gms | German Medical Science

Objective: The evolution of endovascular therapy led to development of flow diversion. A permeable stent reduces flow near the aneurysm neck while preserving flow into the parent vessel and neighboring branches. This provides delayed thrombosis of the aneurysm and causes shrinkage of the sac as the thrombus organizes, followed by growth of new endothelium that excludes the aneurysm from the circulation. Unexpected incomplete aneurysmal occlusion and onset of late hemorrhagic complications in few patients, through mechanisms that are not fully understood, led investigators to analyze flow changes related to stent implantation. Based on optical flow algorithms (OFA), a novel angiographic tool called AneurysmFlow describes flow patterns qualitatively and quantifies relative flow changes in the aneurysm before and after stent implantation (MAFA ratio). MAFA ratio was already correlated with aneurysm occlusion after flow diversion. In this study we describe the technique and an initial experience with 15 patients with intracranial aneurysm.

Methods: The OFA is based on both 2D digital subtraction angiography (DSA) and 3D-rotational angiography (3DRA). After 3DRA scan, a DSA projection avoiding overlapping of the aneurysm with other branches is chosen to acquire flow sequences with 60 frames per second after dye injection. The contrast fluid is transported in a pulsatile fashion from heart activity. This pulsatile dye progression is analyzed for the Optic Flow calculation. The OFA is an image-processing algorithm that extracts velocity and flow data analyzing temporal and spatial variations of pulsatile dye attenuation. The 2D DSA sequence is than projected on 3DRA data to generate a 2D flow map, a representation of detector velocity fields. A monodimensional Optic Flow scheme is then applied iteratively on the contrast wave map to extract the velocity profile. Mean Aneurysm Flow Amplitude (MAFA) ratio is derived by comparing the average projected velocity of the blood flow in the aneurysm before and after flow diverter implantation.

Results: In all patients, we were able to qualitatively describe flow both into the aneurysm and in parent vessels and neighboring branches. Direction of flow was represented with a vectorial map and velocity was superimposed using a color scale. Temporal resolution during pulsatile cardiac activity produced cyclical variation of the vector´s direction and color intensity.

Conclusion: AneurysmFlow is an innovative angiographic tool, with the ability to qualitatively analyze cerebral flow and quantitatively compute cerebral flow variations. Further investigations may help to optimize the use of flow diverters and flow information provided using this technique may be applied in current neurosurgical practice.