Plaques in the descending aorta – a new risk factor for stroke? Visualization of embolization pathways in the thoracic aorta by 4D MRI at 3T

Background: High-risk plaques of the descending aorta (i.e., ≥4mm, thrombi) have received limited attention as a source of embolic stroke. Furthermore, a detailed in vivo analysis of local vascular hemodynamics at the site of aortic thrombi has not yet been performed. Thus, we aimed to visualize individual aortic flow channels in patients with aortic high-risk plaques by time-resolved 3D MRI.

Methods: 77 acute stroke patients were prospectively included. On a 3T system (TRIO, Siemens, Germany) plaques were localized by T1-weighted 3D gradient echo sequence (isotropic spatial resolution 1mm) covering the thoracic aorta. Global and local hemodynamics were evaluated by time-resolved flow-sensitive 4D MRI, visualized by a commercially available software (EnSight, CEI, USA) and characterized in consensus reading by time-resolved 3D particle traces (path of virtual mass-less particles tracing the measured time-resolved velocity vector field originating from a used defined emitter plane). Maximum extent of retrograde flow in cm was measured from inter-actively positioned emitter planes distal to the aortic bulb, brachiocephalic trunk and origin of the subclavian artery.

Results: In 25 patients with high-risk plaques 3D blood flow was analyzed in 11 so far. Figures 1 and 2 illustrate systolic and early diastolic blood flow characteristics in a patient (figure 1B) compared to a healthy volunteer (figure 1A). During systole both volunteer and patient exhibit high and unidirectional flow from the ascending aorta (AAo) into the descending aorta (Dao). In contrast, figure 1B reveals exceeding early diastolic retrograde flow throughout the entire thoracic aorta compared to the volunteer indicating potential embolization pathways in case of thrombus rupture. Similar results are shown for a patient with a large plaque in the proximal Dao. Additionally, a retrograde flow channel originating from the DAo directed towards the left subclavian artery can clearly be identified (figure 2). Results from all evaluated patients are summarized in table 1.

Discussion: Our findings demonstrate the potential of 4D MR imaging to evaluate individual embolization pathways from the site of the aortic high-risk plaque towards the brain supplying arteries in stroke patients. Most noticeably, the results indicate that even a thrombus located several centimeters distal to the left subclavian artery outlet needs to be considered as a high-risk source for embolic stroke.

Tab.1: Retrograde flow analysis for 11 patients

	Retrograde Flow [cm]
Plaque location	AAo	Arch	DAo
AAo (n=l)	1,0	5,5	2,4
Arcus (n=4)	0,3±0,5	4,0±1,5	2,1±0,8
Proximal DAo (n=4)	1,5±1,0	4,5±1,3	1,9±1,5
Distal DAo (n=2)	1,2±0,3	4,1±0,7	3,5±0,2
Total (n=ll)	0,9±0,8	4,3±1,2	2,3±1,1

Fig.1: Aortic blood flow in a healthy volunteer (A) and in a patient with a thrombus of the descending aorta (B).

Fig.2: Time resolved 3D particle trace visualization in a patient with a plaque >4mm in the descending aorta.