Endovascular aortic aneurysm repair (EVAR) is a less invasive and less morbid treatment than open surgical repair (OSR) of abdominal aortic aneurysms (AAA), but post-EVAR imaging surveillance is an essential part of good patient care.¹ Early or late complications after EVAR continue to be a concern if the aneurysm is not completely excluded by the endograft. For successful protection from aneurysm-related complications (rupture, death), the aortic endograft needs to separate blood flow and pressure from the aneurysm wall. If the aneurysm sac is pressurized by an endoleak, a persistent or recurrent connection to the arterial system, there is risk of aneurysm growth and rupture. Endoleak types are summarized in Table 1.

Table 1: Endoleak types after endovascular repair of infrarenal AAA

Type I	Incomplete seal at the proximal (a) or distal (b) seal zone
Type II	Retrograde arterial branch flow from (a) inferior mesenteric artery or (b) lumbar arteries
Type III	Incomplete seal between components, component separation, fabric erosion, or tear
Type IV	Leak due to fabric porosity (generally only present at the time of operation)
Type V	Endotension, sac enlargement without a discernable endoleak (uncommon with current grafts)

An early type I endoleak represents a primary treatment failure, usually due to incorrect graft selection, deployment into diseased "landing zones," or other technical error. A late type I endoleak can occur from loss of fixation due to graft migration or progressive aneurysmal degeneration of the landing zone. This occurs more frequently when EVAR is performed with implantation into "disadvantaged" seal zones (aneurysmal, angulated, uneven, or with plaque or thrombus).

Type III endoleaks can sometimes be due to technical failure at the time of initial procedure, but movement due to hemodynamic forces or aneurysm remodeling, as well as degradation of components over time, can create a type III endoleak.

Type I and III endoleaks need treatment.

Type II endoleaks are the most common type. They may be recognized at the time of EVAR in 15-25% of patients, but immediate treatment is not required. Spontaneous thrombosis of the endoleak occurs in approximately half of type II endoleaks. These patients do not need treatment.

While type II endoleaks may be benign, their natural history varies. Persistence of a type II endoleak at six months is seen in 10% to 15%. The aneurysm sac size may shrink (25%), remain stable (50%-70%), or expand (25%) during follow up.^2-4 In some cases, a type II endoleak may be detected during follow up, even after one or more imaging studies did not find an endoleak.⁵ Thus, the requirement for serial imaging cannot be safely ignored. Type II endoleaks are generally treated when associated with aneurysm expansion.

Imaging surveillance is necessary after EVAR, since secondary interventions may be needed. Baseline imaging should be done soon after the procedure (e.g., within 30 days). If there is no endoleak or other concerning finding, annual imaging surveillance is recommended. When there is a type II endoleak for which surveillance rather that repair is planned, imaging should be done at six-month intervals.

Triple-phase CT scanning is an excellent modality for post-EVAR evaluation, with 1) imaging prior to contrast administration; 2) image acquisition during the arterial phase after IV contrast injection; and 3) delayed imaging sequences to detect endoleaks that fill through small defects or collateral arteries. CT scanning, however, has disadvantages, including risks associated with renal injury from contrast administration and the cumulative effects of radiation exposure. Because of these risks and the higher cost of CT compared to ultrasound imaging, duplex scanning may be an alternative for post-EVAR follow up.⁶

Though color duplex scanning is effective for post-EVAR follow up, some have found it less sensitive than contrast-enhanced CT scanning for endoleak detection.⁷ This may not be a clinically significant disadvantage, as endoleaks missed by duplex scanning are generally small type II endoleaks that do not need treatment.⁸ Ultrasound imaging can provide accurate serial measurements of aneurysm sac size, which is key, as treatment of type II endoleaks is generally reserved for aneurysms that enlarge. Size matters, and ultrasound is effective for detecting aneurysm growth. AAA diameter measurements (with either CT or ultrasound imaging) are most reliable and reproducible when standardized measurement techniques are used and the maximum transverse dimension measured orthogonal to the vessel axis is reported.

Duplex scanning can also detect other complications after EVAR, including compromise of renal artery origins, kinking of graft limbs that threaten patency, and access vessel injuries, including iliofemoral dissections or puncture site pseudoaneurysms.⁹ Further evaluation and treatment is considered when an aneurysm is found to have increased in size under surveillance or when a significant arterial stenosis is demonstrated.

While the imaging capabilities and color Doppler sensitivity of modern duplex scanners are sufficient to identify most endoleaks, contrast-enhanced ultrasound (CEUS) further increases the examination's sensitivity, helping to find small endoleaks and to characterize their sources.^7,8,10 There are three second-generation ultrasound contrast agents available for use in the USA, including lipid microspheres with perflutren (Definity®, Optison®) or sulfur hexafluoride (Lumason®), but CEUS for post-EVAR follow up remains an off-label use of these agents. In practice, CEUS is selectively used for EVAR follow up, but it requires additional time and resources (personnel, supplies). Of note, the added costs associated with CEUS may not be reimbursed.

The duplex ultrasound findings associated with endoleaks are summarized in Table 2.

Table 2: Sonographic features of endoleaks

B-mode (gray-scale) imaging	Contents of aneurysm sac may be appear heterogeneous
Color Doppler imaging	Flow detected within the aneurysm sac Flow in lumbar or inferior mesenteric arteries may be seen
Power Doppler imaging	Flow detected within the aneurysm sac
Pulsed Doppler (spectral Doppler)	Low velocity flow within the aneurysm sac, typically with to-fro pattern
Contrast-enhanced imaging	Enhancement of blood in aneurysm sac after contrast injection Seen with B-mode or harmonic imaging

It is best to have duplex scanning performed by a qualified individual using standardized techniques, e.g., a credentialed vascular technologist in an accredited vascular laboratory. In this setting, duplex scanning for EVAR follow up is reliable, cost-effective, and well accepted by patients. As such, compliance with scheduled follow up may be improved. The occasional incomplete examination (due to technical limitations, overlying bowel gas, or patient habitus) can be recognized by the technologist and reported.

If a good vascular laboratory is available, a practical strategy is to get both a contrast-enhanced CT and a duplex scan when the first post-EVAR imaging is done.¹¹ These findings can be compared, confirming that the baseline findings from both imaging modalities correlate. Duplex scanning can then be used for surveillance. Follow up imaging with CT scan can be used selectively, when additional information for endoleak characterization is needed.

References

1. Chaikof EL, Brewster DC, Dalman RL, et al. SVS practice guidelines for the care of patients with an abdominal aortic aneurysm: executive summary. J Vasc Surg 2009;50:880-96.
2. Kray J, Kirk S, Franko J, Chew DK. Role of type II endoleak in sac regression after endovascular repair of infrarenal abdominal aortic aneurysms. J Vasc Surg 2015;61:869-74.
3. Beeman BR, Murtha K, Doerr K, McAfee-Bennett S, Dougherty MJ, Calligaro KD. Duplex ultrasound factors predicting persistent type II endoleak and increasing AAA sac diameter after EVAR. J Vasc Surg 2010;52:1147-52.
4. Rayt HS, Sandford RM, Salem M, Bown MJ, London NJ, Sayers RD. Conservative management of type 2 endoleaks is not associated with increased risk of aneurysm rupture. Eur J Vasc Endovasc Surg 2009;38:718-23.
5. Zhou W, Blay E Jr, Varu V, et al. Outcome and clinical significance of delayed endoleaks after endovascular aneurysm repair. J Vasc Surg 2014;59:915-20.
6. Pandey N, Litt HI. Surveillance imaging following endovascular aneurysm repair. Semin Intervent Radiol 2015;32:239-48.
7. Mirza TA, Karthikesalingam A, Jackson D, et al. Duplex ultrasound and contrast-enhanced ultrasound versus computed tomography for the detection of endoleak after EVAR: systematic review and bivariate meta-analysis. Eur J Vasc Endovasc Surg 2010;39:418-28.
8. Guo Q, Zhao J, Huang B, et al. A systematic review of ultrasound or magnetic resonance imaging compared with computed tomography for endoleak detection and aneurysm diameter measurement after endovascular aneurysm repair. J Endovasc Ther 2016;23:936-43.
9. Blom AS, Troutman D, Beeman B, Yarchoan M, Dougherty MJ, Calligaro KD. Duplex ultrasound imaging to detect limb stenosis or kinking of endovascular device. J Vasc Surg 2012;55:1577-80.
10. Karthikesalingam A, Al-Jundi W, Jackson D, et al. Systematic review and meta-analysis of duplex ultrasonography, contrast-enhanced ultrasonography or computed tomography for surveillance after endovascular aneurysm repair. Br J Surg 2012;99:1514-23.
11. Nagre SB, Taylor SM, Passman MA, et al. Evaluating outcomes of endoleak discrepancies between computed tomography scan and ultrasound imaging after endovascular abdominal aneurysm repair. Ann Vasc Surg 2011;25:94-100.

Keywords: Endoleak, Aortic Aneurysm, Abdominal, Sulfur Hexafluoride, Mesenteric Artery, Inferior, Renal Artery, Aneurysm, False, Aneurysm, Porosity, Microspheres, Vulnerable Populations, Constriction, Pathologic, Fluorocarbons, Albumins, Ultrasonography, Doppler, Duplex, Ultrasonography, Tomography, X-Ray Computed, Thrombosis, Treatment Failure, Hemodynamics, Endovascular Procedures, Lipids

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