A Pictorial Review of Current Approaches in Endoleak Imaging

Abstract

Endoleak is a significant complication of endovascular aortic repair, associated with adverse long-term outcomes. This review discusses the classification, mechanisms, and imaging diagnosis of endoleaks. Five types of endoleaks are described, each with distinct characteristics and management approaches. Imaging modalities for endoleak detection include computed tomography, magnetic resonance imaging, ultrasonography, and angiography, each with unique advantages and limitations. Computed tomography remains the gold standard, but magnetic resonance imaging and contrast-enhanced ultrasound show promise in specific scenarios. The article details imaging findings for each endoleak type, emphasizing the importance of multimodality imaging for accurate diagnosis. While computed tomography is essential for early postoperative evaluation and reintervention planning, a tailored approach using various imaging techniques may optimize long-term surveillance. Future research should focus on establishing cost-effective, radiation-minimizing protocols for lifelong post-endovascular aortic repair monitoring.

Introduction

EVAR has gained increasing popularity as a treatment for AAA due to its less invasive nature and lower perioperative mortality compared to open surgical repair (OSR). However, long-term outcomes remain problematic, with higher rates of reintervention, aneurysm rupture, and aneurysm-related mortality reported in post-EVAR patients compared to those undergoing OSR [1-5]. Closely associated with these adverse events is endoleak, a complication unique to EVAR. Although the frequency of certain types of endoleaks has decreased due to improvements in stent-graft materials, construction, and interventional procedures, the problem is far from resolved. Therefore, accurate evaluation and proper management of postoperative endoleaks may contribute significantly to improving the long-term outcomes of EVAR.

This article reviews the current understanding and diagnostic approaches for endoleaks following EVAR.

Classification of Endoleaks

Endoleak is a specific complication in which blood flow persists within the aneurysmal sac despite stent-graft placement. The term “endoleak” was first coined by White et al. [6] in 1996, after which endoleaks were classified into five categories based on their mechanism of occurrence. Subsequently, Golzarian et al. [7] proposed subtypes of these endoleaks based on hemodynamic considerations. The classification of endoleak types is summarized in Table 1. The timing of endoleak occurrence is also clinically significant and can be categorized as either early or late.

Table 1.

Classification of Endoleaks.

Type	Subtype	Mechanism	Key features
Type I	Ia	Inadequate sealing between stent-graft and vessel wall (proximal fixation site)	High-pressure, requires additional treatment
	Ib	Inadequate sealing between stent-graft and vessel wall (distal fixation site)	High-pressure, requires additional treatment
Type II	IIa	Retrograde blood flow from one branch vessel	Low-pressure, often resolves spontaneously
	IIb	Retrograde blood flow from multiple branch vessels	Low-pressure, complex flow pattern
Type III	IIIa	Blood flow through stent-graft junction	High-pressure, can occur early or late
	IIIb	Blood flow through graft fabric tear	High-pressure, mainly occurs late
Type IV	-	Leakage due to graft porosity	Low-pressure, resolves spontaneously within 30 days
Type V	-	Aneurysm expansion without visible endoleak (endotension)	Unclear cause, requires individualized treatment

Type I endoleak

Inadequate sealing at the fixation site of the stent graft causes blood flow to enter the aneurysm through the gap between the stent graft and the vessel wall, with an incidence of 5% in EVAR cases during the 5-year follow-up [3]. There are two subtypes of this endoleak: Ia, which originates from the proximal sealing site, and Ib, which arises from the distal sealing site. Any persistent type I endoleaks (TIEL) are high-pressure endoleaks that directly apply aortic pressure and require additional treatment due to the risk of sac enlargement and rupture.

The incidence of type Ia endoleaks (TIaELs) detected on completion angiography ranges from 3% to 21% of EVAR cases, with higher rates possibly attributed to more advanced intraoperative imaging techniques [8, 9]. Several risk factors have been identified for TIaELs, including large neck diameter, short neck length, angulation, calcification, and thrombus in the proximal neck [8, 10]. When detected intraoperatively, various management options are available, such as balloon molding, proximal cuff extension, Chimney technique, and embolization techniques. However, it is important to note that 75%-80% of these endoleaks may persist despite adjunctive procedures. Interestingly, many small intraoperative TIaEL resolves spontaneously on first follow-up imaging. This observation has led to the consideration of conservative management in selected cases [8, 11]. Nevertheless, persistent TIaELs require close surveillance and often necessitate reintervention, which may involve further endovascular techniques or OSR [8, 10, 11]. The long-term outcomes of untreated type Ia endoleaks are not well characterized, but they are believed to increase the risk of aneurysm growth and rupture [10, 12].

Type Ib endoleaks (TIbELs) are observed on completion angiography in 1-5% of EVAR cases [13, 14]. The incidence of late TIbELs ranges from 2.3% to 5% [15, 16]. Risk factors for developing type Ib endoleaks include short distal landing zones (<20 mm), large common iliac artery diameter (>18 mm), and marked iliac tortuosity [15, 17]. Treatment of TIbELs usually involves endovascular techniques to extend the distal seal zone. This commonly includes the placement of additional stent graft components into the external iliac artery, with embolization of the internal iliac artery if needed [18]. More complex options like iliac branched devices may be used to preserve internal iliac flow. Embolization techniques can be considered if standard endovascular approaches are not feasible [19].

Type II endoleak

Type II endoleak (TIIEL) is the most common endoleak, caused by retrograde blood flow into the aneurysmal sac from aortic or iliac side branches, and occurs in 20-40% of cases after EVAR [3, 4, 6]. Responsible vessels include the internal iliac artery, accessory renal artery, inferior mesenteric artery, lumbar artery, and median sacral artery. It is classified into type IIa, in which only one branch is involved and the direction of blood flow is to-and-fro, and type IIb, in which multiple branches are responsible and have inflow and outflow vessels. TIIELs are low-pressure endoleaks that are not directly pressurized by the aorta and often resolve spontaneously [6]. However, persistent TIIELs are associated with reintervention, sac enlargement, rupture [20, 21], and aneurysm-related mortality. Therefore, they are considered candidates for reintervention when an increase in aneurysm diameter of at least 5 mm over 6 months is observed [22].

Recent studies have identified risk factors for TIIEL persistence and sac growth after embolization. Larger initial aneurysm sac diameter (>55 mm) at the time of embolization was found to be a significant predictor of continued sac enlargement [23]. This suggests earlier intervention for smaller TIIELs may be beneficial. The presence of multiple patent aortic branch vessels and moyamoya endoleak morphologies on imaging may also predict poorer outcomes [24].

TAE is a common first-line treatment for TIIELs. Typical approaches include embolizing the endoleak nidus and feeding vessels using coils and/or liquid embolic agents [19]. However, recurrence rates after TAE are high, with studies reporting recurrent TIIELs in 63%-79% of patients on follow-up imaging [23]. Alternative techniques include translumbar direct sac puncture, transcaval embolization, and surgical ligation of feeding vessels. A meta-analysis suggested that translumbar embolization may have higher technical success and lower recurrence rates compared to transarterial approaches [25]. However, high-quality comparative studies are lacking.

In contrast, due to the poor outcomes of reintervention for TIIELs [24], preemptive embolization of aortic side branches has been increasingly performed [26-29]. This preventive approach aims to reduce the incidence of TIIELs and potentially improve long-term outcomes after EVAR.

Type III endoleak

Type III endoleaks (TIIIELs) are observed in 0.4%-1% of completion angiographies [30-32], while they account for 0.1%-4% of all endoleaks detected during follow-up after EVAR [33-35]. These endoleaks include blood flow through the stent-graft junction and through graft fabric tears. The former is subdivided into type IIIa (TIIIaEL) and the latter into type IIIb (TIIIbEL), which occur early and late, respectively. Early TIIIaELs are attributed to insufficient overlap between stent-graft junctions or inadequate stent-graft expansion, while late TIIIaELs are generally attributed to conformational changes in the aneurysm sac causing stent-graft migration or component separation. TIIIbELs are rarely observed early in modern devices, mostly due to the late occurrence of stent-graft fabric injury associated with stent fracture or interference with another stent implanted in the stent graft [36]. Similar to TIELs, all TIIIELs are high-pressure types and are eligible for additional treatment.

The standard treatment for TIIIELs is relining the endograft by deploying a new endograft within the preexisting graft [37, 38]. This approach addresses both component separation and fabric defects. For TIIIELs due to component separation, placement of additional endograft components to bridge the disconnected segments is typically performed [38]. This may involve inserting a new iliac limb for iliac component separation or a new bifurcated device for more proximal separations. TIIIbELs from fabric tears can be managed by placing a new endograft to cover the defect [38]. If the location of the fabric tear is difficult to access, consideration can be given to relining the entire endograft.

Type IV endoleak

Type IV endoleak (TIVEL) is defined as a low-pressure endoleak due to stent-graft porosity observed within 30 days after EVAR. TIVELs typically resolve spontaneously and do not require therapeutic intervention, as they are self-limiting and tend to seal off on their own within a short period after the procedure [39]. The incidence of TIVELs is reported to be between 0.5% and 4% [14, 40, 41]. Polyester stent grafts are more prone to this type of endoleak.

Type V endoleak

This endoleak type, also known as endotension, is defined as sac enlargement with no visible endoleak on imaging. There are several theories as to the mechanism, including increased graft permeability and direct pressure transmission through the stent-graft fabric [42]. It should be noted that cases diagnosed as type V endoleaks can include minor type I-IV endoleaks and vasa vasorum hemorrhage that cannot be identified on imaging. Treatment for endotension is individualized based on aneurysm expansion and patient risk [43]. Observation is recommended for <5 mm growth, while close imaging follow-up is advised for 5-10 mm expansion. Intervention is considered for >10 mm growth [44]. Primary treatment options include stent graft relining, especially effective for older grafts [45]. For high-risk patients or relining failures, open sac plication is an alternative [46]. Open conversion with graft explantation is the most definitive treatment for suitable surgical candidates.

Imaging Modalities for Endoleak Detection

Imaging surveillance after EVAR is generally performed prior to discharge, at 6 months, at 1 year, and annually thereafter if no further problems arise. CT scan remains the gold standard for monitoring endoleaks, though MRI and ultrasonography have partially replaced CT in current practice. Below, we describe the use of each modality, including its advantages and disadvantages.

CT

CT is most frequently used in imaging surveillance after EVAR. While sac diameter and stent graft configuration can be evaluated with plain CT, contrast-enhanced CT (CECT) is required for the detection and differentiation of endoleaks. The sensitivity and specificity of endoleak detection with multiphasic imaging in early and late phases have been reported to be 70%-90% and 90%-100%, respectively [47-51]. Overall, four-dimensional CT (4D-CT), which involves continuous imaging of multiple phases after contrast enhancement, is also useful for identifying the direction of blood flow and the location of endoleaks [52, 53].

CT has the advantage of being an objective imaging modality with short scan times and the ability to investigate surrounding organs in detail, allowing evaluation of potential issues such as infection and abscesses. However, the use of contrast media carries the risk of contrast nephropathy and should be avoided in chronic kidney disease patients. Contrast-induced nephropathy can occur in up to 7%-12% of patients undergoing CECT despite pre-hydration protocols using saline and sodium bicarbonate [54, 55]. Additionally, contrast media can cause hypersensitivity reactions with a frequency of 0.5%-3% and is generally contraindicated for patients who have experienced contrast media allergy or have a higher risk of such allergies [56-58]. If these contrast media-related complications are a concern, consideration should be given to diagnosing endoleaks using other imaging modalities. Furthermore, repeated CT scans may increase radiation exposure and the risk of cancer, especially in younger patients [59, 60].

MRI

Unlike CT scans, MRI scans do not involve radiation exposure. However, they are time-consuming and contraindicated for patients who are claustrophobic or have implanted ferromagnetic devices such as pacemakers. Contrast-enhanced MRI (CE-MRI) using gadolinium (Gd) is useful in identifying endoleaks, with a very high sensitivity of 90%-100% and specificity of 82%-100% [47, 48, 61]. Although the spatial resolution of MRI is inferior to that of CT, CE-MRI can identify small, faint endoleaks that are not recognized with CECT due to higher tissue resolution. In particular, CE-MRI outperforms CECT in sensitivity for detecting TIIELs, suggesting that CE-MRI may be considered in cases where endoleaks are not visualized on CECT [61, 62]. Furthermore, 4D-MRI, which involves continuous scanning of multiple phases after contrast media injection, is useful for identifying the direction of blood flow and the location of endoleaks, similar to 4D-CT [63].

It should be noted that the use of Gd is contraindicated for patients with chronic kidney disease and those on dialysis due to the concern of nephrogenic systemic fibrosis. Alternatively, CE-MRI using superparamagnetic iron oxide (SPIO) is also helpful for endoleak detection and can be indicated as an alternative agent in patients with chronic kidney disease, but is not reimbursed for surveillance after EVAR [64].

Ultrasonography

Ultrasonography, including color-duplex ultrasound (CDUS) and contrast-enhanced ultrasound (CEUS), has an inherent advantage in real-time visualization and evaluation of blood flow, offering information on hemodynamics without radiation exposure. While the sensitivity and specificity of CDUS are 62%-83% and 90%-97%, those of CEUS are 90-98% and 88-93%, respectively, indicating that CEUS is superior to CDUS in detecting endoleaks [25, 65-69].

Contrast agents used in ultrasonography include perfluorocarbon and sulfur hexafluoride, which act as microbubbles. These agents have been proven to be quite safe and can be indicated for patients with chronic kidney disease, although caution should be exercised in patients who have recently experienced acute coronary syndrome [70-72]. The microbubbles are covered by a phospholipid outer wall, which causes echo reflections that can be visualized. While repeated boluses enable multiple scans, CEUS has drawbacks including a limited field of view and short scanning time [71-73]. Furthermore, the accuracy of diagnosis depends on the operator's skill and patient factors such as obesity, vessel diameter, and distribution of intestinal gas. The lack of reproducibility in the scan area is another issue.

Angiography

Angiography enables the differentiation of endoleaks based on their location and timing. Aortography performed near the proximal or distal ends of the stent graft can provide detailed visualization of type Ia and Ib endoleaks. Type II endoleaks can be accurately evaluated through selective angiography of the inferior mesenteric and iliolumbar arteries that feed the responsible vessels. Type III and type IV endoleaks can be detected by angiography focused on the stent-graft's modular junctions or areas of potential graft fabric tears.

While angiography offers superior temporal resolution compared to other imaging modalities, it is more invasive and thus unsuitable for routine follow-up. However, angiography is recommended in cases where aneurysm sac enlargement is observed without obvious endoleaks on other imaging modalities, or when the precise location of an endoleak remains unclear.

Findings of Endoleak on Follow-up Imaging

Type I endoleak

TIELs typically appear as early contrast enhancement adjacent to the proximal or distal ends of the stent graft during the arterial phase [72, 74] on CECT (Fig. 1-2). The contrast accumulation is usually seen centrally within the aneurysm sac and is often continuous with one of the attachment sites [11]. On MRI, TIELs demonstrate early enhancement times, with a mean of 0.28 seconds (± 0.83) after contrast administration [45]. CEUS can also detect TIELs, showing real-time flow dynamics with the contrast agent entering the aneurysm sac from the attachment site [75]. The sensitivity for detecting TIaELs is generally high across imaging modalities, with CECT, CE-MRI, and CEUS all demonstrating sensitivities over 90% [52]. Aortic neck dilatation (AND) and stent-graft sealing length reduction are strongly associated with TIaELs after EVAR [9, 76, 77]. AND occurs in about 25% of patients and correlates with adverse outcomes, including TIaEL and reinterventions [76]. The shortest apposition length <10 mm on initial postoperative CT is a significant predictor of late TIaELs [8]. Larger neck diameters and shorter preoperative neck lengths also increase risk [8, 10]. Therefore, vigilant monitoring of neck dilatation and sealing length changes during follow-up imaging is essential.

Figure 1.

Type Ia endoleak.

a-c: CT demonstrated a type Ia endoleak, with contrast leaking from the proximal attachment site of the stent-graft into the aneurysm sac, leading to aneurysm rupture (arrow: type Ia endoleak).

d, e: Additional aortic extension resolved a type Ia endoleak.

Figure 2.

Type Ib endoleak.

a-c: Dilation of the left common iliac artery with contrast within and surrounding the stent-graft’s limb, indicative of a type Ib endoleak (arrow: type Ib endoleak).

d-f: To address the type Ib endoleak following EVAR, an iliac limb extension was deployed into the external iliac artery. This intervention successfully eliminated the endoleak. Post-procedural CT confirmed the absence of any residual endoleak.

Type II endoleak

TIIELs generally appear as contrast enhancement within the aneurysm sac that is not directly adjacent to the stent graft on CECT [12, 74] (Fig. 3). The enhancement is often observed in a peripheral or posterior location within the sac [78]. TIIELs may be visible on arterial phase imaging but are more commonly detected on delayed phase imaging, usually 2-5 minutes after contrast administration [74, 79]. TIIELs are detected as hyperintense signals within the aneurysm sac on contrast-enhanced sequences on MRI [61]. Time-resolved MRA can demonstrate delayed filling of the sac via collateral vessels [80]. On ultrasound, TIIELs appear as echogenic flow within the aneurysm sac that is separate from the stent graft [11]. CDUS may show bidirectional flow within the sac [12]. CEUS improves the detection of TIIELs compared to standard ultrasound, demonstrating enhancement within the sac that persists longer than arterial phase enhancement [72]. A key feature of TIIELs is their connection to patent branch vessels, most commonly the inferior mesenteric artery, lumbar arteries, and medial sacral artery [74, 78]. The feeding vessels may be visualized on CECT or CE-MRI, particularly with three-dimensional reconstructions [79, 80]. A recent study introduces a crucial distinction in TIIELs: “well-defined” versus “moyamoya” endoleaks. Moyamoya endoleaks, characterized by heterogeneous contrast opacity with indistinct borders on CECT, show significantly poorer outcomes after TAE compared to well-defined endoleaks. This classification provides important prognostic information, suggesting that moyamoya endoleaks may require alternative management strategies, potentially favoring early surgical intervention over TAE. Distinguishing between these endoleak types is crucial for guiding treatment decisions and improving post-EVAR outcomes [24].

Figure 3.

Type II endoleak.

a, b: CT revealed contrast enhancement at the periphery of the aneurysm sac, distant from the stent graft, which was suggestive of a type II endoleak (arrow: type II endoleak).

c, d: Right iliolumbar artery angiogram demonstrating a type II endoleak via the right 4th lumbar artery. The aneurysm sac has been embolized with NBCA, and the right 4th lumbar artery has been occluded with coils.

e, f: Type II endoleak was visualized via a retrograde flow of IMA, which was embolized with coils.

g, h: Follow-up axial CT scan 1 year after embolization. The image demonstrates a noticeable reduction in the size of the aneurysmal sac.

Type III endoleak

TIIIELs are observed as blood flows into the aneurysm sac near a defect in the endograft material or junctional separation between graft components (Fig. 4-5). On CECT, TIIIELs manifest as collections of contrast material centrally within the aneurysm sac, usually distant from the attachment sites with the native vessels [12, 74]. These are frequently large collections that opacify densely with contrast. The contrast enhancement is often seen during the arterial phase, indicating high flow [12]. TIIIELs are revealed as areas of high signal intensity within the aneurysm sac on contrast-enhanced sequences in MRI scans [74]. Time-resolved MRA can demonstrate early filling of the aneurysm sac, consistent with the high-flow nature of TIIIELs [12]. CDUS may show a high-velocity jet arising from the mid-portion of the stent graft in TIIIELs [74]. On CEUS, TIIIELs typically demonstrate rapid contrast filling of the aneurysm sac [12, 79]. Overall, the central location of contrast enhancement within the aneurysm sac, early arterial phase filling, and high-flow characteristics help distinguish TIIIELs from other endoleak types on multimodality imaging [12, 25, 74]. Further observing the temporal changes in graft configuration can aid in the early detection of potential TIIIaEL, which is often caused by separation between stent graft components. Regular imaging follow-up allows for the identification of subtle changes in the relative positions of stent graft modules, potentially indicating an increased risk of TIIIaELs development before it becomes clinically apparent.

Figure 4.

Type IIIa endoleak.

a, b: CT reveals a type IIIa endoleak at the junction of stent graft components, where separation has occurred (arrow: type III endoleak).

c: No endoleak was identified after additional stent-graft placement to provide a new sealing zone.

d, e: CT 1 year after stent-graft relining showed shrinkage of the aneurysmal sac.

Figure 5.

Type IIIb endoleak.

a, b: A linear area of contrast enhancement was visualized near the flow divider of stent-graft, which was suspected of type IIIb endoleak (arrow: type IIIb endoleak).

c: Angiography within the stent graft identified a type IIIb endoleak through the graft fabric near the flow divider (arrow: type III endoleak). Stent graft relining successfully eliminated the endoleak.

d, e: Resolution of endoleak was confirmed in post-procedural CT.

Type IV endoleak

TIVELs are typically seen immediately after stent-graft placement on the first post-procedural angiogram [78]. On imaging, TIVELs appear as a diffuse blush of contrast within the aneurysm sac without a focal source [12, 25]. They are usually self-limiting and resolve spontaneously as the graft material becomes less porous over time [11, 74] (Fig. 6).

Figure 6.

Type IV endoleak.

a, b: Completion angiography showed contrast with a diffuse haziness surrounding the stent-graft, indicative of a type IV endoleak.

c, d: While the endoleak persisted 1 week after the procedure, it spontaneously resolved within 1 month.

Type V endoleak

Type V endoleak, also known as endotension, is characterized by aneurysm sac enlargement without evidence of a detectable endoleak on conventional imaging modalities. On CT angiography, endotension appears as an increase in aneurysm sac size without visible contrast extravasation into the sac (Fig. 7). MRI may show T2 hyperintensity within the aneurysm sac, suggesting the presence of fluid or thrombus, despite the absence of a visible endoleak [42]. CEUS can be valuable in detecting endotension, as it may reveal slow flow within the aneurysm sac that is not apparent in other imaging modalities. However, even with CEUS, no definite source of endoleak may be visualized in cases of true endotension. In some instances, delayed CT imaging up to 300 seconds after contrast administration may demonstrate a very slow accumulation of contrast within the aneurysm sac in cases of endotension [52]. This highlights the importance of delayed phase imaging in the evaluation of potential endoleaks. Ultimately, the diagnosis of endotension relies on the combination of aneurysm sac enlargement on follow-up imaging studies without evidence of a definite endoleak, despite thorough evaluation with multiple imaging modalities [43].

Figure 7.

Type V endoleak (endotension).

a, b, c: Although EVAR appeared successful (a, b), the 1-year follow-up revealed unexpected sac enlargement with no signs of endoleak (c).

d: No obvious endoleak was identified during direct visualization of the stent graft and aneurysmal sac. Following a thorough examination, aneurysm sac plication was performed to address the persistent sac enlargement.

e: Sac shrinkage was achieved after aneurysmorrhaphy.

Summary and Future Direction

While CECT remains the gold standard for endoleak detection and surveillance after endovascular aortic repair, other imaging modalities have emerged as valuable complementary or alternative techniques. CEUS demonstrates high sensitivity for endoleak detection, potentially surpassing CECT for identifying low-flow endoleaks. It offers the advantages of no radiation exposure and no nephrotoxic contrast, making it suitable for more frequent follow-up. In contrast, CDUS, widely available and inexpensive, has lower sensitivity compared to CECT and is highly operator-dependent. MRI also shows high sensitivity, particularly for small endoleaks, but is limited by availability and compatibility issues with some endografts.

For lifelong surveillance, a multimodality approach tailored to individual patient factors may be optimal. CECT remains essential in the early postoperative period and for planning reinterventions. CEUS or CE-MRI could be integrated into surveillance protocols to reduce cumulative radiation and contrast exposure, especially in younger patients or those with renal impairment. Regardless of the modality used, accurate measurement and comparison of aneurysm sac diameter over time is crucial. Ultimately, larger prospective studies are needed to establish the most effective and cost-efficient long-term surveillance protocols incorporating these various imaging techniques.

Conflict of Interest

None