CT angiography helps plan endovascular aneurysm repair

CT angiography of the abdominal aorta is a wellaccepted modality in the evaluation of both aneurismal and occlusive disease. In patients with abdominal aorta aneurysms, it has a major role in all stages of evaluation.

Even though its functions in the diagnosis and surveillance of AAA overlap with those of ultrasound, CTA is the first choice in pretreatment evaluation and sizing as well as in the post-endovascular repair examination for possible endoleaks.

Endovascular aneurysm repair is an alternative to open surgical aneurysm repair.^1-3 Endovascular exclusion of aneurysm via EVAR has expanded the number of patients who are safely and successfully treated by excluding pressurized blood flow within the aneurysm sac and avoiding aneurysm growth and subsequent rupture.

The ultimate goal of EVAR is the prevention of AAA rupture and death. Planning an endovascular repair of an abdominal aneurysm relies on CTA imaging to assess not only the aneurysm dimensions but also the vessel morphology.

DIAMETER MEASUREMENTS

Multislice CT images are acquired isotropically, allowing accurate, high-resolution detail in all planes. This enables confident diagnosis and accurate measurements in all dimensions. Because of the direction of the abdominal aorta, vessel diameter size can usually be measured on axial images. Important measurements evaluated on axial images include aneurysm diameter (Figure 1), proximal neck diameter, and common and external iliac artery diameters.⁴ These measurements assess both the proximal and distal fixation of the endograft to the native vessel wall as well as the size of the aneurysm and access vessels (Figure 2). Tortuous anatomy may require centerline reconstructions to accurately measure a true vessel diameter.

Along with the diameter measurements, the morphology of the aneurysm and the access vessels is also important. Calcification and thrombus within the fixation site may decrease the effectiveness of the endovascular repair by limiting the ability of the grafts to properly seal, thereby increasing the risk of a type 1 endoleak. Calcifications within the access vessels may decrease vessel elasticity, increasing the risk of rupture during endovascular repair.⁵

The amount of tortuosity within the iliac (access) vessels increases the risk of complications during endograft deployment.^6,7 Catheter angiography may underestimate the size of an aneurysm by not demonstrating the associated thrombus; CTA, by allowing the evaluation of the lumen as well as the vessel wall, is more complete.

LENGTH MEASUREMENTS

Vessel length measurements are essential for proper endograft selection. In the early EVAR experience, most patients underwent angiography with a calibration catheter to properly assess the length from the proximal landing zone (infrarenal neck) to the expected distal landing zone. Most of these length measurements are now obtained from CTA.

The most common length measurements include the following:

• Length of the proximal neck: determines the length of the proximal landing zone;
• Length from the lowest renal artery to the aortic bifurcation: determines the amount of room that exists for the endograft and the contralateral limb to be exposed;
• Length of both common iliac arteries: determines the distal attachment zone; and
• Length from the lowest renal artery to the subsequent common iliac artery bifurcation: determines the length of the device to preserve the origin of the hypogastric artery on the respective iliac side.

Proper limb measurements prevent placement of a long main body that will deploy the contralateral limb within the ipsilateral common iliac artery. Undersizing the length measurements to the aortic bifurcation may place both limbs within the endosac, making subsequent canulation difficult. Overestimating the overall length of an endograft may lead to inadvertent exclusion of a hypogastric artery.