Basics of Intravascular Ultrasound An Essential Tool for the Endovascular Surgeon

Basics of Intravascular Ultrasound:An Essential Tool for theEndovascular SurgeonBy Jason T.Lee and Rodney A.WhiteThe concept of catheter-based ultrasound imaging wasfirst introduced in the early1970s.Since its inception,intravascular ultrasound(IVUS)technology has become more user-friendly because of improvements in both the catheters and computer-driven imaging platforms.IVUS catheters enable luminal and transmural cross-sectional imaging of coronary and peripheral blood vessels with high-dimensional accuracy and detailed information about lesion morphology.With the advent of endovascular techniques in both the coronary and peripheral vasculature,IVUS has emerged as a useful and necessary adjunct.In addition to providing diagnostic information,IVUS enables optimal choice of appropriate angioplasty technique,endovascular device guidance,and controlled assess-ment of the efficacy of interventions.In this review we discuss the design and function of available IVUS catheters,imaging techniques and interpretation,and the present and future clinical utility in peripheral endovascular interventions.©2004Elsevier Inc.All rights reserved.I NTRAVASCULAR ULTRASOUND(IVUS)was developed when miniaturized piezoelectric transducers were positioned at the tip of intralumi-nal catheters to allow for high-resolution,ultra-sonic imaging of various cardiac,vascular,and hollow organ structures.IVUS catheters enable luminal and transmural cross-sectional imaging of blood vessels with high-dimensional accuracy and detailed information about lesion morphology(Ta-ble1).Since their inception,coupling the IVUS catheters with real-time computerized processing systems have transformed the technology into a user-friendly and invaluable tool.The concomitant rapid expansion of minimally invasive endovascu-lar techniques in both the coronary and peripheral vasculature has added numerous new roles for IVUS.Precise knowledge of lesion morphology and accurate visualization of vessel wall anatomy are required for endovascular procedures,and be-cause angiography is limited by its ability to dis-play only the outline of the vessel lumen,IVUS has emerged as a useful and often necessary adjunct.In addition to providing diagnostic information, IVUS enables optimal choice of appropriate angio-plasty technique,endovascular device guidance,and controlled assessment of the efficacy of inter-ventions.Further acceptance and more widespread implementation of this modality relies upon the effectiveness of IVUS to improve outcomes and minimize periprocedural complications when com-pared to alternative imaging modalities.In this article we discuss the design and function of avail-able IVUS catheters,imaging techniques and in-terpretation,and the present and future clinical utility in peripheral endovascular interventions.DESIGN AND FUNCTIONThefirst IVUS prototypes were used to measure intracardiac dimensions and cardiac motion in the 1950s,using A-mode transducersfixed to large intraluminal catheters.Various devices(A-,B-, and M-mode)were developed for both intravascu-lar and transesophageal imaging of vascular struc-tures,but not until1972was intraluminal,cross-sectional imaging of vessels reported,using a multielement array transducer.1To obtain a360-degree cross-sectional image,the ultrasound beam is scanned through a full circle and the beam direction and deflection on the display is synchro-nized.This is achieved by one of three means:a rotating transducer within the catheter connected to a motor in the ultrasound unit,a rotating mirror around afixed transducer,or phased-array trans-ducer,where multiple transducers(up to64in a linear array of stationary elements)are electroni-cally switched.Current multielement IVUS catheters operate in a high-resolution B mode,and use frequencies in the range of10to40MHz,with the higher fre-From the Division of Vascular Surgery,Harbor-UCLA Med-ical Center,Torrance,CA.Address reprint requests to Rodney A.White,MD,Chief,Division of Vascular Surgery,Harbor-UCLA Medical CenterBox11,1000West Carson Street,Torrance,CA90509.©2004Elsevier Inc.All rights reserved.0895-7967/04/1702-0005$30.00/0doi:10.1053/j.semvascsurg.2004.03.009110Seminars in Vascular Surgery,Vol17,No2(June),2004:pp110-118quencies providing higher resolution but at the cost of decreasedfield of view and depth of penetration. As an example,current8-MHz to30-MHz IVUS transducers used to image large-to medium-sized vessels have a resolution of approximately0.1to 0.15mm(120to150␮m)and a radial depth of penetration from40to10mm,respectively.For aortic type procedures,8-MHz to12-MHz cathe-ters are normally required to visualize the entire aortic circumference.Twenty and30-MHz cathe-ters are normally uses for iliac and cardiac vessels, respectively.Newly developed40-MHz to 50-MHz transducers provide resolution as to less than100␮m and are normally limited to coronary type of procedures due to their limited depth of penetration.With this spectrum of visualization capabilities,the technology is rapidly advancing to provide accurate imaging in all vascular beds.A problem of the early phased-array devices was the electronic noise caused by the multiple wires within the catheter itself because each of the ele-ments was an independent mini-transducer needing its own connections.This problem was later over-come by incorporating a miniature integrated cir-cuit at the tip of the catheter,which provided sequenced transmission and reception without the need for numerous electrical circuits traveling the full length of the catheter.In addition to reducing the electric noise,this modification simplified the manufacturing complexity and improved theflex-ibility and length restrictions of the catheter.A problem of these imaging catheters,common to all high-frequency ultrasound devices to some extent,is the inability to image structures in the immediate vicinity of the transducer(ie,in the “nearfield”).Because the imaging crystals in a phased-array configuration are in almost direct contact with the structure being imaged,a bright circumferential artifact known as the ring down surrounds the catheter.The ring-down artifact can be electronically removed with software adjust-ments,but structures within the masked region are not imaged.This is not usually a problem in medium to large diameter vessels as the clinician can usually rotate the catheter or reposition the guidewire to guide the catheter off the vessel wall.METHODS AND TECHNIQUES AccessThe IVUS catheters can be introduced either percutaneously through a standard vascular access sheath(5Fr to10Fr)or via an arteriotomy or venotomy during an open procedure.The most commonly used catheter for aorta-iliac interven-tion is the8.2Fr,10-MHz catheter.This catheter tracks over a0.035-inch(up to0.038in)guidewire and can be quickly prepared,introduced and/or exchanged with other catheters.If large vessels proximal to the arteriotomy are to be imaged(ie, aortoiliac imaging via a femoral cutdown),intro-duction through a hemostatic sheath reduces blood loss and prevents catheter damage during insertion. In most situations,a retrograde common femoral artery puncture provides access to the aortoiliac segments,entire thoracic and abdominal aorta,and the coronary vasculature.Percutaneous brachial or axillary puncture provides access to upper limb vessels and may be more convenient for interro-gating the thoracic aorta and aortic arch during the treatment of dissections.IVUS catheters are available in lengths up to 125cm,with size ranges for noncoronary catheters ranging from6.2Fr to8.0Fr(Table2).The larger 10-MHz to12-MHz catheters can image up to6 cm in diameter,whereas smaller3Fr wire config-urations have a more limited lateral resolution.Of the various catheters currently available,our insti-tution has utilized the Volcano catheter(Jomed, Rancho Cordova,CA)and the Atlantis PV and SoniCath(Boston Scientific,Natick,MA)(Fig1).Table1.Lesion Morphology:Data Acquired by IVUS1.Luminal diameters and cross-sectional area2.Wall thickness3.Lesion length,shape,and volume4.Lesion position within the lumen:concentric vs.eccentric5.Lesion type:fibrous(soft)vs.calcific(hard)6.Presence and extent offlap,dissection,or ulceration7.Presence and volume of thrombusTable2.Specifications of Commonly Used IVUS CathetersSize (Fr)Frequency(MHz)Length(cm)IntroducerSheath(Fr)Guidewire(in) Volcano Therapeutics,Inc.3.42013560.018 8.2*109090.038Boston Scientific3.22013560.018 6*12.59580.035 6209580.035 8*159580.035*Indicates peripheral vascular use.111BASICS OF INTRAVASCULAR ULTRASOUNDImage AcquisitionIVUS are advanced over a 0.025-in or 0.035-inch guidewire,which allows more controlled ma-neuvering within the lumen of the vessel,particu-larly in tortuous or tightly stenotic vessels.Because of the con figuration of the older mechanical rotat-ing catheters,a central guidewire channel is not possible and these catheters have a variety of monorail and coaxial lumen options for over-the-wire applications.Multielement array devices use a central guidewire channel,which offers advantages for catheter delivery.Careful positioning of the catheter tip within the vessel and appropriate size matching of the deviceto the artery caliber are essential to optimal visu-alization.Image quality is best when the catheter is parallel to the vessel wall and the ultrasound beam is directed at 90degrees to the luminal surface.Eccentric positioning of the catheter within the vessel cross section causes the wall closer to the imaging chamber to appear more hyperechoic than the distant wall,resulting in an artifactual differ-ence in wall thickness.Catheter centering is espe-cially dif ficult in tortuous vessels,and rotational alignment may also be partly lost as the catheter meanders through the vessel.The best-quality im-ages are generally obtained as the catheter is with-drawn through the lumen rather than during ad-vancement.Gray scale,real-time images are displayed on a monitor and able to be recorded digitally.Measure-ments of vessel dimensions,luminal diameters,and cross-sectional areas can be performed by an on-line processing unit that calculates the area of the lumen outlined on still images on the monitor.A digitized pad can also be used to calculate the area from calibrated photographs of the images.A longitudinal gray-scale image is an option on most of the newer IVUS units.The longitudinal image is obtained by mechanically withdrawing the catheter through the vessel at a controlled rate.The cross-sectional images are then stacked by the processing unit and turned sideways to produce a longitudinal view,similar to an angiogram,of the vessel being interrogated.Distances can then be measured from one point to another.Unfortu-nately,this option is currently limited to cardiac applications due to the slow speed of current pull back devices.Newer pullback mechanisms need to be developed by the manufacturers,to pull the catheters over longer distances,at slightly greater speeds,for most aortic endovascular procedures.An important feature of two-dimensional (2D)longitudinal reconstruction is its display of the transmural wall morphology.An image of the entire length of the vessel is produced,similar to contrast angiography.However,rather than only the luminal pro file that contrast angiography pro-vides,the 2D reconstruction provides detailed cross sectional wall morphology alongside the lon-gitudinal image.The differentiation of plaque,nor-mal tissue,thrombus,dissections,and flaps is often much better appreciated in the 2D longitudinalimage.Fig monly used peripheral vascular IVUS catheters.(A)Visions PV (Volcano Therapeutics,Inc.,Rancho Cordova,CA);(B)Atlantis PV (Boston Scienti fic,Natick,MA).(C)Soni-cath Ultra (Boston Scienti fic).Note the two types of con figu-ration,over the wire (A)and (B)and the monorail system (C).C,catheter;GW,guidewire;F,flush port.112LEE AND WHITEColor-Flow IVUSOne important limitation to standard IVUS cath-eters is its inability to clearly delineate blood flow.A new computer software program ChromaFlo (Endosonics,Rancho Cordova,CA)is able to de-tect differences in the position of echogenic blood particles between images to determine flow states and colorizes the images.The software was eval-uated in a study of 100interventions and it allowed for better recognition of the true lumen and blood flow after angioplasty and stenting.2The origin of larger aortic arch branches is also more clearly outlined during interventions of the thoracic aorta.Searching for an entry site during endovascular treatment of chronic dissections or pseudoaneu-rysms has been aided by the color-flow IVUS (Fig 2).Being able to detect flow would also be of obvious theoretical advantage during surveillanceof aortic stent grafts in looking for an endoleak,although this has yet to be well studied.CLINICAL UTILITYDiagnostic ApplicationsTwo-dimensional images produced by IVUS not only outline the luminal and adventitial surfaces of vessel segments,but also discriminate between normal and diseased components.In muscular ar-teries,the media appears as an echolucent layer sandwiched between the more echodense intima and adventitia,and de fines the outer limits of any noncalci fied plaque with the luminal circumfer-ence de fining the inner limits (Fig 3).This allows accurate localization and measurement of the thickness of the plaque.Small lesions,such as intimal flaps or tears,are well visualized because of their high fibrous tissue content and the con-trasting echoic properties of surrounding blood,as well as the pulsatile blood-flow variation seen because IVUS is performed in real-time.Intralu-minal thrombus can be distinguished from under-lying vessel wall and typically appears as a highly echogenic,homogenous mass with varying image attenuation beyond its location.IVUS devices are also sensitive in differentiating calci fied and non-calci fied vascular lesions.Because theultrasoundFig 2.(A)Color-flow IVUS image of proximal pseudoaneu-rysm of previously repaired thoracic aortic aneurysm showing breakdown of anastamosis.(B)Stent graft coverage of pseu-doaneurysm.Arrow indicates previous entry site now with-out flow fromlumen.Fig 3.Image of super ficial femoral artery and distinct wall layers of muscular arteries.Note on the color-flow image a large pseudoaneurysm.This patient was treated with a stent to cover the neck of the pseudoaneurysm.I,intima;M,media;A,adventitia.113BASICS OF INTRAVASCULAR ULTRASOUNDenergy is strongly reflected by calcified plaque,it appears as a bright image with dense acoustic shadowing behind it.Luminal dimensions and wall thickness deter-mined by IVUS of normal and minimally diseased arteries both in vitro and in vivo are accurate to within0.05mm.Determination of outer vessel diameter may be less accurate,with error margins up to0.5mm.Additional studies have compared contrast angiography and IVUS for determining luminal dimensions of human arteries.3The lumi-nal cross-sectional areas calculated from biplanar angiograms and measured from IVUS correlate well for normal or minimally diseased arteries,as well as mildly elliptical lumens.In severely dis-eased vessels,however,angiography tends to un-derestimate the severity of atherosclerosis in the wall compared to IVUS.4IVUS has also been used in the diagnostic as-sessment of several other pathologic vascular le-sions.Accelerated intimal thickening in the coro-nary arteries of cardiac transplant recipients has been documented by IVUS when angiograms ap-pear normal.IVUS has also been used to determine candidacy for pulmonary thromboendarterectomy as treatment for pulmonary hypertension in pa-tients with chronic pulmonary thromboembolic disease.Intravascular tumors,such as vena caval extensions of renal cell carcinoma,can also be localized by IVUS to aid in planning resection. IVUS has been utilized in the trauma setting, accurately determining whether patients after blunt thoracic trauma with equivocal aortograms have true operable lesions.5In our own experience,we treated a hemodynamically stable patient after a high-speed motor vehicle accident with an aortic stent graft when the angiogram was normal but the IVUS showed the aortic disruption that required operative intervention(Fig4).Therapeutic InterventionsLong-term success of vascular procedures re-quires restoration of near-normal hemodynamic conditions across diseased vessel segments by ad-equate enlarging luminal diameters.Conventional angiography is somewhat limited in its ability to provide sensitive data regarding the effects of en-dovascular therapies.For meaningful critical as-sessment of catheter-based methods,plaque extent and consistency and the distribution of residual lesions following an intervention must be known.The advantage afforded by IVUS in evaluating lesion morphology involves accurate assessment of not only luminal dimensions but also transmural lesion characteristics.Delineation by IVUS of the spatial distribution of the lesion in a concentric or eccentric pattern and the presence of a soft(fi-brous)or hard(calcified)plaque may influence the choice of endovascular therapy as well as predict the risk of immediate or late complications(ie, perforation,thrombosis,restenosis).Evaluation of lesion volume before and after the procedure by IVUS provides a quantitative method to estimate the amount of lesion debulking or displacement and a reference point from which to assess the lesion recurrence/restenosis.IVUS also fulfills many of the necessary requirements of a guidance system for endovascular procedures,namely pre-cise delivery and positioning of devices within target lesions.IVUS is particularly helpful in as-sessing the relationship of the ostia of branch vessels to the lesion that can be used as landmarks during procedures.Percutaneous Transluminal Angioplasty Adjunctive use of IVUS during percutaneous transluminal angioplasty(PTA)has provided a useful perspective in treating coronary and periph-eral arterial occlusive disease.In a study of PTA in 16patients with lesions of the superficial femoral artery,IVUS accurately detected the presence of dissections,plaque fractures,internal elasticlam-Fig 4.IVUS of the descending thoracic aorta near the ligamentum arteriosum in a high-speed motor vehicle acci-dent victim who had a normal angiogram but periaortic he-matoma seen on the spiral computed tomography scan.Note the dissectionflap and the aortic disruption.The patient had an AneuRx stent graft placed and recovered rapidly.114LEE AND WHITEina ruptures,and thinning of the media that oc-curred during PTA.6IVUS showed in these pa-tients that luminal enlargement after PTA is produced by stretching of the arterial wall while the volume of the lesion remains relatively con-stant.The risk of restenosis after percutaneous coro-nary angioplasty(PTCA)has also been correlated to IVUSfindings.Early restenosis following PTCA is associated with luminal thrombus,exten-sive dissection,and oversized balloon dilatation, while late restenosis correlates with residual steno-sis greater than30%,small residual lumen,under-sized balloon use,concentricfibrous plaque,ab-sence of dissection,and absence of calcification.7 These factors are all readily identified by IVUS and illustrate the potential that IVUS has for en-hancing PTCA procedures by allowing periproce-dural decisions to be made regarding the need for additional interventions.Balloon size for PTA is often underestimated when selection is made using quantitative angiography alone,and optimal bal-loon size is more accurately determined by IVUS.8 More recently,improved outcomes were docu-mented when utilizing IVUS to delineate post-PTA dissections that were significant enough to require stents(Ͼ60%residual stenosis).9 Intravascular StentsCommon indications for stent deployment after angioplasty are deep arterial wall dissections,elas-tic recoil,residual stenosis,a significant residual pressure gradient across the lesion,or plaque ul-ceration with local thrombus accumulation.Intra-vascular stents increase the patency of arterial occlusive lesions that have undergone angioplasty by reducing technical failure and restenosis rates, but placing stents is not without risk.Inadequate stent expansion can lead to early thrombosis or stent migration,whereas overexpansion can result in excessive intimal hyperplasia or vessel perfora-tion.IVUS has been useful in establishing the need for stenting as well as guiding stent deployment for both coronary and peripheral lesions.Studies have demonstrated that adequate stent expansion is noted by IVUS in only13%to20%of treated coronary lesions(satisfactory result by angiogra-phy),which required further stent expansion by balloon dilatation.10In peripheral vessels,angiog-raphy-guided stent deployment results in incorrect positioning or expansion in as many as20%to 40%of cases.A recent long-term follow-up study of52patients who underwent balloon angioplasty and stenting of iliac stenoses documented im-proved long-term patency by defining the appro-priate angioplasty diameter endpoint and adequacy of stent deployment.11Residual lumen area is known to be an important variable predicting long-term patency of endovascular procedures,and op-timizing luminal dimensions by stenting is crucial. IVUS has also been studied in carotid stenting, and found to be complementary to angiography by more accurately visualizing stent placement and vessel wall morphology,which is information crit-ical to deciding whether to end the procedure or to proceed with further stenting.12IVUS can quanti-tatively provide accurate cross-sectional area that defines percent area stenosis at the level of the lesion.Long-term studies are still lacking as to whether IVUS will improve outcomes or decrease complication rates from carotid stenting. Abdominal Aortic Aneurysm Endovascular Grafts We believe the most important application of IVUS is in the endoluminal exclusion of abdomi-nal and thoracic aortic aneurysms.The success of these interventions depends upon appropriate pa-tient selection,accurate preoperative and intraop-erative visualization of the anatomy of the aorta, and the proper physical deployment of the devices. Experimental studies have shown that IVUS is extremely useful for choosing the site for stent graft deployment by accurately identifying branch arteries and determining the luminal dimensions of the aorta.13IVUS also can detect whether full stent graft expansion has occurred,and can provide information regarding surface topography,align-ment,and movement of the graft material in the aortic lumen.In addition,several observations are frequently apparent only on IVUS,such as incom-plete stentfixation(as evidenced by independent arterial wall pulsation at the stent interface),fold-ing of unstented portions of a particular prosthesis, or motion with arterial pulsations(Fig5).Identifi-cation of such technical problems allows immedi-ate intraoperative troubleshooting and the possibil-ity of balloon dilatation or placement of an additional modular piece.In our own experimental series,14incomplete proximal balloon-expandable stent expansion was determined by IVUS in20%of cases of endograft115BASICS OF INTRAVASCULAR ULTRASOUNDdeployment with no apparent abnormality seen on angiography.This is an important observation be-cause underexpansion at the proximal or distal fixation sites can lead to endoleaks or device mi-gration.Thirty to forty percent of abdominal aortic aneurysm (AAA)patients treated with endovascu-lar grafts have been found to require a different diameter stent-graft on the basis of IVUS measure-ments over preoperative computed tomography an-giography.15The more accurate sizing and length measurements IVUS provides has been suggested to lead to fewer endoleaks and secondary interven-tions.16Using certain devices,it is possible to place devices by IVUS guidance alone.Although cine-fluoroscopy and IVUS are complementary in en-abling expedient deployment of stent-grafts,an additional important bene fit of IVUS is the poten-tial to reduce fluoroscopy time and contrast,min-imizing the radiation exposure to both personnel and the patient,and the risk of renal failure.Some centers that use IVUS during aortic endovascular interventions have no longer routinely performed angiography,and have not noted increased com-plications.17We currently use mostly IVUS to guide our aortic stent-graft procedures,with con-trast angiography used as an adjunct in patients with normal renal function to con firm the lack of a proximal or distal endoleak.Longer-term studies are certainly necessary to show that routine use ofIVUS during AAA repair improves outcomes and is cost-effective.Aortic DissectionsEndovascular interventions for acute and chronic type B dissections provide an appealing alternative to open techniques that are plagued by high morbidity and mortality.18The deployment of an endovascular stent graft can cover an entry site,obliterate a dissection flap,and restore blood flowFig 5.IVUS clearly demonstrates incomplete stent graft expansion as a separation of the stent from the arterywall.Fig 6.(A)IVUS of aortic dissection delineating left renal artery (L)coming off true lumen (T).(B)Right renal artery (R)noted to come from false lumen (F).Renal vein (V)also clearly visualized.Fenestration of dissection flap performed on this patient rather than stent graft to occlude flow to false lumen.116LEE AND WHITEto the true lumen.IVUS confirms placement of the guidewire within the true lumen,accurately sizes proximal and distal necks,and accurately identifies the anatomic landmarks of the dissection,includ-ing determining the true and false lumen(Fig6). Limiting contrast in these patients is often desired, as renal compromise may be part of the pathophys-iology of the dissection,and repair of these lesions has been guided by IVUS alone.19As the spectrum of treating these devastating disorders broadens with newer endovascular techniques,the ability of IVUS to accurately delineate the anatomy will remain a necessary adjunct in the treatment of aortic dissections.Venous InterventionsIVUS has also been studied in endovascular interventions of the venous system.Transfemoral venography for iliac vein obstruction has numer-ous limitations,and IVUS imaging yieldsfindings not obvious on venography,such as intraluminal webs or external compression and subsequent de-formity.More importantly,IVUS provides an ac-curate assessment of the degree of iliac vein ste-nosis,as venography underestimates by30%.20 This allows more appropriately sized venous stents to be placed after venoplasty.The placement of vena cavalfilters for pulmo-nary embolus prophylaxis has also been optimized by the use of IVUS.Critically ill patients that are not candidates for transport to an angiographic suite can undergo bedside inferior vena cava(IVC)filter placement with IVUS guiding the accurate deployment of thefilter.21This obviates the need for cumbersomefluoroscopic machinery at the pa-tient’s bedside,and reduces the need for contrast in critically ill patients who may have underlying renal dysfunction.We have also performed IVC filter placement under IVUS guidance in morbidly obese patients unable tofit on the standardfluo-roscopy table in our interventional suite.Placement of IVCfilters under IVUS is currently an attractive alternative to traditional methods of insertion,and the cost of the IVUS may be offset by the avoid-ance of angiographic suite costs.FUTURE DEVELOPMENTS Miniaturization,cost-effective manufacturing, and development of user-friendly IVUS devices are needed to enhance the utility of IVUS during vascular interventions.Development of endovas-cular devices with combined interventional com-ponents and IVUS transducers in the stent delivery system capable of real-time imaging during the procedure would simplify catheter exchanges,in-terventional techniques,and lesion assessment.Al-terations in ultrasound probe placement are being developed to improve upon resolution in markedly tortuous vessels.22Finally,refinements in com-puter software and hardware will further reduce processing times and improve2D and3D image quality.In summary,IVUS has developed rapidly from a purely diagnostic imaging modality to an impor-tant tool capable of guiding and assessing a mul-titude of endovascular interventions.As interven-tions become increasingly complex,success will be related to the degree of accuracy of the guidance system employed during the procedure.IVUS is an integral component of current and future endovas-cular interventions at many centers,and although its use is currently somewhat limited by cost,its demonstrated utility may rapidly justify its clinical use.REFERENCES1.Bom N,Lancee CT,Van Egmond FC:An ultrasonic intracardiac scanner.Ultrasonics10:72-76,19722.Irshad K,Reid DB,Miller PH,et al:Early clinical expe-rience with color3-D IVUS in peripheral interventions.J Endovasc Ther8:329-338,20013.Tabbara M,White R,Cavaye D,et al:In vivo human comparison of intravascular ultrasonography and angiography. 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