IN VITRO QUANTIFICATION OF THE SIZE DISTRIBUTION OF INTRASACCULAR VOIDS LEFT AFTER ENDOVASCULAR COILING OF CEREBRAL ANEURYSMS

Abstract

Purpose: Endovascular coiling of cerebral aneurysms remains limited by coil compaction and associated recanalization. Recent coil designs which effect higher packing densities may be far from optimal because hemodynamic forces causing compaction are not well understood since detailed data regarding the location and distribution of coil masses are unavailable. We present an in vitro methodology to characterize coil masses deployed within aneurysms by quantifying intra-aneurysmal void spaces.

Methods: Eight identical aneurysms were packed with coils by both balloon- and stent-assist techniques. The samples were embedded, sequentially sectioned and imaged. Empty spaces between the coils were numerically filled with circles (2D) in the planar images and with spheres (3D) in the three-dimensional composite images. The 2D and 3D void size histograms were analyzed for local variations and by fitting theoretical probability distribution functions.

Results: Balloon-assist packing densities (31±2%) were lower (p=0.04) than the stent-assist group (40±7%). The maximum and average 2D and 3D void sizes were higher (p=0.03 to 0.05) in the balloon-assist group as compared to the stent-assist group. None of the void size histograms were normally distributed; theoretical probability distribution fits suggest that the histograms are most probably exponentially distributed with decay constants of 6-10 mm. Significant (p<=0.001 to p=0.03) spatial trends were noted with the void sizes but correlation coefficients were generally low (absolute r<=0.35).

Conclusion: The methodology we present can provide valuable input data for numerical calculations of hemodynamic forces impinging on intra-aneurysmal coil masses and be used to compare and optimize coil configurations as well as coiling techniques.

Keywords: Balloon; Exponential distribution; Histogram; Porous media; Silicone model; Stent.

Fig. 1

A) Geometry representative of the rabbit elastase-induced aneurysm used for the study; B) silicone replica of the geometry after stent-assisted coiling; the lumen has been filled with epoxy as a fixative

Fig. 1

A) Geometry representative of the rabbit elastase-induced aneurysm used for the study; B) silicone replica of the geometry after stent-assisted coiling; the lumen has been filled with epoxy as a fixative

Fig. 2

Top left to bottom right: image of the sanded face of a sample, a stack of such slices through the sample, and the empty space within the aneurysm lumen maximally filled with circles

Fig. 3

Empty space within the entire aneurysm volume maximally filled with spheres

Fig. 4

Mean packing density of the balloon-assist and stent-assist coil groups (* statistically different, t test p=0.04)

Fig. 5

Variation in the average radius of voids in the neck-to-dome direction for A) the balloon-assist group and B) the stent-assist group. Regression lines (solid) with 95% confidence intervals (dotted) are shown along with correlation coefficients

Fig. 5

Variation in the average radius of voids in the neck-to-dome direction for A) the balloon-assist group and B) the stent-assist group. Regression lines (solid) with 95% confidence intervals (dotted) are shown along with correlation coefficients

Fig. 6

Average histograms of 2D void sizes for the A) balloon-assist and B) stent-assist groups. Voids with diameter greater than 1.2 mm are not included in the plot

Fig. 6

Average histograms of 2D void sizes for the A) balloon-assist and B) stent-assist groups. Voids with diameter greater than 1.2 mm are not included in the plot

Fig. 7

Average histograms of 3D void sizes for the A) balloon-assist and B) stent-assist groups. The bin size is wider for the 3D voids data because the images had to be reduced in size to conserve computational resources in order to calculate the 3D volumetric void locations. Voids with diameter greater than 1 mm are not included in the plot

Fig. 7

Average histograms of 3D void sizes for the A) balloon-assist and B) stent-assist groups. The bin size is wider for the 3D voids data because the images had to be reduced in size to conserve computational resources in order to calculate the 3D volumetric void locations. Voids with diameter greater than 1 mm are not included in the plot

Fig. 8

Average Weibull distribution fit (solid line) for the balloon-assist 2D void size distribution superposed over the data set. The dashed lines are ±1 standard deviation