BrHeart3' 1994;71:572-578

TECHNOLOGY

Intravascular ultrasound imaging ofangiographically normal coronary arteries: a

prospective study in vivo

Junbo Ge, Raimund Erbel, Thomas Gerber, Gunter Gorge, Lothar Koch,Michael Haude, Jurgen Meyer

Department ofCardiology,University of Essen,GermanyJ GeR ErbelG GorgeM HaudeII Medical Clinic,University ofMainz,GermanyT GerberL KochJ MeyerCorrespondence to:Dr J Ge, Department ofCardiology, University ofEssen Hufelandstrasse 55,4300 Essen, Germany.Printed in part at the ThirdNational Scientific Sessionof the Academic Society ofEchocardiography, Boston,June 16, 1992.

Accepted for publication11 October 1993

AbstractIntravascular ultrasound imaging (IVUS)was performed to elucidate the discrep-ancy between clinical history and angio-graphic findings and to measure thediameter and area of the lumen of thenormal left coronary artery in 55 patientswho presented with chest pain but hadnormal coronary angiograms. The leftcoronary artery (LCA) was scanned witha 4-8F, 20 MHz mechanically rotatedultrasound catheter at 413 sites. Athero-sclerotic lesions were identified at 72(17%) sites in 25 patients. The mean (SD)(range) plaque area was 5'55 (3-56) mm2(2-26 mm2) and it occupied 28-8 (9'6)%(13-70%) of the coronary cross sectionalarea. Calcification was detected at 24(33%) atherosclerotic sites in ninepatients. The correlation coefficients forthe lumen dimensions measured at nor-mal sites by IVUS and by angiographywere r = 0-93 (SEE = 0-43) mm for lumendiameter and r = 0'89 (SEE = 4.27) mm2for lumen area (both p < 0.001). 16 of the30 patients in whom no atheroscleroticplaques were detected in the LCA lumenby IVUS had no risk factors of coronaryartery disease. The cross sectional areaof 90 consecutive images of left maincoronary artery (LMCA), proximal leftanterior descending coronary artery(proximal LAD), and mid LAD was mea-sured in these 16 subjects. The mean(SEM) areas at end diastole were LMCA17'33 (7.98) mm2; proximal LAD 13'56(5-85) mm2; mid LAD 9-75 (4.67) mm2.During the cardiac cycle the cross sec-tional area changed by 10-2 (4-0)% in theLMCA, by 8'3 (4-7)% in the proximalLAD, and by 9'8 (4.0)% in the mid LAD.In 11 patients with plaques the change incross sectional area in plaque segments(5-8(3-1)%) was significantly lower thanin the segments from patients withoutplaques (p < 0.001). Lumen area reacheda maximum in early diastole rather thanin late diastole.IVUS can image atherosclerotic lesions

that are angiographically silent; it alsoprovides detailed information aboutplaque characteristics. The variation incoronary cross sectional area duringthe cardiac cycle should not be ignored

during quantitative analysis. Maximumdimensions in normal segments arereached in early diastole. Further studiesare needed to clarify the clinical signifi-cance of atherosclerosis detected byIVUS in patients presenting with chestpain but normal coronary angiography.

(Br HeartJY 1994;71:572-578)

Cardiologists have relied on contract coronaryangiography to evaluate vascular morphologyand architecture in patients with coronaryheart disease. But angiography can onlydefine the contour of the vessel lumen andmajor structural alterations or abnormalitiesof the vessels.'-3 Even with biplane imaging,angiography gives little information on arterialwall thickness and the three dimensional con-figuration of the vessel lumen. Furthermore,discrepancies between cineangiographic andpostmortem findings are considerable, show-ing that complex and eccentric coronaryatherosclerotic lesions are often not identifiedby angiography.45

There has been considerable interest inintravascular ultrasound imaging (IVUS), acatheter-based ultrasound technique, becauseit offers in vivo information on vascularanatomy, physiology, and pathology that hasnot been available until now.6'0 Preliminaryin vitro and in vivo studies showed that IVUSwas a safe, feasible, and accurate method forevaluating vascular morphology. 11-14 Cor-relations for assessing vessel wall thickness,lumen area, lumen diameter, and perimeter invitro were excellent,"-'4 but results in vivoremain controversial.9-" 15

The purposes of this study were: to com-pare IVUS with coronary angiography in theevaluation of the morphology of the left coro-nary artery (LCA); to assess the accuracy andfeasibility of IVUS for measuring the dimen-sions of angiographically normal coronaryarteries; to determine the dimensions of thenormal LCA; and to study changes in thedimensions of the normal LCA during thecardiac cycle.

Patients and methodsPATIENTSWe studied 55 consecutive patients (28 men

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Intravascular ultrasound imaging of angiographically normal coronary arteries: a prospective study in vivo

and 27 women aged (mean (SD)) 42-70 years(55 9 (7 2)) who underwent diagnostic coro-nary angiography for suspected ischaemicheart disease and were found to have angio-graphically normal coronary arteries. Eachgave their informed consent to intracoronaryultrasound imaging. After coronary angio-graphy each patient was given 3000 IU ofintracoronary heparin. An 8F or a 9F guidingcatheter was used to position the ultrasoundcatheter for IVUS imaging. A 0-014 inchguidewire was placed in the left anteriordescending coronary artery (LAD) and weadvanced the imaging catheter over the wireusing a monorail technique to obtain coaxialimages. The IVUS catheter was then drawnback slowly while the images were recorded inreal time and stop frames (every 3-5 mm).The position of the IVUS probe was recordedon an x ray film at the points at which the stopframe IVUS images were taken.

INTRAVASCULAR ULTRASOUND DEVICEWe used an IVUS catheter with a 4-8Fcatheter sheath and a 20 MHz ultrasoundtransducer inside the catheter (Sonicath,Boston Scientific Corporation, Watertown,MA, USA). The transducer was mechanicallyrotated within the catheter at 900 rpm toprovide cross sectional images on a 512 x 512pixel ultrasound diagnostic imaging console(Diasonics, Milpitas, CA, USA). A simulta-neous electrocardiogram was recorded belowthe IVUS images. The axial resolution of thecatheter is about 150 ,um and the lateral reso-lution is about 300 um. The images wererecorded on half inch S-VHS videotape foroff-line analysis.

IVUS IMAGE ANALYSISAll the IVUS images were digitised (32 framesin series) into a 512 x 512-pixel matrix by animage processing computer (ECHO-COM,PPG Hellige, Freiburg, Germany). The imageswere stored on a one gigabyte erasable opticaldisk (Maxtor, The Netherlands). Theseimages were reviewed to identify the bestframe, which was magnified by the built-inzoom function to measure the lumen diameterand cross sectional area.

Sixteen of the patients we examined, wereregarded as truly normal subjects (see results).Ninety consecutive IVUS images of theLMCA, of the proximal LAD, and of the midLAD were digitised onto a 256 x 256 pixelformat. A semi-automatic software programbased on PV-wave (Precision Visuals,Boulder, CO, USA) was developed for theevaluation of the luminal area. Digitisedimages were evaluated on a SUN SPARC-2workstation (Mountain View, CA, USA). Theprogram shifted frames to compensate forcatheter movement caused by cardiac motion.A region of interest including both the lumenand the vessel wall was defined interactively.Both the vessel wall and lumen were seg-mented by thresholding. After segmentationthe contours of the lumen were superimposedon the original images. If there was a mis-match, for example, caused by signal dropout,

we repeated the procedure after correcting thethreshold or region of interest. The area of thesegmented lumen was measured after theremoval of the catheter and guidewire arte-fact. We displayed the pulsatile variation ofthe cross sectional area of the lumen over atleast three cardiac cycles. For each cardiaccycle we measured the maximum area, mini-mum area, and ECG-triggered end diastolicarea. The IVUS images of the plaque segmentin 11 patients were suitable for analysis of thecross sectional area. These values were com-pared with those of the normal segments in 16controls.

ANGIOGRAPHIC MEASUREMENTThe cineangiogram was projected onto a20 x 28 cm screen by an angiogram projectionsystem (CAB-35B, Weinberger, Zurich,Switzerland). The LAD was magnified anddrawn. The positions at which the IVUSimages had been taken were then superim-posed on the drawing. The diameter of thecontrast free guiding catheter (8F = 2-67 mm,9F = 3 mm) was used for calibration; thediameter of the vessel was measured by animage processing computer (Kontron,Cardio-500, Germany). The luminal area wascalculated as:

A = 1/4.7r.D2where A is lumen area and D is lumendiameter.

STATISTICAL ANALYSISThe values of lumen dimensions were pre-sented as mean (1 SD). We used linear regres-sion and correlation coefficients to examinethe relation between the lumen dimensionsmeasured on the IVUS images and thosemeasured by angiography. We also calculatedinter and intra observer variation for 30 IVUSimages. We used the Paired t test to evaluatethe variation in the cross sectional area duringthe cardiac cycle.

ResultsMORPHOLOGICAL OBSERVATIONSWe examined 413 sites in 55 patients. IVUSimages of normal coronary segment showed acircular or eliptical lumen with a smooth sur-face (fig 1). We identified atheroscleroticplaques at 72 (17%) sites in 25 (45%)patients. The plaque area was 5-55(3.56)mm2 (2-26 mm2), and it occupied 28-8(9 6)% (13-70%) of the coronary cross sec-tional area. Calcification was detected at 24(33%) sites in nine patients, (fig 2). Most ofthe plaques (84%) were eccentric (fig 3).None of the patients had obvious stenoses onthe coronary angiograms but in one patientwith typical angina and a positive exerciseECG, pressure at the catheter tip droppedwhen it was placed in the left main coronaryostium. There were no obvious abnormalitieson angiograms in any projection. IVUS wasperformed immediately after the angiographyin examination during which ischaemic ECGchanges occurred. IVUS showed a large

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Figure 1 Angiogram and IVUS images at corresponding sites ofLCA. No abnormalitieswere shown by either angiography or IVUS.

plaque in the ostium of the left main coronaryartery (fig 4). This finding and the abnormalexercise ECG prompted bypass surgery in thispatient.

CORRELATION BETWEEN DIMENSIONSMEASURED BY ANGIOGRAPHY AND BY IVUSFor lumen diameter the intraobserver correla-tion for 30 IVUS images was r = 0-99 (SEE

Figure 2 IVUS images ofeccentric plaques showingcalcified deposits as brightechoes with acoustic

shadowing (arrows).

0<16 mm) and the interobserver correlationwas r = 0 99 (SEE 0<17 mm). For lumencross sectional area the intraobserver correla-tion was r = 0 99 (SEE 1-39 mm2) and inter-observer correlation was r = 0-98 (SEE 1 81mm2).The lumen diameter measured at 341

normalities was 2-7-7-6 mm by IVUS and2-6-7-3 mm by angiography. Linear regres-sion of the lumen dimensions measured at100 randomly selected sites showed a closecorrelation between IVUS and angiographyfor both lumen diameter and area (fig 5Aand B).

NORMAL VALUES OF THE LEFT CORONARYARTERYSixteen (8 men and 8 women, aged 42-68(mean (SD) 53 6(7-4)) of the 30 patients inwhom no plaques in the LCA were detectedby IVUS and who had no risk factors forcoronary artery disease including systemichypertension, cigarette smoking, hyperchol-esterolaemia, and diabetes mellitus weredeemed to be truly normal subjects after theirnegative IVUS examination. Table 1 lists theclinical characteristics of the patients. Thecross sectional area of the lumen in group I,who were regarded as normal subjects, wasmeasured frame-by-frame for three to five car-diac cycles. The mean (SD) cross sectionalareas of LMCA, proximal LAD, and midLAD at end diastole (at the beginning ofQRS complex) were 17-33 (7-98) mm2

Table 1 Clinical characteristics of the study population

Group I Group II Group IIICharacteristic (n = 16) (n = 14) (n = 25)Age (yrs) (mean (SD)) 53 6 (7 4) 55-9 (8-0) 58 8 (6 6)Men/women 8/8 4/10 16/9Positive rest ECG 2/16 12/14 17/25Positive stress ECG 7/16 9/13 12/19Positive thallum test 6/11 2/4 9/11

Group I = No plaques detected by IVUS and no risk factorsfor coronary artery disease. Group II = No plaques detectedby IVUS; patients had risk factors for coronary artery disease.Group III = Plaques detected by IVUS.

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Figure 3 Comparison ofan angiogram, showing noabnormality, and IVUSimages showing eccentricplaques in the LMCA(position 5) and mid LAD(position 2) in one patient.

Figure 4 In this 50 year old patient the angiogram was normal but IVUS showed a large eccentric plaque in the ostium (position 3 and 4).

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8 A (range 9d12-30-71), 13-56 (5 85)mm2 (range6 42-20 05), and 9-75 (4-67)mm2 (range

r=0-930*6* 4-53-22-01). The maximum areas and the

6 SEE =0.43 mm end diastolic areas were significantly differentp < 0.001 (table 2).

PUSATILE VARIATION IN THE CROSS SECTIONAL4_ , / *.AREA OF THE LUMEN

v') The pulsatile variations during the cardiac>. cycle of the normal patients were 10-2 (4 0)%

in LMCA, 8-3(4 7)% in the proximal LAD,2-and 9-8 (4 0)% in mid LAD. Figure 6 showsthe changes in cross sectional area on 90 con-secutive frames of 3 to 5 cardiac cycles in the

o-| § , ,LMCA, the proximal LAD, and the mid-0 2 4 6 8 LAD. The pulsatile variation in 11 patients

Angio (mm) with plaques (5-8 (3-1)%) was much lowerthan in segments in the normal group (p <

50 B 0001) (fig 7).

y=0.85x++3.640- r=0.89 2DiscussionSEE = 4.27 mm2 Davidson et al reported that IVUS might be

P< 0 001 useful in demonstrating angiographically

E 30 "silent" plaques." Davies et al used IVUS toE show a left main stem stenosis in a patientvn * .:who had chest pain at rest but no angio-> 20- * .- * * * graphic evidence of stenosis.16 The case

shown in fig 4 resembles the case reported byDavies et al. We found atherosclerotic plaques

10- / *:in the LCA at 17% of the angiographicallynormal sites in 46% of our patients. These

o- results suggest that IVUS can provide evi-lo10 20 30 40 50 dence of coronary atherosclerosis in patients

with symptoms who have normal or inconsis-Angio (mm ) tent angiographic findings. None the less, a

Figure 5 Relation between angiographic (Angio) and ultrasonic measurements (IVUS) cause and effect relation cannot be assumed.of lumen dimensions at normal sites. (A) Lumen diameter; (B) lumen area. Such a relation is more likely, however, when

there is corroborating functional evidence ofischaemia from stress echocardiography andthallium scintigraphy or evidence of impairedcoronary flow reserve. Moreover, IVUS givesinformation about the plaque characteristics

Figure 6 Pulsatile 14 IFvariation in the crosssectional area of theLMCA (A), proximalLAD (B), and of the midLAD (C) in 90consecutive frames.12A

B

E 10

8

60

Frame number

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Intravascular ultrasound imaging of angiographically normal coronary arteries: a prospective study in vivo

14r--- r--T- --LA

12'-

10

114

X End of T waver + Begining of QRS complex

20 40

12k-

Cl,

10 r

20 40

Figure 7 Comparison of the pusatile variation of the cross sectional area of a normal (A)coronary artery and ofa coronary artery with an eccentric plaque (B).

(concentric, eccentric, calcified). These find-ings might be helpful for patient managementand predicting prognosis.

Preliminary in vitro studies showed a closecorrelation between intravascular ultrasonicand anatomical measurements of vesseldimensions,'2-'4 but results of in vivo studieswere not consistent.9-"1 15 Davidson et alreported that the correlation coefficientsbetween ultrasonographic and digital subtrac-tion angiographic measurements were 097for vessel diameter and 095 for vessel area."Nissen et al and Ge et al reported similarresults.'015 On the other hand, Tobis et alreported a correlation coefficient of 026 forIVUS and angiographic measurements forvessel area.9 In the present study, however,there was a close correlation between IVUSand angiographic measurements for lumendiameter and area at normal sites (r = 093,SEE = 043 mm, p < 0-001; and r = 089,SEE = 4-27 mm2, p < 0-001, respectively).An experimental study in our laboratoryshowed that incorrect programming of theIVUS device can result in misrepresentationof vessel dimensions.'7 The poor correlationbetween IVUS and angiographic measure-

ments in vivo might be caused by (a) difficul-ties in using the catheter tip to calibrate forquantitative angiography; (b) difficulties inmatching the sites measured by IVUS andangiography; (c) assessment of coronarydimensions in different phases of the cardiaccycle; and (d) errors resulting from incorrectsoftware calibration of the IVUS device.The dimensions of normal human coronary

arteries have been measured in several post-mortem studies; some of the measurementswere obtained from sections made using pres-sure perfusion techniques.1820 However, post-mortem measurements may not accuratelyreflect coronary dimensions during the car-diac cycle.20 Data on the normal values ofcoronary arteries in living subjects are limitedand all are based on coronary angiography.3-2'Area determination by quantitative angiogra-phy does not take account of the markedluminal irregularities sometimes found in dis-eased coronary arteries. We found that 17%of the angiographically normal sites in coro-nary arteries were not really normal.Moreover, the lumen dimensions varied dur-ing the cardiac cycle. We suggest that thedimensions obtained by IVUS are more accu-rate than those measured by angiography andpostmortem studies. We showed that thecross sectional area of the lumen reached amaximum in early diastole. This accords withstudies of the circumflex coronary artery inanaesthetised dogs.24 The change in the crosssectional area of the lumen of normal coro-nary arteries during the cardiac cycle was 102(4 0)% in the LMCA, 8&3 (4 7)% in the prox-imal LAD, and 9-8 (4 0)% in the mid LAD.This change in area was much less in seg-ments containing plaques in 11 patients withatherosclerosis. IVUS detected atherosclerosisin almost half of the patients in our study withnormal angiograms. The clinical implicationof these results remains to be clarified by long-term clinical follow up. Repeat catherisationand IVUS may well provide important patho-physiological and clinical information inpatients with evidence of continuing coronaryischaemla.We also measured the pulsatile variation in

cross sectional area during the cardiac cycle.We established that this variation is too largeto be ignored when these dimensions are mea-sured. This finding shows that the pulsatilevariation in dimensions must be taken intoaccount when measurements derived from theIVUS image are compared with other imagingmethods. Moreover, the pulsatile variation ofthe cross sectional area may offer a new indexin evaluating the vascular function.

Table 2 Cross sectional areas ofnormal LCA during cardiac cycle (mean (SD))

Maximum Minimum End diastole Pulsation(mm2) (mm2) (mm2) (0/%)

LMCA 18-33 (8-18) 16-49 (6.77)* 17-33 (7.98)* 10-2 (4 0)Range 9-74-36-02 8-83-29-95 9-12-30-71

Proximal LAD 14-01 (5 54) 12-95 (5 08)* 13-56 (5 85)t 8.3 (4 7)Range 6-91-26-21 6-34-23 61 6 42-26-10

Mid LAD 10-70 (4 47) 9-78 (4 24)* 9-75 (4.67)* 9-8 (4 0)Range 4-78-22-74 4-49-21-47 4-53-22-01

*p < 0-001; tO0005. LMCA, left main coronary artery; LCA, left coronary artery.

LIMITATIONS OF THE STUDYWith IVUS, as with transthoracic M mode orcross sectional echocardiography, the cardiaccycle causes the portion of the transducer onthe chest wall to move in relation to the heart.We found that although the position of theIVUS catheter was fixed by the "Y" adaptorand guidewire, its position in relation to thevessel wall probably changed during thecardiac cycle. Thus the pulsatile variation of

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the vessel lumen may be affected by a shift inthe frame caused by cardiac events.Furthermore, long term follow up study willbe needed to analyse the clinical significanceof the plaques detected by ultrasound. Weused a 20 MHz transducer. New studiesshowed that transducers with higher fre-quency may improve the resolution25 and mayprovide further insights into the atheroscle-rotic process.

We thank Mrs Ruth Katz, Dr Thomas Roth, and Dr PeterKearney for help with preparing this paper.

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