With the development of effective treatment options for AAA, accurate diagnostic methods became more important. The first reported ultrasound visualization of the abdominal aorta in the 1960s introduced ultrasound as a potential method of AAA detection [21]. Continued advancement in ultrasound technology during that time period with the introduction of B-mode imaging allowed for two-dimensional visualization of the aorta and assessment of aortic cross-sectional diameter. A comparison between AAA diameter measurements made by lateral plain x-ray, B-mode ultrasound, and intraoperative direct aortic measurement demonstrated ultrasound to be a more accurate method of AAA size determination [22]. At about the same time, lateral plain film x-ray was shown to overestimate AAA size by 1 cm on average, while B-mode ultrasound was within 0.5 cm of true aortic diameter in 75% for patients [23]. These early findings led to ultrasound being the dominant test for AAA screening and long-term pre-intervention surveillance (Fig. 53.2). Ultimately, ultrasound became the standard for the UK Small Aneurysm Trial.

The use of computed tomography for axial cross-sectional imaging of the body was a significant advancement in diagnostic imaging in many clinical areas including AAA. Initially, CT scanning was used for the diagnosis of intracranial pathology but quickly advanced as a method to image other areas of the body [24]. Today advanced cardiac-gated CT scanning produces remarkable high-quality images (Fig. 53.3), but there are increasing concerns related to overall radiation exposure to the patient [25].

In addition to radiation exposure, CT scanning for aortic evaluation requires the use of intravenous contrast [26]. While non-contrast CT can be used to assess aortic size or potential AAA rupture, more complex issues such as visceral vessel location and aortic suitability for EVAR may require the use of intravenous contrast [27]. In an increasingly older patient population with many comorbid medical issues, patients with chronic renal insufficiency are frequently being evaluated for aortic intervention. As a result, repeated contrast CT scanning as a surveillance method for their AAA pretreatment or following EVAR may expose the patient to prohibitive renal toxicity.

As CT technology continued to improve, the data generated became a continuous data set with the advent of “spiral” CT. This continuous data set allows for images to be reformatted in any plane and allows 3D reconstructions. These 3D images provide a more accurate assessment of aortic diameter, neck angulation, location of renal and visceral vessels, and tortuosity of iliac vessels [28]. The ability to manipulate the 3D images to correct for aortic angulation allows the true aortic diameter perpendicular to the aortic center line to be determined. This measurement has become critical to assessing the aortic neck, particularly with severe degrees of neck angulation (Fig. 53.4). Currently, three-dimensional reconstruction programs such as M2S (New Lebanon, NH) or TeraRecon (Foster City, CA) allow for more accurate EVAR planning. Furthermore, virtual aortic endograft placement can be performed on these CT reconstructions allowing assessment preoperatively for potential endograft options and any issues of iliac artery access.

With the current resolution of aortic CT angiography, many centers have eliminated catheter-based angiography prior to aortic surgery. Several studies have confirmed that a high-quality CT scan is all that is needed for endograft sizing prior to EVAR [8, 27]. Patients with favorable anatomy and a suitable aneurysm for EVAR may not require advanced image reconstruction. In patients with complex aneurysm issues, three-dimensional imaging has become a valuable tool for operative planning (Fig. 53.4). These high-resolution images now allow AAAs originally felt to be a poor candidate for EVAR to benefit from endovascular repair.

Over the past few years, new methods of imaging aneurysms have been developed to attempt to define the aneurysms at risk for expansion or rupture. While traditional imaging has used aneurysm size as a surrogate for risk, other parameters may also help define the aneurysm in need of closer surveillance. Generally these techniques have focused on two areas: wall strain analysis and functional metabolic imaging.

Finite element analysis has been used to assess AAAs for many years [29, 30], but its predictive ability remains unclear. While initial efforts at assessing wall strain were performed using CT scans, more recently ultrasound has been used for the same purpose. By adding wall motion to static imaging, wall strain can be mapped in real time allowing assessment of strain to be included with size and growth to better predict which aneurysms might benefit from intervention [31–33]. While predictive data remains elusive, these newer parameters may allow better prediction of aneurysm behavior in the future.

Functional imaging of AAAs centers on the ability of some imaging modalities to measure AAA wall inflammation as a marker for rupture risk. While this research remains early, current efforts center on developing correlations with wall inflammation that may correlate with AAA growth or rupture risk. Both positron emission tomography and functional magnetic resonance imaging have been used for this purpose [34, 35], but clinical correlations with inflammatory changes on scans remain to be defined.

In the United States, asymptomatic abdominal aortic aneurysms are frequently first diagnosed from a CT scan obtained in evaluation of unrelated medical problems [36]. In addition, CT scanning is the primary operative planning method used for endograft sizing and determination of suitable aortic anatomy using endovascular aortic reconstruction. Due to this common association of AAA diagnosis and EVAR evaluation, patients are frequently evaluated by serial CT scanning until the 5.5 cm threshold is reached, justifying repair. In the UK Small Aneurysm Trial, ultrasound was used as the standard imaging modality to determine AAA diameter. In 2009, the OVER trial looked at the US VA population and randomized patient to open versus endovascular aortic repair once the AAA reached 5.0 cm in diameter. This study protocol allowed for the use of aortic measures from ultrasound, CT scans, or MRI [37]. All modalities were viewed as equivalent for entry into this trial.

In the United States, CT has been considered by some to be the “gold standard” in aortic diameter measurement. Recent studies have suggested that CT tends to significantly overestimate true aortic size, especially in severely angulated aortas. In 2001, a direct comparison of core lab ultrasound and CT data from the original Ancure endograft trial was performed [38]. In that study, 334 paired scans were evaluated and 95% of CT scan measurements were larger than the paired ultrasound images (Fig. 53.5). The average difference was 9.5 mm (P < 0.01). From this original study, it was unclear why there was such a significant difference between imaging modalities. Subsequently, further study evaluated 3D CT reconstruction (M2S, Lebanon, NH) to measure maximal aortic diameter perpendicular to center line flow and compared this diameter to matched ultrasound maximal diameter measurements. When CT diameter was corrected for a diameter perpendicular to centerline flow, the difference between ultrasound and CT decreased to 0.9 mm, which was not statistically significant. Further subgroup analysis showed that an aortic angulation of less than 25° resulted in good correlation between ultrasound and axial CT scan, but once the angulation was greater than 25°, CT significantly overestimated aortic diameter (Fig. 53.6) [8]. Ultrasound aortic diameter measurement is more likely to measure the true aortic diameter since technologists are routinely taught to adjust the imaging plane in real time to produce an image perpendicular to the aortic center line axis, resulting in a round cross section rather than an oval (Fig. 53.7). As a result, in patients with extremely angulated aortas, CT may significantly overestimate the true aortic diameter.

Ultrasound is not without potential drawbacks. Ultrasound is frequently criticized for being an operator-dependent imaging modality, while CT scanning is commonly perceived as more objective. Vascular laboratories undertaking aortic studies should be equipped with the most up-to-date instrumentation, and standardized scanning protocols should be used to produce the highest quality aortic examination. The important issues of patient preparation and scanning technique will be discussed further below.