doppler measurements for the assessment ofthe velocity pattern in the human ascending aortaR. JENNI, A. VIELI, K. RUFFMANN, H.-P. KRAYENBUEHL AND M. ANLIKERMedical Policlinic, Cardiology and Institute for Biomedical Engineering, University of Zurich and SwissFederal Institute of Technology, Zurich, SwitzerlandKEYWORDS: Velocity pattern in the human ascending aorta, multigate Doppler.The velocity pattern in the ascending aorta of 15 healthy adults was measured quasisimultaneously from theDoppler-shifts produced in 16 gates distributed equally within the cross-section along a narrow ultrasoundbeam which centrally traversed the vessel upstream of the brachiocephalic trunk. A comparison between thetime integrals of the velocities in gates 9 (centre line), 4 and 13 (off centre) and the time integral of theweighted mean of the velocities of all gates correlated with r = 0-90, SEE= 105 (gate 9), r = 0-90, SEE 0-88(gate 4) and r = 0-92, SEE 0-94 (gate 13). A better correlation (r = 0-96, SEE=0-60) was found betweenthe linear mean of all gates and the weighted mean.These results show that Doppler measurements in single small gates are not appropriate to determine theaverage cross-sectional blood flow velocity in healthy adults.IntroductionAlthough the potential of utilizing a combi-nation of ultrasonic imaging data and Dopplershift determinations in the ascending aorta toevaluate cardiac output has been recognized""31,no clinical procedure has been generally agreedupon so far. Major problems impeding thedevelopment of such a procedure include thequantitation of all three spatial velocity com-ponents, the evaluation of the cross-sectional areaof the vessel lumen and the determination of thecross-sectional mean blood flow velocity from thespatial velocity distribution along a single beamdirection through the centre of the lumen141. Thepurpose of this study is to examine the differencesone obtains for the values of the volume flow ratewhen one varies the position of a single gate withinthe lumen and interprets the instantaneous velocitywithin the small volume corresponding to this gateas the instantaneous cross-sectional mean velocity.The methods applied until now to assess cardiacoutput by means of Doppler echocardiography areReceived for publication on 23 May 1984 and in revised form 24August 1984.Address for correspondence- Rolf Jenni, M.D.. M.S.E.E.. MedicalPoliclinic, Cardiology, Uimeraty Hospital. Raemistrassc 100,CH-8091 Zurich. Switzerlandbased on placing the gate in the centre of the aorticlumen and on defining the observed velocity asthe cross-sectional mean value11 "31. Invasivestudies based on multigate Doppler131 or hot filmanemometry16-71 have shown that the velocityprofiles in the ascending aorta are flat but skewedin dogs.Fisher et a/.'1' have addressed the questionwhether the position of a single sampling volumeand its length along the beam axis is crucial tothe calculation of cardiac output in dogs withopen chests. Their study showed excellent correla-tions between the results obtained by single gateDoppler measurements and those by beaker andstopwatch for each sampling volume and each site.In contrast to Fisher1", we investigated healthyhuman subjects noninvasively with a multigateDoppler instrument providing quasisimultaneousvelocity information from 16 gates distributedequally along the beam axis within the cross sec-tion of the vessel (Fig. 1). 'Quasisimultaneously'refers to the fact that there is a time shift betweenthe sampling of gates 1 to 16. For all practical pur-poses this time shift is negligible. The 16 velocitiesmeasured define an instantaneous spatial velocityprofile and permit simultaneous volume flow ratedeterminations for 16 different individual gatelocations. Three representative individual results0195-658X84 110948 + 06 S02.00 0 © 1984 The European Society of CardiologySingle gate and multigate ultrasonic Doppler measurements 949Figure 1 Echogram of the aortic arch. The Doppler beam is placed upstream of the brachiocephalic trunk(TR) in the ascending aorta (AO ASC). The range in which the 16 sampling volumes are distributed ismarked with a white bar (AP = pulmonary artery).are normalized with respect to the mean volumeflow rate found by integrating the instantaneousspatial velocity profile along the beam directionover the cross-section of the aorta as illustrated inFig. 2.We thereby assume that in each individual casethe angle of incidence and the vessel lumen remainconstant throughout the cardiac cycle. Thus we donot calculate the true stroke volume but merely thetime integrals of the measured average velocity (i.e.the relative volume flow rate) within the vesselcross-section over the heart cycle.MethodsPATIENTSOur noninvasive investigations were carried outin 15 normal subjects, 8 females (mean age 25years) and 7 males (mean age 26 years). Each sub-ject was placed in supine position and the trans-ducer was positioned in the suprasternal notch sothat the echogram depicted the ascending aorta,the aortic arch and the great vessels (Fig. 1).Figure 2 Ring approach for the calculation of the instan-taneous mean velocity. The vessel lumen is subdivided intohalf annular segments (A )^ with their width correspondingto the distance between adjacent gates. Each velocity value(vj is considered to be representative for one of these halfannuli and therefore weighted with the correspondingarea. The resulting Q, represents the flow rate through thearea segment A,. These segmental flow rates are summedand normalized with respect to area in order to yield theinstantaneous mean velocity.950 R. Jenni et al.Figure 3 Velocity patterns in the ascending aorta. The plot of the results containsthe full set of velocity profiles of a 30-year-old healthy female subject on the leftside. The x-axis corresponds to the depth divided into 16 gates, the y-axis to thetime within the cardiac cycle and the z-axis to the velocity. On the right thevelocity waveforms as a function of time are plotted for the sixteen single-gatevelocities and for the weighted instantaneous mean velocity (Qrtl).INSTRUMENTATIONA Diasonics CV-3400R Sector Scanner with anintegrated multigate Doppler subsystem was usedwith its standard 2-25 Mhz transducer. Beamprofile data kindly supplied by Diasonics Inc.,Milpitas, CA, U.S.A., indicate that the half max-imum beam width in the waterbath is of the orderof 3 to 7 mm in the depth range of 4 to 10 cm andat an angle of less than 30 degrees which includesthe region within which the Doppler measurementswere made. In addition the axial length of the sam-pling volume is given as being 2 mm. The 16 gatesthe interspace of which is adjustable were dis-tributed equally along the Doppler beam axisbetween the anterior and the posterior aortic wall(Fig. 1). If the distance between the anterior andthe posterior aortic wall is more than 32 mm thisresults in a gap between adjacent gates, if the dis-tance is less than 32 mm the gates overlap partially.The pulse repetition frequency for the Dopplerbeam was between 3-7 kHz and 2-35 kHz depend-ing on the depth of the posterior wall. The velocityinformation from each gate was sampled by anexternal microcomputer (Motorola, 6800) and theinstantaneous velocity profiles were displayed onthe monitor of the CV-3400R. The microcomputerdigitises the velocity information from each of thegates 1 to 16 with a frequency of MHz and aresolution of 8 bits. Furthermore, it averages thesedata for each of the gates at corresponding instantsin the heart cycle over 8 successive heart beats.During the data acquisition the instantaneousmaximum velocity versus time is displayed on amonitor for each heart cycle separately. Thisallows the operator to eliminate data sets contain-ing velocity information associated with ectopicbeats and excessive motion of the vessel relative tothe transducer. The averaged profiles are stored onfloppy disc for offline analysis.DATA ACQUISITIONAfter echographic visualization of the ascendingaorta and the aortic arch in their maximumdiameter (Fig. 1), the Doppler beam was placedupstream of the brachiocephalic trunk in such away that (1) the angle of incidence near the anter-ior aortic wall was nearly the same as near theposterior aortic wall and (2) no aliasing occurred.Then, gate number 1 was placed near the aorticwall closer to the transducer, and gate number 16near the one farther away from it. Thereafter thetwo-dimensional image was frozen to allow for theSingle gate and multigate ultrasonic Doppler measurements 951weiocny ratio velocity ratio( » 4 / v , ) at cnar.r.ei 4 vv l 3 /v 9 ) at channel 13Figure 4 Anterior-posterior skewness of the velocity pro-files. For 30% ( • ) , 50% (O) and 70% (A) systolic ejectionperiod (SEP) the velocities of channel 4 (left side) and 13(right side) are plotted normalized to the centre line velocity(gate 9). At all three instants a statistically significant skew-ness with increased velocities near the posterior anddecreased velocities near the anterior vessel wall can beobserved.higher acquisition rate of the Doppler signal. In allsubjects the beam could be placed such that theDoppler signal did not exhibit aliasing.DATA ANALYSISFive approaches were used to determinethe cross-sectional average of the instantaneousvelocities:(1) Each velocity value within a profile is assumedto be representative of the corresponding semi-annular segment of the vessel cross-section (Fig. 2).Based on this assumption each of the measuredvelocities is weighted by the area of its segment.Their sum is normalized with respect to the circularcross-section denned by gates 1 and 16 to yield aninstantaneous mean velocity. This method is calleda ring approach. The corresponding M-shapedweighing function has its minimum at the gates 8and 9 in the centre of the vessel and increaseslinearly with the distance from the vessel axis. Thering approach is taken as a reference for allmeasurements as it is based on the total availablevelocity information.(2) The velocity measured in gate 9 (Fig. 3) nearthe centre of the vessel is interpreted as theinstantaneous mean velocity.(3), (4) Velocity measurements in the gates 4 and 13(Fig. 3) in the anterior and posterior segment of thecross-section, respectively, are interpreted as theinstantaneous mean velocity analogues toapproach (2).(5) The velocities measured in gates 1 through 16were averaged to yield the cross-sectional linearmean.Fig. 3 shows a typical plot of these velocities as afunction of time. The skewness of the profile wasevaluated at different instants with the systolicejection period (SEP) by comparing the velocitiesmeasured in gates 4 and 13 to the centre linevelocity (gate 9). To allow for a comparison of thestroke volumes one would obtain by interpretingthe individual values of the five velocities as thecross-sectional average velocity, each of the fivevelocity-time curves is integrated over time duringthe heart cycle. Each of the five time integralsrepresents an estimate for the stroke volume exceptfor a factor depending on the angle of incidenceand the cross-sectional area.ResultsVELOCITY PROFILESIn Fig. 4 the average skewness of the velocityprofiles of the 15 subjects is characterized by therelative deviation from the centre line velocity.Gate 4 anterior to the centre line shows at 30%SEP a deviation of - 6 % to-22%, at 50% SEPone of +4% to - 4 0 % and at 70% SEP one of+ 14% to - 3 8 % . In contrast, gate 13 posteriorto the centre line yields at 30% SEP a deviation of- 1 8 % to +33%, at 50% SEP one of 9% to+ 27% and at 70% SEP one of - 6 % to +71%.Accordingly the velocity profiles in the ascendingaorta are skewed insofar as the velocities near theanterior wall are lower than in the centre of thevessel and higher near the posterior wall.TIME INTEGRALS OF THE MEAN VELOCITYFig. 5 illustrates the differences betweenapproach 1 and approaches 2 to 5. Based on thecentre line velocity (gate 9) the time integral orvolume flow rate is overestimated by 7% to 45%(mean 14%, r = 0-90, SEE =1-05). The timeintegral based on the velocity in gate 4 yields valuesbetween 19% under- and 12% overestimation(mean - 5 % , r = 0-90, SEE = 0-88) as compared952 R. Jenni et al.141210o" 86420o)y = l-676x-0-2062^ = 0-90SEE = 1-05/V = 15141210= 8642( c)•, ' ' y = H824x + 0-655lSEE = 0-94i i 1 • • 12 4 6 8 10 12 14 2 4 6 8 10 12 14412108642( b)• y/,f y = 0-974U-0-l3633EE = 0-88^=158 10 12 144208642( d )/ •• ///y = 1-132 x-0-9177SEE = 0-60_i 1 1 i i0 2 4 6 8 10 12 14Q».,_.Figure 5 Correlation between the time integrals of v9, v4> v13, vUn and vril l |. The plots (a),(b), (c) and (d) respectively summanze the values of the time integrals of v9, v4, v1 3 and vllnversus the ones of vrln§ for 15 healthy subjects. The time integrals are symbolized by Qv, indi-cating a measure of the stroke volume per unit area. For both coordinates the same arbitraryunits are used. The line of identity is indicated by a dashed line. The line corresponding to theregression equation given in the lower right corner of each figure is drawn solid, r representsthe correlation coefficient, SEE the standard error of the estimate and N the number of sub-jects. The deviations of Qv, from Q,rlmt range from + 7 % to +45% (mean + 14%), the onesof Qv< range from - 1 9 % to + 12% (mean - 5 % ) the ones of g,M range from + 14% to+ 45% (mean +27%) and the ones of Q,nm range from —26% to + 19% (mean +3%) .with the nng approach. The comparison of thetime integral of gate 13 and the ring approachresults in an overestimation of 14% to 45% (mean21%, r = 0-92, SEE = 0-94).The comparison of the time integral of the linearmean and the ring approach yields results between- 2 6 % and +19% (mean = 3%, r = 0-96,SEE = 0-60).DiscussionThe goal of our study was similar to that ofFisher et a/."1 in that we also investigated theinfluence of the sampling site on estimations of thetime integral of the velocity. However, in contrastto providing results obtained in open chest dogs,our data were recorded noninvasively from healthyhuman subjects. Our results differ to some extentfrom Fisher's insofar as we find a variation of thetime integral of the velocity with the location of thesampling volume within the lumen of the ascend-ing aorta. Reasons for this difference may includethe following factors. The transducer position andorientation with regard to the flow direction usedby Fisher et al., differed from ours. Due to theirsmaller angle of incidence the different gate pos-Single gate and multigate ultrasonic Doppler measurements 953itions are selected more along the vessel axis thanacross the vessel diameter. It is recognized that themost sizable error in the ultrasonic measurementof the stroke volume probably arises from inac-curacies in the echographic determination of thecross-sectional area of the vessel and the directionof the flow relative to the ultrasound beam but thisproblem was neither addressed in our study norfully explored in ref*11. We have assumed that ineach individual case this angle is the same in allpoints of the cross-section. Within the frameworkof this assumption our different approaches incaculating the time integral of the instantaneousmean velocity become independent of the truedirection of flow.It may be argued that the angle of incidencetends to be smaller in gate 13 than in gate 4,which would result in higher velocities in gate 13.However, to produce a skewness of the measuredamount with an angle of incidence of roughly50 degrees, the angular difference between theanterior and the posterior vessel wall would needto be about 20 degrees, which is considerably morethan what we could observe in our echograms.The result of Paulsen and Hasenkam'61 showthat there is a left to right skewness in addition tothe antero-posterior one. Although our beamdirection was not exactly the same as the one ofPaulsen and Hasenkam161, our results can roughlybe compared with their antero-posterior skewnesswhich appears to be predominant.By placing a single gate proximal to ourmeasurement site13' additional problems dueto motion of the aorta relative to the fixed gateposition are encountered. During the cardiaccycle different portions of the vessel diameter areinterrogated.!t is of interest that the best correlation isobtained between the linear mean and the ringapproach. From this it can be concluded that a'giant' gate including the whole vessel diametergives the best single gate estimate of the cross-sectional mean velocity. It should be noted that thevelocity profiles in the ascending aorta can becomemuch more irregular as a consequence of diseaseprocesses as illustrated in ref181. This can com-pound the error of the stroke volume determi-nations based on velocity measurements in a singlesmall gate. In summary our results indicate thateven in healthy subjects flow estimates based onsingle gate Doppler measurements in the ascendingaorta depend on the sampling site within thelumen.References[1] Fisher DC, Sahn DJ, Friedman MJ et al. The effect ofvariations on pulsed Doppler sampling site on calcula-tion of cardiac output: An experimental study in openchest dogs. Circulation 1983; 67: 370-6.[2] Steingart RM, Meller J, Barovick RN, Patterson R,Herman MV, Teichholz LE. Pulsed Doppler echo-cardiographic measurement of beat-to-beat changes instroke volume in dogs. Circulation 1980; 62: 542-84.[3] Goldberg SJ, Sahn DJ, Allen HD, Valdes-Cruz LM.Heonecke H, Carnahan Y. Evaluation of pulmonaryand systemic blood flow by two-dimensional Dopplerechocardiography using fast Fourier transform spectralanalysis. Am J Cardiol 1982; 50: 1394-460.[4] Schuster AH, Nanda NC. Doppler echocardiographicmeasurement of cardiac output: comparison with anon-golden standard. Am J Cardiol 1984; 53: 257-9.[5] Perroneau PA, Sandmann W, Xhaard M. Blood flowpatterns in large arteries. In: White DN, Brown RE,eds. Ultrasound in medicine, Vol 3B. New York:Plenum Press, 1977: 1193-208.[6] Paulsen PK, Hasenkam JM. Three-dimensional visu-alization of velocity profiles in the ascending aortain dogs, measured with a hotfilm anemometer. JBiomechanics 1983; 16: 201-10.[7] Seed WA, Wood NB. Velocity patterns in the aorta.Cardiovasc Res 1971; 5: 319-29.[8] Jenni R, Vieli A, Anliker M, Krayenbuehl HP. Dopplerechocardiography; its advantages and limitations inadult cardiology. In: Roelandt J, ed., The practice ofM-mode and two-dimensional echocardiography. DenHaag: Martinus Nijhoff, 1983: 90-7.

A comparison between single gate and multigate ultrasonic Doppler measurements for the assessment of the velocity pattern in the human ascending aorta

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