Experimental study of the characteristics of various types of filling coils for intracranial aneurysm embolisation

Abstract

Background

In endovascular embolisation for an intracranial aneurysm, after framing coil deployment, soft coils (often called filling coils) are usually selected to fill inside the cage of previous coils. Various kinds of filling coils are available, although each coil has its own characteristics. Understanding their differences to ensure proper coil selection is important to achieve successful embolisation. The purpose of this study was to investigate the characteristics of various filling coils.

Materials and methods

The authors developed a radiolucent coil to evaluate the performance of coils under conditions simulating the course of embolisation. Experimental embolisation was performed by using a silicone aneurysm filled with radiolucent coils. Indices including area, circularity, centroid position and coefficient of variation were investigated by analysing the figures of the filling coils after being inserted into the radiolucent coil under fluoroscopy.

Results

The characteristics of each coil depended on the coil design. The helical coil had the highest circularity and centroid position scores and lowest area score. Therefore, it tended to develop a compacted mass. The low shape-memory coil had the lowest circularity, second-highest centroid position and highest coefficient of variation scores. Therefore, it tended to develop irregularly shaped distribution with low reproducibility. Complex coils generally had higher area and circularity scores. Therefore, they tended to provide a balanced distribution with relatively expanded mass and less small compartmentation.

Conclusions

The evaluated characteristics of various filling coils should be useful for appropriate selection of filling coils.

Introduction

Dense coil packing is a crucial factor for prevention of recurrence of aneurysm after endovascular coil embolisation for intracranial aneurysm.^1–4 It is important to select a coil properly that is appropriate for a particular situation. The process of coil embolisation is generally divided into three stages: framing, filling and finishing. After framing coil deployment, soft coils are usually selected to fill inside the cage of previously placed coils. A soft coil is often called a ‘filling coil’, and various kinds are presently available.

Each filling coil has its own characteristics, and understanding their differences is important for achieving better embolisation results and successful outcomes. However, coil selection is usually based on catalogue specifications and the individual’s experience. Some papers on evaluations of coil characteristics, including those of filling coils, have been published,^5–11 but their analyses have been limited regarding coil physical properties. Therefore, the actual performance of filling coils inside framing coils has not been well investigated.

Filling coils are inserted into a previously placed coil mass, and visualisation of the inserted coil is disturbed by the radiopaque coils. Hence, it is difficult to observe the behaviour and distribution of the inserted coils on X-ray monitors and microscopic images. The authors developed a radiolucent coil for experimental study that has similar characteristics to those of typical platinum coils. When using radiolucent coils, only the newly inserted coils are radiopaque and visualised on X-ray imaging. They are clearly distinguished from the previously placed radiolucent coil mass.¹²

In this study, the characteristics of various types of filling coils were investigated by using a silicone aneurysm model filled with radiolucent coils simulating the filling process of embolisation.

Materials and methods

Aneurysm model and radiolucent coil

The silicone aneurysm model had the shape of a round aneurysm located at the T-shaped bifurcation of a parent artery. The dome size was 8 mm in diameter, the neck was 1.5 mm and the parent artery diameter was 3 mm (Figure 1(a)). A metal ball with a diameter of 8 mm was used as a reference marker. The experimental coil was made of thin nylon thread, had no radiopacity and was not visible under fluoroscopy (Figures 1(b) and (c)). The primary coil wind was 0.014 inch, and the radiolucent coil stiffness simulated that of an 18-type standard coil. Details of the radiolucent coil are omitted here, as they have been reported elsewhere.¹² In this experiment, these radiolucent coils were used to simulate the framing coils.

Figure 1.

The aneurysm model is made of silicone and resembles a round-shaped aneurysm located at an arterial bifurcation (a). The silicone aneurysm is filled with radiolucent coils to simulate the filling stage of embolisation. Radiolucent coils are visualised in the microscopic image (b). However, they are not visualised in fluoroscopic images (c). Fluoroscopic image after filling coil insertion showing that the inserted coil is only visualised (d). The circumference of the coil mass is plotted, and the software then automatically calculates area, circularity and centroid position (e).

Experimental embolisation

The silicone aneurysm was filled with radiolucent coils to count an embolisation ratio of approximately 15%, which simulated the filling stage of embolisation. The experimental embolisation was performed by a machine at a constant speed. The coil insertion speed was set at 1.0 mm/s, and the inserted coil length was 8 cm. The data were measured five times for each coil. The mass of radiolucent coils was repeatedly prepared. The tip of the micro-catheter (Excelsior SL-10 STR; Stryker, Fremont, CA) was placed at the centre of the aneurysm.

The coils used in this study were as follows: Target 360 Soft 4 mm–8 cm (Stryker), Cashmere 4 mm–8 cm (Codman Neurovascular, Raynham, MA), Orbit Galaxy Xtrasoft 4 mm–8 cm (Codman Neurovascular), Axium 3D 4 mm–8 cm (Covidien, Irvine, CA), Cosmos 10 4 mm–12 cm (MicroVention Terumo, Tustin, CA), ED Coil Extrasoft 4 mm–8 cm (Kaneka Medics, Osaka, Japan) and ED Coil Infini Extrasoft 16 mm–10 cm (Kaneka Medics).

Target 360 Soft, Cashmere, Orbit Galaxy, Axium 3D and Cosmos 10 were three-dimensional coils. ED Coil Extrasoft was a coil with a helical loop.¹³ ED Coil Infini was almost a straight-shaped soft coil without substantial shape memory.¹⁴ These coils were used as filling coils in the actual coil embolisation in our institution under similar conditions.

Evaluation of inserted filling coil figure

The movements of the filling coil during insertion were observed on an X-ray monitor and microscopic image. The behaviours of the filling coils were assessed by experienced interventionists. However, objective analyses of the coil characteristics were somehow difficult from the video findings. Therefore, to evaluate the coil characteristics objectively, the filling coil performances were assessed by observing the figures of the filling coils after being inserted into the radiolucent coil under fluoroscopy (Figure 1(d)). Figures for each fluoro image were obtained using the flat-panel detector with one direction.

Three indices were analysed – area, circularity and centroid position of the inserted coil figure – by using image analysis software (Pict Area, Inet, Osaka, Japan). Pict Area analyses the graphics file and calculates the various data for the surrounding territory in the graphics, including area, various lengths, perimeter, centroid position, circularity and so on. The circumference of the coil mass on each fluoro image was manually plotted, and then the software automatically calculated the above-mentioned three indices for plotted coil territory (area, circularity and centroid position; Figure 1(e)). Values calculated by Pict Area were based on the pixels, and thus these were converted to the value in the metric system.

Area (mm²) is a field inside the circumference of the coil mass. This index represents the extent to which the coil expands and covers the residual space of the radiolucent coils.

Circularity is a value that expresses the complexity of the two-dimensional figure. The maximum circularity (for a perfect circle) is 1, and it becomes small when a figure is complicated. This index means how roundly the coil makes a distribution.

Centroid position was evaluated by measuring the distance (mm) from the aneurysm neck to the centroid of the plotted coil area. This index indicates how far the coil disperses from the tip of the micro-catheter.

Additionally, the coefficients of variation of area and circularity were evaluated, which were calculated using Microsoft® Excel 2010 (Microsoft Corp., Redmond, WA). The coefficient of variation is the ratio of the magnitude of the data variation to the average, and its value is calculated according to the following formula: standard deviation/mean value.

The coefficient of variation indicates the reproducibility of the inserted coil figure. The data for area, circularity, centroid position and coefficient of variation were analysed for each filling coil.

In addition, the data for the above indices were analysed according to the primary coil diameters, which were: Target 360 Soft: 0.0095; Cashmere: 0.0135; Orbit Galaxy Xtrasoft: 0.0125; Axium 3D: 0.012; Cosmos 10: 0.012; ED Coil Extrasoft: 0.010; and ED Coil Infini Extrasoft: 0.010. Statistical analysis was performed by using IBM SPSS Statistics for Windows v17.0 (IBM Corp., Armonk, NY). Kruskal–Wallis analysis was used for analysis of variance among the seven coils in multiple comparisons. p-Values <0.05 were considered as indicative of statistical significance.

Results

Representative images of each coil are shown in Figure 2. The results of area, circularity, centroid position and coefficient of variation are shown in Figures 3 and 4, respectively.

Figure 2.

Representative fluoroscopic images after coil insertion demonstrate the coil distributions of seven filling coils. Target 360, Cashmere, Orbit Galaxy, Axium 3D and Cosmos 10 are complex coils. ED Coil Extrasoft is a helical coil. ED Coil Infini is a low shape-memory coil.

Figure 3.

The area (a), circularity (b) and centroid position (c) results of the filling coils. Multiple comparison analysis showed significant differences among the coils (p ≤ 0.02). Data were presented in mean value (M ± SE).

Figure 4.

The coefficients of variation for the area (a) and circularity (b) of the filling coils.

Area

Figure 3(a) shows the inserted coil area results (M ± SE). The coil areas ranged from 13.0 to 20.3 mm², which indicated that the coil extended part was 25.9–40.4% of the experimental aneurysm area (50.2 mm²). In the analyses of area, there were significant differences in multiple comparisons (p = 0.02). The area was higher for Target, Axium, Galaxy and Cashmere, followed by Cosmos and ED Coil Infini, and was the lowest for ED Coil.

Circularity

Figure 3(b) shows the circularity results for the inserted coil (M ± SE). The circularity values ranged from 0.44 to 0.77. In the analyses of circularity, there were significant differences in the multiple comparisons (p < 0.01). The circularity values were higher for ED Coil, Cosmos and Target, followed by Axium, Cashmere and Galaxy, and was lowest for ED Coil Infini.

Centroid position

Figure 3(c) shows the results of the centroid position of the inserted coil (M ± SE). The centroid positions of the coil mass ranged from 4.1 to 5.4 mm from the neck. There were significant differences in the multiple comparisons (p < 0.01). The centroid positions of Cashmere, Galaxy and Axium were close to the centre (i.e. catheter tip), followed by Cosmos and Target. In the ED Coil and ED Coil Infini, the centroid positions were located at a relatively far side.

Coefficient of variation

Figure 4(a) and (b) show the coefficients of variation for area and circularity, respectively. The coefficient of variation for area ranged from 0.042 to 0.260. The scores were higher for ED Coil Infini and ED Coil, followed by Galaxy, Cashmere, Target and Cosmos, and was lowest for Axium. The coefficient of variation for circularity ranged from 0.051 to 0.256. The score was higher for ED Coil Infini, followed by Target, Cashmere, ED Coil, Cosmos and Galaxy, and was lowest for Axium.

Relationship between each evaluation index and primary coil diameter

The correlations among the five evaluation indices above and the primary coil diameter were examined. Only centroid position and primary coil diameter demonstrated a significant correlation (p = 0.04; Figure 5).

Figure 5.

Relationship between primary coil diameter and centroid position. A negative correlation is shown (p = 0.04).

Discussion

Appropriate coil selection is important to achieve successful endovascular embolisation of an intracranial aneurysm. Recently, many types of coils have been developed, and various kinds of filling coils have been introduced by medical device companies.

Filling coils are used to fill inside the coil cage and increase the embolisation rate, and they are applied to an aneurysm that is partially embolised. Therefore, to investigate the characteristics and differences between filling coils, it is mandatory to examine them after some coils have already been inserted into the aneurysm. When using radiolucent coils, subsequently inserted coils are only radiopaque and can be clearly distinguished from the coil mass. Ota et al. previously studied the behaviour of finishing coils in the final stage of embolisation.¹² In the present experimental study, the authors used radiolucent coils for filling coil evaluation.

The coil performances of the tested coils in this study were assessed to determine the characteristics of each filling coil. The results obtained here will be useful for selecting filling coils appropriate for various conditions. Figure 6 shows radar charts demonstrating the characteristics of each filling coil. Scores were plotted according to the ranking of seven filling coils in each index.

Figure 6.

Radar charts demonstrate the characteristics of each filling coil. The scores are plotted according to the ranking of seven filling coils in each index.

Coil characteristics

The filling coils used in this study were divided into three types according to the coil design: (1) helical coil (ED Coil Extrasoft), (2) complex coils (Target 360 Soft, Cashmere, Orbit Galaxy, Cosmos 10 and Axium 3D) and (3) low shape-memory coil (ED Coil Infini). Hence, characteristics were found to depend on the coil design.

Helical coil: ED Coil Extrasoft

The ED Coil Extrasoft had the highest circularity and centroid position scores and the lowest area scores score. Therefore, it tended to develop a compacted coil mass ahead of the micro-catheter tip. From the video acquired during coil insertion, coil loops were successively accumulated in the remaining spaces. This characteristic is provided by a coil design with a helical loop. The ED Coil Extrasoft appeared to be better adapted to embolising compact spaces.

Low shape-memory coil: ED Coil Infini Extrasoft

The ED Coil Infini Extrasoft had the lowest circularity, second-highest centroid position and highest coefficient of variation scores. Therefore, it tended to develop an irregularly shaped coil distribution and low reproducibility. In the video acquired during coil embolisation, the ED Coil Infini coil loop sometimes entered through the gap of the radiolucent coils and randomly crawled within the remaining remote space. These results suggest that the ED Coil Infini is suitable for embolising an irregular space or remote remnant part.

Complex coil: Target 360 Soft, Cashmere, Orbit Galaxy, Cosmos 10 and Axium 3D

Complex-shaped coils are most widely used during the filling stage of embolisation in actual clinical practice. Thus, five complex-shaped filling coils were investigated in this study. The findings for all were similar, but some differences were evident. The complex coils generally had higher area and circularity scores. Therefore, they are likely to provide a balanced distribution with a relatively expanded coil mass and less small compartmentation. These findings indicate that complex filling coils are appropriate for homogenously embolising inside the frame.

In the analysis of the five complex filling coils, Target had relatively high area and centroid position scores, and Galaxy had a low circularity score, although the differences were not significant. Axium had the lowest area and circularity coefficient of variation scores, which indicated high reproducibility. Cashmere and Cosmos had mostly average scores in all indices.

Primary coil diameter

In this study, correlations among the area and circularity scores and the primary coil diameter were not observed and might have been influenced by the coil shape design. In contrast, the centroid position was correlated with the primary coil diameter (Figure 5). When the primary coil diameter was small, centroid position score was high. Thus, filling coils with a small primary coil diameter is likely to distribute a deeper component part and to pass through the gap of the framing coils. The primary coil diameters of Target Coil and ED Coil were 0.095 and 0.01 inch, respectively. ED Coil, ED Coil Infini and Target had higher centroid position scores, which meant that they had a tendency to distribute more deeply from the micro-catheter tip. Besides, softness of filling coils can also relate to the distribution. However, the difference of softness in each filling coils were not actually prepared because various types of filling coils were manufactured by various companies.

Limitations

This study had some inherent limitations. First, the experimental aneurysm used was a silicone aneurysm without pulsatile blood flow, and the radiolucent coil was made of nylon thread. Therefore, the friction associated with the silicone aneurysm and nylon coil may have been different from that during an actual coil embolisation. So, the coils might have behaved differently.

Second, data analyses of filling coil distribution were obtained from the X-ray detector in one direction, although analyses with a biplane detector were better for investigating the coil performance three-dimensionally.

Third, the possibility of ununiformity and variation in framing coil distribution were taken into account when interpreting the data obtained in this study. However, the radiolucent coil was relatively stiff compared to the filling coils. Therefore, it seemed that the deformation of framing radiolucent coils caused by the repeated insertions of filling coils rarely affected the results.

Additionally, because the conditions for the micro-catheter and the framing coils were almost fixed in this study, the position of the micro-catheter tip and the distribution and density of the framing coils might have influenced the filling coil performances in actual clinical practice. However, the results obtained here should be close to those that would be obtained under actual clinical conditions that interventionists experience. Given that only a limited number of filling coils were evaluated in this study, data obtained from a greater variety of coils may prove valuable in future investigations.

Conclusions

The characteristics of various filling coils were investigated in a silicone aneurysm with radiolucent coils. It was found that each filling coil had specific characteristics. Understanding their characteristics should contribute to selecting the most appropriate filling coil for a given situation and provide reliable and sufficient embolisation of intracranial aneurysms.

Declaration of conflicting interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship and/or publication of this article: S.M. received research grants from Kaneka Medix while conducting the study. The other authors have no personal or financial interest in any of the materials or devices described in this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.