Neuroradiology (2011) 53:169–175
DOI 10.1007/s00234-010-0722-5

INTERVENTIONAL NEURORADIOLOGY

Evaluation of the characteristics of various types of coils
for the embolization of intracranial aneurysms
with an optical pressure sensor system
Noriaki Matsubara & Shigeru Miyachi & Yoshitaka Nagano & Tomotaka Ohshima &Osamu Hososhima & Takashi Izumi & Arihito Tsurumi & Toshihiko Wakabayashi &Akihito Sano & Hideo Fujimoto

Received: 8 September 2009 / Accepted: 14 May 2010 / Published online: 3 June 2010
# Springer-Verlag 2010

Abstract
Introduction In coil embolization for an intracranial aneurysm, it is important to appropriately choose the coil mostsuitable for coping with various unforeseen situations.
Additionally, because dense coil packing of the aneurysm
sac is the most important factor to avoid a recurrence, properly
selecting the coil is essential. In this article, the authors
measured the coil insertion pressure of various types of coilswith a newly developed sensor system, and coil characteristics
were investigated.
Methods The sensor consists of a hemostatic valve connectedto the proximal end of a microcatheter. The sensor principle isbased on an optical system. Using this, an experimental
silicone aneurysm embolization was performed automaticallyat constant speed. The pattern of the insertion pressure and theN. Matsubara (*) : S. Miyachi : T. Ohshima : O. Hososhima :T. Izumi : A. Tsurumi : T. Wakabayashi
Department of Neurosurgery,
Nagoya University Graduate School of Medicine,
65 Tsurumai-cho, Showa-ku,
Nagoya, Aichi 466-8550, Japane-mail: mnoriaki0817@yahoo.co.jp
Y. Nagano : H. Fujimoto
Department of Computer Science and Engineering,
Nagoya Institute of Technology Graduate School of Engineering,
Nagoya, Aichi, Japan

maximum insertion pressure (MIP) were analyzed for thevarious types of coils. The sensor continuously monitored themechanical force during the insertions.
Results The sensor adequately recorded the coil insertion
pressure during embolization in each coil. MIP was
generally ranked in order of the coil type. The soft type coils
required relatively less insertion pressure than standard/helicaland 3D type. As for the patterns of coil insertion pressure,each coil presented a saw-like pressure pattern, though weobserved some slight differences. 3D type coils showed peakpressure at the moment of “painting”. Coil loop diameters
barely affected MIP. However, as to the patterns of pressure,larger size coils more often presented the peak.
Conclusions Coil characteristics were well evaluated. The
results obtained here reflected some actual clinical experience.
Furthermore, collecting the in vivo study is mandatory, which
may provide clinically useful data.
Keywords Sensor . Coil characteristic . Embolization .
Intracranial aneurysm
Abbreviations
ES
ExtraSoftMIP Maximum insertion pressure
US
UltraSoft3D
Three-dimensional
2D
Two-diameter

A. SanoDepartment of Engineering Physics, Electronics and Mechanics,
Nagoya Institute of Technology Graduate School of Engineering,
Nagoya, Aichi, Japan

Introduction

Y. Nagano
New Product Development R&D Center, NTN Corporation,
Iwata, Shizuoka, Japan

In endovascular coil embolization for an intracranial aneurysm,
it is important to choose the most suitable coil for various

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situations. Additionally, in coil embolization, although thedevelopment of various new devices has continued, including
new types of coil, balloon catheters, and microstents, dense coilpacking of the aneurysm sac remains the most important factor
to avoid an aneurysm recurrence [1–4]. When packing the coils
as densely as possible, it is important to ensure that the various
coils (shape, stiffness, loop size, and length) should beproperly selected to fit the circumstance [5].
The authors developed a new sensor device to measure
the coil insertion pressure via an optical system alreadyreported [6–9]. In this article, the authors measured the coilinsertion pressure of various types of coils with that opticalpressure sensor system. The coil characteristics were
investigated by analyzing the data obtained from each coil.

Materials and methods
The new optical coil insertion pressure sensor consists of ahemostatic valve connected to the proximal end of a microcatheter (Fig. 1a, b). The principle of the pressure sensor isbased on an optical system. The optical line sensor detects
the position of the delivery wire by measuring the shadow
cast, i.e., the degree of the bend, according to the coilinsertion pressure via a coil delivery wire from the surgeon’s
fingertip (Fig. 1c, d). Positioning data of the bend areautomatically translated into an objective insertion pressure
value. We omit here further technical details of theengineering physics of the device; please refer to our alreadypublished reports elsewhere [7, 8].
Experimental aneurysm embolization was performed using
the aforementioned optical pressure sensor, which continuously monitored the mechanical force during the insertion of
coils into the aneurysm (a silicone dummy aneurysm wasused for this study). The complete experimental system is
depicted in Fig. 2a, with panel d showing a schematic. The
Fig. 1 a Photographs illustrating optical pressure sensor. b
Schematic showing opticalpressure sensor. c, d Schematics
showing optical system. cWithout insertion pressure, thecoil delivery wire does not bend.
d With insertion pressure, it
does. Sensor optically detects
position of bend in wire [6]Neuroradiology (2011) 53:169–175

silicone dummy simulates the shape of a round aneurysm
with no branches (Fig. 2b, c). The dome size was 3 mm indiameter, and the neck length and parent artery diameter
were both 2 mm. The experimental embolization was
performed automatically at a constant speed, since manual
coil insertion by a surgeon might adversely affect the result.
The insertion speed was set at as 1.0 mm/s, which wasconsidered the most appropriate speed under that conditionin the preliminary experiment. The tip of microcatheter wasplaced at the center of the silicone aneurysm dome. The
catheter was placed in an almost linear position to minimize
the influence of the frictional pressure exerted when a coilpassed inside the lumen of the microcatheter. Movements ofthe coil during insertion were observed with a microscope,with the microscopic image reflected on a monitor.
The coils used in this study were as follows: GDC (Boston
Scientific Neurovascular, Natick, MA, USA) 10 UltraSoft(US): 3 mm–6 cm, GDC10 US: 4 mm–8 cm, GDC10 Soft 2D
SR 3 mm–6 cm, GDC10 3D 3 mm–6 cm, GDC10 360 SR
4 mm–8 cm, GDC10 Standard 4 mm–8 cm, MicroPlex(Micro Vention, Aliso Viejo, CA, USA) 10 Helical 3 mm–
6 cm, MicroPlex10 Helical 4 mm–8 cm, MicroPlex10
Complex 3 mm–7 cm, and ED coil (Kaneka Medix, Osaka,
Japan) 10 ExtraSoft (ES) 3 mm–6 cm. The diameter of the coilloop was 3 or 4 mm, the length range is 6–8 cm, and helical(including 2D) or complex shaped coils were used. When
deciding the most proper diameter of the coil loop, any loop
over 5 mm was considered so oversized that the coilfrequently protruded outside the aneurysm during embolization. Thus, a 3- or 4-mm coil loop was selected.
Coil insertion pressure was measured five or moretimes for each coil using the optical pressure sensor. The
data obtained during the final 10 s of a coil insertion wasomitted because the stiffness of the detach point between
the coil and coil delivery wire might adversely affect thepressure. The data obtained initial 10 s were also omittedNeuroradiology (2011) 53:169–175

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Fig. 2 a Photograph showing
experimental system for measuring coil insertion pressure with
optical sensor system during
embolization in silicone aneurysm model. b, c Photograph ofexperimental silicone aneurysm.
Aneurysm dome size was 3 mm.
d Schematic showing theexperimental systembecause the distal tip of the coil, i.e., the first coil loop,
sometimes adhered to the aneurysm wall, which mightcause a rise in the insertion pressure. It took 60 s toinsert the entire 6-cm coil at a speed of 1.0 mm/s.
Therefore, the data obtained from 10 to 50 s wereanalyzed. To compare the performance under the same
situation, even in longer 6-cm coils, only the data from10 to 50 s were analyzed. The pattern of the insertion
pressure and maximum insertion pressure (MIP) values

were analyzed. To simplify the analysis, we studied only
the results of the first coil insertion.
Statistical analyses were performed using Ekuseru-Toukei
2006 (Social Survey Research Information Co., Ltd. Tokyo,
Japan). ANOVA was used for multiple comparisons among
the coils. Bonferroni analysis was used for post hoc analysis.
Additionally, these coils were classified into three coil typegroups (soft, standard/helical, or 3D type). Student’s t test wasused for the comparison among each group.

Neuroradiology (2011) 53:169–175

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Results
The new optical sensor adequately recorded the coil insertion
pressure during embolization of a silicone aneurysm in eachcoil; the result was shown in Figs. 3, 4, 5, and 6.
Distribution of the maximum coil insertion pressure wasshown in Fig. 3. MIP was generally ranked in order of thecoil type (soft, standard/helical, or 3D) though no statisticallysignificant difference was found between each of them (p=0.0487 in ANOVA but no significance in Bonferroni
analysis). The soft type coils (ED coil 10 ES, GDC10 US,
and GDC10 Soft 2D) exerted less insertion pressure than thestandard/helical type (MicroPlex10 Helical and GDC10Standard) and 3D type (GDC10 3D, GDC10 360, andMicroPlex 10 Complex) (p=0.0020 and 0.0017, respectively)(Fig. 4). There was no statistical significant difference
between standard/helical and 3D type coils (p=0.468). No
differences were found between GDC US and GDC Soft 2D
coils. GDC10 US 3 mm–6 cm showed the wider range of
95% confidence interval in comparison with other coils. That
was probably caused by the sticking of coil distal tip to theaneurysm wall. Stuck coil distal tip might induce a rise in theinsertion pressure (Fig. 5b). Meanwhile, in GDC Soft 2D,
the first 2D loop might control a rise of the following force(Fig. 5c).
As for the pattern of coil insertion pressure, each coilproduced a saw-like pressure pattern, though we notedsome slight variations (Figs. 5 and 6). Coil loop diameters(3 or 4 mm) barely affected the MIP in this experimental
situation. However, as to pressure patterns, compared to the
3-mm coils showing few saw-like peaks, the 4-mm coils
showed repeated peaks.

Fig. 3 Representative graph demonstrating maximum insertion
pressure (MIP) of each coil (95% CI). MIP was generally ranked inorder of coil type (soft, standard/helical, or 3D) though no statisticallysignificant difference was found among the coils (p=0.0487 in
ANOVA but no significance in Bonferroni analysis)Fig. 4 The soft type coils (ED coil 10 ES, GDC10 US, and GDC10Soft 2D) exerted less insertion pressure than the standard/helical(MicroPlex10 Helical and GDC10 Standard) and the 3D type (GDC10
3D, GDC10 360, and MicroPlex10 Complex) (p=0.0020 and 0.0017,
respectively, Student’s t test). There was no statistical significant
difference between standard/helical and 3D type coils (p=0.468)

Discussion
Compared to neck clipping of the aneurysm, coil embolization
poses a potential risk of aneurysm recurrence due to coilcompaction or recanalization, especially in the case of a larger
aneurysm [10, 11]. To address that problem, various kinds ofcoils, such as 3D/complex coils [12, 13], extremely soft coils[14, 15], bioactive coil [16, 17], and hydrogel-coated coil
[18, 19], are continuously developed. Although bioactive
coils or hydrogel-coated coils are expected to overcome thedrawback of a recurrence, no satisfactory evidences havebeen forthcoming [20].
The dense coil packing of the aneurysm sac is the mostimportant feature to avoid a recurrence. To achieve coilpacking as densely as possible, it is important that thevarious coils (shape: helical or complex, stiffness: standard
or soft, loop diameter, and length) must be appropriately
selected to cope with various unforeseen situations [5]. To
clearly determine the characteristics of various coils is a key
factor in ensuring better coil embolization and a successful
outcome. White et al. reported fundamentals of coil designand coil technology [21]. However, they basically justanalyzed the coil physical properties and did not analyze
the coil insertion pressure. With that in mind, the authors
investigated the coil insertion pressures of various types ofcoils using the optical pressure sensor system.
In analysis of the MIP, soft type coils proved lesspressure than standard/helical and 3D type coils. The
difference between soft and standard/helical type coils

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Fig. 5 Representative graphsdemonstrating insertion pressure
patterns measured by the opticalsystem during embolization insilicone aneurysm (data of soft
type coils and MicroPlex Helicalcoils). Compared to 4-mm coils,
3-mm coils showed few pressure
peaks

probably depends on the coil stiffness itself. MicroPlex10
Helical 3 mm–6 cm showed relatively high MIP comparedto other coils. One of the explanations was that thecharacteristics of MicroPlex coils influenced the force.
However, MicroPlex10 Helical 4 mm–8 cm did not show
similar result. Further investigation is necessary.
Fig. 6 Representative graphsdemonstrating insertion pressure
patterns measured by the opticalsystem during embolization insilicone aneurysm (data of GDCStandard coil and 3D type coils).
The repeated saw-like pressure
patterns were observed

By microscopically observing coil movements during
insertion, characteristic movements of the coils could beobserved. In general, the pressure accumulates gradually as
coil insertion proceeds and is released at the moment of“painting”, i.e., the tip of the microcatheter moves like apendulum when the coil slides smoothly along and is rolledNeuroradiology (2011) 53:169–175

174

against the aneurysm wall. This movement enables a sawlike pressure pattern to be created during the coil insertion.
However, there was some difference noted between thehelical type coils and 3D type coils. The former types wereable to slide smoothly along the aneurysm wall during“painting,” whereas the latter types did not slide readilyalong the wall, so that the coil mass rolled abruptly against
the wall at the moment of “painting.” This abrupt movement
caused the pressure peaks. These results suggested that 3D
type coils could potentially exert undue insertion pressureduring the first coil insertion.
Another interesting finding from this experiment was thatcoil loop diameters of 3 or 4 mm barely affected the MIP. For
that reason, the 3-mm coil generally slid smoothly along theaneurysm wall with less stress, though occasionally it failed to
do so, which elevated the MIP, presenting the peak pattern. In
contrast, the 4-mm coil often exerted the peak pressure. These
results suggested that larger size coils often produce the peakpressure. Overall, the results obtained in this study reflected
the actual clinical experience and the impression of surgeons
in daily interventions.
Although several clinical papers investigating coilcharacteristics have already published [22, 23], they could
not objectively investigate the coil insertion pressure itself
feeding back to the surgeon’s hand. This optical system canevaluate the pressure at the proximal portion of the
microcatheter and can objectively measure and record thecoil insertion pressure, so that, hereafter, collecting in vivo
data would be highly desirable. A main in vivo factor of
coil insertion pressure varies depending of the vessel curves
and kinking within the microcatheter access to theaneurysm. This factor might also be an interesting objective
for future investigations with this optical system in a
tortuous vessel model.
In this study, only the pressure exerted by the first coilinsertion was evaluated, allowing for considerable remaining
space within the aneurysm, so that the difference among
several coils may not be especially remarkable. Therefore, thetheme of further detailed research will be to examine undervarious situations when some coils have already been inserted
into the aneurysm. Additionally, because only a limited
variety of coils were available in this study, data obtained
from more coil types may prove valuable.

Conclusions
The characteristics of various coils could be evaluated withthe optical coil insertion pressure sensor system. A saw-likepressure pattern was presented in each coil. Soft type coils
potentially exert lower insertion pressures. 3D type coils
showed the peak pressures at the moment of “painting”.
Larger size coils more often present the peak pressure. The

results obtained here reflected some actual clinical experience.
We consider an in vivo study to be mandatory, which may
therefore provide clinically useful data.
Acknowledgments The study was the result of a collaboration
among Nagoya University, Nagoya Institute of Technology, and NTN
Corporation, Osaka, Japan. This study was financially supported by
NTN Corporation. The authors wish to thank Takeshi Kinkori,
Takehiro Naito, Tsuyoshi Ichikawa, and Kenichi Haraguchi, membersof “The Neuroendovascular Team” of Nagoya University Graduate
School of Medicine, for their generous assistance in this study.
Conflict of interest statement Y. Nagano is an employee of NTN
Corporation and this study was financially supported by NTN
Corporation.

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