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1. IntroductionIn diagnostic ultrasound equipment, the role of the trans-

ducer is to convert energy from an electrical to an ultrasonic signal and vice versa. It is thus a significant component influencing the performance of the system. Probe tech-nologies have improved considerably in recent years, and probes used in clinical practice offer high performance and functionality, contributing greatly to improved diagnostic ef-ficiency. Hitachi has developed a new probe with ultra-wide frequency bandwidth by harnessing CMUT technology. The CMUT is an important component of the probe serving as its engine.

CMUT is a Micro Electro Mechanical Systems (MEMS) based transducer, a micro mechanical device made using semiconductor technology (Figure 1). CMUT is fabricated by modelling countless micro sensors (CMUT cells) onto a silicon substrate. The structure of each CMUT cell consists of an electrode at the top and bottom, enclosing a sealed vacuum inside an insulating material. By applying voltage to the upper and lower electrodes, electric charge accumulates. For transmission of the ultrasonic wave, a voltage is applied between the electrodes, and an electrostatic force is generat-ed between them causing the membrane above the vacuum space to vibrate, thereby radiating ultrasonic waves. During reception, the membrane is displaced by the reflected echo

causing vibrations which change the accumulated electric charge which can be detected as an electrical signal.

In 2009, Hitachi became the world’s first company to put probes employing CMUT technology into practical use, and has continued with research and development of the CMUT since that time. This experience has culminated in the launch of a diagnostic ultrasound system presenting evo-lutionary CMUT and other cutting edge technologies that bring a new level of image resolution. This report introduces these new technologies.

Development of 4G CMUT (CMUT Linear SML44 probe)

1) Engineering R&D Division 1, Diagnostic R&D Division, Hitachi, Ltd. Healthcare Business Unit2) Center for Technology Innovation, R&D Group, Hitachi, Ltd.

Key Words: 4G CMUT, MEMS, High-power Cell, True Pulse Shaping

Tsuyoshi Otake1) Hiroki Tanaka2)

Akifumi Sako1) Makoto Fukada1)

Kengo Imagawa1) Masahiro Sato1)

In 2009, Hitachi commercialized “Mappie*1, the world’s first Capacitive Micro-machined Ultrasound Transducer (CMUT)

using semiconductor based technology. It generated high quality diagnostic images of mammary glands, thanks to its broad-

band characteristics1). This year, the 4th generation CMUT (4G CMUT) “SML44” has been brought to the market, achieved

using advanced design and precise control of the fabrication process. When combined with new imaging technologies avail-

able with the ARIETTA*2 850, the SML44, in addition to excellent image quality, offers commonly used modalities and func-

tions such as Tissue Harmonic Imaging (THI), Color Flow Mapping (CFM), Real-time Tissue Elastography*3 (RTE), and

Real-time Virtual Sonography*4 (RVS). This report introduces the latest technology adopted in the 4G CMUT design.

Technical Report

Probe

CMUTtransducer

CMUT chip CMUT cell

Vibrating membrane(insulating material)

Highvoltage

Silicon wafer( made using semicon

ductor technology )

*nm=1/10 9 m

Cross section ofCMUT cell

Vacuum cavity (Height ~ nm*)

Figure 1: Structure of CMUT

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2. Acoustic CharacteristicsFigure 2 shows the difference in acoustic characteristics

between the conventional piezoelectric ceramic transducer and that applying CMUT technology. In order to conduct ultrasonic waves to soft biological tissue from the hard piezo-electric ceramic transducer, there is a need to add a material of intermediate hardness between them, called the acoustic matching layer. This acoustic matching layer is designed to allow resonance with an optimal thickness matched to the ultrasonic wavelength. If there is a deviation from the appropriate frequency bandwidth, the ultrasonic propagation efficiency will drop rapidly. As a result, the frequency band-width for piezoelectric ceramics is restricted, and out-of-band frequency components reverberate inside the acoustic matching layer. This results in pulse waveform tailing, and the frequency characteristic becomes relatively irregular in shape.

By comparison, the CMUT vibrating membrane is thin and light, with acoustic characteristics that resemble those of biological tissues. Thanks to this feature, the CMUT can transmit an ideal ultra-short pulse waveform without tailing with a smooth ultra-wide frequency bandwidth character-istic, realizing excellent resolution and favorable penetra-tion. In particular, the smooth bandwidth characteristics of CMUT are maintained with depth despite the effects of the attenuation of the ultrasonic waves which becomes greater with higher frequencies. Thus, it has the advantage of main-taining excellent imaging properties at depth2).

3. Leading-edge technologies employed for 4G CMUT

The recently commercialized 4G CMUT is the culmina-tion of continuing advancements in Hitachi’s unique CMUT technology. 4G CMUT is compatible with the latest premium class diagnostic ultrasound systems that attain high reso-lution and can maximize the performance of the ultra-wide bandwidth properties of CMUT (Figure 3). Imaging modes such as Tissue Harmonic Imaging (THI) which is difficult to apply to conventional CMUT, and Color Flow Mapping (CFM) allowing application to many diagnostic areas, are

also supported. The following section introduces some ex-amples of the technologies adopted.

3.1 THI Technology for 4G CMUT

With the THI method, ultrasonic waves are transmitted into the body at comparatively high amplitude, whilst only weak harmonic signals are extracted from the body. THI is one of the core modalities for generating high quality imag-ing for ultrasound diagnosis. For applying THI to CMUT, 4G CMUT adopts two new techniques, namely High-power Cell and True Pulse Shaping (Figure 4).

High-power Cell technology optimizes the CMUT cell structure through meticulous cell design and process con-trol. Applying sophisticated semiconductor manufacturing technology, the width of the vacuum gap is controlled with high precision to ensure optimum balance between trans-mission and reception. Furthermore, a fine nano-structured spacer is provided on the vibrating membrane to ensure operational stability during contact of the membranes. This technology allows both the high sensitivity and high ampli-tude drive pulse required for THI3) 4).

True Pulse Shaping is a technology allowing ideal pulse waveform transmission even from the CMUT which re-sponds with complicated movements with respect to the in-put electrical waveform. In the case of THI, if the distortion of the ultrasonic wave signal generated from the probe is mixed into the received signal, separation from the harmon-ic signals generated in the body becomes difficult, resulting in image noise. Using a unique algorithm, the True Pulse Shaping function predicts the special response characteris-tics of CMUT from the input electrical signals with high pre-cision, and cancels out the distortion generated by CMUT in signal transmission.

These sophisticated technologies realized for the first time by fine control of the CMUT cell structure are indis-pensable for applying CMUT to THI.

Since the CMUT vibrating membrane is thin and light, with an acoustic impedance similar to that of the body, an acoustic matching layer is not required and an ultra-short pulse with ultra-wide bandwidth can be obtained.

Piezoelectric ceramic

CMUT Human body

Human body

Piezoelectric ceramic

Soft(≈1.5MRayl)

Hard (≈30MRayl)

Waveform

Time

Time

Frequency

Frequency

Bandwidth

Waveform

Acousticmatchinglayers

Acousticlens Bandwidth

Figure 2: Difference in acoustic characteristics between conventional probe and that applying CMUT technology

CMUT linear SML44 probe

Figure 3: 4G CMUT (Left) and ARIETTA 850 (Right)

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3.2 Matrix arrayThe ultrasonic beam from a conventional probe has a

fixed focus in the elevation (short axis) direction determined by the acoustic lens, and it is inevitable that the resolution will be poorer at all points away from the focal region. In con-trast, the CMUT chip adopted for 4G CMUT consists of tens of thousands of CMUT cells in a matrix shape (grid pattern) and their operation can be controlled with a high degree of freedom. This enables tuning of the ultrasonic beam in the elevational plane to create multiple focal zones to optimize beam width with depth in the diagnostic image, thereby improving spatial resolution (Figure 5). With ARIETTA 850, the short axis beam shape is automatically controlled, to ob-tain high resolution imaging at all depths.

4. Examples of Images

Figures 6 and 7 are examples of images taken by the ARIETTA 850 diagnostic ultrasound system applying 4G CMUT. In the phantom image in Figure 6, 4G CMUT has a finer speckle pattern compared to the conventional probe, and produces images with very good wire target distance and high azimuth resolution. Figure 7 shows several clinical images. High resolution and excellent penetration is demon-strated from near field diagnostic areas, such as superficial

High-power Cell

TimeTime

CMUTOptimal input waveform

( positive and negative components are asymmetrical )

Nano-structure spacer

CMUT cell cross-section

High precision control of vacuum cavity

Wide amplitude of vibrating membrane

Transmission signal(electric)

Ideal transmission pulse waveform

( positive and negative com ponents are symmetrical )

True Pulse Shaping

Transmission signal(ultrasonic wave)

Wire targets

2cm

4cm

4cm

8cm

（a）

（c）

（e）

（b）

（d）

（f）

ShallowMiddle

Deep

Elevati

onal

(short

axis)

direc

tion

Long axis direction

CMUT cell

Figure 4: THI Technology adopted for 4G CMUT

Figure 6: Examples of phantom images (Left: Conventional probe, Right: 4G CMUT)

Figure 7: 4G CMUT clinical images(a) Superficial tissue (shoulder), (b) Mammary gland (Elastogram), (c) Carotid artery(d) Lower limb vein, (e) (f) Abdominal images (Arrows denote 10 mm of depth)

Figure 5: Matrix array adopted in 4G CMUT

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tissues and mammary glands, to areas at greater depth such as in the abdomen.

eFocusing, a new development for ARIETTA 850, enhanc-es both the azimuth and contrast resolution throughout the whole field of view. Together with the synergistic effects of the CMUT matrix and ultra-wide bandwidth characteristics, the resolution is enhanced in three-dimensions. This con-tributes further to spatial resolution optimization, realizing a new level of image quality. In contrast to the conventional use of multiple probes to cover the range of clinical applica-tions, 4G CMUT offers a “one probe solution”, a linear probe that can be employed across a wide range of ultrasound applications and paves the way for new areas of ultrasound diagnosis.

5. Conclusion

The 4G CMUT probe which applies evolutionary CMUT technology supports multiple modalities to realize high quality imaging for ultrasound diagnosis combined with the advanced functionality delivered by the new ultrasound system. As a result, 4G CMUT is able to provide the exam-iner with more precise information, whilst at the same time, offering a “one probe solution” for multiple clinical areas. Whatever the clinical application, this ultra-wide bandwidth probe is expected to bring numerous advantages such as im-proved objectivity, reduced physical stress on the examiner, enhanced exam efficiency, and more.

The CMUT technology will continue to evolve and its use extended to other types of probes, contributing to further medical technological innovation.

6. AcknowledgmentsThe authors would like to thank all those involved for

their support in putting 4G CMUT into clinical practice, in-cluding Daisuke Ryuzaki, the leader and principal research-er, Shuntaro Machida, principal researcher, Taiichi Takeza-ki, principal researcher, Hiroaki Hasegawa, researcher, and Yasuhiro Yoshimura, principal researcher, from the Hitachi R&D Group’s.

*1 Mappie, *2 ARIETTA, *3 Real-time Tissue Elastography, and *4 Real-time Virtual Sonography are registered trade-marks or trademarks of Hitachi, Ltd. in Japan and other countries.

References

1) Akifumi Sako, et al.: “Development of Ultrasonic Trans-ducer “Mappie” with cMUT technology”, MEDIX 51, 31-34, 2009.

2) Kunio Hashiba: “Medical ultrasound transducers using micro-electro-mechanical systems technology”, Acoustical Science and Technology No. 71-5, 239-246, 2015.

3) S. Machida, et al.: “Highly reliable CMUT cell structure with reduced dielectric charging effect”, in Proc. IEEE Ultrasonics Symp., 2015.

4) H. Tanaka, et al.: “Acoustic characteristics of CMUT with rectangular membranes caused by higher order modes”, in Proc. IEEE Ultrasonics Symp., 2009.

4G CMUT

Thyroid Carotid

Breast

Superficial/Orthopedics

Abdomen

Vein/Artery

Figure 8: “One probe solution” “4G CMUT”