Photoacoustic imaging (PAI) is a hybrid imaging modality that uses ultrasound waves generated from light absorbing tissue chromophores to provide high spatial resolution and depth-resolved molecular information. However, conventional PAI setups involve complicated arrangement of optical components surrounding opaque ultrasound transducers to achieve a co-aligned optical illumination and ultrasound receiving field. This opacity of traditional ultrasound transducers impedes the miniaturization of the imaging head, besides precluding integration with other imaging modalities. To overcome these limitations, we recently fabricated a single element transparent ultrasound transducer (TUT) window using indium tin oxide (ITO) coated lithium niobate (LiNbO3) piezoelectric material and demonstrated its application for endoscopy and microscopy PAI applications. Extending on this work, we report new developments of TUTs to improve their detection bandwidth, sensitivity, and signal to noise ratio (SNR) while maintaining sufficient transparency. This includes investigating LiNbO3 and PMN-PT as transparent piezoelectric materials with different matching layer designs. Fabricated TUTs were characterized using pulse echo and electrical impedance analysis. The PAI performance of the fabricated TUTs were characterized using photoacoustic A-line signals from light absorbing targets. The proposed TUTs are low cost, easy to fabricate, and can be scaled and easily integrated into different PAI geometries such as: endoscopy, microscopy, and computed tomography systems for high-throughput imaging applications.
Photoacoustic imaging (PAI) maps functional and molecular optical contrasts of tissue at ultrasonic spatial resolution and imaging depth. To generate detectable PA signals from deeper regions, expensive, bulky and high-energy class IV lasers are conventionally employed. Light emitting diodes (LED) have recently emerged as an alternative excitation source for PA imaging offering many advantages including portability, affordability, speed, multi-wavelength excitation, and eye/skin safety. Although the output energy of LED’s is far lower than lasers, high pulse repetition rate offers possibility to average more frames and thus improve the SNR. In this work, we performed controlled experiments on tissue-mimicking phantoms to compare the PAI performance of laser and LED light sources comprehensively. Our studies demonstrate that the LED based PA systems are ideal for low resource and point-of-care settings where the required depth of penetration is within 2-3 cms., whereas a high-energy laser is found to be more effective for higher penetration depths (<3 cm). In addition, it is clear from our results that LED-based PA imaging offers higher frame rate with similar spatial resolution and decent signal to noise ratio, which is comparable to conventional laser-based photoacoustic imaging.
Photoacoustic computed tomography (PACT) has been widely explored for studying human diseases as well as response to therapies. Most PACT systems employ a large footprint, bulky, and high-cost lasers. Light emitting diodes (LEDs) based B-mode photoacoustic imaging systems have emerged as a low cost and compact alternative, offering a unique opportunity to expedite the widespread adoption of photoacoustic imaging in clinical and resource-poor settings. The high pulse repetition rate of LEDs facilitates signal-to-noise ratio improvements through averaging in spite of lower pulse energy. Here, we present the development of first low-cost LED-based PACT system that uses multiple LED arrays and a linear ultrasound transducer to generate three-dimensional structural, functional and molecular images of the object. Similar to OPO based lasers, our LED-PACT system allows for the multi-wavelength photoacoustic imaging vital for mapping functional and molecular information. Our experiments demonstrate that this study will enable clinical and pre-clinical applications such as imaging human arthritis and whole body mouse imaging.
Conventional photoacoustic imaging (PAI) systems use bulky and high-cost laser sources to derive functional and molecular information of the tissue. Recently, light emitting diodes (LED) have emerged as an affordable and compact alternative illumination source for PAI. Despite their low energies, LEDs have provided sufficient photoacoustic contrast for in vivo imaging of mice and for certain clinical applications. This is largely due to PA signal averaging allowed by higher repetition rates of the LEDs without compromising on video frame rate photoacoustic imaging. In this work, using multiple in vivo and phantom experiments, we demonstrate the potential of LED-based photoacoustic and ultrasound imaging (2-D and 3-D) for real-time functional, molecular and structural characterization of tissue. This includes photoacoustic derived functional oxygen saturation information and mapping molecules such as melanin, methylene blue and indocyanine green, and ultrasound derived anatomical information of tissue. These results demonstrate that LED-based PA and US imaging hold strong potential for accelerating several pre-clinical and clinical applications, especially in resource-poor settings.
Infant brain imaging is highly challenging but necessary for diagnosing various prevalent disorders including vascular malformations, encephalitis, and abusive head trauma. Conventional brain imaging technologies such as MRI, CT, and PET are not suitable for repeated use on neonates due to the use of ionizing radiation (CT and PET), need for patient transport, uncomfortable environment, high cost, and bulky equipment. A wearable photoacoustic imaging (PAI) hat can be an ideal candidate for this application. However, its practical realization suffers from many system design problems such as complex assembly, unviability of full-hat rotation around the neonatal head, ultrasound coupling, and requirements of <3,000 ultrasound data acquisition channels to cover the whole brain. Here, we present a modular photoacoustic imaging (PAI) hat solution that uses an innovative modular design approach, making it realizable by assembling individual working units while minimizing the challenges of back-end electronics. The modular photoacoustic hat consists of multiple PAI disc modules of 2 inches in diameter that conform to the shape of the local head surface and assembled on a hat to cover the whole neonatal brain. Each PAI disc is integrated with optical fibers for light excitation of brain tissue. For photoacoustic detection, the discs are either densely packed with ultrasound elements to eliminate the need for rotation or can have fewer ultrasound elements (usually in trapezoidal shape) on the rotating disc to overcome large number of data acquisition channels. In this article, we have demonstrated the design, integration and initial results of the proposed wearable PAI-hat.
In recent years, conventional ultrasound (US) imaging devices have been adapted with the photoacoustic (PA) imaging capabilities to simultaneously provide both anatomical and molecular optical contrasts of soft biological tissues. To help optimize the design parameters of such dual modality imaging devices, we present a numerical simulation approach for Bmode beamformed US and multispectral PA imaging using a linear ultrasound transducer array surrounded by a light source. We combined the finite element based simulation platforms for ultrasound and light propagation, K-wave and NIRFast respectively, to model the ultrasound and photoacoustic effects in deep tissue, and created an effective hybrid platform for simulating US and multispectral PA imaging of different configurations. We also developed and applied a spectral unmixing algorithm on multispectral photoacoustic images, obtained from multiple optical wavelengths, to map different molecules (e.g., Indocyanogreen (ICG), Deoxyhemoglobin (Hb), and Oxyhemoglobin (HbO2)) present inside the tissue background. The multi-spectral plots and unmixed spectral images clearly delineated the molecular contrast arising from different regions inside the tissue. The presented simulation platform allows for optimization of key design parameters of both US and PA imaging devices, such as the size of ultrasonic transducer array, and size and the distribution of light sources. Our results demonstrate that the ability to mimic the imaging performance of such dual modality deep tissue-imaging device will help to achieve high molecular sensitivity for the targeted clinical application, thus functioning as a powerful tool for medical device design.
The first successful validation of a forward-looking Piezoelectric Micromachined Ultrasound Transducer (PMUT) ring array designed for photoacoustic endoscopic imaging applications is presented. PMUT ring arrays were fabricated with a 0.5 mm inner diameter, to allow insertion of an optical fiber for light delivery, and ~2.5 mm overall outer diameter. Each ring array consisted of 6 elements, with a total of 102 PMUT cells, or 17 cells per element. Each PMUT cell has a 100 μm diameter multi-layered diaphragm having a ~700 nm thick c-axis oriented aluminium nitride (AlN) thin film as the piezoelectric layer over a Si (handle layer) / SiO2 (1 μm) / Si (10 μm) / SiO2 (100 nm) / TiO2 (40 nm) / Pt (150 nm) substrate to act as an ultrasound receive element. The resonant frequency was ~ 6 MHz in water. The output end of an optical fiber, coupled to a pulsed laser diode (PLD), was fitted with a 2.5 mm ferrule. The PMUT ring array was concentrically mounted on the ferrule to obtain a miniaturized endoscopic PAI device. The observed photoacoustic bandwidth was ~75%, and a strong photoacoustic signal of ~13 mV peak-to-peak output was observed from a light absorbing target kept 5 mm away from the PMUT array. 2D and 3D photoacoustic images of the targets were obtained via raster scanning of the phantom sample. In the future, the functionality of the PMUT ring array will be enhanced through multichannel acquisition to obtain variable acoustic focus and limited view photoacoustic images in real time.
Inadvertent cuts to blood vessels and nerves poses a significant risk during percutaneous needle procedures, often leading to serious injuries and even death. We propose a computer-assisted photoacoustic imaging-based device that is able to detect these vascular structures and robotically guide the surgeons in avoiding them. A fiber-coupled pulsed laser diode capable of generating photoacoustic signals is attached through a ferrule, where a 2.5 mm diameter ultrasound ring transducer receives the corresponding photoacoustic waves. The integrated device is secured on an XYZ axis linear translational stage configuration, and robotically navigated through vessel-modelling phantoms to reach a targeted region of interest. A steering feedback algorithm calculates the relative position of the device with respect to each vessel, generates a 2D map of the navigational plane, and controls the stages to steer the device accordingly towards the target while avoiding the vessels. We first ran the algorithm in a water phantom to demonstrate feasibility, and then in a milk solution to model real tissue scattering. Our proposed device successfully avoids the phantom blood vessels in both cases through photoacoustic detection, and the corresponding 2D navigational path and plane through the phantom is mapped and recorded. Our results demonstrate that a computer-assisted photoacoustic imaging-based device is a viable method of intraoperatively guiding percutaneous needle procedures. The ability of our proposed guidance device to detect and avoid damage to blood vessels and nerves can further be used to optimize biopsies and tumor removal procedures in various parts of the body.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.