Although CT scanning techniques may vary by institution, some basic principles apply. Unenhanced images are obtained primarily to evaluate the degree of aortoiliac wall calcification. CTA is then performed with thin sections and precise contrast timing from the lung bases to the pubic symphysis. At our institution we use a standardized dose of 100 mL iodinated nonionic contrast medium (iohexol 350 mg I/mL) injected at 4–5 mL/s with bolus tracking at 150 HU over baseline on the aorta at the celiac trunk. We use 0.625-mm axial sections for 3D reconstruction and 1.25- to 2.5-mm images for primary review at the workstation. A 60-second delayed scan may be acquired to evaluate for suspected inflammatory aneurysm [11].

Several radiation dose reduction tools are available on modern CT scanners and should be used for EVAR imaging. Automatic exposure control (AEC) adjusts the radiation dose according to patient size and tissue attenuation. With z-axis AEC modulation software, the mAs is varied along the length of the patient according to the topographic image. Angular AEC adjusts the dose in the xy plane as the x-ray tube rotates around the patient. With these techniques, use of AEC modulation software can reduce the dose up to 40–50% [15]. Although not widely available as of this writing, automated tube potential selection reduces the peak kilovoltage and increases the tube current–time product to maintain image quality and reduce dose. Several modern CT scanners have replaced or combined filtered back projection with iterative techniques of image reconstruction. Use of iterative reconstruction loops reduces image noise and improves resolution. As a result, diagnostic-quality studies can be obtained at a significantly lower dose [15, 16].

Dual-energy CT technology is another option for dose reduction. This technique allows simultaneous acquisition of CT data with two different photon energy levels, which results in different degrees of x-ray attenuation. With this method, iodine can be subtracted and virtual unenhanced images can be created to replace the standard unenhanced series and reduce CT dose. Using a dual-phase dual-energy CT protocol can reduce the radiation dose 19.5% compared with that of a standard triphasic CT examination and yield diagnostic-quality images [17].

Several advanced reconstruction techniques should be used to assure accurate measurements, including 2D multiplanar reformation, maximum intensity projection, curved planar reformation on lumen center-line, and 3D reconstruction (Fig. 2). Diameters may be overestimated on axial images, and tortuosity must be taken into account to perform accurate measurements (Fig. 3). Use of 2D multiplanar reformation and 3D reconstructed images can improve interobserver agreement and decrease variability in preoperative measurements [18]. The greatest benefit of 3D volume-rendered imaging is the depiction and precise measurement of angulation in aneurysms with marked tortuosity. Standard and oblique multiplanar reformation, maximum-intensity-projection, and curved planar reformation images, however, can be used for accurate and reproducible diameter and length measurements [19].

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Fig. 2A —Reconstructed images are useful for assessing tortuosity and understanding angulation that may be underestimated on planar images. A, 77-year-old man with abdominal aortic aneurysm. Three-dimensional volume-rendered CT image shows overview of morphologic features of aneurysm. It is especially useful for visualizing tortuosity and involvement of landing zones.

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Fig. 2B —Reconstructed images are useful for assessing tortuosity and understanding angulation that may be underestimated on planar images. B, Same patient as in A. Curved planar reformation CT image constructed on vessel centerline shows accurate measurement of aneurysm length.

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Fig. 2C —Reconstructed images are useful for assessing tortuosity and understanding angulation that may be underestimated on planar images. C, 71-year-old man with abdominal aortic aneurysm. Maximum-intensity projection (MIP) image shows overview of degree of calcification.

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Fig. 2D —Reconstructed images are useful for assessing tortuosity and understanding angulation that may be underestimated on planar images. D, Same patient as in C. Postprocedural MIP image clearly shows metallic stent framework and placement. In this example, uncovered portion of stent extends above renal arteries to assist in proximal fixation.

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Fig. 3A —78-year-old woman with abdominal aortic aneurysm and tortuous anatomy. Cross-sectional diameters must be measured in plane perpendicular to long access of vessel. A, Axial contrast-enhanced CT image shows how tortuosity of aorta leads to overestimation of maximal aortic neck diameter. In measurement of tortuous arteries that are not properly reformatted, shortest axial dimension may be reported to avoid overestimation.

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Fig. 3B —78-year-old woman with abdominal aortic aneurysm and tortuous anatomy. Cross-sectional diameters must be measured in plane perpendicular to long access of vessel. B, Curved planar reformation CT image constructed on vessel centerline shows accurate measurement of neck diameter obtained perpendicular to long axis of aorta.

Gadolinium-based contrast agents used for 3D contrast-enhanced MRA are associated with low risk of anaphylactic reaction. Nephrogenic systemic fibrosis in patients with renal insufficiency is rare but may be seen in patients with a glomerular filtration rate less than 30 mL/min and acute renal failure. Standard gadolinium agents can be used to acquire first-pass arterial angiographic images, whereas blood-pool contrast agents, such as gadofosveset trisodium, have a prolonged intravascular circulation time. This allows generation of high-resolution 3D multiplanar reformatted images similar to those obtained with CTA. A study comparing blood-pool MRA images with CTA images [14] showed lower subjective image quality for MRA, but the preprocedural measurements resulted in selection of the same stent-graft components based on the MRA or CTA images.

Unenhanced MRA can yield measurements of equal accuracy to CTA measurements and avoids the risk of nephrogenic systemic fibrosis in patients with chronic kidney disease [12]. The imaging protocol consists of steady-state free precession sequences based on a low-flip-angle gradient echo with a short TR. Flowing blood has inherent high signal intensity compared with background tissue. This sequence allows accurate measurement of vessel diameters without use of gadolinium. Respiratory triggered 2D steady-state free precession sequences on a 1.5-T system with a 16-element phased-array body coil are obtained in the axial, sagittal, and coronal planes. The axial sequence parameters used in a 2013 study by Goshima et al. [12] were as follows: TR/TE, 2.8/1.38; flip angle, 80°; matrix size, 192 × 256; FOV, 40 × 26 cm; section thickness, 6 mm with 2-mm overlap. This axial sequence may be performed in a short acquisition time of approximately 5 minutes. Studies [12, 13] showed no difference in the stent size and configuration chosen when unenhanced MRA technique was compared with CTA.

MRA has lower sensitivity than CTA for detecting small vessels and may not show accessory renal arteries smaller than 2 mm in diameter. However, it is likely that these small renal arteries do not supply substantial renal parenchyma and would not affect stent-graft selection [13]. A second limitation is the inability to describe the degree of intimal calcification on MR images. This may increase the risk of secondary complications and affect treatment planning. For this reason, an unenhanced CT scan should be acquired in conjunction with the planning MRA to evaluate the degree of calcification in the stent-graft landing zones. MRA is a useful imaging alternative in certain cases, but CT remains the reference standard for preprocedural planning.

Several important characteristics of the aneurysm must be accurately described in the imaging report for standard stent-graft sizing. The aneurysm is described in terms of the proximal landing zone, the characteristics of the aneurysm sac, the distal landing zone, and the vascular access [3, 9] (Table 1). Following the convention of the sample planning template in Figure 4, diameters are designated D and lengths L. Reported diameters should include the aorta at the level of the most inferior renal artery (D1), the aortic neck 15 mm distal to the lowest renal artery (D2), the aorta at the bifurcation (D3), the largest aneurysm sac dimension (D4), and the common iliac arteries (D5 and D6). The length of the aneurysm neck (L1) should be given, as should the length from the lowest renal artery to the aortic bifurcation (L2) and the length of the aneurysm sac (L3). The length of the distal landing zone is described as the distance from the aortic bifurcation to the common iliac artery bifurcation (L4 and L5). Minimal diameters should be recorded in the distal landing zone (D7 and D8) and external iliac artery access vessels (Fig. 4).

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TABLE 1: Unfavorable Anatomic Characteristics

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Fig. 4 —Diagram shows example of planning template. Diameters D1–D8 and lengths L1–L5 must be accurately reported for appropriate sizing of endograft components. (Courtesy of Endologix, Inc.)

The proximal landing zone consists of the region from the inferiormost renal artery to the beginning of the aneurysmal dilatation. Proper fixation is necessary to avoid stent-graft migration, kinking, and attachment site leak (type 1 endoleak). Several characteristics of the aneurysm neck must be accurately described. The maximal acceptable neck diameter is 32 mm, which is best evaluated on properly oriented multiplanar reformatted images. The length of the neck should be at least 15 mm (although one device allows a 7-mm neck). A short proximal neck is the most common excluding factor for EVAR [20]. The angle between the superior portion of the aneurysm neck and the suprarenal aorta is preferably less than 60°. However, endografts have been successfully placed in angulated necks greater than 60° when the neck length was at least 15 mm. Neck angulation progressively decreases after EVAR, likely because of remodeling of the aneurysm sac [21]. A reverse-tapered or conical neck configuration occurs when the neck increases in diameter as it extends distally toward the main body of the aneurysm. This unfavorable configuration may lead to an improper seal and higher complication rate [11] (Fig. 5).

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Fig. 5A —72-year-old woman with abdominal aortic aneurysm. Evaluation of proximal landing zone. A, Three-dimensional CT image shows neck curvature of approximately 80°, measured between proximal neck and suprarenal aorta.

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Fig. 5B —72-year-old woman with abdominal aortic aneurysm. Evaluation of proximal landing zone. B, Curved planar reformation CT image constructed on vessel centerline shows reverse tapered neck. Distal neck diameter is larger than proximal diameter, predisposing to inadequate seal and endoleak.

Aneurysm neck thrombus should be described in terms of the percentage circumference of the aortic wall involved and the maximal thickness of the thrombus. Heavy calcification and thrombus involving more than 90° of the neck circumference are traditionally associated with greater risk of migration, endoleak, and renal artery embolization. However, a 2012 study comparing outcomes in patients with and those without neck thrombus [22] showed no clinically significant complications in short-term and midterm results. Active proximal fixation helps protect from stent-graft migration even in the presence of neck thrombus. For example, many endografts are constructed with an uncovered metallic framework that extends above the renal arteries with hooks and barbs to provide additional fixation [23]. The measured volume of neck thrombus appears to decrease over time, and despite the initial presence of heavy neck thrombus, the endograft eventually becomes closely apposed to the aortic wall [22].

Approximately 20–30% of patients have proximal neck anatomy traditionally considered unfavorable for stent-graft placement [24, 25]. Several studies of procedures performed with hostile neck anatomy outside the recommended device-specific instructions for use have shown an increased rate of late type 1 endoleaks and secondary interventions, although no difference was found in 5-year mortality, migration, sac expansion, or rupture [24, 26]. Another group [27] found that endograft placement outside the recommended anatomic guidelines is associated with increased risk of aneurysm sac expansion. If neck anatomy is treated outside the device-specific instructions for use, vigilant surveillance is necessary to accurately diagnose complications and identify aneurysm growth.

Juxtarenal abdominal aortic aneurysms have a short proximal landing zone that does not allow proper sealing of the standard endograft device. The chimney graft technique is an endovascular treatment alternative for these patients. In this technique, stents placed within the renal arteries or superior mesenteric artery create a conduit that runs parallel to the main endograft and retains blood flow to the stented aortic branch [28] (Fig. 6). This technique can be used during accidental deployment to salvage a covered branch artery or for intentional placement of the endograft above the renal arteries to obtain an appropriate seal. The chimney graft technique is performed with a combination of currently available devices, which limits cost and time associated with preparing custom devices. A downside is the necessity for brachial artery access and the associated complications. Several studies [28–31] have shown excellent technical success without persistent early endoleak. The complexity of these procedures requires detailed preprocedural imaging to accurately size the components and assure good wall apposition to limit postprocedural complications.

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Fig. 6A —75-year-old man with abdominal aortic aneurysm and endograft covering renal arty ostia. Chimney graft technique for preservation of renal arteries. A, Fluoroscopic image shows deployment of proximal cuff and bilateral renal artery stents (arrows).

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Fig. 6B —75-year-old man with abdominal aortic aneurysm and endograft covering renal arty ostia. Chimney graft technique for preservation of renal arteries. B, Coronal postprocedural maximum-intensity projection CT image shows renal artery stents (arrows) positioned parallel to proximal cuff of aortic stent-graft.

Fenestrated stent-grafts are another option for endovascular treatment when the proximal neck anatomy is not suitable for standard EVAR. These devices have fenestrations for the ostia of the major vessels to be preserved. Bare metal or covered stents are placed through these fenestrations to maintain blood flow [32, 33]. Fenestrated devices are tailored to each specific patient and require meticulous preoperative planning with a manufacturing delay of 4–6 weeks. The procedure is expensive and technically demanding. A 2012 literature review [34] revealed a 30-day mortality rate of 2.1% and a reintervention rate of 17.8%, similar to those for standard EVAR. The branch vessel patency during a mean follow-up period of 15 months was 93.2%.

The diameter of the aneurysm sac should be reported, although a strict maximal diameter does not exist for exclusion purposes (Fig. 7). Study results suggest higher perioperative risk and greater risk of endoleak for aneurysms larger than 5.5–6.5 cm [35, 36]. The shape of the aneurysm can be described as saccular or fusiform. The residual lumen through the aneurysm should measure approximately 18 mm to allow passage and proper deployment of the device [11]. Narrowing of the distal aorta at the bifurcation risks impingement or kinking of the iliac extension limbs. This predisposes to graft limb thrombosis and occlusion. Certain patients with a narrow distal aorta may be treated with an aortomonoiliac stent-graft device with occlusion of the contralateral iliac artery and placement of a femoral to femoral artery bypass. Inflammatory aortic aneurysms, associated with retroperitoneal fibrosis, are considered a relative contraindication to endovascular repair. EVAR avoids open surgery in the inflamed, fibrotic retroperitoneum and reduces the size of the aneurysm sac [37]. However, because of persistent periaortic fibrosis, especially in the presence of ureteral obstruction, additional interventions, including open surgical revision, may be required.

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Fig. 7 —59-year-old man with abdominal aortic aneurysm. Oblique sagittal 2D multiplanar reformation CT image shows proper measurement of maximal aneurysm diameter in plane perpendicular to long axis of aorta.

It is helpful to describe the extent of thrombus and calcification of the aneurysm wall. A standard measurement to quantify thrombus does not exist, and volumetric analysis may be useful [38]. The implications of aortic thrombus are unclear, and reports are conflicting. One study [39] showed that thrombus is associated with lower wall stress, which suggests a lower probability of aneurysm expansion and rupture. However, other authors [40, 41] correlate greater thrombus burden with increased risk of aneurysm rupture due to promotion of proteolysis and wall thinning.

The preferred distal landing zone is the common iliac artery. Evaluation is similar to that of the proximal neck with assessment of diameter, length, tortuosity, and degree of calcification and thrombus. The common iliac artery diameter should not be larger than 25 mm, and at least 10 mm of length is required for an adequate seal. However, aneurysms often extend into the common iliac artery. Concurrent common iliac artery aneurysms are present in approximately 20–30% of patients [42–44]. Besides open surgical options, several advanced endovascular techniques exist for treating these combined aortoiliac aneurysms.

A commonly performed procedure consists of extending the stent-graft limb past the aneurysmal common iliac artery and into the external iliac artery to obtain a distal seal (Fig. 8). This two-stage procedure consists of embolizing the ipsilateral internal iliac artery to prevent retrograde filling of the aneurysm. Unilateral internal iliac embolization is generally considered safe, although buttock claudication or sexual dysfunction may result in 23–50% of cases [44]. Bilateral occlusion is traditionally considered to carry a higher risk of severe pelvic ischemia. However, a recent report [45] showed no significant difference in unilateral versus bilateral embolization or simultaneous versus sequential embolization in terms of ischemic side effects. Preprocedural angiography may be useful for assessing the pelvic collateral vasculature before internal iliac artery embolization [42, 46].

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Fig. 8A —87-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Treating combined aortoiliac aneurysm with graft extension into external iliac artery. A, Three-dimensional reconstructed CT image shows common iliac artery aneurysm (arrow) within expected distal landing zone.

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Fig. 8B —87-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Treating combined aortoiliac aneurysm with graft extension into external iliac artery. B, Posttreatment 3D CT image shows extension of iliac limb (arrow) into external iliac artery to obtain adequate distal seal. In this case, ipsilateral internal iliac artery was embolized to prevent endoleak.

The bell-bottom technique consists of obtaining a distal seal with a flared iliac limb. Bell-bottom grafts are useful in patients with contralateral internal iliac artery occlusion and common iliac artery diameters up to 25–30 mm [43, 47]. A study comparing the internal iliac artery embolization and external iliac artery extension technique with the bell-bottom technique [47] showed a lower incidence of perioperative complications and reinterventions with the bell-bottom technique. Long-term durability of the bell-bottom graft may be limited by continued expansion of the iliac artery aneurysm, retrograde migration of the stent, and eventual loss of the distal seal.

The snorkel graft technique consists of maintaining flow to the internal iliac artery by deploying a covered stent-graft into the internal iliac artery that runs parallel to the iliac extension limb of the main endograft device [42, 43] (Fig. 9). This technique allows endovascular treatment of patients with common iliac artery aneurysms and tortuous or short distal landing zones. With traditional endograft devices, brachial artery access is required to place the iliac snorkel graft [43]. However, if a bifurcation-sparing main endograft device is used, the snorkel stent may be placed from the contralateral femoral access by use of an up-and-over technique that avoids brachial access and the associated complications. Initial reports are promising, showing good graft patency and no evidence of aneurysm growth on surveillance images [43, 44].

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Fig. 9A —70-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Snorkel graft technique for treatment of combined aortoiliac aneurysms. A, Oblique coronal 2D multiplanar reformation CT image shows aneurysmal dilatation extending into right common iliac artery (arrow).

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Fig. 9B —70-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Snorkel graft technique for treatment of combined aortoiliac aneurysms. B, Intraoperative fluoroscopic image obtained during placement of snorkel graft (arrow) into internal iliac artery shows snorkel extending parallel to stent-graft iliac extension limb and maintaining perfusion to internal iliac artery.

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Fig. 9C —70-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Snorkel graft technique for treatment of combined aortoiliac aneurysms. C, Coronal postprocedural CT image shows snorkel graft (arrow) adjacent to iliac artery extension limb.

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Fig. 9D —70-year-old man with abdominal aortic aneurysm and common iliac artery aneurysm. Snorkel graft technique for treatment of combined aortoiliac aneurysms. D, Postprocedural 3D CT image shows covered snorkel stent (arrow) within internal iliac artery.

Branched endograft devices are under investigation but are not widely available. The procedure requires technical expertise and may not be possible in small, tortuous iliac vessels. Initial short-term results are promising: No mortality or major complications were reported after 6 months in a study of eight patients [48], and 100% technical success with no iliac branch thrombosis or aneurysm expansion after 14.7 months was reported in a study of nine patients [49]. A larger study of 37 patients [50] showed 85.4% cumulative primary patency and 87.3% secondary patency after 22 months. Two of the five patients with iliac side branch occlusion had associated symptoms of internal iliac artery occlusion. Although advanced endovascular techniques require extensive preprocedural planning and operator expertise, they may eventually replace open surgical and hybrid techniques for treating these complex cases. A detailed and accurate preprocedural imaging assessment is necessary to appropriately size the stent-graft components in the planning of a complex aortoiliac aneurysm repair.

Iliofemoral access must be adequate to deliver the stent-graft device. The external iliac artery should be at least 6 mm in diameter to accept the delivery sheath. The devices are stiff and may not traverse tortuous iliac vessels. Common or external iliac artery curvature greater than 90° is a relative contraindication to EVAR [11] (Fig. 10A). In certain cases, tortuous vessels may be straightened with a stiff wire or the stiff introducer sheath. Intravascular ultrasound may be useful at the completion of the procedure to evaluate for endograft limb kinking in tortuous arteries that may not be evident on angiograms [51].

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Fig. 10A —75-year-old woman with abdominal aortic aneurysm. CT assessment of vascular access. A, Three-dimensional volume-rendered CT image shows 180° curvature of right external iliac artery (arrow), which may complicate device passage.

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Fig. 10B —75-year-old woman with abdominal aortic aneurysm. CT assessment of vascular access. B, Axial CT image shows heavily calcified access vessel (arrow). Imaging reports should include statement describing degree of calcification because it can complicate access and use of closure devices.

Most procedures are performed percutaneously with vascular closure devices. This total percutaneous EVAR procedure avoids the surgical risks of a groin cutdown procedure. The femoral access must be carefully evaluated. Circumferential, heavily calcified vessels are nonpliable and increase the risk of rupture with device passage (Fig. 10B). Anterior calcification of the common femoral artery of greater than 50% circumference is associated with a higher likelihood of closure device failure [52]. When the access vessels are inadequate, angioplasty and stenting may be performed, or an iliac artery conduit may be used to bypass the diseased or small distal arteries and provide access for stent-graft placement. Conduits are typically 10-mm diameter Dacron polyester and may be left in the patient for future secondary procedures [51].

A complete imaging report should also include a general vascular evaluation with a description of the celiac artery, superior mesenteric artery, inferior mesenteric artery, and renal arteries. It is important to note the number, size, and location of accessory renal arteries. The pelvic vasculature should be evaluated, and any thrombosis or variant anatomy noted. The size and number of patent lumbar arteries and the patency of the inferior mesenteric artery should be described because a larger number of patent arteries is associated with higher risk of postprocedural endoleak [53]. Study results suggest a decreased incidence of type 2 endoleak when preoperative embolization of these collateral vessels is performed [54]. Coils placed in a position proximal to the left colic artery maintain collateral pathways to the inferior mesenteric artery and reduce the risk of mesenteric ischemia. A study of 108 patients showed a 15% reduction in type 2 endoleak, decreased incidence of aneurysm sac enlargement, and fewer secondary procedures when inferior mesenteric artery embolization was performed before EVAR [55].