Development of Compact Near-Surface Ultrasound Biomedical Imaging System Based on CMUT

Abstract

Ultrasound imaging is an attractive imaging technique used to diagnose conditions inside the human body, owing to its noninvasive characteristics in distinguishing soft tissues and various organs. Various ultrasound imaging systems have been developed, from mechanical scanning systems with a single transducer to non-scanning systems with a 2-D array transducer for real-time 3-D volumetric imaging. Despite the development of ultrasound imaging systems, conventional ultrasound images are cut off in the near-surface range because of technical difficulties, such as electrical and acoustical interference, whereby the transmission source directly affects the receiving channels. The interference signals are overlaid with the reflected ultrasound signals in the near-surface range. The realization of a near-surface imaging system requires several tasks: fabrication of a miniaturized transducer to reduce the near-field range of the transducer and remove the acoustical and electrical interference due to the transmission signals. This thesis presents the feasibility of ultrasound near-surface imaging systems. To validate near-surface imaging capability, fingerprint imaging and in-vivo targeting experiments were conducted.

Fingerprint-imaging systems are constructed using various methods, such as the ultrasound pulse-echo, photoacoustic (PA), and impediography methods. In the ultrasound pulse-echo method, the fingerprint images were obtained with a bulk system by using a focused transducer. The pulse-echo experiments demonstrate an ultrasound beamforming method under glass. The possibility of near-surface imaging was verified by a 2-D array transducer based on numerical analysis. This study focuses on the feasibility of an under-glass measurement of a dry finger with a dense array transducer for compact device application. In addition, glass is shown to act as a coupling layer with rigidity-dependent and thickness-dependent characteristics. The focused transducer-based system was extended to the PA method by using a tilted irradiated light source. The experimental PA image indicates that the ultrasonic signal generated by the irradiated light can distinguish the ridges and valleys of the fingerprint.

The impediography fingerprint-imaging systems are divided into direct-contact-based and waveguide-based systems, 0.5 mm in height. The systems are constructed by capacitive micromachined ultrasonic transducers (CMUTs) to secure high resolution. The CMUT, fabricated by the local oxidation of silicon (LOCOS), has a 6.25 MHz center frequency and 50 V collapse voltage. The waveguide is made from quartz glass to protect the device protection and improve the image resolution. The proposed technique distinguishes the ridges and valleys of a fingerprint based on electrical impedance differences at the resonant frequency. The experimental fingerprint images are compared with the finite-element analysis (FEA) results. In addition, the waveguide system, in terms of energy loss, is analyzed by numerical simulation.

The near-surface volumetric imaging system in vivo was constructed based on the CMUT using the PA method. PA imaging (PAI) is an excellent near-surface imaging method because it does not require transmission of the transducer. The proposed system allows near-surface volumetric imaging, owing to the following advanced features: backward light irradiation, low-profile ultrasonic receiver unit (< 1 cm), and short measurement time of the 64-element CMUT (< 1 s). The ring-array CMUT used in this experiment has a 10.4 MHz center frequency in air and 6.2 mm diameter. A 6 mm polydimethylsiloxane (PDMS) layer is used as the matching layer. Phantom-based tests are constructed by imaging the point and line targets of pencil lead (solid model) and red ink (liquid model) to validate the system performance. In-vivo experiments targeting vessels and moles within the hand and wrist are performed. The wrist vein and mole are successfully imaged, and the depth of the vessel from the skin is detected according to the position of the hand.

The proposed final system in this thesis is a dual-mode imaging system capable of pulse-echo and PAI. The main concept is to use an optical ultrasound generated by a carbon nanotube (CNT)-PDMS composite irradiated with light as an ultrasonic pulser for the pulse-echo method. With a thin fabricated CNT-PDMS composite, the semi-optical-absorption CNTs generate optical ultrasound, and semi-optical-penetrated light is delivered to objects. The CNT thickness of < 1 μm provides a maximum acoustic pressure of 154 kPa, and the ultrasonic receiver of the CMUT has a 2 MHz center frequency in PDMS. The system consists of an intermediate layer of 6 mm PDMS; thus, the ultrasonic signals are transmitted into the PDMS layer. The experiments are performed with point and line targets composed of CNTs, red inks, and air gaps. In addition, the particle spread test is conducted in soft tissue. Therefore, this thesis presents the feasibility of ultrasound imaging systems, ranging from the contact-based system to the near-surface in-vivo ultrasound pulse-echo and PA systems.