CLINICAL STUDIES
Serial volumetric (three-dimensional) intravascular ultrasound analysis of restenosis after directional coronary atherectomy
Evelyn A. de Vrey, MD*,
Gary S. Mintz, MD, FACC*,
Clemens von Birgelen, MD ,
Takeshi Kimura, MD, FACC ,
Masakiyo Noboyoshi, MD, FACC,
Jeffrey J. Popma, MD, FACC*,
Patrick W. Serruys, MD, PhD, FACC and
Martin B. Leon, MD, FACC*
* Intravascular Ultrasound Imaging and Cardiac Catheterization Laboratories, Washington Hospital Center, Washington, DC, USA
Kokura Memorial Hospital, Kitakyushu, Japan
Thoraxcenter, University Hospital Rotterdam Dijkzigt, The Netherlands
Manuscript received May 20, 1998;
revised manuscript received July 17, 1998,
accepted August 6, 1998.
Address for correspondence: Dr. Martin B. Leon, Director of Research and Education, Cardiology Research Foundation, 110 Irving Street NW, Suite 4B1, Washington, DC 20010
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Abstract
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Objectives. We report the use of three-dimensional (volumetric) intravascular ultrasound (IVUS) analysis to assess serial changes after directional coronary atherectomy (DCA).
Background. Recent serial planar IVUS studies have described a decrease in external elastic membrane (EEM) area following catheter-based intervention as an important mechanism of late lumen renarrowing.
Methods. Thirty-one patients with de novo native coronary lesions treated with DCA in the Serial Ultrasound Restenosis (SURE) Trial and in Optimal Atherectomy Restenosis Study (OARS) were enrolled in this study. Serial IVUS was performed before and after intervention and at 6 months’ follow-up. In a subgroup of 18 patients from the SURE trial, IVUS was also performed at 24 h and at 1 month postintervention. Segments, 20-mm-long (200 image slices), were analyzed using a previously validated three-dimensional, computerized, automated edge-detection algorithm. The EEM, lumen, and plaque+media (P+M = EEM–lumen) volumes were calculated.
Results. At follow-up, lumen volume was smaller than at postintervention (159 ± 69 mm3 vs. 179 ± 49 mm3, p = 0.0003). From postintervention to follow-up, there was a decrease in EEM volume (377 ± 107 to 352 ± 125 mm3, p < 0.0001), but no change in P+M volume (p = 0.52). The  lumen volume correlated strongly with EEM volume (r = 0.842, p < 0.0001), but not with P+M volume. In the 18 patients from the SURE Trial, the decrease in lumen and EEM volumes occurred late, between 1 month and 6 months of follow-up.
Conclusions. Volumetric IVUS analysis demonstrated that late lumen volume loss following DCA was a result of a decrease in EEM volume. This was a late event, occurring between 1 and 6 months’ postintervention.
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Abbreviations and Acronyms
| | CSA | = cross-sectional area | | DCA | = directional coronary atherectomy | | DS | = diameter stenosis | | EEM | = external elastic membrane | | IVUS | = intravascular ultrasound | | MLD | = minimum lumen diameter | | P+M | = plaque+media | | QCA | = quantitative coronary angiography |
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Restenosis remains the major problem after catheter-based interventions such as directional coronary atherectomy (DCA) (1,2). Animal models, human necropsy studies, and analyses of retrieved atherectomy specimens originally suggested that an exaggeration of the normal reparative processes following angioplasty-induced local vessel trauma leads to uncontrolled smooth muscle cell proliferation and restenosis (3–6). Conversely, recent studies using histopathologic analysis in animals and planar intravascular ultrasound (IVUS) imaging in humans have suggested that a decrease in arterial area may be more important than neointimal hyperplasia as a mechanism of restenosis (7–14).
Using IVUS allows transmural, tomographic imaging of coronary arteries in humans in vivo, providing insights into the pathology of coronary artery disease by defining vessel wall geometry and major components of the atherosclerotic plaque. However, when performing natural history studies using cross-sectional area (CSA) IVUS analysis, subjective decisions regarding image slice selection may bias the results (5,6,14). Also, previous studies assessing long-term effects of catheter-based procedures typically included patients treated with different techniques (5,6). In the current study we used independent serial volumetric IVUS analysis to determine the relative contributions of changes in plaque and arterial volumes to late lumen loss after DCA. Volumetric analysis was performed with a previously validated and published automated, three-dimensional (volumetric) contour detection system (15,16).
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Methods
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Patient population.
Serial volumetric IVUS was used to study de novo native coronary lesions in 31 patients (one lesion per patient) treated with successful directional coronary atherectomy (DCA). Patients were selected from two serial atherectomy studies. The Optimal Atherectomy Restenosis Study (OARS) enrolled a total of 200 patients and included serial IVUS pre-DCA, post-DCA, postadjunct percutaneous transluminal coronary angioplasty (PTCA) and at follow-up. The purpose of the IVUS substudy of OARS was to 1) measure the contribution of tissue removal to lumen improvement after DCA and after adjunct PTCA, 2) determine the optimum IVUS and angiographic end points of DCA, and 3) assess the mechanism of restenosis after DCA. The results of OARS and its IVUS substudy have been published (14,17). The Serial Ultrasound Restenosis (SURE) Trial enrolled 61 patients treated with DCA or PTCA and included serial IVUS preintervention, postintervention, 24 h postintervention, 1 month postintervention, and 6 months’ postintervention. Its purpose was to 1) confirm the existence and 2) define the time course of remodeling (change in arterial dimensions) after DCA or PTCA. The results of SURE have also been published (8).
The patient cohort in the current study was selected after examining the IVUS and angiographic studies from OARS and SURE to identify all of the patients who met the following criteria: 1) successful DCA, 2) high-quality IVUS images with limited target lesion calcium throughout the length of the lesion, 3) angiographically noncurved target segments, 4) nonostial lesion location, and 5) the same 20-mm-long segment of artery imaged at each time point with at least 5 mm of vessel proximal and distal to the minimum lumen area preintervention, postintervention, and at follow-up. The threshold for excluding cases based on excessive calcification was one quadrant (90°) of uninterrupted calcium over a length of  5mm. Because volumetric IVUS analysis was not a planned part of either trial, most patients were excluded from the current analysis because of the last criteria. There were 25 men and 6 women (mean age 61 ± 9 years); lesion location was left anterior descending artery in 20, left circumflex artery in 4, and right anterior descending coronary artery in 7 patients.
The DCA procedure was performed according to standard protocols using an average number of 27.0 ± 9.6 cuts. In three patients predilatation was performed before DCA (but after the initial IVUS run). Adjunct balloon angioplasty was performed in 21 patients (68%) at a maximal pressure of 11.9 ± 4.9 atm using a mean balloon size of 3.7 ± 0.5 mm; this was then followed by the final IVUS run. Patient and lesion characteristics of the subgroup of 31 individuals showed no significant difference compared to the complete study population. All patients gave written informed consent as approved by the local Medical Ethics committees.
Quantitative coronary angiographic (QCA) analysis.
All cineangiograms were analyzed by an independent core angiographic laboratory using a quantitative automated edge detection algorithm (ARTREK, Quantitative Cardiac Systems). The outer diameter of the contrast-filled catheter was used for calibration. The minimum lumen diameter (MLD), reference diameter, and percent diameter stenosis (DS) were measured from multiple projections, and the results from the "worst view" were recorded.
IVUS imaging protocol.
All IVUS imaging runs were performed after administration of 100 to 200 mcg intracoronary nitroglycerin. The imaging system used incorporated a single-element, 30-MHz beveled transducer within either a 2.9F-long monorail catheter (having a common distal lumen design that accommodated the guide wire or transducer, but not both) or a 3.2F short monorail catheter (CardioVascular Imaging Systems). The imaging catheter was advanced beyond the target lesion, and the transducer was withdrawn slowly within the stationary imaging sheath using a motorized pull-back device at a constant speed of 0.5 mm/s. The accuracy of this motorized transducer pullback device has been validated in vivo (18). All IVUS studies were recorded on 0.5 in. (1.27 cm) high-resolution s-VHS tape for off-line analysis.
Quantitative computerized IVUS analysis.
All IVUS tapes from OARS and SURE were submitted for independent analysis. This individual was experienced in the use of the computerized three-dimensional analysis system and was blinded to the angiographic and clinical results of both trials as well as to the previously published planar IVUS findings. This individual was responsible for screening the IVUS studies to exclude those that did not meet all inclusion criteria (see preceding discussion). All segments analyzed had a length of 20 mm (equivalent to 40 s of videotape at a pullback speed of 0.5 mm/s). The segments analyzed included the minimal lumen area at preintervention, postintervention, and follow-up with at least 5 mm of vessel proximal and distal. By using one or more reproducible axial landmarks (for example, the aorto-ostial junction, large proximal and/or distal side branches, or unusually shaped calcium deposits) and a known pullback speed, identical 20-mm-long segments on serial studies could be identified for comparative analysis. If necessary, the serial studies were analyzed side-by-side to ensure that the same 20-mm-long segments were measured.
The IVUS image analysis has been validated previously (19–22). The external elastic membrane (EEM) is a reproducible measure of the arterial boundary and was identified by the leading edge between the hypoechoic media and the hyperechoic adventitia. Because media thickness cannot be measured accurately, plaque+media (P+M) was used as a measurement of atherosclerotic plaque (23).
The IVUS analysis was performed off-line using a previously validated computerized, quantitative analysis system developed at the Thoraxcenter Rotterdam (15,16). Validation of this algorithm has been reported both 1) in a tubular phantom and 2) within atherosclerotic human coronary artery segments studied in vitro in which there were good correlations with the corresponding histologic sections (r = 0.94, 0.88 and 0.80 for areas and 0.98, 0.91 and 0.83 for volumes, respectively) and with manual tracing of IVUS images (r = 0.97 for CSA and 0.99 for volume) and showed low mean differences of ±3.7%. In vivo, there were high intraobserver and interobserver correlations for both area measurements (r = 0.95 to 0.98) and volume measurements (r = 0.99) with small mean differences (–0.87% to 1.08%). Standard deviations for lumen, P+M and arterial measurements did not exceed 7.3%, 10.8% and 4.4% (CSA) and 2.7%, 2.8% and 0.7% (volume), respectively.
Using this algorithm, 200 IVUS images were digitized from the videotapes at a frame rate of 10/s. The reconstructed segment length (20 mm) was thus defined by the speed of the motorized transducer pullback device during image acquisition (0.5 mm/s) and by the digitization frame rate (10/s).
The automated processing technique was based on the interaction between longitudinal and cross-sectional contours. Longitudinal tracings of the lumen and media-adventitia interfaces facilitated automated contour detection on the cross-sectional IVUS images. To perform the analysis, the automated boundary detection algorithm first approximated the lumen-intima and media-adventitia interfaces on all 200 image slices. Two longitudinal planes were then constructed and manually edited. This updated information was used to perform a second automatic contour detection sequence. Subsequently, the set of 200 cross-sectional images was visually checked and again manually edited where necessary. Finally, the volumetric results were computed from the edited cross-sectional image data set (Fig. 1). This procedure has been reported previously in detail (15,16,24,25).
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Figure 1 Schematic drawing of automated contour detection of lumen-tissue and media-adventitia boundaries in a coronary segment. The processing technique is based on the concept that longitudinal contours facilitate the automated contour detection on the cross-sectional intravascular ultrasound (IVUS) images by defining the center and the range of the boundary-searching process. After digitalization of a motorized IVUS video recording, two longitudinal sections (X,Y) are processed. The tissue-lumen and media-adventitia boundaries in these longitudinal sections are automatically traced. Subsequently, this information is used as points to define regions-of-interest in the cross-sectional image to guide a second automatic contour detection process. Finally, the volumetric results are calculated from the contour data of the cross-sectional IVUS images.
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Statistical analysis.
StatView 4.5 was used for the statistical analysis. Categorical variables are presented as frequencies and compared using chi-square statistics. Continuous variables are presented as mean values ± 1 SD. Continuous variables are compared using regression analysis. An ANOVA for repeated measurements was used to evaluate potential differences in volumetric measurements over the time course of the study. Post hoc, paired Student’s t-test with the Bonferroni correction was used to compare paired measurements of EEM, lumen and P+M volumes. A p < 0.05 value was considered to be statistically significant, except for the post hoc comparisons when statistical significant required a p < 0.05/number of comparisons (p < 0.05/3 or p < 0.017 for the overall cohort of 31 patients and p < 0.05/5 or p < 0.01 for the 18 patients studied preintervention, postintervention, at 24 h, at 1 month, and at 6 months) (26,27).
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Results
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Serial angiographic results.
The MLD increased significantly from 1.04 ± 0.30 mm preintervention to 2.61 ± 0.20 mm postintervention (p < 0.0001). The DS decreased from 66 ± 12% to 20 ± 7% (p < 0.0001). At 6 months’ follow-up, the overall MLD had decreased to 1.81 ± 0.82 mm (p < 0.0001) and diameter stenosis increased to 43 ± 25% (p < 0.0001). Eleven lesions (35%) had a diameter stenosis >50% at follow-up.
Serial volumetric IVUS measurements.
The lumen volume increased significantly from 115 ± 44 mm3 preintervention to 175 ± 44 mm3 postintervention (p < 0.0001). Both tissue removal (P+M decrease from 201 ± 85 to 177 ± 77 mm3, p < 0.0001) and vessel expansion (EEM increase from 315 ± 97 to 351 ± 98 mm3, p < 0.0001) contributed to lumen improvement.
At follow-up, mean lumen volume was smaller than at postintervention (145 ± 61 mm3 vs. 175 ± 44 mm3, p = 0.0003). There was a decrease in EEM volume (from 351 ± 98 mm3 to 318 ± 112 mm3, p = 0.0001), but no change in P+M volume (177 ± 77 vs. 174 ± 72 mm3, p = 0.52) (Fig. 2). A single example of the output of the analysis system is shown in Figure 3. Figure 4 shows that the change in lumen volume correlated with the change in EEM volume from postintervention to follow-up (r = 0.842, p < 0.0001), but not with the change in P+M volume (r = 0.244, p = 0.186).
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Figure 2 Plots showing individual IVUS measurements of external elastic membrane (EEM), lumen, and plaque+media (P+M) volumes postintervention (Post) and at follow-up (6 mo FU) in the entire cohort of 31 patients.
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Figure 3 A serial (preintervention, postintervention, and 6-month follow-up) three-dimensional IVUS analysis is shown. Lesion length is plotted on the x-axis. The external elastic membrane (EEM) cross-sectional area over the length of the lesion is shown by the upper line, and the lumen cross-sectional area over the length of the lesion is shown by the lower line, and the plaque+media (P+M) cross-sectional area over the length of the lesion is shown by both the gray area and by the dark black line. Preintervention, the minimum lumen cross-sectional area, is at the 5-mm length marker. In this example, the increase in lumen during the directional atherectomy procedure (DCA) is almost entirely the result of a decrease in P+M cross-sectional area at the center of the lesion. At follow-up, the stenosis recurs with minimum lumen cross-sectional area again located at the 5-mm length marker. There has been a reduction in EEM cross-sectional area over the length of the lesion, but especially from the 0-mm to the 15-mm length marker. There has been no change in P+M cross-sectional area over the length of the lesion. Thus, recurrence was entirely the result of negative arterial remodeling (decrease in EEM).
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Figure 4 In the overall cohort of 31 lesions, the change in lumen volume correlated with the change in EEM volume from postintervention to follow-up (r = 0.842, p < 0.0001), but not with the change in P+M volume (r = 0.244, p = 0.186).
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At follow-up, the EEM volume was less than at preintervention in 17 lesions: in 9/11 restenotic lesions (82%) and in 8/20 nonrestenotic lesions (40%, p = 0.0252). At follow-up, 16/31 lesions had an increase in P+M volume (compared to postintervention). An increase in P+M volume (postintervention to follow-up) was similar in restenotic vs. nonrestenotic lesions (p = 0.3205).
SURE trial substudy.
Table 1 shows the volumetric IVUS results of the 18 patients studied preintervention, postintervention, at 24 h, at 1 month, and at 6 months. Of note, the decrease in lumen and EEM volumes occurred between 1 month and 6 months of follow-up (both p < 0.0001). Lumen volume changes correlated with changes in EEM volume at each interval during the follow-up period (r = 0.903, p < 0.0001), but not with changes in plaque volume (r = 0.246, p = 0.073) (Fig. 5).
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Figure 5 In the 18 lesions from the SURE Trial over the time course of the study (representing three intervals per lesion for a total of 54 observations), the change in lumen volume (from postintervention to 24 h, from 24 h to 1 month, and from 1 to 6 months) correlated with the change in EEM volume (r = 0.903, p < 0.0001), but not with the change in P+M volume (r = 0.246, p = 0.073).
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Discussion
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The current volumetric IVUS analysis substantiated the previously published planar IVUS findings implicating a decrease in arterial dimensions as an important mechanism of restenosis in nonstented lesions. The main results of the current study were 1) changes in lumen volumes after DCA correlated with the changes in EEM volumes, not with the changes in P+M volumes, and 2) late lumen volume loss (and, therefore, restenosis) occurred between 1 month and 6 months’ postintervention when the decrease in EEM volume occurred. No significant increase in plaque volume occurred at any time during this study.
Previously published planar IVUS findings.
The initial report of a decrease in EEM CSA postintervention included 221 nonstented lesions treated with a variety of catheter-based techniques alone or in combination (7). In this initial report, 70% to 75% of late lumen loss was attributed to a decrease in EEM CSA; and the change in lumen CSA correlated more strongly with the change in EEM CSA (r = 0.751) than with the change in P+M CSA (r = 0.284). There appeared to be one exception to these findings: diabetic patients (28). In diabetic patients, there was a trend toward a significant increase in P+M CSA (1.3 ± 2.6 mm2 vs. 0.6 ± 2.5 mm2 in nondiabetic patients, p = 0.0720). This was especially true when just the restenotic lesions were considered: 2.3 ± 2.8 mm2 vs. 0.9 ± 2.5 mms (diabetic vs. nondiabetic patients, p = 0.0191).
The importance of a decrease in EEM CSA was confirmed in the planar IVUS analysis from OARS, a study of lesions treated only using DCA (14). The decrease in EEM CSA was greatest at the lesion site, but also appeared to extend into contiguous reference segments. The decrease in EM CSA at the reference segment correlated with the lesion site (r = 0.608).
In the SURE Trial, 26 patients were treated with DCA, and 35 patients were treated with PTCA. In the planar analysis, there was an early (within 1 month) increase in EEM CSA followed by a late (1 to 6 months) decrease in EEM CSA (8). Because the decrease in EEM CSA was shown to be a late event (and preceded by an increase in EEM CSA), it was shown to be distinct from early passive elastic recoil. Throughout the duration of the SURE Trial, the change in lumen CSA paralleled the changes in EEM CSA (r = 0.789), not the changes in P+M CSA (r = 0.176).
Current analysis.
There was one consistent methodologic limitation in these previous studies. The results were dependent on both the accurate identification of the same anatomic cross section on serial ultrasound studies and on the selection of the image slice for analysis—that is, the image slice with smallest lumen CSA preintervention vs. postintervention vs. follow-up. In three of these studies, the image slice with the smallest lumen CSA at follow-up was analyzed; and in one study the measurements from the image slice with the smallest lumen CSA preintervention and at follow-up were averaged throughout the time course of the study. The selection of the image slice for analysis is important because the axial location of the minimum lumen CSA has been shown to shift significantly among these time points (14). The current three-dimensional (volumetric) analysis was designed to address this limitation.
Early volumetric IVUS studies required laborious manual analyses of many two-dimensional ultrasound images (29–33). More recently, algorithms for automated quantification of the lumen and arterial (EEM) volumes have been introduced (15,16,24,34). The three-dimensional (volumetric) IVUS analysis system used in the current study had been validated extensively. It is more rapid than multiple planar measurements. Tissue-lumen and media-adventitia borders are detected automatically and edited manually in both cross-sectional and longitudinal sections. This permits measurement of arterial, lumen, and plaque volumes from a relatively larger number (n = 10) of planar ultrasound images per millimeter arterial length, or 200 images over the length of artery analyzed.
The independent, blinded three-dimensional IVUS analysis in the current study confirmed the importance of the decrease in EEM as a mechanism of restenosis. The current analysis also confirmed previous planar and volumetric IVUS reports of the relative contributions of tissue removal and vessel expansion to lumen enlargement during DCA procedures (17,24,29).
Nomenclature and limitations of IVUS.
Serial IVUS analysis can only measure net changes in P+M. Therefore, it cannot isolate cellular proliferation, matrix deposition, atherosclerosis progression/regression, or plaque stabilization/apoptosis from the overall net quantitative changes.
The IVUS analysis can only detect two borders accurately: the lumen-intima interface and the media-adventitia interface. Because the leading edge of the media cannot be detected consistently, media thickness cannot be measured accurately. Because the outer edge of the adventitia cannot be detected, adventitial thickness cannot be measured. Finally, except for venous structures and side branches, there are no consistent peri-arterial structures or interfaces. Therefore, IVUS cannot separate primary changes in EEM dimensions from changes in EEM dimensions secondary to peri-vascular processes. In fact, the decreases in EEM volumes reported in the current study may actually reflect an increase in adventitial thickness or the impact of peri-arterial scar formation.
Study limitations.
This was only a subset of the overall cohort of patients in OARS and the SURE Trial; however, there were no differences between this subset and the entire cohorts with regards to clinical, lesion site and procedural variables. Also, there were only a few restenotic lesions at follow-up. Global volumetric values over the 20-mm-long segments could hide discordant localized behavior in individual cases with or without restenosis.
Differences in vascular tone could have contributed to measurements of arterial and lumen dimensions. However, all patients were studied only after administration of significant doses of intracoronary nitroglycerin; and differences in vascular tone should not have affected measurement of P+M.
There were also potential limitations in the analysis system and methodology employed. The movement of the IVUS catheter during the cardiac cycle and the systolic-diastolic vessel changes could have induced artifacts in the longitudinally reconstructed views. Electrocardiographic (ECG) gated acquisition is a promising, novel approach that can select sequential images at the same time of the cardiac cycle, thereby avoiding these longitudinal artifacts (24); however, this technique was not available for the present study. Linear three-dimensional analysis systems do not consider vessel curvatures (35). Therefore, there may have been underestimation of plaque at outer curvatures and overestimation of inner curvatures. Combined use of data from angiography and IVUS can overcome this problem (36), but this approach is not yet feasible for clinical use. To surmount these limitations, our study included only coronary segments with, at most, mildly curved segments.
Conclusions.
This serial three-dimensional (volumetric) IVUS analysis confirmed previously published planar IVUS analyses indicating that a decrease in dimensions is an important mechanism of restenosis. Plaque regrowth did not play a significant role in restenosis in the current study.
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Footnotes
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Dr. de Vrey is the recipient of a scholarship of the Dutch Heart Association (The Hague, The Netherlands). Dr. von Birgelen is the recipient of a fellowship of the German Research Society (DFG, Bonn, Germany).
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