Introduction

Intravascular ultrasound (IVUS) was first described as a diagnostic adjunct to coronary angiography [1,2,3]. Intravascular imaging, including IVUS and intracardiac echocardiography), has since proved to be a valuable diagnostic and interventional tool with an established safety profile [4]. Intracardiac echocardiography is a distinct modality referring to the use of endovascular US probes for visualizing cardiac structures. Various endovascular US probes can be used for visualizing intra- and/or extracardiac structures. For the purposes of this discussion, we focus on the extracardiac uses of these probes and use the term “IVUS” to refer to the general concept of imaging with intravascular US catheters including both radial/rotational and phased-array/“side-firing” types (Fig. 1).

Fig. 1
figure 1

Types of intravascular ultrasound (IVUS) catheters. a Rotational/radial IVUS catheter (Visions PV.035; Philips, Andover, MA). b Phased-array IVUS catheter (AcuNav; Siemens, Malvern, PA)

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The American College of Cardiology (ACC) and the European Society of Cardiology (ESC) have published guidelines regarding the use of IVUS in the setting of percutaneous coronary intervention [5,6,7]. Data support the use of IVUS for coronary interventions with a demonstrated reduction in major adverse events and mortality with its application [8,9,10]. There is also evidence that IVUS-guided peripheral vascular interventions are associated with improved outcomes and comparable overall cost, relative to angiography alone [8, 11]. IVUS utilization is also associated with decreased procedural time, contrast dose and patient radiation exposure [12]. Relative to cardiac interventions, however, data pertaining to the efficacy and safety of IVUS for extracardiac interventions are limited and consist of case reports, small case series and topical reviews.

Intravascular US guidance for the creation of a direct intrahepatic portosystemic shunt (DIPS) was first described in 2001 [13], and the utility and safety of IVUS has been described for extracardiac procedures, including portal interventions (e.g., transjugular intrahepatic portosystemic shunt (TIPS) [14, 15] catheter-directed portal vein thrombolysis [16]), aortic and peripheral arterial interventions [2, 11], venous interventions (e.g. bedside inferior vena cava filter placement [17], stent reconstruction for iliac vein compression syndromes [12], sharp recanalization with total venous reconstructions for iliocaval occlusion [18,19,20]), transbiliary biopsies in people with complex post-surgical anatomy [21] and neurovascular interventions (e.g., cerebral venous sinus stent placement [22], carotid artery stent placement [23]). There is, however, no consensus or standardized guideline for interventional radiologists on the use of IVUS.

While the use of IVUS has become increasingly prevalent in adult interventions [12], it remains an infrequently used modality in the pediatric setting. The reason for low IVUS utilization rates among pediatric interventional radiologists is likely multifactorial. Because IVUS utilization is less common in pediatric IR, it might be difficult for some institutions to justify the cost of the equipment and training necessary to implement IVUS in a meaningful way. Because pediatric-specific safety profiles have not been established, some radiologists might avoid IVUS when risks related to IVUS equipment access (particularly size relative to accessed vessel) subjectively outweigh the benefits of its inclusion. Pediatric interventionalists might be unfamiliar with the equipment, and there might be concerns about the compatibility of the equipment with pediatric anatomy. For this reason, we present a review of the equipment, techniques and potential clinical applications of IVUS for pediatric interventionalists.

Equipment and techniques

Compared to the two-dimensional projection of a vessel provided by conventional angiography, IVUS provides superior anatomical detail with real-time cross-sectional imaging from inside the vessel, which allows for an accurate depiction of the vessel wall and lumen morphology, measurements of vessel dimensions, and guidance for stent placement. It also allows for imaging of adjacent extravascular structures (e.g., other vessels, solid organs), which can provide an invaluable perspective for procedural guidance. Two types of IVUS catheters are available: radial/rotational IVUS catheters and phased-array/“side-firing” IVUS catheters. Several widely available systems are summarized in Table 1. IVUS catheters contain a piezoelectric transducer that produces sound waves. Radial IVUS uses a single-element, rotating piezoelectric transducer. Phased-array IVUS uses a 64-element stationary transducer. In both types of catheters, the transducer is mounted at the end of the catheter [24].

Table 1 Common rotational and phased-array intravascular ultrasound systems
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In radial IVUS catheters, a rotating transducer is driven by a flexible drive cable to create cross-sectional images in a 360° radial plane perpendicular to the long axis of the catheter. These transducers operate at higher imaging frequencies and allow for high near-field resolution (up to 6–8 cm), but have more limited depth penetration relative to the phased-array counterparts [24]. Phased-array IVUS consists of a transducer mounted on the end of an 8–10-French steerable catheter. Wedge-shape images, similar to those of conventional US with a curvilinear probe, are produced with a greater depth of penetration (up to 21 cm), allowing for high-quality far-field imaging of adjacent vessels and extravascular structures. In addition to greater depth of penetration, other traditional advantages of phased-array IVUS include increased maneuverability secondary to the steerable nature of the transducer and the ability to acquire color Doppler imaging [24]. While a rotational-type catheter is typically positioned over a guidewire, a phased-array catheter is usually advanced via a vascular sheath positioned in the area of interest.

Rotational IVUS allows for a tomographic assessment of the vessel wall and also offers automated pullback, which facilitates accurate measurement of lesion length and selection of an appropriate-size stent [5]. IVUS can identify significant lesions that might be subtle or obscured on planar angiography. Catheters with pulse-tracing capabilities allow for further evaluation of hemodynamic significance. Three-dimensional assessment might reveal areas of extrinsic compression or intraluminal pathology, such as vascular webs, that might be occult on venography [25]. Systems typically offer intraprocedural multi-directional diameter, circumference, and cross-sectional area measurements. IVUS can lead to significantly larger luminal diameter measurements when compared to angiography alone in both pediatric and adolescent patients [26]. Therefore, IVUS usage might allow for more accurate balloon and stent selection (Fig. 2). Conversion of cross-sectional area to cylindrical diameter and stent selection is provided in Table 2. Once the stent is placed, IVUS allows for direct visualization of the stent struts and their relationship to the vessel wall, as well as the radial expansion of the stent. Appropriate stent size is paramount in maintaining stent patency because suboptimal stent wall apposition is a well-documented cause of in-stent thrombosis [12, 27].

Fig. 2
figure 2

Cavernous transformation of the portal vein and splenorenal shunt stenosis in a 2-year-old girl. a Anteroposterior digital subtraction venogram demonstrates severe stenosis at the anastomosis of the splenic and renal veins (arrow). b Rotational intravascular ultrasound (IVUS) shows a patent splenic vein upstream to the splenorenal anastomosis (luminal cross-sectional diameter of 5–5.7 mm was measured prior to selection of an appropriate-size angioplasty balloon). c IVUS image at the anastomosis demonstrates significant luminal narrowing. d Anteroposterior spot image demonstrates inflation of a 5-mm balloon catheter performed across the anastomosis

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Table 2 Conversion of elliptical vascular cross-sectional area determined by intravascular ultrasound (IVUS) to cylindrical diameter and corresponding suggested stent selection
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While rotational IVUS enables precise measurement of lumen size and other vascular characteristics, phased-array IVUS can be used to guide extravascular needle positioning for various procedures with components outside the vessel within which the IVUS catheter resides.

Arterial diseases

Intravascular US guidance can be useful in myriad arterial interventions. At our institution, it has been utilized in the treatment of renovascular hypertension. Percutaneous transluminal renal artery angioplasty can be a safe and effective treatment of pediatric renovascular hypertension in appropriately selected patients [28]. IVUS allows for precise identification of luminal narrowing, and vessel diameter measurements obtained with IVUS are significantly larger when compared to angiography alone [26]. Therefore, IVUS might allow for selection of a more accurate-size angioplasty balloon. Additionally, pulse tracing capabilities might confirm the hemodynamic significance of the lesion prior to treatment (Fig. 3).

Fig. 3
figure 3

Renovascular hypertension in a 16-year-old girl. a Anteroposterior digital subtraction angiogram depicts the area of focal narrowing in the left renal artery (arrow). b Rotational intravascular ultrasound (IVUS) image with transducing element within the angiographic stenosis confirms luminal narrowing to 1.5–2 mm within the left renal artery. c IVUS image demonstrates post-stenotic dilation with cross-sectional measurements of the luminal diameter at 4.8–4.9 mm. d Pulse tracing feature used to obtain spectral Doppler waveforms in the post-stenotic segment of the left renal artery (Eagle Eye; Philips, San Diego, CA). The waveforms show low resistance and delayed systolic upstrokes, consistent with post-stenotic flow dynamics. e, f Despite modest angiographic luminal diameter in the anteroposterior projection following balloon angioplasty (e), completion pulse tracing demonstrates waveform normalization (f)

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Venous interventions and vascular compression syndromes

Iliac vein compression syndrome (May–Thurner syndrome)

May–Thurner syndrome is caused by compression of the left common iliac vein between the right common iliac artery and spine with resultant venous congestion and/or deep vein thrombosis (DVT). Researchers have tried to determine the optimal size for iliac venous stents and made recommendations regarding stent sizing for the adult and pediatric populations [25, 29]. Anticipating the “normal” venous caliber, however, is patient-specific and varies depending on patient age, body habitus and other anatomical factors. Therefore, it is important to accurately measure the vessel lumen and choose the appropriate stent size on a case-by-case basis. This requires an accurate assessment of lesion location, length and severity. In the setting of pediatric and adolescent vascular compression syndromes, IVUS provides a more accurate representation of vessel compression and diameter compared to angiography [26]. In one study of 155 limbs with iliofemoral venous stenosis, the authors found that venography, when compared to IVUS, substantially missed significant stenotic lesions, underestimated their severity, and misidentified the location of the iliocaval confluence and distal landing zone [30]. Murray’s law might be used to estimate the expected diameter of daughter vessels based on the parent vessel diameter, and this concept might be applied in iliac vein stenting [31, 32]. This method is dependent on accurate measurement of the parent vessel, which can be done with greater accuracy with IVUS guidance (Fig. 4). Additionally, IVUS assessment of the unaffected common iliac vein can guide optimal stent selection, assuming symmetry. Determining normal venous diameter with anteroposterior projection angiographic measurement of a vein that is compressed in the same axis leads to overestimation of the vessel size, inappropriate stent sizing and associated morbidity (Fig. 5).

Fig. 4
figure 4

Iliac vein compression syndrome post thrombolysis of left lower extremity deep venous thrombosis in an 11-year-old girl. a Anteroposterior (AP) digital subtraction venogram demonstrates effacement of the central left common iliac vein (arrowhead) with collateral outflow (arrows). b, c Rotational intravascular ultrasound (IVUS) positioned at the level of the stenosis of the left common iliac vein (arrow in b; AP projection) provides fluoroscopic demarcation and confirms compression of the left common iliac vein (arrows in c) beneath the aortic bifurcation (L left, R right). d, e AP fluoroscopic image shows repositioning of the catheter to the unaffected segment (arrow in d) assists stent length selection and corresponding IVUS assesses nominal diameter of the common iliac vein (e). f Dual-injection AP iliac venography following placement of the 14 × 60-mm self-expanding bare metal stent confirms both appropriate diameter and cephalad extent (arrows)

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Fig. 5
figure 5

Iliac vein compression syndrome in a 16-year-old boy. a Anteroposterior (AP) venogram from the left external iliac vein demonstrates prior stent placement with fluoroscopic guidance alone that resulted in inappropriately large diameter (20-mm) and cephalad extension of a left common iliac vein stent contacting the right lateral wall of the inferior vena cava (IVC) (arrow), resulting in in-stent stenosis (star) and contralateral deep venous thrombosis. b AP dual-injection venogram following reconstruction with bilateral 12-mm stent extending from bilateral external iliac veins (gray arrows) into the IVC in a double-barrel configuration (white arrows) demonstrates re-established normal venous diameter

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Intravascular US is frequently used in the management of iliac vein compression syndrome because it allows for assessment of the vessel wall and valves as well as the proximity of adjacent arteries and the degree of associated venous compression [12, 33]. Additionally, it can be helpful in determining the acuity of the lesion [12, 34].

Nutcracker syndrome

Nutcracker syndrome results from compression of the left renal vein between the superior mesenteric artery and spine leading to elevated venous pressures, back pain and hematuria. Left renal vein stenting has emerged as an alternative to renal venous bypass and autotransplant [35]. Similar to iliac vein compression syndrome, anteroposterior compression makes measurement via planar venography problematic. Furthermore, stent migration to the IVC and heart is more common than previously thought, up to 6.7% in one large series where stent undersizing might have been a causative factor [35]. The procedure can be effectively and safely performed in an adolescent population, but with meticulous attention to appropriate stent sizing. Cronan and colleagues [36] demonstrated excellent clinical efficacy and no instances of stent migration in 10 adolescents for whom stents were sized on the basis of both venographic and rotational IVUS measurements. When possible, the renal vein should be assessed without and with Valsava maneuver [37].

Portomesenteric interventions

A DIPS can be created utilizing IVUS guidance as described by Petersen and colleagues [13, 38]. Early experience with DIPS involved IVUS probes capable only of producing images in a transaxial plane; however, subsequent advances in IVUS technology enabled visualization of the entire needle path by a side-firing probe capable of depicting cross-sectional images of the liver. This allows for direct visualization of the portal vein and needle along its entire course, thus facilitating difficult transhepatic portal access (Fig. 6). Central hepatic artery and biliary branches can be identified for avoidance.

Fig. 6
figure 6

Direct intrahepatic portosystemic shunt (DIPS) creation for a 19-year-old man with Budd–Chiari syndrome and failing hepatic venous outflow stent. a Phased-array intravascular ultrasound (IVUS) image in an oblique coronal plane shows the portal vein target (arrowhead) and trans-caudate needle approach (arrow). The IVUS catheter was positioned in the intra-hepatic inferior vena cava (IVC) to obtain this image. b Anteroposterior (AP) projection digital subtraction portal venogram following DIPS access confirms portal venous access. c AP completion venogram confirms successful DIPS creation from right portal vein (white arrow) to the IVC (open arrow)

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These same concepts are applied when utilizing IVUS to assist in creating TIPS (Fig. 7). IVUS guidance has been shown to reduce the number of access attempts, procedure time and radiation dose [39, 40]. While IVUS guidance is not always used in TIPS creation for adults, in a review of pediatric TIPS creation the authors strongly recommended routine use of IVUS because of the inherently small patient size and other anatomical challenges [15]. Additional benefits of IVUS cited in that review included the utility of IVUS for confirming wire access into the portal vein, obviating the need for the dual-injection portogram by fluoroscopically marking the hepatic vein confluence for stent measurement, and evaluating stent positioning before complete deployment. In a review of the clinical effectiveness of TIPS creation in 59 pediatric and adolescent patients, the overall technical success rate was 93.4%. IVUS was used in 21 of these patients with 100% technical success [41].

Fig. 7
figure 7

Transjugular intrahepatic portosystemic shunt (TIPS) creation for a 15-year-old girl with idiopathic hepatic fibrosis. a Phased-array intravascular ultrasound (IVUS) image in an oblique coronal plane shows the portal vein target (arrowhead) and the needle (arrow). b In the same plane, the needle is visualized along its entire course as it enters the portal vein (arrowhead). c Anteroposterior (AP) fluoroscopic image shows successful wire passage into the portal vein with the IVUS catheter (arrow) positioned appropriately from a femoral approach. d AP completion venogram confirms successful TIPS creation with reversal of flow in the left portal vein (arrow) and brisk portal diversion through the TIPS from the main portal vein to the right atrium (star)

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Catheter-directed thrombolysis is effective and safe treatment for portomesenteric venous thrombosis [42]. Selecting a transjugular intrahepatic approach for portal venous access can mitigate bleeding risk when compared to a transhepatic approach [16]. IVUS reduces access attempts and provides confirmation of portal venous access when blood aspiration confirmation is infeasible because of thrombus (Fig. 8).

Fig. 8
figure 8

Trichobezoar complicated by portal venous thrombosis in a 12-year-old girl. a Axial contrast-enhanced CT of the abdomen demonstrates portal thrombosis (arrow). b Phased-array intravascular ultrasound (IVUS) with Doppler in an oblique coronal plane demonstrates echogenic thrombus within the main portal vein (arrows). c In the same plane, IVUS-guided transjugular intrahepatic portosystemic shunt (TIPS) approach to portal venous access, from the middle hepatic vein (arrowhead) to left portal vein (arrow), is performed. d Anteroposterior (AP) portal venogram confirms portal venous entry and abrupt cessation of flow from the main portal vein (star) to the portal bifurcation and intrahepatic branches (arrowheads). The IVUS probe is situated within the intrahepatic inferior vena cava (arrow). e AP digital subtraction portal venogram following single-session pharmaco-mechanical thrombolysis demonstrates residual nonobstructive thrombus adjacent to the portal vein bifurcation (arrow) but improved intrahepatic portal venous flow

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Endovascular aspiration and biopsy

Image-guided percutaneous biopsy is one of the procedures most frequently performed by the interventionalist. Ultrasound-, CT- and less frequently MRI-guided biopsies are commonplace. In the case of thoracic lesions requiring biopsy, endobronchial US-guided biopsy and mediastinoscopy capabilities are limited in some pediatric centers. For lesions inaccessible by these approaches but abutting or within large venous structures, IVUS-guided technique might be considered (Fig. 9) [43, 44]. One study of 50 IVUS-guided transvenous biopsies of perivascular targets showed high diagnostic accuracy, although the authors noted that complication rates might be slightly higher compared to percutaneous needle biopsy [45]. IVUS guidance might facilitate endovascular liver biopsy when a transcaval approach is necessitated by Budd–Chiari syndrome or post-surgical anatomy precluding hepatic vein access [46].

Fig. 9
figure 9

Transvenous biopsy performed with intravascular ultrasound (IVUS) guidance in a 17-year-old girl with a mediastinal mass. a Axial contrast-enhanced CT of the chest demonstrates a large central mass (arrow) insinuating between the azygos veins and ascending aorta and effacing the superior vena cava (SVC). b Phased-array IVUS from a femoral approach with the tip positioned in the SVC in an oblique coronal plane identifies the mass (arrowheads) and a safe transvascular approach. c, d Hybrid oblique coronal IVUS (c) and anteroposterior fluoroscopic imaging (d) monitors the femoral approach passage of a 19-gauge endovascular biopsy needle (arrows). Biopsy confirmed non-Hodgkin lymphoma

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Conclusion

The use of IVUS has become increasingly prevalent in adult interventional radiology and has proved to be a safe and effective tool a myriad of situations. While IVUS is not as commonly employed in the pediatric setting, it can be used effectively to expand the procedural repertoire of the pediatric interventionist. Use of IVUS is feasible and safe in various procedures in children, with distinct advantages that extend beyond the standard angiographic techniques. Further study is needed to obtain longitudinal data regarding patient outcomes and overall costs.