Endoleaks: Classification, Diagnosis, and Treatment

Diagnosis of Endoleaks with Computed Tomography

The diagnostic CT criterion for an EL is a contrast-enhancing area outside the stent-graft lumen but inside the aneurysmal sac. For correct classification of an EL, the location of the contrast-enhancing area, its relationship to the proximal and distal sealing zones, and the presence of possibly perfused side branches have to be evaluated. Additional desirable information includes stent-graft integrity, deformity, and migration.

Typically, multiphase examinations are performed with multislice CT scanners, including an unenhanced, arterial, and delayed-phase scan. Unenhanced images may be helpful in differentiating small perigraft leaks from areas of calcified thrombus or metallic structures of the stent-graft. Typically the arterial-phase acquisition clearly shows perigraft flow if present. However, Rozenblit et al. showed that a delayed CT acquisition in addition to the arterial-phase scan can help detect additional ELs ( Fig. 23.5 ). These authors also showed that the number of indeterminate findings can be reduced when an additional delayed-phase CT is acquired, so a triphasic CT acquisition protocol, including a delayed-phase scan acquired 50 seconds after injection of contrast material, has now been adopted in many institutions.

In the case of a type I EL, direct communication of the parent vessel with the sac lumen may be depicted. Sometimes this communication can be displayed best in a multiformatted reconstruction (see Fig. 23.1 ).

The diagnostic clue to a type II EL is seeing contrast-enhancing side branches with direct communication to the area of contrast enhancement in the sac ( Fig. 23.6 ).

In type III ELs, the EL is usually in direct communication with the stent-graft, and direct communication between the lumen of the stent-graft and sac can often be seen. Disintegration of the stent-graft and metallic strut fractures is a strong indicator of a type III EL. Although the metallic skeleton of the stent-graft is typically evaluated by plain abdominal films, the same information can be provided by modern multislice CT scanners.

Type IV ELs do not play a clinically significant role, as already mentioned.

Type V ELs, by definition, are characterized by increasing diameter of the sac without a detectable EL.

Although CT has been advocated as the gold standard for EL detection, it has several drawbacks. One is that a considerable amount of potentially nephrotoxic contrast medium is required. Depending on the multislice CT scan protocol, 80 to 120 mL of contrast medium per examination is used. Over a 5-year period, Azizzadeh et al. found impaired renal function in 83% of their 398 patients who underwent EVAR. The authors conclude that periprocedural renal protection and the use of alternative contrast agents during follow-up might be beneficial in selected patients.

The second drawback of multislice CT is radiation exposure. Although several technical features are implemented in modern CT scanners to reduce radiation exposure, multislice CT remains a radiologic examination responsible for more than two-thirds of the total patient radiation dose associated with medical imaging. Based on an equation published by the International Commission for Radiation Protection, the lifetime risk for induction of cancer is 280 per million patients after a single abdominal CT scan. White and Macdonald estimate a total effective radiation exposure of 145 to 205 mSv for 70-year-old patients, including planning CT, EVAR, and surveillance CT at 1, 3, 6, and 12 months and yearly thereafter. In their calculation, this leads to a lifetime cancer risk of 0.42%, or 1 in 240. These numbers become more worrisome with decreasing patient age. For a 50-year-old patient, the cancer risk would be 0.73%, or 1 in 140. However, a mitigating factor is that we are dealing with an elderly patient population usually well above the age of 65, and it has been well documented that radiation risk is significantly lower in the elderly.

Nevertheless, contrast medium load and radiation exposure are important concerns when serial CTs are performed for long-term follow-up of patients after EVAR. For this reason, two studies evaluated whether the commonly used multiphase CT protocol can be reduced by eliminating the early arterial phase or the unenhanced or delayed phase or both. The authors of the first study concluded that the arterial phase may not be necessary for routine EL detection, whereas the authors of the latter study found that the combination of unenhanced and arterial-phase CT affords the highest positive predictive value for detection of ELs after 1 month, and that the delayed phase does not significantly increase the sensitivity for detection of ELs, although ELs not seen on early-phase CT may be seen on delayed-phase CT. In another study, Bley et al. showed that serial volumetric analysis of nonenhanced CT may serve as an adequate screening test to identify ELs that cause a volumetric increase of more than 2%. Recently, dual-energy CT has been evaluated for EL detection after EVAR. With dual-energy CT, only one single delayed scan is acquired with a dual-source CT scanner. From this dataset, a virtual nonenhanced scan is calculated. Compared with a triphasic CT protocol, there was no significant difference in the numbers of ELs detected, but the radiation dose was up to 61% less.

The third drawback of CT is that it cannot display flow dynamics. In some cases, correct EL classification depends on evaluating the direction of perigraft flow and the direction of flow in aneurysmal side branches. This problem is discussed in the sections describing the value of intraarterial digital subtraction angiography and US in patient follow-up after EVAR.

The ideal CT protocol has not yet been defined. Although it is of utmost importance to apply the CT protocol with the best trade-off between radiation and contrast medium exposure and diagnostic yield, proper timing and selection of the imaging modality used during follow-up have the largest influence.

Diagnosis of Endoleaks with Ultrasonography

Ultrasonography is being used more frequently as an alternative to multiphase CT in patient follow-up after EVAR. It is less expensive, and no radiation or nephrotoxic contrast material is required ( Fig. 23.7 ). The reliability of US in detecting ELs has been evaluated in numerous studies. In an early study, Sato et al. reported a 74% specificity and 97% sensitivity of color-coded US (ccUS) for detection of ELs. Better results were reported by Zannetti et al., who found an encouragingly high 91.7% sensitivity and 98.4% specificity of ccUS for EL detection, with CT serving as the standard of reference. In contrast, Raman et al. found only 42.9% sensitivity of ccUS in detecting ELs. Its specificity was 96%, and the positive predictive value was 53.9%, with only modest correlation of the results of ccUS and CT. Based on their experience with 281 patients, the authors concluded that ccUS cannot effectively replace CT after EVAR.

In more recent studies, contrast-enhanced US (CEUS) has been compared with CT. Encouraging results have been reported by Giannoni et al. and Napoli et al. In both studies, CEUS showed ELs not detected by other imaging modalities. One potential advantage of US over other imaging modalities is its ability to characterize blood flow dynamics. For instance, a persistent EL is typically associated with an inflow and outflow situation. In contrast to CT, these flow phenomena can be evaluated with US. In a small but interesting series of 18 patients, Bargellini et al. used CEUS to analyze the hemodynamic flow pattern of type II ELs and found a correlation of type II EL flow dynamics and AAA size. Type II ELs with wash-in and washout times less than 100 and 520 seconds, respectively, were called hyperdynamic ELs , in contrast to hypodynamic ELs , which had longer wash-in and washout times. The authors showed that a volume increase of 1 mL per month or greater might be associated with hypodynamic type II ELs. They hypothesized that insufficient outflow from the sac in a hypodynamic EL could lead to pressurization of the aneurysm and possible enlargement. On the other hand, in hyperdynamic type II ELs, a widely patent inflow and outflow tract might allow free flow through the aneurysm without significant effects on pressure. In a later study, Beeman et al. also tried to determine Doppler sonographic flow parameters, which may be associated with the persistence of type II ELs and an increase in AAA sac diameter. In contrast to the Bargellini study, they found no correlation between the intrasac flow velocity and increased AAA size after EVAR. However, the presence of multiple type II ELs and a bidirectional intrasac flow were identified as possible predictors of an increase in sac diameter. In another study, Parent et al. showed that a to-and-fro flow pattern in a type II EL is more often associated with spontaneous sealing of the EL than is the case with a monophasic or biphasic flow pattern.

Despite these interesting observations, the roles of ccUS and CEUS are still under evaluation. In a systematic review performed by Ashoke et al. and based on eight published and two unpublished studies, the sensitivity and specificity of US were 69% and 91%, respectively. The authors concluded that US does not have sufficient diagnostic accuracy for detection of all ELs in routine clinical practice. However, this conclusion was only partially confirmed by Mirza et al., who performed a more recent and more extensive systematic review. Twenty-one studies comparing ccUS with CT and seven studies comparing CEUS with CT were included. Similar to Ashoke’s analysis, sensitivity for ccUS (77%) was rather low, but specificity was 94%. However, CEUS had a sensitivity and specificity of 98% and 88%, respectively.

Another important parameter during patient follow-up after EVAR is sac diameter. US was thought to be less reliable than CT for sac measurement, but several recent studies have shown them to be equally reliable.

The fact that CEUS may detect ELs not shown by CT angiography (CTA) explains why CTA can no longer be considered the standard of reference. Most endoleaks are shown by both imaging modalities, but some are only seen on CTA and some only by CEUS.

In general, ccUS and especially CEUS are valid methods for patient follow-up after EVAR, and in many centers, US has replaced CTA as the standard method.