2.4. Aortic Remodelling 
Aortic remodelling is a term which describes the desirable morphologic changes in the aortic anatomy following TEVAR. The parameters used to identify remodelling are false lumen (FL) thrombosis, FL regression, true lumen (TL) expansion, and maximum aortic diameter stability [1, 27]. Several studies have validated the surveillance of these morphologic characteristics as significant predictors of outcomes in TBAD [16, 17, 27, 44]. Whilst there is evidence suggesting pre-emptive TEVAR in patients with un-TBAD [5, 45], assessment of aortic remodelling is vital in monitoring the post-procedural course of the dissection process and predicting outcomes [46, 47].
The FL is the most dangerous element in the process of aortic dissection. Its patency and hence perfusion (with elevated intraluminal pressure) contribute to the aortic expansion.. Furthermore, it may compress the TL and impair the blood flow to distal organs, causing malperfusion [48-50]. Endo-graft placement by TEVAR seals the primary entry tear and decreases the pressure within the FL, hopefully leading to both FL thrombosis and TL expansion [16]. FL thrombosis has a significant clinical value in the follow-up of patients with un-TBAD following TEVAR [16, 17]. Any FL thrombosis can be assessed by the absence of contrast media by a Computerised tomogram. However, Clough et al. [51] challenged this modality for assessment of FL and showed that magnetic resonance imaging with a blood pool agent has more accuracy in detecting thrombosis in FL.
Following TEVAR, the surveillance of FL thrombosis is critical in evaluating aortic stability and remodelling. Persistent FL perfusion or aneurysmal expansion might be an indication of stent graft failure, and warrant further reintervention [52, 53].
Different segments in the dissected thoracic and abdominal aorta exhibit variable degrees of remodelling. The proximal anatomical zones demonstrating the greatest favourable remodelling following TEVAR [54]. The FL thrombosis was shown to be more prominent in the thoracic dissecting aorta with less remodelling observed in the abdominal aorta irrespective of the timing of TEVAR [55]. Kamman et al. [44] showed that FL complete thrombosis in the distal zones (presented as >20 cm from LSA origin) could be as low as 22%. Similar findings were seen by Yuan et al. [5], who showed a superior FL thrombosis rate in the thoracic aorta compared to abdominal segments, especially in patients treated in the chronic phase of the disease. The overall occurrence of FL thrombosis in the dissected thoracic aortic segment increases with time after TEVAR [54]. Table (3) shows the degree of FL thrombosis reported in different studies according to the follow-up period.