This paper reports a new micromachined two-axis water-immersible micro scanning mirror using BoPET (biaxiallyoriented polyethylene terephthalate) hinges. A new fabrication process based on lithography, wafer-bonding, and reactive ion etching on a hybrid silicon-polymer substrate was developed to enable high-resolution patterning, miniaturization and also batch fabrication capability. For demonstration, a prototype scanning mirror was designed, fabricated and tested. Its overall size was reduced to 5×5×5 mm3, which is comparable to that of a typical siliconbased micro scanning mirror. The testing results show good raster scanning performance. This approach could enable the miniaturization and batch fabrication of water-immersible scanning mirrors for different scanning optical and acoustic imaging applications in liquid environments.
Photoacoustic endoscopy offers in vivo examination of the visceral tissue using endogenous contrast, but its typical B-scan rate is ∼10 Hz, restricted by the speed of the scanning unit and the laser pulse repetition rate. Here, we present a transvaginal fast-scanning optical-resolution photoacoustic endoscope with a 250-Hz B-scan rate over a 3-mm scanning range. Using this modality, we not only illustrated the morphological differences of vasculatures among the human ectocervix, uterine body, and sublingual mucosa but also showed the longitudinal and cross-sectional differences of cervical vasculatures in pregnant women. This technology is promising for screening the visceral pathological changes associated with angiogenesis.
In this paper, we report a new electromagnetically-driven two-axis water-immersible scanning mirror with single inductor coil actuator. By making the resonance frequencies significantly different, the scanning motion of the two axes can be decoupled due to the dynamic structural filtering effect. For verification, a prototype micro scanning mirror was designed, fabricated and tested. The resonance frequencies of the fast and slow axes in air and water were determined to be 720/72 Hz and 690/55 Hz, respectively. The optical tilting angle of both fast and slow axes at the resonance frequencies was characterized, which shows good linearity with the amplitude of the driving current. With both axes driven simultaneously at frequencies close to their resonance, stable and repeatable 2D raster scanning was successfully achieved in both air and water, respectively. 2D B-mode ultrasound microscopy was also demonstrated by using this micro scanning mirror.
KEYWORDS: Acoustics, Silicon, Polymers, Photoacoustic tomography, Transducers, Signal attenuation, Acquisition tracking and pointing, 3D printing, Data acquisition, Electronics, Ultrasonography, Receivers, Signal detection
Recent studies have shown that micromachined silicon acoustic delay lines can provide a promising solution to achieve real-time photoacoustic tomography without the need for complex transducer arrays and data acquisition electronics. However, as its length increases to provide longer delay time, the delay line becomes more vulnerable to structural instability due to reduced mechanical stiffness. In addition, the small cross-section area of the delay line results in a large acoustic acceptance angle and therefore poor directivity.
To address these two issues, this paper reports the design, fabrication, and testing of a new silicon acoustic delay line enhanced with 3D printed polymer micro linker structures. First, mechanical deformation of the silicon acoustic delay line (with and without linker structures) under gravity was simulated by using finite element method. Second, the acoustic crosstalk and acoustic attenuation caused by the polymer micro linker structures were evaluated with both numerical simulation and ultrasound transmission testing. The result shows that the use of the polymer micro linker structures significantly improves the structural stability of the silicon acoustic delay lines without creating additional acoustic attenuation and crosstalk. In addition, a new tapered design for the input terminal of the delay line was also investigate to improve its acoustic directivity by reducing the acoustic acceptance angle. These two improvements are expected to provide an effective solution to eliminate current limitations on the achievable acoustic delay time and out-of-plane imaging resolution of micromachined silicon acoustic delay line arrays.
Fast multiaxis scanning is useful for not only optical but also acoustic microscopic imaging. Although they have been used for optical scanning, the application of (MEMS) scanning mirrors in acoustic microscopy is still very limited due to their small mirror plate size, and more importantly, inability to operate in liquids (as ultrasound coupling media). A microfabricated two-axis water-immersible scanning mirror for optical and acoustic microscopy is reported. It has an optical and acoustically reflective mirror plate (6 mm×4 mm) to provide numerical aperture for ultrasound beam steering. Electromagnetic and mechanical analysis and simulation were conducted to estimate the mechanical tilting angle and resonance frequency of both fast and slow axes, which matches well with the measurement results. The fast axis has a resonant frequency of 320 Hz in air and 220 Hz in water, which is more than 10 times higher than that of the slow axis (24 Hz in air and 14 Hz in water). Under a 100-mA driving current, the scanning angles of the fast axis reached ±9.5 deg in both air and water at the resonance frequency, respectively. The scanning angles of the slow axis reached ±15 deg in air and ±12.5 deg in water at resonant frequencies, respectively. Raster scanning of a collimated laser beam was achieved by driving both axes simultaneously close to their own resonance frequencies. The feasibility of using the two-axis water-immersible scanning mirror in scanning acoustic microscopy was also demonstrated.
Optical-resolution photoacoustic microscopy (OR-PAM) offers label-free in vivo imaging with high spatial resolution by acoustically detecting optical absorption contrasts via the photoacoustic effect. We developed a compact handheld OR-PAM probe for fast photoacoustic imaging. Different from benchtop microscopes, the handheld probe provides flexibility in imaging various anatomical sites. Resembling a cup in size, the probe uses a two-axis water-immersible microelectromechanical system mirror to scan both the illuminating optical beam and resultant acoustic beam. The system performance was tested in vivo by imaging the capillary bed in a mouse ear and both the capillary bed and a mole on a human volunteer.
Fast scanning is highly desired for both ultrasound and photoacoustic microscopic imaging. Limited by water environment required for acoustic propagation, traditional mircoelectromechanical system (MEMS) scanning mirrors could not be widely used. In this paper, a new water-immersible scanning mirror microsystem has been designed, fabricated and tested. Polymer hinges were employed to achieve reliable under water performance. Two pairs of high strength neodymium magnet disc and three compact RF choke inductor were used to actuate mirror module. Experimental results show that the fast axis can reach a mechanical scanning angle of ±15° at the resonance frequency of 350 Hz in air, and ±12.5° at the resonance frequency of 240 Hz in water, respectively. The slow axis can reach a mechanical scanning angle of ±15° at the resonance frequency of 20 Hz in air, and ±12.5° at the resonance frequency of 13 Hz in water, respectively. The two scanning axes have very different resonance frequencies, which are suitable for raster scanning.
Micro scanning mirrors that can operate reliably under water is useful in both ultrasound and photoacoustic microscopic imaging, where fast scanning of focused high-frequency ultrasound beams is desired for pixel-by-pixel data acquisition. This paper reports the development of a new micro-fabricated water-immersible scanning mirror with a small form factor. It consists of an optically and acoustically reflective mirror plate, which is supported onto two flexible polymer hinges and driven by an integrated electromagnetic micro-actuator. It can achieve one-axis scanning of ±12.1° at a resonant frequency of 250Hz in air and 210Hz in water, respectively. By optimizing the design and enhancing the fabrication with high-precision optical 3D printing, the overall size of the scanning mirror module is less than 7 mm × 5 mm × 7 mm. The small form factor, large scanning angle, and high resonant frequency of the new water-immersible scanning mirror make it suitable for building compact handheld imaging probes for in-vivo high-speed and wide-field ultrasound and photoacoustic microscopy.
Microscanning mirrors that can operate reliably under water are useful in both ultrasound and photoacoustic microscopic imaging, where fast scanning of focused high-frequency ultrasound beams is desired for pixel-by-pixel data acquisition. We report the development of a new microfabricated water-immersible scanning mirror with a small form factor. It consists of an optically and acoustically reflective mirror plate which is supported by two flexible polymer hinges and driven by an integrated electromagnetic microactuator. It can achieve 1-axis scanning of ±12.1 deg at a resonant frequency of 250 Hz in air and 210 Hz in water, respectively. By optimizing the design and enhancing the fabrication with high-precision optical three-dimensional printing, the overall size of the scanning mirror module is less than 7 mm×5 mm×7 mm. The small form factor, large scanning angle, and high-resonant frequency of the new water-immersible scanning mirror make it suitable for building compact handheld imaging probes for in vivo high-speed and wide-field ultrasound and photoacoustic microscopy.
Melanoma, one of the most common types of skin cancer, has a high mortality rate, mainly due to a high propensity for tumor metastasis. The presence of circulating tumor cells (CTCs) is a potential predictor for metastasis. Label-free imaging of single circulating melanoma cells in vivo provides rich information on tumor progress. Here we present photoacoustic microscopy of single melanoma cells in living animals. We used a fast-scanning optical-resolution photoacoustic microscope to image the microvasculature in mouse ears. The imaging system has sub-cellular spatial resolution and works in reflection mode. A fast-scanning mirror allows the system to acquire fast volumetric images over a large field of view. A 500-kHz pulsed laser was used to image blood and CTCs. Single circulating melanoma cells were imaged in both capillaries and trunk vessels in living animals. These high-resolution images may be used in early detection of CTCs with potentially high sensitivity. In addition, this technique enables in vivo study of tumor cell extravasation from a primary tumor, which addresses an urgent pre-clinical need.
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