Thin polymer etalon arrays for high-resolution photoacoustic imaging

Towards non-contact photoacoustic imaging [review]

Photoacoustic imaging (PAI) takes advantage of both optical and ultrasound imaging properties to visualize optical absorption with high resolution and contrast. Photoacoustic microscopy (PAM) is usually categorized with all-optical microscopy techniques such as optical coherence tomography or confocal microscopes. Despite offering high sensitivity, novel imaging contrast, and high resolution, PAM is not generally an all-optical imaging method unlike the other microscopy techniques. One of the significant limitations of photoacoustic microscopes arises from their need to be in physical contact with the sample through a coupling media. This physical contact, coupling, or immersion of the sample is undesirable or impractical for many clinical and pre-clinical applications. This also limits the flexibility of photoacoustic techniques to be integrated with other all-optical imaging microscopes for providing complementary imaging contrast. To overcome these limitations, several non-contact photoacoustic signal detection approaches have been proposed. This paper presents a brief overview of current non-contact photoacoustic detection techniques with an emphasis on all-optical detection methods and their associated physical mechanisms.

Fast photoacoustic imaging systems using pulsed laser diodes: a review

Photoacoustic imaging (PAI) is a newly emerging imaging modality for preclinical and clinical applications. The conventional PAI systems use Q-switched Nd:YAG/OPO (Optical Parametric Oscillator) nanosecond lasers as excitation sources. Such lasers are expensive, bulky, and imaging speed is limited because of low pulse repetition rate. In recent years, the semiconductor laser technology has advanced to generate high-repetitions rate near-infrared pulsed lasers diodes (PLDs) which are reliable, less-expensive, hand-held, and light-weight, about 200 g. In this article, we review the development and demonstration of PLD based PAI systems for preclinical and clinical applications reported in recent years.

Backward-Mode Photoacoustic Imaging Using Illumination Through a CMUT With Improved Transparency

In this paper, we describe a capacitive micromachined ultrasonic transducer (CMUT) with improved transparency for photoacoustic imaging (PAI) with backside illumination. The CMUT was fabricated on a glass substrate with indium-tin oxide bottom electrodes. The plate was a 1.5- silicon layer formed over the glass cavities by anodic bonding, with a 1- silicon nitride passivation layer on top. The fabricated device shows approximately 30%-40% transmission in the wavelength range from 700 to 800 nm and approximately 40%-60% transmission in the wavelength range from 800 to 900 nm, which correspond to the wavelength range commonly used for in vivo PAI. The center frequency of the CMUT was 3.62 MHz in air and 1.4 MHz in immersion. Two preliminary PAI experiments were performed to demonstrate the imaging capability of the fabricated device. The first imaging target was a 0.7-mm diameter pencil lead in vegetable oil as a line target with a subwavelength cross section. A 2-mm-diameter single CMUT element with an optical fiber bundle attached to its backside was linearly scanned to reconstruct a 2-D cross-sectional PA image of the pencil lead. We investigated the spurious signals caused by the light absorption in the 1.5- silicon plate. For pencil lead as a strong absorber and also a strong reflector, the received echo signal due to the acoustic excitation generated by the absorption in silicon is approximately 30 dB lower than the received PA signal generated by the absorption in pencil lead at the wavelength of 830 nm. The second imaging target was a "loop-shape" polyethylene tube filled with indocyanine green solution ( ) suspended using fishing lines in a tissue-mimicking material. We formed a 3-D volumetric image of the phantom by scanning the transducer in the - and -directions. The two experimental imaging results demonstrated that CMUTs with the proposed structure are promising for PAI with backside illumination.

Optical Detection of Ultrasound in Photoacoustic Imaging

OBJECTIVE

Photoacoustic (PA) imaging emerges as a unique tool to study biological samples based on optical absorption contrast. In PA imaging, piezoelectric transducers are commonly used to detect laser-induced ultrasonic waves. However, they typically lack adequate broadband sensitivity at ultrasonic frequency higher than 100 MHz, whereas their bulky size and optically opaque nature cause technical difficulties in integrating PA imaging with conventional optical imaging modalities. To overcome these limitations, optical methods of ultrasound detection were developed and shown their unique applications in PA imaging.

METHODS

We provide an overview of recent technological advances in optical methods of ultrasound detection and their applications in PA imaging. A general theoretical framework describing sensitivity, bandwidth, and angular responses of optical ultrasound detection is also introduced.

RESULTS

Optical methods of ultrasound detection can provide improved detection angle and sensitivity over significantly extended bandwidth. In addition, its versatile variants also offer additional advantages, such as device miniaturization, optical transparency, mechanical flexibility, minimal electrical/mechanical crosstalk, and potential noncontact PA imaging.

CONCLUSION

The optical ultrasound detection methods discussed in this review and their future evolution may play an important role in PA imaging for biomedical study and clinical diagnosis.

Non-Contact Photoacoustic Imaging Using a Commercial Heterodyne Interferometer

Most current photoacoustic imaging (PAI) systems employ piezoelectric transducers to receive photoacoustic signals, which requires coupling medium to facilitate photoacoustic wave propagation and are not favored in many applications. Here, we report an all-optical non-contact PAI system based on a commercial heterodyne interferometer working at 1550 nm. The interferometer remotely detects ultrasound-induced surface vibration and does not involve any physical contact with the sample. The theoretically predicated and experimentally measured noise equivalent detection limits of the optical sensor are about 4.5 and 810 Pa over 1.2 MHz bandwidth. Using a raster-scan PAI system equipped with the non-contact design, stereotactic boundaries of an artificial tumor in a pig brain were accurately delineated. The non-contact design also enables the tomographic PAI of biological tissue samples in a non-invasive manner. The preliminary results and analyses reveal that the heterodyne interferometer-based non-contact PAI system holds good potential in biomedical imaging.

Dual Modality Noncontact Photoacoustic and Spectral Domain OCT Imaging

We developed a multimodal imaging system, combining noncontact photoacoustic imaging and optical coherence tomography (OCT). Photoacoustic signals are recorded without contact to the specimens' surface by using an interferometric technique. The interferometer is realized within a fiber-optic network using a fiber laser at 1550 nm as source. The fiber-optic network allows the integration of a fiber-based OCT system operating at a wavelength region around 1310 nm. Light from the fiber laser and the OCT source are multiplexed into one fiber using wavelength-division multiplexing. The same focusing optics is used for both modalities. Back-reflected light from the sample is demultiplexed and guided to the respective imaging systems. As the same optical components are used for OCT and photoacoustic imaging, the obtained images are co-registered intrinsically in lateral direction. Three-dimensional imaging is implemented by hybrid galvanometer and mechanical scanning. To allow fast B-scan measurements, scanning of the interrogation beam along one dimension is executed by a galvanometer scanner. Slow-axis scanning, perpendicular to the fast axis, is performed utilizing a linear translational stage. We demonstrate two-dimensional and three-dimensional imaging on agarose phantoms.

All-optical photoacoustic microscopy

Three-dimensional photoacoustic microscopy (PAM) has gained considerable attention within the biomedical imaging community during the past decade. Detecting laser-induced photoacoustic waves by optical sensing techniques facilitates the idea of all-optical PAM (AOPAM), which is of particular interest as it provides unique advantages for achieving high spatial resolution using miniaturized embodiments of the imaging system. The review presents the technology aspects of optical-sensing techniques for ultrasound detection, such as those based on optical resonators, as well as system developments of all-optical photoacoustic systems including PAM, photoacoustic endoscopy, and multi-modality microscopy. The progress of different AOPAM systems and their representative applications are summarized.

Multimodal noncontact photoacoustic and optical coherence tomography imaging using wavelength-division multiplexing

We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.

Miniaturized fiber-optic ultrasound probes for endoscopic tissue analysis by micro-opto-mechanical technology

A new Micro-Opto-Mechanical System (MOMS) technology for the fabrication of optoacoustic probes on optical fiber is presented. The technology is based on the thermoelastic emission of ultrasonic waves from patterned carbon films for generation and on extrinsic polymer Fabry-Perot acousto-optical transducers for detection, both fabricated on miniaturized single-crystal silicon frames used to mount the ultrasonic transducers on the tip of an optical fiber. Thanks to the fabrication process adopted, high miniaturization levels are reached in the MOMS devices, demonstrating fiber-optic emitters and detectors with minimum diameter around 350 and 250 μm respectively. A thorough functional testing of the ultrasound emitters mounted on 200 and 600 μm diameter optical fibers is presented, in which the fiber-optic emitter with a diameter of 200 μm shows generated acoustic pressures with peak-to-peak value up to 2.8 MPa with rather flat emission spectra extended beyond 150 MHz. The possibility to use the presented optoacoustic sources in conjunction with the fiber-optic acousto-optical detectors within a minimally invasive probe is also demonstrated by successfully measuring the ultrasonic echo reflected from a rigid surface immersed in water with various concentration of scatterers. The resulting spectra highlight the possibility to discriminate the effects due to frequency selective attenuation in a very wide range of frequencies within a biological medium using the presented fiber-optic probes.

High resolution three-dimensional photoacoutic tomography with CCD-camera based ultrasound detection

A photoacoustic tomograph based on optical ultrasound detection is demonstrated, which is capable of high resolution real-time projection imaging and fast three-dimensional (3D) imaging. Snapshots of the pressure field outside the imaged object are taken at defined delay times after photoacoustic excitation by use of a charge coupled device (CCD) camera in combination with an optical phase contrast method. From the obtained wave patterns photoacoustic projection images are reconstructed using a back propagation Fourier domain reconstruction algorithm. Applying the inverse Radon transform to a set of projections recorded over a half rotation of the sample provides 3D photoacoustic tomography images in less than one minute with a resolution below 100 µm. The sensitivity of the device was experimentally determined to be 5.1 kPa over a projection length of 1 mm. In vivo images of the vasculature of a mouse demonstrate the potential of the developed method for biomedical applications.

Continuous real-time photoacoustic demodulation via field programmable gate array for dynamic imaging of zebrafish cardiac cycle

A four dimensional data set of the cardiac cycle of a zebrafish embryo was acquired using postacquisition synchronization of real time photoacoustic b-scans. Utilizing an off-axis photoacoustic microscopy (OA-PAM) setup, we have expanded upon our previous work with OA-PAM to develop a system that can sustain 100 kHz line rates while demodulating the bipolar photoacoustic signal in real-time. Real-time processing was accomplished by quadrature demodulation on a Field Programmable Gate Array (FPGA) in line with the signal digitizer. Simulated data acquisition verified the system is capable of real-time processing up to a line rate of 1 MHz. Galvanometer-scanning of the excitation laser inside the focus of the ultrasonic transducer enables real data acquisition of a 200 by 200 by 200 pixel, volumetric data set across a 2 millimeter field of view at a rate of 2.5 Hz.

Non-contact photoacoustic imaging using a fiber based interferometer with optical amplification

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications contact with the tissue should be avoided. As alternatives to contacting transducers interferometric means can be used to acquire photoacoustic signals remotely. In this paper we report on non-contact three and two dimensional photoacoustic imaging using an optical fiber-based Mach-Zehnder interferometer. A detection beam is transmitted through an optical fiber network onto the surface of the specimen. Back reflected light is collected and coupled into the same optical fiber. To achieve a high signal/noise ratio the reflected light is amplified by means of optical amplification with an erbium doped fiber amplifier before demodulation. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain reconstruction algorithm. We present remote photoacoustic imaging of a tissue mimicking phantom and on chicken skin.

Photoacoustic imaging using an adaptive interferometer with a photorefractive crystal

In this paper we report on remote three-dimensional photoacoustic imaging without the need for a coupling agent or other aids (e.g. detector foils) by utilizing a two-wave mixing interferometer (TWMI). In this technique the beams coming from sample and reference are brought to interfere in a Bi₁₂SiO₂₀ photorefractive crystal (PRC). Such a setup allows the measurement of ultrasonic displacements on rough sample surfaces. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain synthetic aperture focusing technique (FSAFT). We present three-dimensional imaging of a hair ribbon phantom, on biological samples with embedded artificial blood vessels or pig bristles, and measurements on a human forearm in-vivo.

Self-referenced composite Fabry-Pérot cavity vapor sensors

We develop a versatile, self-referenced composite Fabry-Pérot (FP) sensor and the corresponding detection scheme for rapid and precise measurement of vapors. The composite FP vapor sensor is formed by etching two juxtaposed micron-deep wells, with a precisely controlled offset in depth, on a silicon wafer. The wells are then coated with a vapor sensitive polymer and the reflected light from each well is detected by a CMOS imager. Due to its self-referenced nature, the composite FP sensor is able to extract the change in thickness and refractive index of the polymer layer upon exposure to analyte vapors, thus allowing for accurate vapor quantitation regardless of the polymer thickness, refractive index, and light incident angle and wavelength. Theoretical analysis is first performed to elucidate the underlying detection principle, followed by experimental demonstration at two different incident angles showing rapid and consistent measurement of the polymer changes when the polymer is exposed to three different analytes at various concentrations. The vapor detection limit is found to be on the order of a few pico-grams (~100 ppb).

An integrated optoacoustic transducer combining etalon and black PDMS structures

An integrated optoacoustic transducer combining etalon and black polydimethylsiloxane (PDMS) structures has been designed and developed. The device consists of an 11-μm-thick black PDMS film confined to a 2-mm-diameter circular region acting as an optoacoustic transmitter, surrounded by a 5.9-μm Fabry-Perot polymer etalon structure serving as an optoacoustic detector array. A pulsed laser is focused onto a 30-μm spot on the black PDMS film, defining the transmit element, while a CW laser probes a 20-μm spot on the etalon for ultrasound detection. Pulse-echo signals display center frequencies of above 30 MHz with bandwidths of at least 40 MHz. A theta-array is formed for 3-D ultrasound imaging by mechanically scanning the generation laser along a 1-D array and the detection laser around an annular array. Preliminary images with 3 metal wires as imaging targets are presented. Characterization of the device’s acoustical properties, as well as preliminary imaging results, suggest that all-optical ultrasound transducers are potential alternatives to piezoelectric techniques for high-frequency 2-D arrays enabling 3-D high-resolution ultrasound imaging.