Preoperative CT is useful for the assessment of the aneurysm size even in patients where ultrasound imaging is impossible. Diameter measurements can be done without contrast. However, a complete preoperative assessment of the aortic anatomy by the CT should include scans with and without contrast and have thin axial reconstruction (≤1 mm). The scan should be in the arterial phase and extend from the skull base to the lesser femoral trochanter. This will allow mapping of the supraaortic circulation in cases of thoracic aneurysms as well as the access vessels. The assessment of the supraaortic circulation is of importance since it allows evaluation of anatomic variations such as separate origin of the left vertebral artery or aberrant right subclavian artery. Moreover it depicts the completeness of the circle of Willis and thereby assists in the decision on the need for revascularization of the left subclavian artery. CTs done for dissections or aneurysms involving the arch and the ascending aorta are preferably done with EKG-gated technique in order to avoid movement artifacts. Furthermore the EKG-gated CT offers the possibility of triple rule-out diagnostic [45]. Retrospective EKG-gated CT scanning allows for dynamic imaging of the aorta, while maintaining the radiation dose comparable to conventional CT. Dynamic CT can also identify pulsatile changes in the thoracic and abdominal aorta during the heart cycle [46]. The significance of these findings and their implications for stentgraft oversizing during planning as well any consequences for the long-term durability are still uncertain. Other applications of CT imaging are computational post-processing for the refinement of the rupture risk using finite element analysis [47].

EVAR relies on the remote insertion of the stent-graft without disrupting the physical integrity of the aneurysm wall. Aneurysm size can, therefore, be used as an indicator of the effectiveness of EVAR in a similar way as it was for the preoperative evaluation of the rupture risk. Aneurysms that become well excluded after EVAR are expected to decrease in size (Fig. 23.3). Consequently, imaging methods have been used for the periodical follow-up after EVAR, evaluating aneurysm size, stent-graft integrity, and/or migration and endoleak status. The use of spiral CT scans for the postoperative follow-up after aortic aneurysm repair was very frequent during the first 15 years after the introduction of EVAR. Currently there has been a trend for changing follow-up after standard EVAR of AAA replacing CTA with ultrasound. However, CT is still the cornerstone for thoracic and more complex EVAR.

Follow-up CTA scans are performed with nonionic contrast medium enhancement as described above for the preoperative work-up. In addition, the scan is repeated 60 s later to obtain a delayed scan. The inclusion of a delayed scan to obtain a late arterial phase has the potential of identifying type II endoleaks that could otherwise remain unnoticed [48]. Even during follow-up, low-dose CT protocols can be used to reduce radiation in the repeated exams, without affecting the diagnostic outcome [49].

The increase of the number of detectors up to 320 rows has increased the amount of information and the time required to obtain it. The other significant development in CT scanners in recent years has been the introduction of dual energy CT also called dual source. This consists of two X-ray tubes that simultaneously transmit at different energy levels (usually 80 and 140 kV). This will give different attenuations at the same time and has the potential to reduce the radiation of the exams during EVAR follow-up since it permits the production of virtual non-enhanced images by subtraction of the contrast medium which will allow the identification of endoleaks without the need of a non-enhanced scan [50–54].

As mentioned above, the introduction of multidetector CT scanning increased the definition of the imaging and allowed for 3D imaging which increased the post-processing possibilities. Axial, coronal, and sagittal reconstructions are currently a standard of CT imaging. The transfer of the DICOM raw images to workstations specifically dedicated to vascular imaging post-processing increases the possibilities of the imaging analysis vastly. This is important when planning for EVAR, especially when more advanced prostheses such as fenestrated and branched endografts are needed. It is important to emphasize the need for good quality raw imaging to be able to perform good post-processing. This means high-resolution imaging with thin axial reconstruction of less than 1 mm with good arterial phase enhancement.

Data post-processing allows the reconstruction of multiple multiplanar reconstructions (MPRs) as well as maximum intensity projections (MIPs) with free rotational ability at different angles allowing detailed assessment of different parts of the anatomy such as aortic branch takeoff. These may be useful to plan for the EVAR procedure including the best projections to visualize the different vessels [55]. Another tool of particular importance for EVAR planning is the center line of flow analysis (CLF, Fig. 23.4). This involves the manual or semiautomatic creation of a CLF and thereafter length measurements along that line with evaluation of orthogonal reconstructions along the CLF. Length measurements along CLF are helpful in determining the length of standard grafts since the aorto-iliac segment tends to elongate and become more tortuous upon aneurysm development which makes it difficult to measure lengths in the traditional axial, coronal, or sagittal reconstructions [56]. Moreover, when planning more complex endografts CLF lengths are also helpful in assessing the distance between the different aortic branches that are included in endovascular repair (Fig. 23.4c). The position of the target vessel fenestrations on the endograft also needs to be planned according to their position on the circumference (clock position) and the orthogonal reconstructions can be very useful in very tortuous cases (Fig. 23.4d, e). All CLF measurements are dependent on the accuracy of the graft following the CLF and this is not always the case, especially in cases of high tortuosity. For this reason, one needs to check the CLF path and manually adjust it whenever needed before performing the measurements.

Post-processing is also very useful in the follow-up after EVAR. Applications such as the creation of multiple MIPs with different window settings and projections will allow assessment of stent-graft integrity and possible dislocation. Fly-through applications that allow virtual angioscopy assist in the assessment of the different branches and possible compression or kinking of stents or stent-grafts [57] CLF is useful even in the assessment of stent-graft migration especially when the endograft is located in a tortuous region such as the distal aortic arch [58]. The use of workstations with the possibility of simultaneous visualization of different exams is very useful during the follow-up after EVAR since it allows the synchronization of images at the same level and in that way evaluation of any changes during follow-up including the evolution of the aneurysm size. This synchronization is also very useful using scans with and without contrast to distinguish between calcifications and endoleaks. Moreover, workstations also provide different methods of measuring the aneurysm size, such as axial diameter, orthogonal diameter, and volume of the aneurysm size. These methods seem to have increasing reliability in the assessment of the evolution of the aneurysm size during follow-up [27, 59, 60], but also require increasing amount of post-processing. A good strategy is therefore to successively take advantage of these different methods in doubtful cases, but using axial diameter measurements in the vast majority. These should be done perpendicular to the longest diameter at the chosen level in order to avoid overestimation caused by tortuosity.