conversion of umbilical arterial doppler waveforms to cardiac cycle triggering signals: a...

PII: S0301-5629(00)00315-X

● Original Contribution

CONVERSION OF UMBILICAL ARTERIAL DOPPLER WAVEFORMS TOCARDIAC CYCLE TRIGGERING SIGNALS: A PREPARATORY STUDY

FOR ONLINE MOTION-GATED THREE-DIMENSIONALFETAL ECHOCARDIOGRAPHY

JING DENG*†, ALEX G. BIRKETT,† KARIM D. KALACHE,* M ARK A. HANSON,*DONALD M. PEEBLES,* A LFRED D. LINNEY,† WILLIAM R. LEES,‡ CHARLES H. RODECK*Departments of *Obstetrics and Gynaecology,†Medical Physics and Bioengineering, and‡Medical Imaging,

University College London, London, UK

(Received5 July 2000; in final form 31 August2000)

Abstract—To remove motion artefacts, a device was built to convert “noisy” umbilical arterial Dopplerwaveforms (UADWs) from an ultrasound (US) system into sharp ECG R-wave-like cardiac cycle triggeringsignals (CCTSs). These CCTSs were then used to gate a simultaneous (online) 3-D acquisition of sectional fetalechocardiograms from another US system. To test the conversion performance, a study was carried out in sheepfetal twins. Pulmonary arterial flow waveforms (PAFWs) from implanted probes were traced, in the meantime,to determine the reference cardiac cycle. Interference caused by running the two nonsynchronised US systemswas controlled to three degrees (not-noticeable, moderate, and severe), together with high (> 40 cm/s) and low(< 40) flow velocities on UADWs. The conversion efficiency, assessed by the percentage of UADWs converted intoCCTSs, was in the range of 83% to 100% for not-noticeable and moderate interference, and 0% to 71% forsevere interference. The triggering accuracy, assessed by [(time lag mean between the onsets of PAFWs andcorresponding CCTSs)2 (its 99% confidence level)]4 the mean, was 90% to 96% for the not-noticeableinterference high- and low-flow groups and for the moderate interference high-flow group; 19% to 93% for themoderate interference low-flow group; and from not obtainable up to 90% for the severe interference groups.The results show that UADWs can be used as a satisfactory online motion-gating source even in the presence ofmoderate interference. The major problems are from severe interference or moderate interference with low-flowvelocity, which can be minimised/eliminated by the integration of the individual systems involved. (E-mail:[email protected]) © 2001 World Federation for Ultrasound in Medicine & Biology.

Key Words: Fetal heart, Ultrasonography; Doppler and echocardiography, Three-dimensional, Four-dimen-sional, Motion artefact, Cardiac gating, Ultrasound interference, Scanner design, Transit-time ultrasound flowmeter, Electrocardiography.

INTRODUCTION

In adult and paediatric four-dimensional (4-D) (i.e.,3-D 1 motion) echocardiographic studies, electrocardi-ography (ECG) has been successfully used to providetemporal information of the cardiac cycle for eliminatingmotion artefacts (Schwartz et al. 1994; Vogel et al.1995). Due to the nonavailability of good fetal ECGsignals from a noninvasive approach, ECG gating isimpractical for prenatal studies. Most fetal studies re-ported have used cyclical cardiac motion or arterial flowinformation on ultrasound (US) recordings as their gat-

ing sources. The shortcomings of these methods werethat the fetal heart was gated either before (Kwon et al.1996) or after (Nelson et al. 1996; Deng et al. 1996) a4-D scan, which could not guarantee an accurate matchbetween prerecorded gating source and the real-timecardiac cycle (with the “before” method) or a reliablegating of the entire heart (with the “after” method).Real-time volumetric imaging has recently been shownto be a feasible and rapid new technique for 4-D fetalheart visualisation (Sklansky et al. 1999). But, whether itcan eventually eliminate the necessity of cardiac gatingand provide high-resolution images needs further inves-tigation (Deng et al. 2001).

Our preliminary studies have shown that online(during a 4-D scan) gating is feasible using two US

Address correspondence to: Dr. Jing Deng, Department of Med-ical Physics, 1st Floor Shropshire House, 11-20 Capper Street, LondonWC1E 6JA UK. E-mail: [email protected]

Ultrasound in Med. & Biol., Vol. 27, No. 1, pp. 51–59, 2001Copyright © 2001 World Federation for Ultrasound in Medicine & Biology

Printed in the USA. All rights reserved0301-5629/01/$–see front matter

51

systems simultaneously (Deng et al. 2000, 2001). Asectional transducer from one system performs a volumescan over the heart to provide structural data for 3-Dreconstruction while a transducer from another systemsamples arterial Doppler waveforms to provide temporalinformation for gating purposes. However, unprocessedarterial Doppler signals are too noisy and have multiplepeaks, so they cannot be used directly as online trigger-ing signals (Fig. 1a). A device was then designed toconvert these signals into sharp, ECG R-wave like sig-nals to act as cardiac cycle triggering signals (CCTSs). Inthe present work, a sheep experiment was carried out toexamine the conversion efficiency and accuracy underdifferent conditions related to clinical online 4-D scan-ning.

MATERIALS AND METHODS

Arterial Doppler signal to cardiac cycle triggeringsignal conversion

A standard US scanner with a 5.0-MHz transducer(Sonotron CFM800A, VingMed, Horten, Norway) wasused to collect arterial Doppler signals. DisplayingDoppler spectra on the monitor as a standard output, thescanner also provides Doppler signals through its AnalogVmean Trace Output that could be used for this study.

Two measures were taken to change the “noisy”

signals into ECG R-wave like signals that would beaccepted by the ECG input channel of any standard USscanner or a 4-D-acquisition system (4D Echo Scan,Tomtec, Munich, Germany). First, the low velocity re-jection (LVR) control on the VingMed scanner wasincreased to such a level that most diastolic componentsof arterial Doppler waveforms were filtered, enhancingtheir pulsatility and reducing the possibility of diastolicsignals being converted into triggering signals by mis-take (Fig. 1b).

Second, the conversion box we designed and devel-oped was used to sample the processed Doppler signalsand generate a single sharp pulse (i.e., a CCTS) fromeach waveform (each cardiac cycle) (Fig. 1c, Fig. 2). Thecircuit functions as an adaptive trigger, and the outputpulse is triggered at the same point on the incomingsignal, independently of the input magnitude. To obtainthis function, the input is compared with a fixed percent-age of the time-averaged input signal. The conversionfunctions (Fig. 2) are:1. Doppler signal acquisition. The incoming amplitude

(maximal velocity) is measured and averaged over auser-defined period (1, 2 or 5 s) to derive a noise-freethreshold for the comparator. A moving average ofthe input is created that has the effect of reducingspurious triggering from amplitude variations and

Fig. 1. Two measures to convert arterial Doppler waveforms into ECG R-wave-like cardiac cycle triggering signals(CCTSs). (a) Pulsed Doppler waveforms were acquired from an adult carotid artery. To compare with standard Dopplerdisplay (upper row), the Doppler Vmean output from the scanner’s rear panel was input onto the scanner’s standard ECGchannel (lower row). The unprocessed signals were noisy and have multiple peaks, hardly showing the pulsitility. (b)Diagnostic components of the waveforms seen in (a) were filtered by the low-velocity rejection (LVR, upper row),making the pulsatility easily visible/convertible (lower row). (c) The LVR was applied to umbilical arterial Dopplerwaveforms (upper row). From these waveforms, a conversion box (Fig. 2) was used to generate a sharp signal (CCTS)out of each “valid” cardiac cycle and input on the same ECG channel (lower row). Note that, due to a change in the flowdirection, the conversion was automatically stopped after the first CCTS until regular beats were re-detected, activatingnew conversion. Because the threshold was set on the down slope, the time delay between the onsets of systolic Doppler

waves and subsequently converted CCTSs was longer in the last four waveforms than in the first one.

52 Ultrasound in Medicine and Biology Volume 27, Number 1, 2001

noise due to ectopic heart beats, fetal or maternalrandom motion artefacts and operator inconsistencies.

2. Conversion threshold setting. This allows the operatorto optimise the conversion to a level at between 10%and 90% of the averaged peak value of the Dopplermagnitude (from minima to maxima), so that triggerpulses can be generated at a time point as consistentas possible throughout all cardiac cycles during one4-D scan.

3. Trigger pulse generation and retrigger lockout. Theoutput from the comparator is fed to a pulse generatorthat has an output pulsewidth of 24 ms and an ampli-tude of 10 mV to suit the ECG lead input. To reducethe generation of spurious trigger pulses due to themultipeak systolic Doppler input, the output rate islimited by a lockout circuit that prevents retriggeringof the pulse generator for a period of# 240 ms(equivalent heart rate:$ 250 beats per min, bpm).The choice of this period is a balance between theshortest cardiac cycle (250 ms, 240 bpm) and thesystolic interval (about 200 to 220 ms in the longestcardiac cycle of 500 ms, 120 bpm) likely to be en-countered. The 10 ms allowance for the shortest car-diac cycle would permit slight heart rate variance notto interrupt the conversion.

Flowmeter probe implantationTransit-time US flowmeter has become the “gold

standard” for measuring instantaneous and continuousblood flow in animal studies (Wong et al. 1991; Giussaniet al. 1993; Dean et al. 1996). Unlike fetal ECG, it is notsubject to interference by the simultaneous pulsed Dopp-ler and 2-D US scans andvice versa. In this study, it wasused for two purposes. Its ability to detect instant flowchanges was used to calculate heart beats and cardiaccycle, and set a reference point for computing triggeringaccuracy in this study. Its ability to measure instanta-neous and continuous flow will be used to provide ref-erence stroke volume and cardiac output needed for acontinuing 4-D study.

The dynamic 3-D fetal echocardiographic projectwas carried out in accordance with Home Office Regu-lation (Animals (scientific procedures) Act, 1986). Underanaesthesia, hysterotomy was performed on a ewe with atwin pregnancy at 120-days gestation. One twin wasexteriorised and a catheter was inserted into one jugularvein. A thoracotomy was then performed, and a size 6flow probe (Transonic Systems Inc., Ithaca, NY) wasimplanted around the main pulmonary artery. After tho-racic closure, the fetus was returned to the uterus in itsoriginal position. A similar procedure was then carriedout on the second twin. Finally, the lost amniotic fluidwas replaced with warm saline and the cavity and ab-dominal wall were closed. The pulmonary arterial flowwaveforms (PAFWs) were then traced by a T201 Tran-sonic flowmeter.

Experimental variablesInterference severity.During online motion-gated

scanning, there is cross talk between two nonsynchro-nised US systems (Deng et al. 2000, 2001). To test theconversion box performance under different interferenceseverity, the cross talk to the umbilical Doppler sampling(with the VingMed scanner) from the 3-D cardiac scan-ning (with another standard scanner, 128XP10, Acuson,Mountainview, CA) was controlled to three degrees:not-noticeable interference (NI), moderate interference(MI) and severe interference (SI). After steady umbilicalarterial Doppler waveforms (UADWs) were obtainedand the VingMed Doppler gain was minimised, the Acu-son transducer was activated in B-mode and movedtoward the VingMed transducer until just before thecross talk from the Acuson transducer became noticeableon the VingMed monitor. This was NI. MI was createdby bringing the transducers closer until the cross talkbecame obvious, but UADWs were still clearly visiblefrom the cross talk (Fig. 3a). SI was produced by movingthe two transducers even closer, until the cross talkbecame dominant and UADWs were scarcely visible inthe background cross talk (Fig. 3b).

UADW maximal velocity.To investigate the re-sponse to different flow velocities of UADWs in earlyand late gestation, UADWs were collected at two levels:high (maximal velocity$ 40 cm/s, Fig. 3a) and low (,40 cm/s, Fig. 3b). This was achieved by intentionallychanging the sampling sites and/or angles without anglecorrection. Samples for each degree of interference se-verity were then divided into two subgroups, such asNI-h (not-noticeable interference with high flow group)and NI-l (not-noticeable interference with low flowgroup). Similarly, other groups were abbreviated asMI-h, MI-l, SI-h and SI-l (Table 1).

Fig. 2. Doppler to cardiac cycle triggering signal (CCTS)conversion unit and timing diagram.

Doppler and 3-D fetal echocardiography● J. DENG et al. 53

Heart rate.To examine if the conversion methodwas able to cope with different heart rates, 20-mL salineinfusion and 5-mL bolus air injectionvia the jugularcatheter were used (and repeated when necessary) toalter the heart rate. The heart rate change would beconsidered significant if thep value of at-test was,0.01 for the mean cardiac cycles (calculated fromPAFWs) between two samples.

Data collection and analysisThe above variables were applied to each twin,

making two sets of data available for analysis. Whilecontinuously tracing PAFWs, UADWs were sampledand converted into CCTSs. Both PAFWs and CCTSswere then recorded simultaneously on a MacLab Chart(V3.3, AD Instruments Inc., Milford, MA). The follow-ing indices were defined to assess the conversion (Fig.4):

1. 20 CCTS time. This is the time needed to obtain 20consecutive CCTSs. This is because 20 imaging sec-tions are usually needed to cover the entire heart for asatisfactory 3-D reconstruction (Deng et al. 2001). Bydefault, each CCTS drives the 4-D system to acquirea set of cardiac frames (images) at one anatomicalcross-section before moving the probe to the nextsection. If 20 consecutive CCTSs could not beachieved after 30 s, the data were classified as notconvertible. This is because, clinically, the possibilityof encountering random fetal and maternal move-ments would be high over a period longer than 30 s.

2. Conversion efficiency was defined as the percentageof 20 (consecutive CCTSs) divided by the number ofUADWs from which the 20 CCTSs were converted.

3. Time lag, defined as the time delay between theonsets of a PAFW and a corresponding CCTS. Be-cause pulse travel velocity is unlikely to change dra-matically during one 4-D scan (sample), the time lagsshould theoretically have been the same if there wereno changes in UADWs and the conversion settings.However, UADWs identical to each other are hardlyobtainable, even within a single scan, not to mentionfrom different scans. Therefore, optimal conversionthresholds (10%, 90%, or so of the magnitude) andoptimal segments (up or down slope of the systolicwave) were usually different between scans, resultingin varying optimal time lags. However, during onescan, the conversion settings were kept unchangedafter steady UADWs were obtained.

4. Triggering accuracy. Because optimal time lags var-ied under the current conversion strategy, it was im-possible to use them directly as a measure of accu-racy. Because the final timing of 4-D data sets wouldtake their own time lags into account, it was alsounnecessary to have the same time lags between dif-ferent scans. It was the constancy of the time lagswithin one scan that mattered. The triggering accu-racy was assessed by the relative variations of timelags.

Triggering accuracy~%! 5 @~Time lag mean!

2 ~99% Confidence level!# 4 ~Time lag mean!

3 100, (1)

in which the time lag mean was the averaged time lags of20 consecutive CCTSs from one sample, and the 99%confidence level was calculated from the sample’s stan-dard deviation (SD) at 99% confidence interval:

Confidence level5 tn,0.01 3 [SD 4 =n] (2)

Fig. 3. Interference severity (see text for definition). (a) Asample from twin 2 with moderate interference and high Dopp-ler maximal velocity (MI-h, T2). (b) A sample from twin 2 withsevere interference and low Doppler maximal velocity (SI-l,

T2).


in which n equalled 20 (consecutive CCTSs used foraveraging the time lag, andtn,0.01 5 2.58). Equation (1)made accuracy comparable between different samples.5. Gating precision, defined as:

Gating precision~%!

5 ~Time lag99% Confidence level!

4 ~PAFW mean cardiac cycle! 3 100 (3)

By comparing 99% time lag confidence levels to corre-sponding mean cardiac cycle determined by PAFWs, thisindex was used to calculate achievable gating precisionin relationship to the cardiac phases (Fig. 5. See nextsection).

RESULTS AND DISCUSSION

Table 1 summarises the results for conversion indi-ces under variable conditions, and two examples weregiven in Fig. 4.

Heart rate variationThe cardiac cycle calculated from PAFWs (Table 1)

ranged from 285 to 400 ms (211 to 150 bpm) and themean6 SD was 3516 45 ms (1716 19 bpm). Among45 possible pairs tested from all 10 convertible samples(Table 2), the variation in the heart rate was statisticallynot significant only in 9 pairs (if the criticalp value wasset to$ 0.01) or in 5 pairs (if thep value$ 0.05). Thisprovided the opportunity of testing the conversionmethod in coping with variable heart rates within acommon range (see below).

Conversion efficiency and scanning durationThe conversion efficiency was 86.96 to 100% and

83.33 to 95.24% in the NI and MI groups, respectively(Table 1). The corresponding 20 CCTS time was 5.48 to8.85 s and 7.00 to 8.32 s.

The results from two SI-h samples showed twoextremes. At one end, the conversion efficiency was ashigh as 71.43% and the 20 CCTS time as short as 8.17 s,which could be considered clinically acceptable. At theother end, the efficiency was down to 29.41% and 20CCTS time was close to 30 s (25.70 s). From the SI-lgroup, no regular CCTSs were converted within 30 s dueto strong interference and weak UADWs.

20 CCTS time obtained from an anaesthetised sheepstudy should be considered relatively shorter than from anonanaesthetised human study. The reasons are twofold.First, under anaesthesia, the sheep fetuses do not moveduring 4-D acquisition, eliminating random fetal motionartefacts and, thus, increasing the conversion efficiency.Second, the average sheep fetal heart rate (150 to 211

Tab

le1.

Sum

mar

yof

the

conv

ersi

onre

sults

Inte

rfer

ence

UA

DW

velo

city

Tw

ins

CC

TS

no.

20C

CT

Stim

e(s

)*P

AF

Wno

.C

onve

rsio

nef

ficie

ncy

(%)*

Tim

e-la

g*T

rigge

ring

accu

racy

(%)*

Car

diac

cycl

eca

lcul

ated

from

CC

TS

Car

diac

cycl

eca

lcul

ated

from

PA

FW

Gat

ing

prec

isio

n(%

)*M

ean

6(C

L 0.9

9m

s)M

ean

(SD

,m

s)B

eats

used

Mea

n(S

D,

ms)

Bea

tsus

ed

NI

2h

T1

2005

.790

2010

0.00

192

(011

)94

.22

305

(021

)19

307

(005

)19

03.6

2T

220

08.8

5023

086.

9621

1(0

10)

95.2

939

7(0

14)

1740

0(0

02)

2202

.49

21

T1

2005

.480

2010

0.00

189

(014

)92

.67

288

(027

)19

285

(013

)19

04.8

4T

220

07.6

4021

095.

2418

4(0

19)

89.7

038

3(0

44)

1838

0(0

02)

2004

.98

MI

2h

T1

2007

.000

2408

3.33

198

(009

)95

.56

304

(018

)17

305

(007

)23

02.8

8T

220

08.3

2022

090.

9118

7(0

11)

93.9

839

6(0

31)

1839

6(0

05)

2102

.83

21

T1

2007

.620

2109

5.24

307

(022

)92

.78

378

(035

)18

378

(004

)20

05.8

7T

220

08.2

0023

086.

9605

4(0

44)

19.2

638

4(0

83)

1737

8(0

04)

2211

.64

SI

2h

T1

2008

.170

2807

1.43

177

(019

)89

.48

308

(040

)14

304

(010

)27

06.1

5T

220

25.7

0068

029.

4121

4(0

48)

77.8

033

5(0

65)

1138

1(0

09)

6712

.48

21

T1

ncN

otob

tain

able

with

in30

sT

2nc

Not

obta

inab

lew

ithin

30s

*One

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ces.

UA

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alar

teria

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pler

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efor

m;

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TS5

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iac

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alco

nver

ted

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W5pu

lmon

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oww

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orm

;sc

anni

ngdu

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n5

23

20C

CT

Stim

e(s

eete

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trig

gerin

gac

cura

cy5se

eeq

n(1

);C

L 0.9

95

99%

confi

denc

ele

vel,

see

eqn

(2);

SD

5st

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viat

ion;

gatin

gpr

ecis

ion5

see

eqn

(3);

NI,

MI,

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oder

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15

high

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axim

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ty;

T1,

T25

twin

1,tw

in2;

nc5

not

conv

ertib

le.


bpm in this study) was much faster than the human one(100 to 180 bpm with 4-D acquisition; Deng et al. 2000,2001), shortening each cardiac cycle.

In addition, 4-D systems usually use one cardiaccycle to do two jobs: acquiring a set of frames at onesection and then moving the transducer to the next sec-tion. Because the trans-section movement (either manu-ally or by step-motor) takes time (although only severalms), better timing can be achieved by making acquisitionand movement in alternate cycles.

From the considerations discussed in the last twoparagraphs, we generally double the lamb’s 20 CCTStime to get an equivalent human scanning duration. Thelongest duration in the NI and MI groups would, then, beequivalent to about 20 s (23 8.85 s). Because the fetuscan usually stay immobile in such a short period, theseresults can be considered acceptable for a clinical 4-Dscan (if the triggering accuracy and gating precision arealso satisfactory; see below). The 20 CCTS time in SI-hgroup could be as long as 50 s (23 25.70 s) in anequivalent human study, which was obviously not clin-ically acceptable.

Triggering accuracy and gating precisionAmong samples from the NI-h, NI-l and MI-h

groups, the triggering accuracy and gating precisionranged from 89.70% to 95.56% and from 2.49% to

4.98%, respectively. However, in the MI-l group, thetriggering accuracy was strikingly different between twin1 (92.78%) and twin 2 (19.26%) although they hadalmost identical heart rate patterns (mean cardiac cy-cle 6 SD were both 3786 4 ms (Table 1) withp 50.884 (Table 2)). The difference in the gating precisionwas less striking, but still considerable (5.87% vs.11.64%).

In the SI-h group, the triggering accuracy was fairlyhigh (77.80 to 89.48%), but the gating precision waspoor (6.15 to 12.48%). Expectedly, the worst resultscame from the SI-l group with no conversion takingplace within 30 s.

Throughout the entire cycle, the movement of ven-tricular walls (including septum) are relatively gradualand smooth, but cardiac valves change their positionsrapidly and sharply during some phasic changes (Fig. 5).For example, it only takes about 5.0% to 7.5% of acardiac cycle for the atrioventricular valves to open/close(when the cardiac phase changes from isovolumetricdiastole/to isovolumetric systole. Therefore, if the gatingprecision were numerically greater than 5%, the rapidvalvular opening/closure might be missed on 4-D acqui-sition. We, then, chose 5% as cut-off point for whether agating precision was acceptable for 4-D acquisition.

Taking the mean cardiac cycle of 351 ms as anexample, a 5% precision would give only a range of6

Fig. 4. (a), (b) PAFWs and CCTSs from NI-l, T2 (a sample from twin 2 with not-noticeable interference and low flow).Only one PAFW, the first in (b), expanded from (a), fails to be converted into CCTSs, resulting in very high conversionefficiency (95.24%, 20 of 21, Table 1). Although time lags 1 and 2 in (b) differ obviously, the overall time lags in (a)are very close to each other, giving high triggering accuracy (89.70%). (c) (continued on the next page) PAFWs andCCTSs from MI-l, T2 (from twin 2 with moderate interference and low flow). Although CCTSs are converted from mostcardiac cycles, the time lags (e.g.,TL1, TL2, and TL3) vary too much with TL35 0 (thicker grey line), resulting in

relatively high conversion efficiency (86.96%), but very low triggering accuracy (19.26%, in Table 1).


17.55 ms (351 ms3 5%) at most about the predictedtriggering points in 99% converted signals (as the 99%confidence interval was chosen in eqn (3)). Hence, theprecision from the NI-h, NI-l and MI-h (all smaller than5%) is acceptable, but that from the MI-l and SI-h groupsis not.

The impact of variables on the conversionIn view of the overall conversion results, it appears

that the UADW flow velocity and interference severity,in particular the latter, were more important factors thanthe heart rate in determining the indices. When the in-terference was severe and the velocity was low (e.g., inthe SI-l group), the conversion was impossible. When theinterference was severe, but the velocity was high, orwhen the interference was moderate, but the velocity waslow, the conversion was unstable (such as the efficiencyin the SI-h group, one high and one low) and unbalanced(such as the efficiency high, but the accuracy low, in theMI-l group). Only when interference was moderate ornot noticeable were all conversion indices satisfactory,no matter whether the velocity was high or low.

The heart rate variation within the range studied hadrelatively less effect on the conversion acceptance. Forinstance, the indices were very satisfactory in all NI andMI-h samples, despite the fact that their heart rates werevery significantly different (cardiac cycles range: 285 to400 ms) with all 15p values but one (0.298) well below0.01 (Table 2). In contrast, there was no difference (p .0.884) in the heart rate pattern between the twin 1 andtwin 2 in the MI-l group, but the accuracy and precisionin twin 2 were far from those in twin 1 (as discussedabove). In fact, the twin 1 accuracy (92.78%) was veryclose to those (93.98% to 95.56%) in the MI-h group.

Although the above conclusion was drawn from thesheep fetal heart rate range of 150 to 211 bpm, webelieve that, when dealing with the human heart rate of100 to 200 bpm, it would be easier to achieve the sameresults. This is because, at a lower rate (longer cycle,therefore, relatively improved temporal resolution), eachwaveform will contain more information for averaging,helping set up an optimal threshold.

Here, we do not stress the individual differencebetween the twins. The reason is that the interference andthe flow velocity were well controlled during the exper-iment. Although the third variable, heart rate, was lesscontrollable, it did not play an important role in deter-mining the conversion indices. However, we did notmanage to obtain conversion results with irregular heart-beats from all groups due to experimental time con-straint. Obviously, more studies are needed to investigateeffects of individuality and irregularity.

Fig. 5. Cardiac phasic changes and gating precision. Twoidentical cardiac cycles of 420 ms each are shown on theM-mode echocardiogram. LVW and RVW5 left and rightventricular walls; IVS 5 interventricular septum; MV andTV 5 (open) mitral and tricuspid valves. Presume that themean time point for conversion is along black line o-p (in therapid filling phase). The impact of different precision is shownby the horizontal (temporal) and vertical (positional) differ-ences between a1, b1, a2, b2 and the reference o, and betweenc1, d1, c2, d2 and the reference p, respectively. If precisionfrom one sample is 5% (6 21 ms), 99% conversion (because99% confidence interval was used) will take place betweena1-c1 and b1-d1. The MV will be acquired neither earlier thanthe beginning of the rapid filling phase (a1, MV opening point)nor later than the end of the phase (b1, widest MV excursion).But, if the precision from a sample is 10% (6 42 ms), 99%conversion will take place between a2-c2 and b2-d2. The MVmay be registered as early as at the beginning of isovolumetricdiastole (a2, as RVW is about to relax, c2) or as late as in themiddle of the reduced filling phase (b2). Similarly, RVW (andLVW and IVS) registration is affected, although the positionaldifferences are less significant due to relatively smoothermovement of the walls than that of the valves as shown in cycle1. (Arterial valves (not shown here) move even faster and

precision of, 2.5% may need to be considered).

Fig. 4. Continued


Tab

le2.

Mea

nca

rdia

ccy

cles

calc

ulat

edfr

omP

AF

Ws,

t-te

sts

agai

nst

each

othe

rgr

oup

tva

lue

NI

MI

SI

2h

21

2h

21

2h

21

T1

T2

T1

T2

T1

T2

T1

T2

T1

T2

T1

T2

pva

lue

NI

2h

T1

----

278

.154

6.76

12

61.1

621.

056

257

.800

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Initial 4-D reconstruction and further workWith this triggering method, gated 3-D images of

the fetal heart can be obtained from both sheep (Fig. 6)and human studies (Deng et al. 2001).

Improvements to the prototype conversion box arecurrently in progress. One of the major developments hasbeen made in setting the trigger point on the rising edgeof the systolic waveform and compensating automati-cally for Doppler signal polarity. This will assist inensuring that the trigger pulse occurs on the initial slopeof the systolic waveform independent of the polarity(blood flow direction).

The integration of two individual US systems and a4-D acquisition system has also been recommended. Thismay minimise or even eliminate interference, the mainsource of unsatisfactory conversion indices. Integrationmay also be a necessary step in solving other problemsinvolved in developing 4-D fetal echocardiography as aclinical tool (Deng et al. 2001).

CONCLUSION

This study has shown that, even with two nonpur-pose-built US systems, the umbilical arterial Dopplerwaveform can be converted into an efficient, accurateand precise online cardiac gating triggering signals. Un-

satisfactory results arising from samples with severeinterference and/or low flow velocity may be overcomeby integration of these systems and improvements inconversion box design, which should, ideally, be part ofthe integration.

Acknowledgements—The authors are grateful to the following person-nel and groups, for suggestions on instrumentation, Drs. C. F. Ruff, S.Mosse and the Electromedical Unit, Medical Physics Department and,for help with animal experiments, Drs. J. Newman, H. Nishina and theFetal Physiology Group, Obstetrics Department, and the Bio-ScienceUnit. J. Deng was supported by a Wellcome Trust grant (052754/Z/97/Z). Sparks and the Birth Defects Foundation provided the fundingfor the purchase of the VingMed scanner and the TomTec system,respectively. Some equipment was contributed by UCL Friends Pro-gramme and Graduate School.

REFERENCES

Dean DA, Jia CX, Cabreriza SE, D’Alessandro DA, Dickstein ML,Sardo MJ, Chalik N, Spotnitz HM. Validation study of a new transittime ultrasonic flow probe for continuous great vessel measure-ments. ASAIO J 1996;42:M671–M676.

Deng J, Gardener JE, Rodeck CH, Lees WR. Fetal echocardiography inthree and four dimensions. Ultrasound Med Biol 1996;22:979–986.

Deng J, Ruff CF, Linney AD, Lees WR, Hanson MA, Rodeck CH.Interference between two ultrasound transducers firing simulta-neously at the same target: A preparatory study for motion-gatedthree-dimensional fetal echocardiography. Ultrasound Med Biol2000;26(6):1021–1032.

Deng J, Yates R, Birkett AG, Ruff CF, Linney AD, Lees WR, HansonMA, Rodeck CH. Online motion-gated dynamic three-dimensionalechocardiography in the fetus—preliminary results. UltrasoundMed Biol 2001;27:43–50.

Giussani DA, Spencer JAD, Moore PJ, Bennet L, Hanson MA. Affer-ent and efferent components of the cardiovascular reflex responsesto acute hypoxia in term fetal sheep. J Physiol 1993;461:431–449.

Kwon J, Shaffer E, Shandas R, et al. Acquisition of three-dimensionalfetal echocardiograms using external trigger source. Am J SocEchocardiogr 1996;9:389.

Nelson TR, Pretorius DH, Sklansky M, Hagen-Ansert S. Three-dimen-sional echocardiographic evaluation of fetal heart anatomy andfunction: Acquisition, analysis and display. J Ultrasound Med1996;15:1–9.

Schwartz SL, Cao QL, Azevedo J, Pandian NG. Simulation of intra-operative visualization of cardiac structures and study of dynamicsurgical anatomy with real-time three-dimensional echocardiogra-phy. Am J Cardiol 1994;73:501–507.

Sklansky MS, Nelson T, Strachan M, Pretorius D. Real-time three-dimensional fetal echocardiography: Initial feasibility study. J Ul-trasound Med 1999;18:745–752.

Vogel M, Ho SY, Buhlmeyer K, Anderson RH. Assessment of con-genital heart defects by dynamic three-dimensional echocardiogra-phy: Methods of data acquisition and clinical potential. Acta Pae-diatr 1995;(Suppl. 410):34–39.

Wong DH, Watson T, Gordon I, Wesley R, Tremper KK, Zaccari J,Stemmer P. Comparison of changes in transit time ultrasound,esophageal Doppler, and thermodilution cardiac output afterchanges in preload, afterload, and contractility in pigs. AnesthAnalg 1991;72:584–588.

Fig. 6. Motion-gated 3-D images of a sheep fetal heart (120days). The heart is sectioned into halves at the level of 4-cham-ber view by “computer surgery”. Left: lower half; right: upperhalf. RV and LV5 right and left ventricular chambers; RA andLA 5 right and left atrial chambers; IAS and IVS5 interatrialand interventricular septa; AO Root5 aortic root; DAO 5descending aorta; TV5 tricuspid valves. (4-D movies avail-able on our web site: www.medphys.ucl.ac.uk/mgi/jdeng/ un-der Fetal Heart entry. Data acquired and reconstructed with an

Echo 4-D Scan, TomTec).


conversion of umbilical arterial doppler waveforms to cardiac cycle triggering signals: a...

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