Bao Yu Hsieh

All-optical scanhead for ultrasound and photoacoustic dual-modality imaging

We propose a new scanhead design for combined ultrasound (US)/photoacoustic (PA) imaging that can be applied to dual-modality microscopy and biomedical imaging. Both imaging modalities employ the optical generation and detection of acoustic waves. The scanhead consists of an optical fiber with an axicon tip for excitation, and a microring for acoustic detection. No conventional piezoelectric device is needed, and the cost of the design makes it suitable for one-time, disposable use. Furthermore, a single laser pulse is employed to generate both US and PA signals. A subband imaging method can be applied to the receiver to enhance the contrast between the US and PA signals. Phantom data demonstrate the feasibility of this approach.

Integrated intravascular ultrasound and photoacoustic imaging scan head

The combination of intravascular ultrasound and intravascular photoacoustic imaging has been proposed for improving the diagnosis of arterial diseases. We describe a novel scan-head design for implementing such multimodality imaging. The proposed device has the potential to achieve a sufficiently small size for clinical intravascular applications. The design aims for efficient image data acquisition for facilitating real-time three-dimensional imaging and reducing the required laser pulse repetition frequency. The integrated scan head consists of a single-element, ring-shaped transducer for sideward ultrasound transmission, a multimode fiber with a cone-shaped mirror for optical illumination, and a single polymer microring with mechanical scanning. The phantom imaging and some experimental results are presented. A microring array can be realized in the future to achieve high-frame-rate intravascular multimodality imaging.

A laser ultrasound transducer using carbon nanofibers-polydimethylsiloxane composite thin film

The photoacoustic effect has been broadly applied to generate high frequency and broadband acoustic waves using lasers. However, the efficient conversion from laser energy to acoustic power is required to generate acoustic waves with high intensity acoustic pressure (>10 MPa). In this study, we demonstrated laser generated high intensity acoustic waves using carbon nanofibers–polydimethylsiloxane (CNFs-PDMS) thin films. The average diameter of the CNFs is 132.7 ± 11.2 nm. The thickness of the CNFs film and the CNFs-PDMS composite film is 24.4 ± 1.43 μm and 57.9 ± 2.80 μm, respectively. The maximum acoustic pressure is 12.15 ± 1.35 MPa using a 4.2 mJ, 532 nm Nd:YAG pulsed laser. The maximum acoustic pressure using the CNFs-PDMS composite was found to be 7.6-fold (17.62 dB) higher than using carbon black PDMS films. Furthermore, the calculated optoacoustic energy conversion efficiency K of the prepared CNFs-PDMS composite thin films is 15.6 × 10−3 Pa/(W/m2), which is significantly higher than carbon black-P...

Strategies to improve phase-stability of ultrafast swept source optical coherence tomography for single shot imaging of transient mechanical waves at 16 kHz frame rate

We present single-shot phase-sensitive imaging of propagating mechanical waves within tissue, enabled by an ultrafast optical coherence tomography (OCT) system powered by a 1.628 MHz Fourier domain mode-locked (FDML) swept laser source. We propose a practical strategy for phase-sensitive measurement by comparing the phases between adjacent OCT B-scans, where the B-scan contains a number of A-scans equaling an integer number of FDML buffers. With this approach, we show that micro-strain fields can be mapped with ∼3.0 nm sensitivity at ∼16 000 fps. The systems capabilities are demonstrated on porcine cornea by imaging mechanical wave propagation launched by a pulsed UV laser beam, promising non-contact, real-time, and high-resolution optical coherence elastography.

All-optical scanhead for ultrasound and photoacoustic imaging—Imaging mode switching by dichroic filtering

Ultrasound (US) and photoacoustic (PA) multimodality imaging has the advantage of combining good acoustic resolution with high optical contrast. The use of an all-optical scanhead for both imaging modalities can simplify integration of the two systems and miniaturize the imaging scanhead. Herein we propose and demonstrate an all-optical US/PA scanhead using a thin plate for optoacoustic generation in US imaging, a polymer microring resonator for acoustic detection, and a dichroic filter to switch between the two imaging modes by changing the laser wavelength. A synthetic-aperture focusing technique is used to improve the resolution and contrast. Phantom images demonstrate the feasibility of this design, and show that axial and lateral resolutions of 125 μm and 2.52°, respectively, are possible.

Image reconstruction in intravascular photoacoustic imaging

Intravascular photoacoustic (IVPA) imaging is a technique for visualizing atherosclerotic plaques with differential composition. Unlike conventional photoacoustic tomography scanning, where the scanning device rotates around the subject, the scanning aperture in IVPA imaging is enclosed within the imaged object. The display of the intravascular structure is typically obtained by converting detected photoacoustic waves into Cartesian coordinates, which can produce images with severe artifacts. Because the acquired data are highly limited, there does not exist a stable reconstruction algorithm for such imaging geometry. The purpose of this work was to apply image reconstruction concepts to explore the feasibility and efficacy of image reconstruction algorithms in IVPA imaging using traditional analytical formulas, such as a filtered back-projection (FBP) and the lambda-tomography method. Although the closed-form formulas are not exact for the IVPA system, a general picture of and interface information about objects are provided. To improve the quality of the reconstructed image, the iterative expectation maximization and penalized least-squares methods were adopted to minimize the difference between the measured signals and those generated by a reconstructed image. In this work, we considered both the ideal point detector and the acoustic transducers with finite- size aperture. The transducer effects including the spatial response of aperture and acoustoelectrical impulse responses were incorporated in the system matrix to reduce the aroused distortion in the IVPA reconstruction. Computer simulations and experiments were carried out to validate the methods. The applicability and the limitation of the reconstruction method were also discussed.

Concurrent photoacoustic-ultrasound imaging using single-laser pulses

Abstract. Conventional ultrasound (US) and photoacoustic (PA) multimodality imaging require the use of a US pulse for US data acquisition and a laser pulse for PA data acquisition. We propose a method for concurrent US and PA data acquisition with a single-laser pulse. A light-absorbing multilayer film that can generate a US pulse based on the thermoelastic effect is used. The selection of appropriate layer thickness, interlayer spacing, and absorption coefficient allows the spectral characteristics of the generated US signal to be adjusted so that it does not overlap with the spectrum of the PA signal generated by the light transmitting through the layer. Thus, the US signal and the PA signal can be generated, received, and separated by using a single-laser pulse combined with spectral filtering. This method is demonstrated using a multilayer film that generates US signals with a center frequency of 24.2 MHz and fractional bandwidth of 26.8%. The synthetic-aperture focusing technique is applied to improve the lateral resolution and the signal-to-noise ratio. A cyst-like phantom and a film phantom were used to demonstrate the feasibility of this method of concurrent PA-US imaging using single-laser pulses.

Design and fabrication of an integrated intravascular ultrasound/photoacoustic scan head

The combination of intravascular ultrasound and intravascular photoacoustic imaging has been proposed. In this study, we propose a scan head design that is sufficiently small to fit in the tip of the catheter. In addition, the design is also suitable for ultra high frame rate imaging. The scan head consists of a single element, ring-shaped transducer for sideward ultrasound transmission. The transducer has a diameter of 3mm. On acoustic detection, we propose the use of a polymer microring array. For demonstration purposes, a single micro-ring is used with mechanical scanning in this study. For optical illumination, a multimode fiber with a cone-shaped mirror is used. Note that only a single ultrasound/laser pulse is required to acquire an ultrasound/photoacoustic image frame. Phantom imaging results are demonstrated.

Application of limited-view image reconstruction method to intravascular photoacoustic tomography

Intravascular photoacoustic (IVPA) imaging that aims to detect atherosclerotic plaques with differential composition is studied computationally and experimentally. IVPA images are usually reconstructed by simply aligning photoacoustic signals with scan conversion, which results in images with severe blurring and increases the difficulty in signal detection. The scanning aperture in IVPA, in contrast to other photoacoustic tomography applications, is enclosed within the imaged object. Consequently, quantitative image reconstruction becomes infeasible, as the data sufficiency condition for stable image reconstruction is not satisfied in such a limited-view scanning. However, useful information regarding certain plaque boundaries can still be reconstructed, which can facilitate plaque detection. In this study, strategies for limited-view reconstruction will be investigated for the IVPA scanning geometry. Computer simulations are carried out to validate the developed method.

All-optical transducer for ultrasound and photoacoustic imaging by dichroic filtering

The ultrasound (US) and photoacoustic (PA) multi-modality imaging has the advantages of combining good acoustic resolution with high optical contrast. An all-optical transducer for both imaging modalities can simplify the entire system design and miniaturize the imaging scanhead for intravascular imaging applications. In this study, we propose and demonstrate an all-optical US/PA transducer using a thin plate for optoacoustic generation for US imaging, a microring resonator for acoustic detection, and a dichroic filter to switch between the two imaging modes. In other words, a dichroic (i.e., a color filter) is used to selectively pass light depending on the imaging mode. When under US imaging, the light hits the color filter to generate US for US imaging. When under PA imaging, the light pass through the color filter and directly hits the image object. The experimental image results indicated that the US and PA imaging modes can be selected by switching the laser wavelengths using a dichroic filter. The imaging resolution and contrast were improved by applying the synthetic aperture focusing technique (SAFT). The axial and lateral resolutions of the implemented imaging system were 125μm and 2.52°, respectively, while application of the SAFT enhanced the contrast by about 5dB.