Residual stenosis determined by intravascular ultrasound and duplex ultrasound after balloon angioplasty of the superficial femoral artery

Copyright

Ultrasound in Med. & Bid., Vol. 22, No. 7, pp. 801-806. 1996 0 1996 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/96 $15.00 + .OO

PII: SO301-5629( %)00081-6

ELSEVIER

*Original Contribution RESIDUAL STENOSIS DETERMINED BY INTRAVASCULAR ULTRASOUND AND DUPLEX ULTRASOUND AFTER BALLOON ANGIOPLASTY OF THE SUPERFICIAL FEMORAL ARTERY GERARD

PASTERKAMP,

ANJJZ

M. SPIJKERBOER,

WILLEM

P. T. M. MALI

and CORNELIUS BORST Heart Lung Institute and the Department of Radiology, Utrecht University Hospital, Utrecht, The Netherlands (Received

20 November

1995; in final

form

11 March

1996)

Abstract-Exact determination of the percentage luminal stenosis after balloon angioplasty is essential when deciding to redilate or not, especially since the percentage luminal stenosis may be a predictor for long-term outcome. Conflicting percentage residual stenosis is frequently observed when anglography is compared with duplex or intravascular ultrasound measurements. The aim of the present study was to compare the percentage luminal stenosis after balloon angloplasty determined by duplex and intravascular ultrasound. In 22 patients, balloon angloplasty was performed ln the superEM femoral artery to treat dlsabllng claudication. Intravascular ultrasound studies were performed immediately after b&loon angioplasty; duplex studies were performed 24-36 h after intervention. Intravascular ultrasonnd percentage lumlnal stenosis was calculated with respect to a proximal reference lumen. Duplex pemx&age iuminal stenosis was determined by two methods: llrst, by assuming that the increase in peak flow velocity is directly related to lumen area; and second, by considering a peak Bow velocity ratio of 1.6 and 2.4 is representative for >30% and >50% diameter stenosis, respectively. The percentage luminal stenosis calculated from duplex measurements was higher compared with intravascular ultrasound measurements (y = 0.3&r + 20.1, r = 0.57). Excluding cross-sections with vascular wall damage (dissection or plaque frac&re) over more than 60” of the circumference improved the slope and correlation coefficient of intravascuhu ultrasound measurements versus duplex measurements 0, = 0.88~ + 7.8, r = 0.70). Thus, after balloon rurgieplesty, conflicting percentage hnnlnal stenosis is frequently observed using intravascular ultrasound and duplex measurements. These differences in percentage hunhml stenosis may partly be explained by the extent of vascular wall damage visualized on the intravascular ultrasound image.

Key Words: Intravascular

ultrasound, Duplex, Balloon angioplasty, Residual stenosis, Dissection.

INTRODUCTION

ameter stenosis (Mewissen et al. 1992; Sacks et al. 1990).

In clinical practice, angiography is the diagnostic tool of first choice to determine the percentage luminal stenosis during balloon angioplasty procedures. Duplex scanning is a noninvasive method to locate the position and to grade the severity of peripheral arterial stenoses. Before angioplasty, duplex quantification of luminal stenosis demonstrates high sensitivity and specificity using angiography as the “gold standard” (Jager et al. 1985; Kohler et al. 1987; Legemate et al. 1989, 1991; Ranke et al. 1992; de Smet et al. 1990). However, after balloon angioplasty, duplex-derived residual flow disturbances may not correlate with angiographic di-

High frequency intravascular ultrasound is a new diagnostic tool used to assess the effect of balloon angioplasty in vitro and in vivo, quantitatively as well as qualitatively (Coy et al. 1992; Gussenhoven et al. 1995; Losordo et al. 1992; The et al. 1992; Tobis et al. 1989). The intravascular ultrasound image depicts the three different layers of the vascular wall in a transverse cross-section in contrast to angiography which visualizes a longitudinal silhouette of the arterial lumen. Several investigators have demonstrated that intravascular ultrasound accurately visualizes the lumen area and plaque area (Gussenhoven et al. 1989; Mallery et al. 1990; Nishimura et al. 1990; Nissen et al. 1990) as well as morphologic characteristics of balloon angioplasty, like dissections and plaque frac-

Address correspondence to: Gerard Paste&, Heart Lung Institute, Utrecht University Hospital, Room GO2-523, Heidelbcrglaan 100, 3584 CX Utrecht, The Netherlands. 801

802

Ultrasound in Medicine and Biology

Volume 22, Number 7, 1996

tures (Coy et al. 1992; Tobis et al. 1989). In contrast to preintervention measurements, after balloon angioplasty, the correlation of lumen diameter observed with angiography and intravascular ultrasound is moderate to poor (Nissen et al. 1990; de Scheerder et al. 1994). Furthermore, compared to intravascular ultrasound, angiography underestimates the incidence of vascular wall damage due to balloon angioplasty, like dissections or plaque fractures (Neville et al. 1991). Intravascular ultrasound is considered to be the new gold standard to visualize lumen stenoses and morphologic vessel wall characteristics. In the present study, we hypothesize that intravascular ultrasound characteristics, quantitative as well as qualitative, may be representative for hemodynamics at the treated lesion site. MATERIALS Intravascular

ultrasound

AND METHODS study

Twenty-two patients (16 male, 6 female; mean age 59 & 10 y) were studied before and after routine balloon angioplasty of the superficial femoral artery to treat disabling claudication. Balloon angioplasty was performed in 28 lesions. Informed consent was obtained from each patient. A 4.2F intravascular ultrasound catheter was used (30 MHz, axial resolution 0.2 mm, penetration depth 10 mm; DuMED, Rotterdam, The Netherlands). The ultrasound transducer rotated 16 times per second. The resulting images were displayed on a monitor by means of a videoscanned memory and recorded on sVHS videotape. In all patients, a series of cross-sectional images was recorded during pull back of the ultrasound catheter. To localize the intravascular ultrasound catheter, a ruler was used as a reference during fluoroscopy. The position of the patella was recorded with respect to that of the radiopaque ruler to match intravascular ultrasound and duplex measurements. Ultrasound images were selected every 0.5 cm of the arterial segment over a length of approximately 15 cm, including the reference site and the cross-section in the lesion site with the smallest lumen.

Fig. 1. Representative example of an intravascular ultrasound image obtained after balloon angioplasty of the superficial femoral artery. The arrows and arrowheads demarcate the boundary of the lumen and echolucent media, respectively.

This reference site was assumed to have a normal lumen. The percentage luminal cross-sectional area stenosis was calculated as (1-[lumen area of the treated site/lumen area of the reference site])*lOO%. In each lesion, the cross-section with maximal luminal narrowing was selected for comparison with duplex measurements. After balloon angioplasty, the presence of dissections or plaque ruptures was documented. Criteria for evaluating the morphologic features of the arterial wall before and after balloon angioplasty with intravascular ultrasound have been described previously (The et al. 1992). In short, a rupture was defined as a radial tear within the plaque, a dissection was defined as a circumferential tear witbin or behind the plaque.

The extent of vascular damage was expressed semiquantitatively in hours (1 h [30”]) of the circumference (maximum = 12 h [360”] per cross-section). Crosssections were divided in two groups: those with minor vessel wall damage (dissection extent 52 h), and those with extensive vessel wall damage. (dissection

Image analysis

Intravascular ultrasound images recorded on videotape were analyzed with a digital video analyzer as described previously (Wenguang et al. 1991) (Fig. 1). We traced the lumen cross-sectional area and the area circumscribed by the inner border of the echolucent media. Plaque area was calculated by subtracting lumen area and the area circumscribed by the echolucent media. The reference was that cross-section proximal of the lesion that contained the least amount of plaque.

extent

>2 h) (arbitrary division). Duplex scanning

One day after balloon angioplasty (always less than 36 h), duplex studies were performed with an Ultramark 9 machine, equipped with color Doppler (Advanced Technology Laboratories, Bothell, WA). A linear array (5 MHz or 5- 10 MHz) transducer probe was used. Velocity measurements were taken starting in the common femoral artery working downward to

Residual stenosis determined by intravascular ultrasound 0 G. PASTERUMP et al.

the popliteal artery. The peak systolic velocity (PSV) was determined from the Doppler spectrum, acquired by positioning the Doppler sampling gate center stream in the flow after Doppler angle correction. The PSV was measured at the angioplasty site and in the norrnalappearing vessel proximal of the lesion (Fig. 2A and B). The PSV ratio was calculated by dividing the PSV at the lesion site by the PSV at the reference site. Two methods were used to calculate percentage luminal stenosis: 1. Peak flow velocity was considered to be linearly related to the lumen cross-sectional area: percentage luminal cross-sectional area stenosis = (l-[PSV reference site/PSV lesion site])*lOO%. 2. A PSV ratio of > 1.6 was considered representative for more than 30% luminal diameter stenosis) (Ranke et al. 1992), (i.e., >50% cross-sectional area stenosis). Also, a PSV ratio of >2.4 was considered representative for more than 50% diameter stenosis (Legemate et al. 1991) (i.e., >75% cross-sectional area stenosis). The percentage luminal stenosis as determined by intravascular ultrasound and duplex (method 1) were classified in four categories: O-25%, 26-50%, 5175% and >75% cross-sectional area stenosis. Care was taken to match the location of lesion site and the reference site as determined by intravascular ultrasound and duplex by using a (radiopaque) ruler which was always related to the patella. The maximal PSV ratio at the lesion site was selected for comparison with the intravascular ultrasound measurements. Statistics Data are presented as mean 2 standard deviation (SD). Correlation coefficients were determined to com-

803

pare percentage luminal stenosis calculated from intravascular ultrasound and duplex measurements using method 1. Specificity and sensitivity of diagnosing a luminal diameter stenosis of >30% and ~50% was calculated for duplex measurements using method 2. The intravascular ultrasound measurement was considered the gold standard. A p < 0.05 was considered a significant difference. RESULTS In all 28 lesions, balloon angioplasty was successful (~50% diameter stenosis as determined by angiography) and duplex was available. Lumen area of the lesion site, reference site and percentage luminal stenosis on intravascular ultrasound was 15.1 + 3.9 mm2 (mean II: SD), 24.3 + 5.9 mm2 and 35.5 2 20.1%, respectively. PSV at the lesion site, reference site and PSV ratio was 213.2 ? 96.8 cm/s, 97.1 ? 20.3 cm/s and 2.3 ? 1.2 cm/s, respectively. Comparison of duplex percentage luminal stenosis (assuming that the increase in peak flow velocity and percentage lumen cross-sectional area reduction are linearly related) and intravascular ultrasound percentage luminal stenosis are shown in Table 1 and Fig. 3A. Regression analysis revealed a weak but significant correlation between percentage luminal stenosis calculated from duplex measurements and percentage luminal stenosis calculated from intravascular ultrasound measurements (r = 0.57, p < 0.05) (Fig. 3A). For 11 of 28 lesions, the duplex revealed a higher classified percentage luminal stenosis compared to intravascular ultrasound. In 9 of 11 of these lesions, vascular wall damage was observed in the intravascular

Fig. 2. Duplex images of (A) a reference segment in a superficial femoral artery and (B) at the lesion site. Peak

systolic velocity in the reference segment proximal of the lesion site is 60 cm/s. The peak systolic velocity in the lesion is 250 cm/s, resulting in a PSV ratio of 4.2.

804

Ultrasound in MedicineandBiology

Volume22, Number7, 1996

Table 1. Classificationof duplex ultrasoundand intravascularultrasoundpercentagecross-sectional area luminal stenosis.Flow velocity increaseand lumenarea decreasewere assumedto be linearly relatedfor duplex percentagestenosiscalculations.

Using the intravascular ultrasound as the gold standard, the specificity and sensitivity of a PSV ratio of > 1.6, as determined by duplex, for a diameter stenosis percentage of >30% (Ranke et al. 1992) (i.e., >50% cross-sectional area stenosis) was 26% and 39%, respectively. The sensitivity of a PSV ratio of >2.4 for a percentage diameter stenosis of >SO% (six lesions) (i.e., >75% cross-sectional area stenosis) was 68%. For nine lesions, a PSV ratio of >2.4 was observed in which intravascular ultrasound revealed a percentage cross-sectional area stenosis of <75%. In four femoral artery segments, duplex revealed a PSV ratio of more than 1.6 proximal or distal to the treated lesion. In all four cases, intravascular ultrasound did not reveal a percentage ltinal cross-sectional area stenosis of more than 50%. Thus, an additional four “lesions” would have been overestimated by duplex measurements if the maximal observed PSV ratio along the entire arterial segment was considered representative for the lesion site.

Duplex O-25%

26-50%

51-75%

76- 100%

lvus O-25% 25-50% 50-75% 75-K@%

2 3 8

2 9 0 0

1 4 3 0

0 2 2 0

ultrasound images (dissections with plaque rupture: n = 8; plaque rupture alone: IZ = 1). Mean extent of vascular wall damage in this group was 3.0 2 1.5 h. In 14 of 28 lesions, duplex percentage luminal stenosis was classified in the same category as the intravascular ultrasound measurement. In 10 of 14 of these lesions, vascular wall damage was observed by intravascular ultrasound (dissections with plaque rupture: n = 9; plaque rupture alone: n = 1). Mean extent of the vascular wall damage in the same lesion classification was 1.2 2 1.2 h (p < 0.05 compared to the nonmatching classifications). A higher correlation between duplex and intravascular ultrasound percentage h.uninal stenosis was observed when the cross-sections with an extent of vascular wall damage >2 h were excluded (arbitrary classification) (Fig. 3B). Figure 3C illustrates the relation between percentage luminal stenosis as determined by duplex and the percentage luminal stenosis as determined by intravascular ultrasound for those cross-sections with an extent of vascular wall damage of more than 2 h (y = 0.23~ + 23.8, r = 0.35).

DISCUSSION The percentage luminal stenosis after balloon angioplasty is an important predictor for long-term outcome (Beatt et al. 1992; Ellis et al. 1989). Therefore, exact determination of the residual luminal stenosis is essential to decide whether or not to redilate. Angiography is considered the gold standard in the lumen diameter measurements during routine balloon angioplasty. The duplex measurement is a well-recognized tool to determine luminal stenosis based on blood flow velocity. Intravascular ultrasound accurately visualizes the arterial lumen and wall and is considered the new gold standard for interpretation of angio-

A

B Dissectionsd

sll lesions 100 60

.I?

60

B : 5 s

2H

dissections>

2H

100

y-0.38x+20.1, z

C

r-O.57

y-0.88x+7.8, *

d *

//kj+

40 20

*

60

.” tl 5

60

iz aR

A

*

2

r-0.70

y-0.23x+28.3

r-0.35

*A.&&L+

40

* 1 ib

20

* 0

L' 0

20

% stenosis

" 40

0 60

Duplex

60

US

100

L’

0

”

20

% stenosis

40

4

60

Duplex

60

US

100

0’ 0

20

% stenosis

40

60

Duplex

60

100

US

Fig. 3. Relationbetweenpercentagestenosisasdeterminedby duplex ultrasoundand intravascularultrasound(A) for all lesion sites,(B) the lesionswith a dissectionon intravascularultrasoundbut with an extent ~60” [s2H (hours)] circumference,and (C) the lesionswith a dissectionextent of >60” [>2H].

Residual stenosis determined by intravascular ultrasound 0 G. PASTERKAMP et al.

plasty results. Both duplex and intravascular ultrasound measurements correlate well with angiography before angioplasty. After angioplasty, however, conflicting findings are frequently observed. In the present study, we compared percentage residual luminal stenosis determined by intravascular ultrasound and duplex. We hypothesized that the hemodynamic outcome of balloon angioplasty of the superficial artery as determined by duplex may be predicted by intravascular ultrasound characteristics. The principal results of this study were: 1. After balloon angioplasty, the percentage luminal stenosis calculated from duplex measurements was higher compared with intravascular ultrasound measurements (gold standard). 2. The relation between duplex and intravascular ultrasound measurements was affected by the extent of vascular wall damage visualized on the ultrasound images. In the present study, two methods were used to calculate percentage luminal stenosis from duplex-derived flow velocities. First, we assumed that peak flow velocity is linearly related to lumen area. However, decrease of lumen area may initially be represented by spectral broadening by duplex ultrasound without subsequent increase of the peak flow velocity. And therefore, secondly, PSV ratios of 1.6 and 2.4 were considered representative for 30% and 50% diameter stenosis, respectively (Legemate et al. 1991; Ranke et al. 1992). There is no consensus on which PSV ratio is representative for a percentage stenosis of >30% and >50%, and our choice is therefore arbitrary (Jager et al. 1985; Kohler et al. 1987; Legemate et al. 1989, 1991; Ranke et al. 1992; de Smet et al. 1990). Both methods revealed higher percentages of stenosis for duplex measurements compared with intravascular ultrasound. The l-day delay for flow measurements may be an explanation for the higher percentage stenosis observed with duplex. Elastic recoil of the arterial wall after coronary balloon angioplasty is frequently observed and could be more pronounced after 1 day. However, serial angiographic measurements have revealed that, on average, angiographic restenosis is minimal 1 day after balloon angioplasty (Nobuyoshi et al. 1988). Vascular wall damage, e.g., dissections or plaque fractures, is frequently observed after balloon angioplasty and is accurately visualized with intravascular ultrasound (Losordo et al. 1992; The et al. 1992; Tobis et al. 1989). The extent of dissections was larger in lesions in which duplex revealed higher percentage luminal stenosis compared to intravascular ultrasound. A large dissection may cause turbulence and alter the peak flow velocity at the lesion site. It is conceivable

805

that, by vascular wall damage, a large thrombogenetic surface is exposed to the flowing blood. Subsequently, thrombus formation within the neolumen of the dissection is likely to occur. This newly formed thrombus may be responsible for the residual luminal stenosis after 1 day which was not yet present immediately after the intervention. The neolumen obliterated by thrombus formation may be the substrate for a permanent neointimal layer, i.e., restenosis. The latter would imply that both intravascular ultrasound and duplex measurements could be predictive for development of restenosis, albeit each associated with a different mechanism. There is no consensus, however, whether the presence of vascular wall damage after balloon angioplasty is predictive for the development of restenosis (The Guide Trial Investigators 1994; Jain et al. 1994; Peters 1995). The present study also demonstrates that, in an arterial segment, peak flow velocity as determined by duplex may in some cases not be interpreted as residual luminal stenosis because the maximal flow velocity was measured proximal or distal of the lesion site. Therefore, exact localization of the treated lesion site is necessary when two methods that measure luminal stenosis are compared. Limitations of the study Duplex measurements were performed 1 day (but ~36 h) after balloon angioplasty. As mentioned earlier, comparison of our duplex and intravascular ultrasound findings may be biased due to thrombus formation or delayed recoil within the first 24 h. Furthermore, flow conditions may have been different during the intravascular ultrasound investigation and the duplex measurements due to the presence of the sheath and the imaging catheter in the vessel. The calculation of percentage lurninal cross-sectional area stenosis applying the continuity equation is based on the assumption that peak flow velocity and mean velocity are linked by a fixed ratio in the presence of a parabolic flow velocity profile. This parabolic profile may be absent in the presence of abrupt changes in vascular diameter. In the results, no discrimination has been made between the extent of plaque ruptures and the extent of dissections. Extensive plaque ruptures might reveal different flow alterations compared to extensive dissections. In the present study, however, the number of cross-sections in which a plaque rupture was observed without a dissection was too small to make a valid comparison. Conclusion The percentage luminal stenosis immediately after balloon angioplasty of the superficial femoral artery

806

Ultrasound in Medicine and Biology

measured by intravascular ultrasound may be a poor predictor of duplex measurements performed 1 day after intervention. A large extent of vascular wall damage might produce increased flow velocities which might lead to overestimation of the percentage luminal stenosis by duplex. REFERENCES Beatt KJ, Serruys PW, Luyten HE, Rensing BJ, Suryapranata H, de Feyter P, van den Brand M, Laarman GJ, Roclandt J. Restenosis after coronary angioplasty: the paradox of increased lumen diameter and restenosis. J Am Coll Cardiol 1992; 19:258-266. Coy KM, Park JC, Fishbein MC, Laas T, Diamond GA, Adler L, Maurer G, Siegel RJ. In vitro validation of three-dimensional intravascular ultrasound for the evaluation of arterial injury after balloon angioplasty. J Am Co11 Cardiol 1992;20:692-700. Ellis SG, Roubin GS, King SB, Douglas JS, Cox WR. Importance of stenosis morphology in the estimation of restenosis risk after elective percutaneous transluminal coronary angioplasty. Am J Cardiol 1989;63:30-34. The GUIDE Trial Investigators. IVUS-determined predictors of restenosis in PTCA and DCA: an interim report from the GUIDE trial, Phase II [abstract]. Circulation 1994 9O(Suppl): 113. Gusset&oven ET, Essed CE, Lancee CT, Mastik F, Frietman P, van Egmond FC, Reiber J, Bosch H, van Urk H, Roelandt J, Born N. Arterial wall characteristics determined by intravascular ultrasound imaging: an in vitro study. J Am Co11 Cardiol 1989; 14~947-952. Gussenhoven EJ, van der Lugt A, Pasterkamp G, van den Berg FG, Sie LH, Vischjager M, The SHK, Li W, Pieterman H, van Urk H. Intravascular ultrasound predictors of outcome after peripheral balloon angioplasty. Eur J Vast Endovasc Surg 1995;10:279288. Jager KA, Phillips DJ, Martin RL, Hanson C, Roederer GO, Langlois YE, Ricketts HJ, Strandness DE. Noninvasive mapping of lower limb arterial lesions. Ultrasound Med Biol 1985; 11:515-521. Jain SP, Jain A, Collins TJ, Ramee SR, White Cl. Predictors of restenosis: a morphometric and quantitative evaluation by intravascular ultrasound. Am Heart J 1994; 128:664-673. Kohler TR, Nance DR, Cramer MM, Vandenburghe N, Strandness DE. Duplex scanning for diagnosis of aortoiliac and femoropopliteal disease: a prospective study. Circulation 1987;76: 1074-1080. Legemate DA, Teeuwen C, Hoeneveld H, Ackerstaff RGA, Eikelboom BC. The potential of duplex scanning to replace aortoiliac and femoro-popliteal angiography. Eur J Vast Surg 1989;3:49-54. Legemate DA, Teeuwen C, Hoeneveld H, Eikelboom BC. Value of duplex scanning compared with angiography and pressure measurement in the assessment of aortoiliac arterial lesions. Br J Surg 1991;78:1003-1008. Losordo DW, Rosenfield K, Pieczek A, Baker K, Harding M, Isner JM. How does angioplasty work? Serial analysis of human iliac

Volume 22, Number 7, 1996 arteries using intravascular ultrasound. Circulation 1992; 86: 1845-1858. Mallery JA, Tobis JM, Griffith J, Gessert J, McRae M, Moussabeck 0, Bessen M, Moriuchi M, Henry WL. Assessment of normal and atherosclerotic arterial wall thickness with an intravascular ultrasound imaging catheter. Am Heart J 1990; 119: 1392- 1400. Mewissen MW, Kinney EV, Bandyk DF, Reifsnyder T. Seabrook GR, Lipchik EO, Towne JB. The role of duplex scanning versus angiography in predicting outcome after balloon angioplasty in the femoropopliteal artery. J Vast Surg 1992; 15:860-866. Neville RF, Yasuhara H, Watanabe BI, Canady J, Duran W, Hobson RW. Endovascular management of arterial intimal defects: an experimental comparison by arteriography, - _ _ angioscopy, and int&vascular ultrasonography. J Vast Surg 1991; 13:4%-502. Nishimura RA. Edwards WD, Wames CA. Reeder GS. Holmes DR. Tajik AJ, Yock PG. Intravascular ultrasound imaging: in vitro validation and pathologic correlation. J Am Co11 Cardiol 1990; 16:145-154. Nissen SE, Grines CL, Gurley JC, Sublett K, Hayne D, Diaz C, Booth DC, DeMaria AN. Application of a new phased-array ultrasound imaging catheter in the assessment of vascular dimensions. Circulation 1990;81:660-666. Nobuyoshi M, Kimura T, No&a H, Mioka S, Ueno K, Yokoi H, Hamasaki N, Horiuchi H, Ohishi H. Restenosis after successful percutaneous transluminal coronary angioplasty: serial angiographic follow-up of 229 patients. J Am Co11 Cardiol 1988; 12:616-623. Peters RJG on behalf on the PICTURE Study Group. Prediction of the risk of angiographic restenosis by intracoronary ultrasound imaging after balloon angioplasty. J Am Co11Cardiol 1995 (suppl):35A. Ranke C, Creutzig A, Alexander K. Duplex scanning of the peripheral arteries: correlation of the peak velocity ratio with angiographic diameter reduction. Ultrasound Med Biol 1992; 18:433440. Sacks D, Robinson ML, Marinelli DL, Perlmutter GS. Evaluation of the peripheral arteries with duplex US after angioplasty. Radiology 1990; 176:39-44. de Scheerder I, De Man F, Herregods C. Wilczek K, Barrios L, Raymenants E, Desmet W, De Geest H, Piessens J. Intravascular ultraound versus angiography for measurement of luminal diameters in normal and diseased coronary arteries. Am Heart J 1994; 127:243-251. de Smet AAEA, Kitslaar PJEHM. A duplex criterion for aorto-iliac stenosis. Eur J Vast Surg 1990;4:275-278. The SHK, Gussenhoven EJ, Zhong Y, Li W, van Egmond F, Pieterman H, van Urk H, Genitsen P, Borst C, Wilson RA, Born N. Effect of balloon angioplasty on femoral artery evaluated with intravascular ultrasound imaging. Circulation 1992;86:483-493. Tobis JM, Mallery JA, Gessert J, Griffith I, Mahon D, Bessen M, Moriuchi M, McLeay L, McRae M, Henry WL. Intravascular ultrasound cross-sectional arterial imaging before and after balloon angioplasty in vitro. Circulation i989;80:873-882. Wenguang L, Gussenhoven WJ, Zhong Y, The SHK, Di Mario C, Madretsma S, van Egmond F, de Feyter P, Pieterman H, van Urk H, Rijsterborgh H, Born N. Validation of quantitative analysis of intravascular ultrasound images. Int J Card Imag 1991;6:247253.