J Neurosurg 111:41–47, 2009

A novel pressure sensor with an optical system for coilembolization of intracranial aneurysms
Laboratory investigation
Noriaki Matsubara, M.D.,1 Shigeru Miyachi, M.D.,1 Yoshitaka Nagano, M.Eng., 2,3
Tomotaka Ohshima, M.D.,1 Osamu Hososhima, M.D.,1 Takashi Izumi, M.D.,1
Arihito Tsurumi, M.D.,1 Toshihiko Wakabayashi, M.D.,1 Masamichi Sakaguchi, Ph.D., 2
Akihito Sano, Ph.D., 2 and Hideo Fujimoto, Ph.D. 2
Department of Neurosurgery, Nagoya University Graduate School of Medicine; 2Graduate School of Engineering, Nagoya Institute of Technology, Nagoya, Aichi; and 3New Product Development R&D Center, NTN
Corporation, Iwata, Shizuoka, Japan
1

Object. In endovascular coil embolization for an intracranial aneurysm, the excessive pressure created during
coil insertion into an aneurysm can cause a catastrophic rupture or dislodge a microcatheter tip from the aneurysm
dome, resulting in insufficient embolization. Such undue mechanical pressure can only be subjectively detected by
the subtle tactile feedback the surgeon experiences. Therefore, the authors of this study developed a new sensor device to measure the coil insertion pressure via an optical system.
Methods. This novel sensor system consists of a hemostatic valve connected to the proximal end of a microcatheter (Y-connector). The sensor principle is based on an optical system composed of a light-emitting diode (LED) anda line sensor. The latter measures how much the coil-delivery wire slightly bends in response to the insertion pressure
by detecting the wire shadow. This information is translated into a given force level. Experimental aneurysm embolization was performed using this optical sensor. A silicone aneurysm model and an in vivo model (porcine aneurysm
model) were used in this study. Several surgeons manually performed the coil insertions. The sensor continuously
monitored the mechanical force during the insertions.
Results. The sensor adequately recorded the coil insertion pressure during embolization. The presence of thesensor did not hinder the embolization procedure in any way. During embolization in the silicone aneurysm model,
a sinusoid pattern of pressure occurred, reflecting actual clinical experience. Similar results were obtained in the invivo study.
Conclusions. This new sensor device adequately measures coil insertion pressure. This system provides potential­
ly safer and more reliable aneurysm embolizations. (DOI: 10.3171/2009.1.JNS081181)

Key Words•
intracranial aneurysm
coil insertion pressure•
sensorI

n recent years, neuroendovascular treatments havebeen widely used with a variety of improved devices
such as microcatheters and microguidewires as wellas an enhanced angiographic system. Coil embolization
for intracranial aneurysms is one such treatment.
Craniotomy with clipping of the aneurysm neckhas been the most conventional therapeutic option for
intracranial aneurysms.16 However, with the reports ofimproved outcomes after coil embolization for ruptured
aneurysms—such as those documented in the International Subarachnoid Aneurysm Trial (ISAT)10,11 following the development of the Guglielmi detachable coil in
19914,5—and with the rising need for less invasive therapies, coil embolization has spread rapidly over the pastAbbreviation used in this paper: LED = light-emitting diode.

J Neurosurg / Volume 111 / July 2009•

coil embolization•

10 years. Furthermore, the development of various new
devices, including 3D coils,2,21 balloon catheters,8,17 andmicrostents,1,3,22 has continued, reducing the difficulties
associated with conventional treatment procedures.
Note, however, that coil embolization is always accompanied by a risk of procedural complications, although it is a less invasive treatment than aneurysm neckclipping. In endovascular coil embolization for intracranial aneurysms, the most fatal complication is catastrophic
rupture of the lesion, which can indeed result in a pooroutcome. Such a rupture is often caused by aneurysm
wall perforation by a microcatheter or microguidewire,
although excessive pressure during coil insertion can also
induce a rupture.6,7,9,20 Moreover, in the final stage of theprocedure, such undue pressure can lead to kickback of
the microcatheter tip, and the microcatheter can be unintentionally dislodged from the aneurysm dome, resulting41

N. Matsubara et al.
in insufficient embolization. To avoid such mishaps, it isimportant to adequately control the coil insertion pressure
exerted through the microcatheter. Thus far, mechanicalstress caused by coil insertion pressure could only be subjectively detected by the subtle tactile feedback experienced by the surgeon inserting the coil.
Because of these risks, we envisioned a system thatwould objectively register and quantify the coil insertion
pressure to promote safer and more reliable coil embolizations. Therefore, we developed a new sensor device incollaboration with Nagoya University, Nagoya Institute ofTechnology, and NTN Corporation. Utilizing an opticalsystem, our sensor measures the coil insertion pressure
from the bend of the coil-delivery wire. Here, we introduce this novel sensor and present the results of some basic experiments.

Methods
Optical Pressure Sensor System

The new optical coil insertion pressure sensor consists of a hemostatic valve connected to the proximal endof a microcatheter (Y-connector; Fig. 1B and C). The sensor principle is based on an optical system equipped withan LED and 128 detectors of optical line sensors. The
optical sensor is set in the space between the portion of
the device connecting to the microcatheter (exit part) andthe portion that accepts the coil-delivery wire (entrancepart). Figure 1D shows a schematic of the device.
We designed a novel shape for the Y-connector. In
the usual Y-connector (Fig. 1A), the space between theentrance and exit portions of the delivery wire is linear.
In contrast, our device creates a bend in the delivery wire
because we intentionally added a little flexure in thisspace of the device (Fig. 2A and B). Inside this space, thelight emitted from an LED casts the shadow of the delivery wire, which synchronizes with the bend in the wire.
The optical line sensor detects the change in the positionof the delivery wire by measuring the shadow cast—that
is, the degree of the bend—according to the coil insertion pressure from a surgeon’s fingertip. The darkest point
is defined as the position of the bending wire. The line
sensor resolution is 63.5 µm (Fig. 2C and D). Positioning
data of the bend are transmitted to a control computerbox, which automatically translates them into an objective insertion pressure value.
The geometric structure of the sensor is very important for measuring the wire flexion. We determined thegeometric shape by experimenting with many prototype
sensors. The important geometric parameters in designing the sensor are shown in Fig. 2E: α is the angle between the center lines of the openings, β is the angle thecenter line makes with the opposing wall, and ρ is thecurvature of the wall. If α is 0, the wire will not bend; so,
α must be > 0. As α increases, the bending of the wire,
due to compressive pressure, increases and the sensitivity
increases. Note, however, that an increase in α also increases the friction between the sensor and the wire. If β
is < 90°, the wire cannot pass through the sensor; it mustbe > 90° to allow the wire to pass through the sensor. The
curvature ρ enables both flexible and hard wires to bemeasured with the same sensor; it prevents flexible wires
from buckling and facilitates bending of hard wires. Notethat α, β, and ρ are interdependent parameters. We determined the optimum geometric parameters by making andmeasuring the prototypes.
At the development stage, the optical sensor was calibrated with a pressure gauge widely used in the engineer-Fig. 1. A: Photograph showing a commercially available Y-connector. B and C: Photographs illustrating the Y-connector
with the optical pressure sensor system. Arrow indicates part of the optical pressure sensor. D: Schematic showing the opticalpressure sensor. 1, part connecting to proximal end of the microcatheter; 2, part connecting to infusion line with normal saline; 3,part to insert into coil-delivery wire; 4, coil-delivery wire; 5, optical pressure sensor.

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A novel pressure sensor for coil embolization

Fig. 2. Schematics showing the optical system. A: Without insertion pressure, the coil-delivery wire does not bend. B:
With insertion pressure, the wire bends. Sensor optically detects the position of the bending wire. C: Optical system of thepressure sensor, consisting of LED, lens, and line sensor. D: Line sensor determines the position of the wire by measuring the
shadow cast. The darkest point is defined as the wire position, which is translated to insertion pressure. Line sensor resolution
is 63.5 µm. E: Geometric structure of the sensor.

ing field to analyze the force of linear objects. The distal
part of the coil-delivery wire (without coil) was connected to the pressure gauge, and coil insertion pressure wasapplied to the proximal part of the delivery wire little bylittle. We collected data on the relationship between theinsertion pressure and the position of the wire shadow,which reflects the wire bend. Note that coil-delivery
wires are made of uniform materials corresponding toeach of the various kinds of coils. These materials havetheir own Young modulus, or modulus of elasticity, which
defines the correlation between the stress and strain ofelastic material. Therefore, the relationship between the
degree of coil flexion and the insertion pressure is not so
difficult to calculate. We omit here other technical detailsof the engineering physics of the device, which have been
described elsewhere.12,13
Indeed, this system can measure coil insertion pressure only at the proximal portion of the wire. Note, however, that according to principles of engineering physics,
the pressure detected at the distal tip of the wire is thesame as the proximal pressure if the wire is laid in a linear position and without friction. Therefore, it is thoughtthat proximal pressure directly and simultaneously reflects the distal pressure in such an ideal situation. Of
course, when any frictional pressure exists, the proximalpressure does not directly reflect the distal pressure. But
the pressure measured by the sensor can accurately reflect
the proximal pressure, that is, the subtle tactile feedback
that the surgeon receives at his or her hand.
J Neurosurg / Volume 111 / July 2009

Although the hardness and diameter of a coil-delivery wire differ with the various types of coils, the pressure calculation program on our sensor can be changed
with the flip of a switch. This device can accommodate
any 10-system coil now available in Japan. Pressure values are translated into an optic monitor (color indicator),
auditory sensor (speaker), and pressure display monitor(digital numerical value or analog graph). Patents arepend­ing for this device.
Experimental Aneurysm Embolization

Experimental aneurysm embolization was performed
using this optical sensor, which continuously monitored
the mechanical force during the insertion of coils into
the aneurysm. For preliminary study, a silicone dummy
aneurysm was used. The whole experimental system is
depicted in Fig. 3A. The silicone dummy simulates theshape of a round aneurysm with no branches (Fig. 3B).
The dome size was 5 mm in diameter, and the neck lengthand parent artery diameter were both 3 mm. Several surgeons performed the experimental embolization. Movements of the coil during insertion were observed with amicroscope, with the microscopic image reflected on amonitor. For the in vivo study, we fabricated a porcine
aneurysm model by surgically transplanting the venous
graft and positioning the distal carotid artery to anotherside of the carotid artery. The dummy aneurysm was ~6.3 mm in diameter (Fig. 4). In this article, we analyzed
the results of only the first coil insertion.
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N. Matsubara et al.

Fig. 3. A: Photograph showing the experimental system for measuring coil insertion pressure with an optical sensor system
during embolization in a silicone aneurysm model. B: Photograph of the experimental silicone aneurysm.

Results

The sensor adequately recorded the coil insertion
pressure during embolization in both a silicone aneurysm
and in an in vivo model. The presence of the sensor did
not hinder the embolization procedure in any way.
In the silicone aneurysm model, sinusoid patternchanges in pressure occurred. When the embolization was
performed successfully, the maximum insertion pressure
did not greatly exceed 0.30 N (Fig. 5). Microscopic imaging was used to monitor coil movement. Coil insertion
could be stopped instantaneously and the advance immediately resumed in response to a push-pull-push movement

Fig. 4. A: Angiogram showing porcine aneurysm model. Venous graft was surgically transplanted, and contralateral carotid
artery was transpositioned at another carotid artery. Aneurysm dome size was 6.3 mm. B–D: Photographs showing in vivo coilembolization with an optical pressure sensor. The presence of the sensor did not hinder conduction of the embolization in any
way. This device could be used in the same way as a normal Y-connector.

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A novel pressure sensor for coil embolization

Fig. 5. Representative graphs demonstrating insertion pressures
measured by an optical system during embolization in a silicone aneurysm model. The aneurysm dome size was 5 mm, and the inserted
coil was 6-mm × 15-cm GDC10-3D. Each panel shows procedures performed by 2 different surgeons. The maximum insertion pressure did
not exceed 0.3 N, and a sinusoid pattern of pressure was observed.

of the surgeon’s fingers. The insertion pressure decreased
just before coil insertion was momentarily stopped. At
the moment that coil advancement was resumed, the pressure was increased by the restart acceleration.
In the in vivo study, changes in the pressure patternwere similar to those in the silicone aneurysm model(Fig. 6).
In some experimental embolizations (both in vitro
and in vivo), the surgeon was able to control the coil-insertion speed by using the feedback system of this sensor.

Discussion
Optical Pressure Sensor System

Actually, in coil embolization of an intracranial aneurysm, the mechanical stress on the lesion due to coilinsertion pressure can be detected only subjectively
through the subtle tactile feedback the surgeon detects
via the coil-delivery wire. The surgeon has only been able
to describe the insertion pressure as “hard,” “stiff,” “soft,”“smooth,” and so forth. Our new sensor device was developed to clarify such ambiguity. It can objectively measure and record the coil insertion pressure via an opticalsystem, which enables the surgeon to objectively evaluate
and quantify the coil insertion pressure received by feedback. In addition, other members of the surgical staff canwitness the feedback pressure previously detected onlyby the surgeon. Better cooperative handling is possible

J Neurosurg / Volume 111 / July 2009

Fig. 6. Representative graphs demonstrating the insertion pressureduring in vivo embolization. The aneurysm dome size was 6.3 mm, andthe inserted coil was a 7-mm × 18-cm MicroPlex10 complex. Each panel shows procedures performed by 2 different surgeons. The maximum
insertion pressure did not much exceed 0.20 N, and a sinusoid patternof pressure was observed. These results were similar to those obtainedwith the silicone aneurysm model.

when 2 surgeons perform the procedure: one performs
the coil insertion while the other controls the microcatheter. Additionally, expert staff can give precise instructions to the surgeon.
The outstanding breakthrough feature of this device
is that the coil insertion pressure can be evaluated in thespace inside the Y-connector. This system is the first tomeasure and display the actual subtle pressure applied by
the surgeon’s fingertips to the very thin wire. In conventional studies, a pressure sensor has been attached to theexperimental aneurysm wall itself14 or to the microcatheter tip.15,18,19 It has been difficult to measure insertion pressure during an in vivo embolization, because putting aprecision sensor on a microcatheter tip is too complicatedand expensive. A microcatheter longer than 150 cm witha small distal tip cannot be equipped with a sensor forsufficient sensitivity. Given that this sensor can evaluate
the pressure at the proximal portion of the microcatheter
far more adequately, it has the potential to revolutionize
in vivo coil embolizations and their actual clinical application in the future.
Both digital and analog displays are available forexpressing the coil insertion pressure. The pressure data
can be transmitted to the control computer and converted
into a sound (auditory sensor) as well as a color indicator(optic sensor). After having tried various methods of indicating the pressure, the surgeon seems to prefer looking
at the radioscopic monitor during the procedure. It wasthought that an auditory sensor would be the most realistic method of keeping the surgeon aware of the coil insertion pressure while simultaneously allowing him to experience the direct feedback in the operation. This device
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N. Matsubara et al.
can also accommodate an alarm system that will alert asurgeon when the insertion pressure spikes dangerously.
The space inside the Y-connector, through which thedelivery wire passes, and the sensor itself are never indirect contact, so the sensor has little influence on the
coil-delivery wire procedure and is not entirely pollutedby blood. Furthermore, the coil-delivery wire remains
sterile because it touches only the sensor’s interior plastic
part but never the optical system itself. In addition, theY-connector on the new sensor could be used in the samemanner as a conventional Y-connector.
Since the sensor part of the Y-connector itself neednot be made of special materials, it can be mass produced
very inexpensively and even intended as a disposable
part.
Experimental Aneurysm Embolization

This device adequately recorded the coil insertion
pressure produced during embolization procedures in botha silicone aneurysm model and an in vivo model (porcine
aneurysm model). The recorded pressure reflected the actual pressure from clinical experience. When embolization of a silicone aneurysm was successfully performed,
the maximum insertion pressure did not greatly exceed
0.30 N. Although the first coil easily slid on the surfaceof the aneurysm wall, the insertion pressure tended to rise
just before the microcatheter was about to “paint”; that is,
the tip of the microcatheter moved like a pendulum when
the coil slid along and was rolled against the aneurysm
wall. The pressure accumulates gradually as coil insertion proceeds and is released at the moment of “painting.”
Moreover, during embolization, a push-pull-push movement of the surgeon’s fingers advances the coil-delivery
wire steadily. With this movement, the coil is advanced,
stopped abruptly, and soon advanced again. The pressure
decreases just before the coil is stopped. Once the coiladvance is resumed, acceleration occurs and pressure is
likely to increase under that acceleration. Thus, in synchrony with the movement of the surgeon’s fingers, increased and decreased forces are generated in turn. This
repetitive movement mainly causes repeated changes inthe sinusoid pressure pattern in the first coil embolization. In the in vivo study, a similar result was observed.
In the present study, a microcatheter was placed in
an almost linear position free of kinks or notable bending
so that the frictional pressure exerted when a coil passed
inside the lumen of the microcatheter would scarcely affect the result. However, in an actual coil embolization, amicrocatheter can often undergo flexures along the routeof access. In that situation, our device may not preciselyreflect the actual stress on an aneurysm. Therefore, a future area of more detailed research will be the relationship between the flexural condition of a microcatheter
and the frictional coil insertion pressure.

Conclusions

We reported on a new coil insertion pressure sensor
based on an optical system that adequately measured suchpressure. This system provides potentially safer, more reliable aneurysm embolizations.
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DisclosureThis study was supported by by NTN Corporation, Osaka,
Japan. Yoshitaka Nagano is an employee of NTN Corporation.
Acknowledgments
This study is the result of a collaboration among Nagoya
University, Nagoya Institute of Technology, and NTN Corporation.
The authors thank Jun Yoshida, Higashi Nagoya National Hospital,
and Takeshi Kinkori and Takehiro Naito, Nagoya University
Graduate School of Medicine, for their generous support in performing this study.
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Manuscript submitted October 6, 2008.
Accepted January 5, 2009.
Please include this information when citing this paper: published
online February 27, 2009; DOI: 10.3171/2009.1.JNS081181.
Address correspondence to: Shigeru Miyachi, M.D., Departmentof Neurosurgery, Nagoya University Graduate School of Medicine,
65 Tsurumai-cho, Showa-ku, Nagoya, Aichi 466-8550, Japan. email:smiyachi@med.nagoya-u.ac.jp.

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