Hao-Chung Yang

Integrated intravascular optical coherence tomography ultrasound imaging system

We report on a dual-modality optical coherence tomography (OCT) ultrasound (US) system for intravascular imaging. To the best of our knowledge, we have developed the first integrated OCT-US probe that combines OCT optical components with an US transducer. The OCT optical components mainly consist of a single-mode fiber, a gradient index lens for light-beam focusing, and a right-angled prism for reflecting light into biological tissue. A 40-MHz piezoelectric transducer (PZT-5H) side-viewing US transducer was fabricated to obtain the US image. These components were integrated into a single probe, enabling both OCT and US imaging at the same time. In vitro OCT and ultrasound images of a rabbit aorta were obtained using this dual-modality imaging system. This study demonstrates the feasibility of an OCT-US system for intravascular imaging, which is expected to have a prominent impact on early detection and characterization of atherosclerosis.

A dual-modality probe utilizing intravascular ultrasound and optical coherence tomography for intravascular imaging applications

We have developed a dual-modality biomedical imaging probe utilizing intravascular ultrasound (IVUS) and optical coherence tomography (OCT). It consists of an OCT probe, a miniature ultrasonic transducer and a fixed mirror. The mirror was mounted at the head of the hybrid probe 45° relative to the light and the ultrasound beams to change their propagation directions. The probe was designed to be able to cover a larger area in blood vessel by IVUS and then visualize a specific point at a much finer image resolution using OCT. To demonstrate both its feasibility and potential clinical applications, we used this ultrasound-guide OCT probe to image a rabbit aorta in vitro. The results offer convincing evidence that the complementary natures of these two modalities may yield beneficial results that could not have otherwise been obtained.

Endoscopic Photoacoustic Microscopy

We present a concept and system implementation for endoscopic photoacoustic microscopy that enables minimally invasive diagnosis of internal organs. The system incorporates an in-house made single element ultrasonic transducer for photoacoustic signal detection and an optical fiber for light delivery into a tubular probe with a mechanical scanning unit. The implemented probe size for the distal end is 4.2 mm in diameter and 48 mm in length. We evaluated the systems performance by imaging a carbon fiber in clear and turbid media. We also demonstrated its ability to image actual biological tissues and its endoscopic applicability, by imaging abdominal surfaces and a large intestinal tract of a rat ex vivo. This study shows the systems potential for in vivo optical biopsy of tissue abnormalities, such as tumors, developed in internal organs.

Novel biomedical imaging that combines intravascular ultrasound (IVUS) and optical coherence tomography (OCT)

We have developed a new biomedical imaging probe combining intravascular ultrasound (IVUS) and optical coherence tomography (OCT). Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) needle ultrasound transducers with an aperture size of 0.6 mm were fabricated. The measured center frequency and -6 dB fractional bandwidth of the PMN-PT needle ultrasound transducer were 35 MHz and 60 %, respectively. A mirror was mounted at the tip of the probe at position 45deg to change the propagation direction of the ultrasound beam and the laser beam. In vitro images of rabbit trachea and aorta forming from this combined probe have been acquired. These results demonstrate that the complementary nature of these two modalities may yield beneficial results that could not be obtained otherwise.

Development of a Flexible Implantable Sensor for Postoperative Monitoring of Blood Flow

We have developed a blood flow measurement system using Doppler ultrasound flow sensors fabricated of thin and flexible piezoelectric‐polymer films. These flow sensors can be wrapped around a blood vessel and accurately measure flow. The innovation that makes this flow sensor possible is the diffraction‐grating transducer. A conventional transducer produces a sound beam perpendicular to its face; therefore, when placed on the wall of a blood vessel, the Doppler shift in the backscattered ultrasound from blood theoretically would be 0. The diffraction‐grating transducer produces a beam at a known angle to its face; therefore, backscattered ultrasound from the vessel will contain a Doppler signal. Flow sensors were fabricated by spin coating a poly(vinylidene fluoride–trifluoroethylene) copolymer film onto a flexible substrate with patterned gold electrodes. Custom‐designed battery‐operated continuous wave Doppler electronics along with a laptop computer completed the system. A prototype flow sensor was evaluated experimentally by measuring blood flow in a flow phantom and the infrarenal aorta of an adult New Zealand White rabbit. The flow phantom experiment demonstrated that the error in average velocity and volume blood flow was less than 6% for 30 measurements taken over a 2.5‐hour period. The peak blood velocity through the rabbit infrarenal aorta measured by the flow sensor was 118 cm/s, within 1.7% of the measurement obtained using a duplex ultrasound system. The flow sensor and electronics operated continuously during the course of the 5‐hour experiment after the incision on the animal was closed.

Volumetric photoacoustic endoscopy of internal organs: a phantom and in situ study

In this study, we further developed our photoacoustic endoscopic system to produce three-dimensional images of internal organs by performing pullback C-scans. Employing the side-scanning photoacoustic endoscopic probe discussed in the Optical Society of Americas journal Optics Letters, we could acquire successive B-scan images by pulling back the probe with a motorized linear stage. We demonstrate the endoscopic systems volumetric imaging ability through imaging of a metal wire phantom and an in situ rat rectum.

Crosstalk reduction for high-frequency linear-array ultrasound transducers using 1-3 piezocomposites with pseudo-random pillars

The goal of this research was to develop a novel diced 1-3 piezocomposite geometry to reduce pulse-echo ring down and acoustic crosstalk between high-frequency ultrasonic array elements. Two PZT-5H-based 1-3 composites (10 and 15 MHz) of different pillar geometries [square (SQ), 45° triangle (TR), and pseudo-random (PR)] were fabricated and then made into single-element ultrasound transducers. The measured pulse-echo waveforms and their envelopes indicate that the PR composites had the shortest -20-dB pulse length and highest sensitivity among the composites evaluated. Using these composites, 15-MHz array subapertures with a 0.95λ pitch were fabricated to assess the acoustic crosstalk between array elements. The combined electrical and acoustical crosstalk between the nearest array elements of the PR array subapertures (-31.8 dB at 15 MHz) was 6.5 and 2.2 dB lower than those of the SQ and the TR array subapertures, respectively. These results demonstrate that the 1-3 piezocomposite with the pseudo-random pillars may be a better choice for fabricating enhanced high-frequency linear-array ultrasound transducers; especially when mechanical dicing is used.

A study of 1–3 pseudo-random pillar piezocomposites for ultrasound transducers

The goal of our research is to develop a new diced 1-3 piezocomposite geometry to reduce pulse-echo ring down and acoustic cross-talk between high frequency ultrasonic array elements. Two PZT-5H based 1-3 composites (10 and 15 MHz) with different pillar geometries (square (SQ), 45° triangle (TR), and pseudo-random (PR)) were fabricated and then made into single element ultrasound transducers. The measured pulse-echo waveforms and their envelopes indicate that the PR composites had the shortest -20 dB pulse length and strongest sensitivity among all the composites. Using these composites, 15 MHz array sub-apertures with a 0.95 λ pitch were fabricated to measure the acoustic cross-talk between array elements. The combined electrical and acoustical crosstalk between the nearest array elements of the PR array sub-apertures (-32 dB @ 15 MHz) was 6 dB and 2 dB lower than that of the SQ and the TR array sub-apertures, respectively. These results demonstrate that the 1-3 piezocomposite with the pseudo-random pillars may be a good choice for fabricating enhanced high frequency linear array ultrasound transducers; especially when mechanical dicing is used.

Bipolar-power-transistor-based limiter for high frequency ultrasound imaging systems

High performance limiters are described in this paper for applications in high frequency ultrasound imaging systems. Limiters protect the ultrasound receiver from the high voltage (HV) spikes produced by the transmitter. We present a new bipolar power transistor (BPT) configuration and compare its design and performance to a diode limiter used in traditional ultrasound research and one commercially available limiter. Limiter performance depends greatly on the insertion loss (IL), total harmonic distortion (THD) and response time (RT), each of which will be evaluated in all the limiters. The results indicated that, compared with commercial limiter, BPT-based limiter had less IL (-7.7 dB), THD (-74.6 dB) and lower RT (43 ns) at 100 MHz. To evaluate the capability of these limiters, they were connected to a 100 MHz single element transducer and a two-way pulse-echo test was performed. It was found that the -6 dB bandwidth and sensitivity of the transducer using BPT-based limiter were better than those of the commercial limiter by 22% and 140%, respectively. Compared to the commercial limiter, BPT-based limiter is shown to be capable of minimizing signal attenuation, RT and THD at high frequencies and is thus suited for high frequency ultrasound applications.

New modified Butterworth Van-Dyke model for high frequency ultrasonic imaging

For ultrasonic transducers, a number of equivalent circuit models including KLM, Mason and Butterworth Van Dyke (BVD) have been developed. To allow them to be incorporated into design tools for complex integrated circuits, KLM or Mason models are not practical because the discrete components of the equivalent circuits have negative values. Therefore, BVD model appears to be more appropriate but it needs to be improved because the resolution of a high frequency ultrasound system may be severely affected by impedance mismatching between the transducer and the system, as well as attenuation due to parasitic impedances of the systems. A new modified BVD model has been developed and the results demonstrate its usefulness in modeling high frequency ultrasonic transducer and its imaging systems.