Po-Hsun Wang

Gold-nanorod contrast-enhanced photoacoustic micro-imaging of focused-ultrasound induced blood-brain-barrier opening in a rat model

In this study, we develop a novel photoacoustic imaging technique based on gold nanorods (AuNRs) for quantitatively monitoring focused-ultrasound (FUS) induced blood-brain barrier (BBB) opening in a rat model in vivo. This study takes advantage of the strong near-infrared absorption (peak at ≈ 800 nm) of AuNRs and the extravasation tendency from BBB opening foci due to their nano-scale size to passively label the BBB disruption area. Experimental results show that AuNR contrast-enhanced photoacoustic microscopy (PAM) successfully reveals the spatial distribution and temporal response of BBB disruption area in the rat brains. The quantitative measurement of contrast enhancement has potential to estimate the local concentration of AuNRs and even the dosage of therapeutic molecules when AuNRs are further used as nano-carrier for drug delivery or photothermal therapy. The photoacoustic results also provide complementary information to MRI, being helpful to discover more details about FUS induced BBB opening in small animal models.

Design and characterization of the immersion-type capacitive ultrasonic sensors fabricated in a CMOS process

This work presents the CMOS micromachined capacitive sensors for ultrasound detection in water. The sensing membranes with a 60 µm diameter are released through small etchant holes of 2 µm × 2 µm by a post-CMOS metal etch and sealed with the thinnest possible silicon dioxide (type A) or parylene-D film (type B). Nine membranes form a single detection unit with a capacitance value of 292.5 fF. Convenient routing, which is desired for making a large two-dimensional array, is allowed with the detection circuits being placed directly beneath the sensing membranes. An alternating voltage bias is applied to the sensing electrodes for stabilizing the sensed signals which would otherwise attenuate over time due to trapped charges between electrodes. Resonant frequencies of type-A and type-B sensors in water are 8.8 and 5.8 MHz, with fractional bandwidths of 0.43 and 0.55, respectively. The measured sensitivities are 151.0 and 369.8 mVpp MPa−1 V−1. The equivalent noise pressures, based on the measured thermal noise, are 3.3 and 1.35 Pa Hz−1/2 at a 1 V membrane bias.

In vivo photoacoustic micro-imaging of microvascular changes for Achilles tendon injury on a mouse model

Since neovascularization has been reported that it is associated with tendinopathy, assessments of vascularity are important for both diagnosis and treatment estimation. Photoacoustic imaging, taking the advantages of good ultrasonic resolution and high optical absorption contrast, has been shown a promising tool for vascular imaging. In this study, we explore the feasibility of photoacoustic micro-imaging in noninvasive monitoring of microvascular changes in Achilles tendon injuries on a mouse model in vivo. During collagenase-induced tendinitis, a 25-MHz photoacoustic microscope was used to image microvascular changes in Achilles tendons of mice longitudinally up to 23 days. In addition, complementary tissue structural information was revealed by collateral 25-MHz ultrasound microscopy. Morphological changes and proliferation of new blood vessels in Achilles tendons were observed during and after the acute inflammation. Observed microvascular changes during tendinitis were similar to the findings in the literatures. This study demonstrates that photoacoustic imaging can potentially be a complementary tool for high sensitive diagnosis and assessment of treatment performance in tendinopathy.

Optical resolution photoacoustic microscopy using a Blu-ray DVD pickup head

Optical resolution photoacoustic microscopy (OR-PAM) has been shown as a promising tool for label-free microvascular and single-cell imaging in clinical and bioscientific applications. However, most OR-PAM systems are realized by using a bulky laser for photoacoustic excitation. The large volume and high price of the laser may restrain the popularity of OR-PAM. In this study, we attempt to develop a compact, portable, and low cost OR-PAM based on a consumer Blu-ray (405 nm) DVD pickup head for label-free micro-vascular imaging and red-blood-cell related blood examination. According to the high optical absorption of the hemoglobin at 405 nm, the proposed OR-PAM has potential to be an alternative for the conventional optical microscopy in the examinations of hematological morphology for blood routine. We showed that the Blu-ray DVD pickup head owns the required laser energy and focusing optics for OR-PAM. The firmware of a Blu-ray DVD drive was modified to allow its pickup head to generate nano-second laser pulses with a tunable pulse repetition rate of >30 kHz and a tunable pulse width ranging from 10 to 30 ns. The laser beam was focused onto the target after passing through a transparent cover slide, and then aligned to be confocal with a 50-MHz focused ultrasonic transducer in forward mode. To keep the target on focus, a scan involving auto-tracking procedure was performed. The measured maximum achievable lateral resolution was 1 μm which was mainly limited by the minimum step size of the used motorized stage. A blood smear was imaged without any staining. The red blood cells were well resolved and the biconcave structure could be clearly visualized. In addition, to verify the in vivo imaging capability of the proposed OR-PAM, the micro-vasculature of a mouse ear was imaged without any contrast agent. The results showed that it performed better than a 200x digital optical microscope in terms of image contrast and vascular morphology. In summaries, the proposed OR-PAM has been demonstrated as a promising tool for label-free blood imaging in both small animal studies and blood examinations, and potentially can be a compact and low-cost OR-PAM platform.

Visualization of microcalcifications using photoacoustic imaging:feasibility study

Recently, photoacoustic imaging has been intensively studied for blood vessel imaging, and shown its capability of revealing vascular features suggestive of malignancy of breast cancer. In this study, we explore the feasibility of visualization of micro-calcifications using photoacoustic imaging. Breast micro-calcification is also known as one of the most important indicators for early breast cancer detection. The non-ionizing radiation and speckle free nature of photoacoustic imaging overcomes the drawbacks of current diagnostic tools - X-ray mammography and ultrasound imaging, respectively. We employed a 10-MHz photoacoustic imaging system to verify our idea. A sliced chicken breast phantom with granulated calcium hydroxyapatite (HA) - major chemical composition of the breast calcification associated with malignant breast cancers - embedded was imaged. With the near infared (NIR) laser excitation, it is shown that the distribution of ~500 μm HAs can be clearly imaged. In addition, photoacoustic signals from HAs rivals those of blood given an optimal NIR wavelength. In summary, photoacoustic imaging shows its promise for breast micro-calcification detection. Moreover, fusion of the photoacoustic and ultrasound images can reveal the location and distribution of micro-calcifications within anatomical landmarks of the breast tissue, which is clinically useful for biopsy and diagnosis of breast cancer staging.

Three-Dimensional Photoacoustic Imaging by a CMOS Micromachined Capacitive Ultrasonic Sensor

In this letter, we demonstrate the feasibility of 3-D photoacoustic imaging using a monolithic CMOS micromachined capacitive ultrasonic sensor. The sensing membranes are released by a post-CMOS metal etch and sealed by silicon dioxide. Nine membranes, each with an inner diameter of 60 μm, form a single detection unit with a capacitance value of 292.5 fF. A 6- μm carbon fiber was employed to evaluate the 3-D photoacoustic imaging capability and resolution of our sensor. A 2-D large-aperture array was emulated by 2-D mechanical scanning of a single sensing element with a scanning step of 120 μm. A 2-D synthetic aperture focusing technique was used for image reconstruction. The measured -6-dB spatial resolutions were 181 μm in axial and 448 μm in lateral, respectively, at the depth of ~ 2.4 cm. The 3-D photoacoustic image of a carbon fiber phantom was successfully produced.

Compact touchless fingerprint reader based on digital variable-focus liquid lens

Identity certification in the cyberworld has always been troublesome if critical information and financial transaction must be processed. Biometric identification is the most effective measure to circumvent the identity issues in mobile devices. Due to bulky and pricy optical design, conventional optical fingerprint readers have been discarded for mobile applications. In this paper, a digital variable-focus liquid lens was adopted for capture of a floating finger via fast focusplane scanning. Only putting a finger in front of a camera could fulfill the fingerprint ID process. This prototyped fingerprint reader scans multiple focal planes from 30 mm to 15 mm in 0.2 second. Through multiple images at various focuses, one of the images is chosen for extraction of fingerprint minutiae used for identity certification. In the optical design, a digital liquid lens atop a webcam with a fixed-focus lens module is to fast-scan a floating finger at preset focus planes. The distance, rolling angle and pitching angle of the finger are stored for crucial parameters during the match process of fingerprint minutiae. This innovative compact touchless fingerprint reader could be packed into a minute size of 9.8*9.8*5 (mm) after the optical design and multiple focus-plane scan function are optimized.

A CMOS MEMS capacitive ultrasonic sensor array for three-dimensional photoacoustic imaging

Three dimensional (3-D) photoacoustic imaging is demonstrated in this study by using a 4 × 4 CMOS (complementary metal oxide semiconductor) micromachined capacitive ultrasonic sensor array. The sensing membranes are fabricated by a post-CMOS metal etch and sealed by parylene with a measured resonant frequency between 3 to 5.1 MHz. The area of a single detection unit is 280 × 280 μm2. A large 2-D aperture was emulated by 2-D mechanical scanning of the array with a scanning step of 1.12 mm. The measured -6-dB spatial resolutions were 381 μm in axial and 1.3 mm in lateral, respectively, at a depth of ~2.1 cm. The 3-D photoacoustic image of a hair phantom was successfully produced.

DVD pickup head based optical resolution photoacoustic microscopy

Optical resolution photoacoustic microscopy (OR-PAM) has been shown as a promising tool for label-free microvascular and single-cell imaging in clinical and bioscientific applications. However, most OR-PAM systems are realized by using a bulky laser for photoacoustic excitation. The large volume and high price of the laser may restrain the popularity of OR-PAM. In this study, we develop a low-cost and compact OR-PAM system based on a commercially available DVD pickup head. We showed that the DVD pickup head have the required laser energy and focusing optics for OR-PAM. The firmware of a DVD burner was modified to enable its laser diode to provide a 13-ns laser pulse with 1.3-nJ energy at 650 nm. Two excitation wavelengths at 650 and 780 nm were available. The laser beam was focused onto the target after passing through a 0.6-mm thick DVD transparent polycarbonate coating, and then aligned to be confocal with a 50-MHz focused ultrasonic transducer in forward mode. To keep the target on focus, a scan involving auto-tracking procedure was performed. The lateral resolution was verified via cross-sectional imaging of a 6-μm carbon fiber. The measured -6 dB width of the carbon fiber was 6.66 μm which was in agreement with optical diffraction limit. The proposed OR-PAM has potential as an economically viable and compact blood screening tool available outside of large laboratories due to its low cost and portability. Furthermore, a better spatial resolution could be provided by using a blue ray DVD pickup head.

Three dimensional photoacoustic imaging using a monolithic CMOS MEMS capacitive ultrasonic sensor

In this study, we demonstrate the feasibility of three dimensional (3-D) photoacoustic imaging using a monolithic CMOS MEMS capacitive ultrasonic sensor. The sensing membranes are released by a post-CMOS metal etch and sealed by silicon dioxide. Nine membranes, each with an inner diameter of 60 µm, form a single detection unit with a capacitance value of 292.5 fF. The measured sensitivity is 151 mVpp/MPa/V. The equivalent noise pressure is 3.3 Pa/√Hz. A photoacoustic method with a 6-µm carbon fiber was employed to characterize the impulse and frequency responses of our CMOS sensor. To evaluate the 3-D imaging capability and the achievable photoacoustic imaging resolution from our CMOS sensor, carbon fiber and hair phantoms were imaged. A 2-D large-aperture array was emulated by 2-D mechanically scanning of a single sensing element with a scanning step of 120 µm. A 2-D synthetic aperture focusing technique was used for photoacoustic image reconstruction. The −6dB spatial resolution provided by our sensor and scanning and reconstruction scheme was 181 µm in axial and 448 µm in lateral, respectively at the depth of ∼2.4 cm and with a synthesized 120-mm by 4.8-mm 2-D array. Future work will focus on fabrication of a true 2-D array and improving the bandwidth and the integrated electronics to offer higher SNR and facilitate real-time imaging.