Elsevier Editorial System(tm) for Ultrasound in Medicine and Biology Manuscript Draft Manuscript Number: Title: Preclinical Testing of Frequency-Tunable Capacitive Micromachined Ultrasonic Transducer Probe Prototypes Article Type: Original Contribution Keywords: medical imaging; intracardiac echocardiography; capacitive micromachined ultrasonic transducer; collapse mode; frequency tunability Corresponding Author: Mr. Martin Pekař, Corresponding Author's Institution: Philips Research First Author: Martin Pekař Order of Authors: Martin Pekař; Alexander F Kolen; Harm Belt; Frank van Heesch; Nenad Mihajlović; Imo E Hoefer; Tamas Szili-Török; Hendrik J Vos; Johan G Bosch; Gijs van Soest; Antonius F. W van der Steen Abstract: In intracardiac echocardiography (ICE) it may be beneficial to generate ultrasound images acquired at multiple frequencies, having the possibility of high penetration or high resolution imaging in a single device. The objective of the presented work is to test two frequencytunable probe prototypes in a preclinical setting: a rigid probe having a diameter of 11 mm and a new flexible and steerable 12-Fr ICE catheter. Both probes feature a forward-looking 32-element capacitive micromachined ultrasonic transducer array (aperture of 2 mm x 2 mm) operated in collapse-mode, which allows for frequency-tuning in the range from 6 MHz to 18 MHz. The rigid probe prototype is tested ex-vivo in a passive heart platform. Images of an aortic valve acquired in high-penetration (6 MHz), generic (12 MHz), and high-resolution (18 MHz) mode combine satisfying image quality and penetration depth between 2.5 cm and 10 cm. The ICE catheter prototype is tested in-vivo using a porcine animal model. Images of an aortic valve are acquired in the three imaging modes with the ICE catheter placed in an ascending aorta at multiple depths. It was found that the combination of the forward-looking design and frequency-tuning capability allows visualizing intracardiac structures of various sizes at different distances relative to the catheter tip, providing both wide overviews and detailed close-ups. Suggested Reviewers: Kai Thomenius Institute of Medical Engineering & Science, MIT [email protected]
Expert in ultrasound imaging, co-author of number of forward-looking intracardiac imaging catheters, e.g. Stephens, D. N., Truong, U. T., Nikoozadeh, A., Oralkan, Ö., Seo, C. H., Cannata, J., Dentinger, A., Thomenius, K., de la Rama, A., Nguyen, T., Lin, F., Khuri-Yakub, P., Mahajan, A., Shivkumar, K., O’Donnell, M. and Sahn, D. J. (2012) ‘First In Vivo Use of a Capacitive Micromachined Ultrasound Transducer Array – Based Imaging and Ablation Catheter’, Journal of Ultrasound in Medicine, 31, pp. 247–256. Available at:
http://www.jultrasoundmed.org/content/31/2/247.short (Accessed: 27 May 2013). Petr Neuzil Cardiology, Homolka hospital [email protected]
Clinical expert in minimally-invasive cardiology. He routinely uses intracardiac imaging catheters and publishes on new devices in his field, e.g. Ho, I. C. K., Neuzil, P., Mraz, T., Beldova, Z., Gross, D., Formanek, P., Taborsky, M., Niederle, P., Ruskin, J. N. and Reddy, V. Y. (2007) ‘Use of intracardiac echocardiography to guide implantation of a left atrial appendage occlusion device (PLAATO)’, Heart Rhythm. Elsevier, 4(5), pp. 567–571. Alessandro Savoia Acoustoelectronics Laboratory, Univeristà degli Studi Roma Tre [email protected]
Expert in capacitive-micromachined ultrasound transducer technology. Coauthor of number of publications on this topic,e.g. Savoia, A. S., Calianov, G. and Pappalardo, M. (2012) ‘A CMUT probe for medical ultrasonography: from microfabrication to system integration.’, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, 59(6), pp. 1127–38. doi: http://dx.doi.org/10.1109/TUFFC.2012.2303. Opposed Reviewers:
20th February, 2017 in Eindhoven, the Netherlands
Dear Editor, It is our pleasure to announce you that we have decided to submit our paper on Preclinical Testing of FrequencyTunable Capacitive Micromachined Ultrasonic Transducer Probe Prototypes to Ultrasound in Medicine & Biology (UMB). The presented study shows in-vivo the unique concept of a frequency-tunable, forward-looking and steerable intracardiac echocardiography catheter prototype, which we hope will be of a great interest to the scientific UMB community. Hereby we state that the manuscript, or specified parts of it, have not been and will not be submitted elsewhere for publication. For a review process of the submitted work we recommend the following three referees:
Kai Thomenius Petr Neužil Alessandro Savoia
MIT, Institute of Medical Engineering & Science Homolka hospital, Department of Cardiology Univeristà degli Studi Roma Tre, Acoustoelectronics Laboratory
74 Vanvranken Road, Clifton Park, NY 12065, USA Roentgenova 2/37, 150 30 Praha 5, the Czech Republic via della Vasca Navale 84, 00146 Roma, Italy
With kind regards and on behalf of the authors, Martin Pekař
001 518 371 2943
00420 257 272 211
*Manuscript Click here to view linked References
Preclinical Testing of Frequency-Tunable Capacitive Micromachined Ultrasonic Transducer Probe Prototypes Martin Pekaˇra,b,∗, Alexander F. Kolena , Harm Belta , Frank van Heescha , Nenad Mihajlovi´ca , Imo E. Hoeferd , Tamas Szili-T¨or¨okc , Hendrik J. Vosb , Johan G. Boschb , Gijs van Soestb , Antonius F. W. van der Steenb a
Philips Research, Royal Philips NV, High Tech Campus 34, 5656 AE Eindhoven, the Netherlands b Erasmus MC, Thorax Center Dept. of Biomedical Engineering, Wytemaweg 80, 3015 CN Rotterdam, the Netherlands c Erasmus MC, Thorax Center Dept. of Cardiology, Wytemaweg 80, 3015 CN Rotterdam, the Netherlands d Utrecht University, Faculty of Veterinary Medicine, Bolognalaan 50, 3508 TD Utrecht, the Netherlands
Abstract In intracardiac echocardiography (ICE) it may be beneficial to generate ultrasound images acquired at multiple frequencies, having the possibility of high penetration or high resolution imaging in a single device. The objective of the presented work is to test two frequency-tunable probe prototypes in a preclinical setting: a rigid probe having a diameter of 11 mm and a new flexible and steerable 12-Fr ICE catheter. Both probes feature a forward-looking 32-element capacitive micromachined ultrasonic transducer array (aperture of 2 × 2 mm2 ) operated in collapse-mode, which allows for frequency-tuning in the range from 6 MHz to 18 MHz. The rigid probe prototype is tested ∗
Corresponding Author: Martin Pekaˇr, Philips Research, Royal Philips NV, High Tech Campus 34, 5656 AE Eindhoven, the Netherlands; Email, [email protected]
; Phone, +31 6 16 89 15 66.
Preprint submitted to Ultrasound in Medicine and Biology
February 20, 2017
ex-vivo in a passive heart platform. Images of an aortic valve acquired in high-penetration (6 MHz), generic (12 MHz), and high-resolution (18 MHz) mode combine satisfying image quality and penetration depth between 2.5 cm and 10 cm. The ICE catheter prototype is tested in-vivo using a porcine animal model. Images of an aortic valve are acquired in the three imaging modes with the ICE catheter placed in an ascending aorta at multiple depths. It was found that the combination of the forward-looking design and frequencytuning capability allows visualizing intracardiac structures of various sizes at different distances relative to the catheter tip, providing both wide overviews and detailed close-ups. Keywords: medical imaging, intracardiac echocardiography, capacitive micromachined ultrasonic transducer, collapse mode, frequency tunability,
Cardiovascular deaths represent 31 % of all global deaths in the past few
years, claiming more lives than all forms of cancer combined (Mozaffarian
et al., 2016). Minimally-invasive procedures have proven to be effective in
improving the patient outcome while minimizing trauma and complexity of
cardiac interventions. Intracardiac echocardiography (ICE) is an established
guidance tool for device closure of interatrial communications and electro-
physiological ablation procedures (Earing et al., 2004; Reddy et al., 2010).
The exploitation of ICE for navigation during other cardiac interventions
and, more importantly, its use as a diagnostic tool is currently limited by
its imaging performance at a distance and the restricted view it typically
provides (Bartel et al., 2014). Our goal is to design a steerable forward-
looking catheter which can change its imaging frequency between 6 MHz
and 18 MHz, allowing for high penetration or high resolution imaging within
a single device. The forward-looking design will offer complementary views
to the conventional side-looking ICE concept. The frequency tunability com-
bined with the catheter maneuverability will allow for both navigation and
detailed close-up imaging.
While the clinical review reports demonstrate the unmet need for a frequency-
tunable forward-looking ICE catheter, there has been published only a single
design of a forward-looking ultrasound transducer array which can change
its operating frequency (Yeh et al., 2006). This design utilized a capacitive
micromachined ultrasonic transducer (CMUT) ring array operated at 8 MHz
and 19 MHz in conventional and collapse mode, respectively. The array could
generate 3-D images of a wire phantom in oil, however, its integration in a 3
steerable catheter shaft and its preclinical imaging capability remains to be
Our design utilizes a single-type CMUT operated solely in collapse-mode,
which has an extra feature that allows changing its operating frequency over
a continuous range as opposed to a discrete change between conventional and
collapse mode. This behaviour is referred to as ”frequency tunability” (Pekaˇr
et al., 2016a). The frequency tunability is investigated in this preclinical
study for forward-looking ICE imaging, addressing the requirement of having
the possibility of high penetration (zoom-out) or high resolution (zoom-in).
The principal objective of the presented work is to test two frequency-
tunable forward-looking probe prototypes in a preclinical setting: a rigid
probe having a diameter of 11 mm and a new flexible and steerable 12-Fr
catheter. The acoustic characterization of the rigid probe has been published
earlier (Pekaˇr et al., 2016b). The following section introduces the develop-
ment of the new 12-Fr catheter. The concept of frequency tunability is first
demonstrated on 2-D images of an aortic valve in a controlled ex-vivo setting
using the rigid probe. Zoom-in and zoom-out capability of the developed
catheter is presented in in-vivo imaging of the aortic valve in a live animal
Materials and Methods
Our design utilizes a 32-element CMUT phased array operated in collapse-
mode Klootwijk et al. (2011), having a 9-Fr active aperture of an octagonal
shape as shown in Fig. 1(a). The CMUT consists of thousands of tiny ca4
pacitors (about 60 µm in diameter and 1.5 µm thick) with movable top
membranes arranged in columns connected electrically in parallel. Static
DC bias voltage (80 V – 160 V) is used to collapse the center area of the
membrane onto the bottom of the cavity. Then AC voltage is applied to
excite the free part of the membrane without releasing the center area from
the bottom of the cavity. The CMUT transducts ultrasound and voltage
as a result of these membrane vibrations, of which the resonance frequency
increases for a high bias voltage magnitude, because the membrane stiffness
and the contact radius of the collapsed portion of the membrane increases
with larger deflection. The transmit pulse style and the bias voltage set-
tings are tailored to the three imaging modes, based on initial resolution
and penetration measurements on a tissue-mimicking phantom (unpublished
The fabricated CMUT array is integrated with front-end electronics into
two probe prototypes: a robust rigid probe having a diameter of 11 mm, and
a steerable 12-Fr catheter. Both devices are equipped with identical CMUT
array and an application specific integrated circuit (ASIC) to integrate the
front-end electronics closely with the CMUT at the tip of the probe. The
ASIC has 16 channels, each of which comprising of a 60-V unipolar pulser,
a transmit and receive switch, a 27 dB amplifier, and a line driver. The
assembled catheter tip is shown in Fig. 1(a).
The 110-cm long catheter shaft offers omnidirectional steering achieved
by manipulating 4 wires connected to an anchor ring near the tip and being
controlled remotely via a mechanical joystick. The reach of the catheter is 5
Figure 1: (a) assembled forward-looking catheter tip with integrated CMUT array and front-end electronics. (b) steerability of the catheter tip by a mechanical joystick.
38 mm when the tip is deflected at 90◦ and the maximum bending radius is
180◦ as shown in Fig. 1(b).
The probe prototypes are coated with a thin layer (about 15 µm) of a
silicone-like material for electrical insulation of the array connections and
passivation of the CMUT cells (Zhuang et al., 2007).
The two probe prototypes are connected to a Verasonics Vantage imaging
system (Kirkland, WA, USA), which is used to generate transmit pulses of
arbitrary frequency (5 MHz – 20 MHz), amplify, digitize (14-bit, 62.5 MHz),
and transfer the received sensor data to a host controller PC for real-time
ultrasound image visualization or data recording. The transmit pulse fre-
quency, number of transmit pulses and the CMUT bias voltage can be set
via a graphical user interface which controls the Verasonics system and a
programmable electronic toolbox.
Conventional line-based sector scanning (von Ramm and Smith, 1983) is
implemented on the Verasonics system for the real-time visualization, which
is used to navigate the probes, whereas the recording option facilitates high 6
frame rate data acquisition for off-line processing (Montaldo et al., 2009).
The beamformed, high frame rate images are filtered in the fast-time do-
main with 50 % band-pass finite impulse response filter (FIR) centered at
6 MHz, 12 MHz, or 18 MHz yielding high-penetration, generic, or high-
resolution imaging performance, respectively. A median-filter is applied on
the consecutive frames to improve an image signal-to-noise ratio and to yield
about 25 FPS, which is at the order of magnitude typically used for cardiac
imaging. Electrocardiogram (ECG) is recorded synchronously with the pulse
transmission via an audio card fitted on the host controller PC.
Passive heart platform
The concept of frequency tunability is first demonstrated by imaging an
aortic valve in a passive beating heart platform (LifeTec Group BV, Eind-
hoven, the Netherlands) which features mechanical pumping of a porcine ex-
vivo heart freshly collected from a local abattoir. Air bubble-free, heparinized
blood is pumped through the left ventricle of the heart. The system allows
precise control and monitoring of hemodynamical parameters and robust per-
formance for up to 12 hours. Earlier validation of this platform shows repro-
ducible physiological hemodynamics, e.g. aortic pressures of 120/80 mmHg
with 5 L/min of cardiac output (Leopaldi et al., 2015).
The rigid probe prototype was inserted and sealed in the aorta facing the
aortic valve (Fig. 2), of which the long-axis images were acquired in the three
Figure 2: Photograph of the ex-vivo testing of the rigid probe prototype using a passive heart platform.
Porcine animal model
Live animal studies were carried out after securing approval from the
Dutch Central Commission for Animal Studies (protocol nr. AVD/115002015205)
and according to the European directives (2010/63/EU) and the Guidelines
for the Care and Use of Laboratory Animals (NIH). The forward-looking
ICE catheter was tested in a porcine model (about 70 kg) under general
anaesthesia with a mixture of midazolam, sufentail and pancuronium, and
low-frequency mechanical ventilation. The animal was heparinized to avoid
thrombus formation. The ECG and oxygen saturation were monitored con-
tinuously throughout the study. Access for the aortic valve imaging was
obtained via a femoral artery or a left common carotid artery. Fluoroscopic
images of the catheter position inside the heart were taken from the left
anterior oblique view (LAO) with cranial angulation.
Ex-vivo passive heart study
The frequency-tunable collapse-mode CMUT array is utilized to gener-
ate high-penetration depth (6 MHz), generic (12 MHz), and high-resolution
(18 MHz) images of an aortic valve in an ex-vivo passive heart platform.
Fig. 3 compares imaging performance of these three modes. The penetration
mode shown in Fig. 3(a, d) allows imaging the heart up to 10 cm, clearly vi-
sualizing the aorta, left atrium, left ventricle and providing a coarse imaging
of the opening and closing of the aortic and the mitral valve. The generic
mode (Fig. 3(b, e)) provides increased resolution clearly depicting the leaflets
of the aortic valve and a portion of the mitral valve up to the depth of about
5 cm. The high-resolution mode (Fig. 3(c, f)) generates the finest detail of
the aortic valve with penetration depth of about 2.5 cm.
The dynamic range in the obtained images is scaled to the peak signal
intensity and optimized subjectively for display. If the Fig. 3(f) would be
displayed at the same dynamic range as Fig. 3(e), it would exhibit lower
contrast. Figs. 3(b, c, e, f) utilize imaging frequency higher than 10 MHz,
at which the spontaneous echo contrast of blood appears. This results in the
white swirl seen above the closed aortic valve in Figs. 3(b, c). The white
pattern throughout the vertical center of the displayed images is common
mode noise which is dominant at steering angles close to 0◦ . To better ap-
preciate the frequency-tunability, a video corresponding to Fig. 3 is included
as an online supplement (Movie 1). This recording shows frequency-tunable
imaging of the ex-vivo aortic valve placed at a fixed distance.
Figure 3: Imaging of an aortic valve using the frequency-tunable probe prototype in an ex-vivo passive heart platform. Closed and open state of the aortic valve is shown in the top and bottom row, respectively. The imaging modes (HPEN indicates high-penetration; GEN, generic; HRES, high-resolution) and the displayed dynamic range are shown in the top-right corner. The white curve at the bottom of each image depicts the recorded ECG signal, of which the cursor indicates the displayed moment in the heart cycle.
Live animal study
To investigate the usability of having the high penetration (zoom-out)
or high resolution (zoom-in) in a single device, the frequency tunability for
forward-looking ICE imaging was studied in an in-vivo animal model. The
catheter prototype was first inserted via a femoral artery to the ascending
part of the aorta facing the aortic valve (Fig. 4(b)). Images of long-axis view
of the aortic valve located at a depth of about 2.5 cm were acquired in the
general mode (Figs. 5(b, e)). The catheter could not be advanced closer to
the aortic valve, due to its bending profile not allowing the advancement
through the tight aortic arc of the porcine heart. It was therefore removed
Figure 4: Fluoroscopic images indicating the position of the catheter’s tip during the zoomin and zoom-out imaging of an aortic valve using the developed prototype of frequencytunable forward-looking ICE catheter in an in-vivo animal model. The catheter is positioned at about (a) 5 cm, (b) 2.5 cm and (c) 1 cm from the aortic valve. These fluoroscopic images correspond to the ultrasound images shown in Fig. 5.
and inserted via the carotid artery (Fig. 4(c)). Thus close-up images of the
aortic valve at a depth of about 1 cm in the high-resolution mode (zoom-in)
have been obtained as shown in Figs. 5(c, f). Subsequently, the catheter was
pulled back to about 5 cm from the aortic valve (Fig. 4(a)) and images in
the penetration mode were recorded (Figs. 5(a, d)).
The zoom-in and zoom-out capability of the developed forward-looking
frequency-tunable ICE catheter is shown in comparison of opened and closed
aortic valve images (Fig. 5) displayed at 28 FPS. The opened and closed state
of the valve can be identified in all three imaging modes. The penetration
mode enables anatomical overview for navigation whereas the high-resolution
mode allows a close-range, more detailed anatomic image of the aortic valve.
The generic imaging mode provides a compromise of the two.
Figure 5: Imaging of an aortic valve using the developed prototype of frequency-tunable forward-looking ICE catheter in an in-vivo animal model. Closed and open state of the aortic valve are shown in the top and bottom row, respectively. The imaging modes (HPEN indicates high-penetration; GEN, generic; HRES, high-resolution) and the displayed dynamic range are shown in the top-right corner. The white curve at the bottom of each image depicts the recorded ECG signal, of which the cursor indicates the displayed moment in the heart cycle.
This study reports on preclinical testing of a novel forward-looking ICE
catheter prototype. The catheter is based on collapse-mode CMUT tech-
nology which features bias voltage-based frequency tuning to generate ultra-
sound images at the center frequencies of 6 MHz, 12 MHz, and 18 MHz, but
any frequency in between is possible and can be changed in realtime.
It was found that intracardiac structures were delineated with satisfying
image resolution or penetration depth. The frequency-tunable catheter can
visualize anatomical structures of various sizes at different distances relative 12
to the catheter tip, enabling both wide overviews and detailed close-ups.
The catheter prototype presented in this study has an active aperture of
9 Fr but outer diameter of 12 Fr, due to the wirebonds which connect the
CMUT array with the front-end electronics. Clinical practise requires the
diameter to be reduced to at least 10 Fr. This could be achieved either by
monolithic integration with the front-end electronics or by utilizing a flex-
to-rigid technology (Khuri-Yakub and Oralkan, 2011; Mimoun et al., 2010).
A decreased shaft diameter would increase the catheter’s deflectability and
steerability, which were found to be crucial for easy navigation in the confined
space of the heart.
The potential gain in navigation efficiency of the forward-looking design,
which is similar to using a flashlight, towards the specific feature of interest
needs to be proven in a future study.
Even though it is common practice to place a commercial ICE catheter
inside the aorta for investigational guidance (Bartel et al., 2014), its restricted
bandwidth and side-looking design limits its diagnostic capability, e.g. for
infective endocarditis. The catheter design presented in this study allows
for a complementary view and its switching to higher operation frequency
exploits the usage of ICE for close-up high frequency diagnosis of the heart
High frequency imaging would also allow for near-by thrombus detection
or guidance of percutaneous biopsies of intra-aortic masses suspected to be
tumours. It is foreseen that frequency-tunable forward-looking ICE catheter
will exploit the use of ICE for diagnosis of heart valves, guiding LAA closure
and real-time lesion visualization during ablation procedures.
A rigid probe and a steerable ICE catheter prototype, both equipped with
collapse-mode frequency-tunable forward-looking CMUT array, have been
successfully tested in a passive heart platform and an animal experiment,
respectively. Images of the aortic valve acquired in high-penetration (6 MHz),
generic (12 MHz), and high-resolution (18 MHz) mode show satisfying image
quality and penetration depth between 2.5 cm and 10 cm for the imaging
aperture of 2 × 2 mm2 . The ICE catheter prototype placed in the ascending
aorta was utilized to image the aortic valve at multiple depths in-vivo. It
was found that the combination of the forward-looking design and frequency-
tuning feature allows visualizing intracardiac structures of various sizes, e.g.
leaflets of the heart valves and the ventricle at different distances relative to
the catheter tip, providing both wide overviews and detailed close-ups. The
promising approach may substantially influence the future role of forward-
looking ICE in the clinical practice.
The authors would like to thank the following people for their effort and
support in this study: Michel van Bruggen, Frank Budzelaar, Geert Gijsbers,
Jeannet van Rens, Marc Notten, Bas Jacobs, Wim Weekamp, Alfons Groen-
land, Ferry van der Linde, Maurice van der Beek, Debbie Rem-Bronneberg,
Wendy Dittmer and Ren´e Aarnink.
This research is in part funded by European Union Seventh Framework Programme project ”OILTEBIA”, grant no. 317526.
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Movie 1: Recording of the aortic valve acquired during one cardiac cycle
with the frequency-tunable probe prototype in the ex-vivo passive heart
platform. (a) high-penetration, (b) generic and (c) high-resolution
mode. The penetration depth is fixed to 10 cm. The white curve
at the bottom of each image depicts the recorded ECG signal, of which
the cursor indicates the displayed moment in the heart cycle.
Figure 1a Click here to download high resolution image
Figure 1b Click here to download high resolution image
Figure 2 Click here to download high resolution image
Figure 3 Click here to download high resolution image
Figure 4 Click here to download high resolution image
Figure 5 Click here to download high resolution image
Movie 1 Click here to download Supplemental Video: 2016-04-21_HPEN_GEN_HRES_forUMBpaper_v2.avi
Movie 1 (still) Click here to download high resolution image
LaTeX Source Files Click here to download LaTeX Source Files: 2017-02-20_PreclinicalFreqTuning_v07_sentToUMB.zip