The utility of the unenhanced phase is to differentiate extraluminal calcification or postendoleak intervention material from extraluminal contrast seen on contrast-enhanced images. Although comprehensive, such a protocol delivers a high radiation dose. Some institutions employ a late delayed phase of 300 seconds to better visualize low-flow endoleaks [72]. With regard to endoleak, type I endoleaks occurring at the proximal (Ia) or distal (Ib) landing zones are the most common cause for reintervention following TEVAR, occurring in up to 15% of cases [75]. In contrast to abdominal aorta endovascular repair where type II endoleaks are most common, type II endoleaks occur in a small percentage of TEVAR patients, likely because of fewer patent collateral vessels in the thorax compared with the abdomen [76]. Furthermore, type II endoleaks are not associated with increased risk of thoracic aorta rupture and are often managed conservatively. Types III, IV, and V endoleaks occur much less frequently, with type III being the only subtype other than type I to require immediate therapy [72]. Thoracic Aorta Interventional Planning and Follow-Up It has been shown that luminal diameter as determined from 3-D measurements correlates well with aortic luminal area during the postoperative period and can be used as a proxy for luminal blood flow [77]. This applies to both true and false lumens in cases of dissection, although the relationship becomes less clear with large false lumen diameters because of the propensity for complex luminal configuration. Serial evaluation of true and false lumen diameters via CTA in the postintervention period is a marker for vascular remodeling. Recent midterm results from the VIRTUE Registry demonstrate similar rates of vascular remodeling in patients with uncomplicated acute and subacute type B dissections.
Thoracic Aortic Aneurysm or Dissection Treatment Planning and Follow Up. The utility of the unenhanced phase is to differentiate extraluminal calcification or postendoleak intervention material from extraluminal contrast seen on contrast-enhanced images. Although comprehensive, such a protocol delivers a high radiation dose. Some institutions employ a late delayed phase of 300 seconds to better visualize low-flow endoleaks [72]. With regard to endoleak, type I endoleaks occurring at the proximal (Ia) or distal (Ib) landing zones are the most common cause for reintervention following TEVAR, occurring in up to 15% of cases [75]. In contrast to abdominal aorta endovascular repair where type II endoleaks are most common, type II endoleaks occur in a small percentage of TEVAR patients, likely because of fewer patent collateral vessels in the thorax compared with the abdomen [76]. Furthermore, type II endoleaks are not associated with increased risk of thoracic aorta rupture and are often managed conservatively. Types III, IV, and V endoleaks occur much less frequently, with type III being the only subtype other than type I to require immediate therapy [72]. Thoracic Aorta Interventional Planning and Follow-Up It has been shown that luminal diameter as determined from 3-D measurements correlates well with aortic luminal area during the postoperative period and can be used as a proxy for luminal blood flow [77]. This applies to both true and false lumens in cases of dissection, although the relationship becomes less clear with large false lumen diameters because of the propensity for complex luminal configuration. Serial evaluation of true and false lumen diameters via CTA in the postintervention period is a marker for vascular remodeling. Recent midterm results from the VIRTUE Registry demonstrate similar rates of vascular remodeling in patients with uncomplicated acute and subacute type B dissections.