Time-Resolved 3-Dimensional Magnetic Resonance Velocity Mapping at 3 T Reveals Drastic Changes in Flow Patterns in a Partially Thrombosed Aortic Arch
A 36-year-old female patient presented with recurrent arterial emboli of both arms and a suspected transient ischemic attack. Routine thoracic contrast-enhanced, 3D magnetic resonance angiography (CE-MRA) at 3 T was performed to rule out additional arterial thrombi. The resulting CE-MRA images and consecutive, curved, multiplanar reconstructions (Figure 1B and 1C) revealed a partially obstructing thrombus (as a signal void in the otherwise-bright aortic lumen) in the aortic arch and the proximal left subclavian artery. In contrast, maximum-intensity-projection analysis (Figure 1A) did not depict these findings, which mimicked normal aortic geometry.
To analyze the hemodynamic consequences of the local geometric changes in the aortic lumen, time-resolved 3D MR velocity mapping was performed. ECG-gated and navigator-controlled data acquisition during free breathing allowed complete spatial and temporal coverage of blood flow in 3D. Various visualization modes of filtered and noise-reduced data included vector graphs and particle traces as well as streamlines. Figure 2 shows the 3D particle-trace visualization of the intricate impact of the thrombus on blood flow in the proximal descending aorta. Particle traces were initiated at a set of emitter points located orthogonal to the aortic arch in the region of the thrombus (yellow dashed line, Figure 1B). The individual images show the temporal evolution of blood flow characteristics at 3 distinct systolic time frames within the cardiac cycle. Next to an initially detected and physiologically expected acceleration of flow through the luminal narrowing (white arrows), considerable complex and vortical flow patterns were observed in the proximal descending aorta (open arrow). These substantial hemodynamic changes may lead to increased strain of the aortic wall and result in secondary morphological alterations.
Flow experiments were performed with a 3 T MR system (Magnetom TRIO, Siemens, Erlangen, Germany) with a radiofrequency spoiled gradient echo sequence with interleaved 3-directional velocity encoding. Measurements were performed during free breathing and prospectively gated to the ECG cycle. Respiration control was achieved with navigator gating and combined with adaptive k-space reordering to improve scan efficiency. Data were acquired with α=15°, velocity sensitivity=150 cm/s, spatial resolution of 3.2×2.1×5.0 mm3, echo time=3.5 ms, repetition time=5.6 ms, and temporal resolution=45 ms. Blood flow visualization was performed with use of a commercially available software package (EnSight, CEI, Apex, NC).
Dr Markl is supported by research grant MA 2383/3-1 by Deutsche Forshungsgemeinschaft. The other authors report no conflicts.
The online-only Data Supplement, which contains a movie, can be found at http://circ.ahajournals.org/cgi/content/full/113/11/e460/DC1.