Fabrication of an Array of Eccentric Sources for Freehand Optical Ultrasound Imaging

Free-hand optical ultrasound (OpUS) imaging is an emerging ultrasound imaging paradigm that utilises an array of fiber-optic sources and a fiber-optic detector to achieve video-rate, real-time imaging, with a flexible probe. Previous designs used multimode fibers to achieve circular OpUS sources that emitted divergent fields propagating away from the imaging plane, resulting in image artefacts and reduced penetration depths. The directivity of the emitted ultrasound field can be optimised by changing the trasnducer shape, moving to eccentric transducers can improve elevational confinement and associated aretefacts. In this work, methods for fabricating suitably eccentric waveguides that can be placed distally to a fiber-bundle array probe are presented. In addition, the scalability of one of these methods is demonstrated by fabricating a ten-element array of waveguides

Minimally invasive photoacoustic imaging:Current status and future perspectives

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality that is based on optical absorption contrast, capable of revealing distinct spectroscopic signatures of tissue at high spatial resolution and large imaging depths. However, clinical applications of conventional non-invasive PAI systems have been restricted to examinations of tissues at depths less than a few cm due to strong light attenuation. Minimally invasive photoacoustic imaging (miPAI) has greatly extended the landscape of PAI by delivering excitation light within tissue through miniature fibre-optic probes. In the past decade, various miPAI systems have been developed with demonstrated applicability in several clinical fields. In this article, we present an overview of the current status of miPAI and our thoughts on future perspectives.status: publishe

High-resolution sub-millimetre diameter side-viewing all-optical ultrasound transducer based on a single dual-clad optical fibre

All-optical ultrasound (OpUS), where ultrasound is both generated and received using light, has emerged as a modality well-suited to highly miniaturised applications. In this work we present a proof-of-concept OpUS transducer built onto a single optical fibre with a highly miniaturised lateral dimension (0.4 MPa and a corresponding bandwidth >27 MHz. Concurrent ultrasound generation and reception from the transducer enabled imaging via motorised pull-back allowing image acquisition times of 4 s for an aperture of 20 mm. Image resolution was as low as ~50 µm and 190 µm in the axial and lateral extents, respectively, without the need for image reconstruction. Porcine aorta was imaged ex vivo demonstrating detailed ultrasound images. The unprecedented level of miniaturisation along with the high image quality produced by this device represents a radical new paradigm for minimally invasive imaging

Fibre-Optic Hydrophone For Detection of High-Intensity Ultrasound Waves

Fibre-optic hydrophones (FOHs) are widely used to detect high-intensity focused ultrasound (HIFU) fields. The most common type consists of an uncoated singlemode fibre with a perpendicularly cleaved end face. The main disadvantage of these hydrophones is their low signal-to-noise ratio (SNR). To increase the SNR, signal averaging is performed, but the associated increased acquisition times hinder ultrasound field scans. In this study, with a view to increase SNR whilst withstanding HIFU pressures, the bare FOH paradigm is extended to include a partially-reflective coating on the fibre end face. Here, a numerical model based on the general transfer-matrix method was implemented. Based on the simulation results, a single-layer, 172 nm TiO2-coated FOH was fabricated. The frequency range of the hydrophone was verified from 1 to 30 MHz. The SNR of the acoustic measurement with the coated sensor was 21 dB higher than of the uncoated one. The coated sensor successfully withstood a peak-positive pressure of 35 MPa for 6000 pulses

PDMS Composites with Photostable NIR Dyes for B-Mode Ultrasound Imaging

All-optical ultrasound has rapidly progressed as an imaging paradigm well-suited to application in minimally invasive surgical scenarios, moving from benchtop to in vivo studies. In this work, we build on previous studies, demonstrating B-mode all-optical ultrasound imaging using a composite comprising a photostable near-infrared absorbing dye and polydimethylsiloxane. This composite is cost-effective and simple to manufacture, and capable of generating broadband (ca. 35 MHz) and high-pressure (ca. 1.5 MPa) ultrasound. The composite was compared with an established ultrasound-generating composite: reduced graphene oxide with polydimethlysiloxane. Both materials were coated on multimode optical fibres and were optically and ultrasonically characterized. Further, the transmitters were coupled with a plano-concave microresonator for ultrasound reception and synthetic aperture B-mode pulse-echo US imaging of a tungsten wire phantom and ex vivo swine aorta tissue was performed. The images acquired using each transmitter were compared in terms of the image resolution and signal-to-noise ratio

Fibre-optic hydrophones for high-intensity ultrasound detection: modelling and measurement study

Background, Motivation and Objective: Fibre-optic hydrophones (FOHs) are widely used to detect and spatially characterise high-intensity focused ultrasound (HIFU) fields. In this context, the most common type of FOH consists of a fibre with a flat-cleaved uncoated tip. The ultrasound (US) field is detected by measuring changes in reflected light intensity due to pressure-induced modulations of the refractive index of the fluid. However, these sensors tend to have a low signalto-noise ratio (SNR) (with a high noise equivalent pressure [typically 2–3 MPa]), which imposes significant dynamic range constraints on field characterisation. In this study, we extend this bare FOH paradigm to include partially-reflective coatings on the fibre end faces, with a view to increase SNR whilst withstanding HIFU pressures. Previously, a limited number of studies have investigated this paradigm. Here, we present a comprehensive elasto-optic numerical model capable of predicting the sensitivity for arbitrary numbers of coatings, and use this model to design and fabricate an FOH comprising a single coating layer using a novel material. / Statement of Contribution/Methods: A simulation method based on the general transfer-matrix method was developed in MATLAB to compute the change of reflectance with respect to pressure (dR/dP, which is proportional to the FOH sensitivity). A single layer coated FOH comprising a quarter-wave layer (172 nm) of deposited TiO2 was fabricated. The FOH was placed in the focus of a HIFU source (diameter: 64 mm, focal length: 63.2 mm; H101, Sonic Concepts). The SNR gain observed experimentally was compared against numerical predictions. Furthermore, the potential of further increasing SNR using a multi-layer sensor configuration was investigated. Results/Discussion The SNR of the US measurement with the single-layer TiO2 coated sensor was found to be 21 dB higher than for an uncoated one (Fig. 1a), corresponding to a sensitivity gain of 11x. (c.f. 8.5x predicted with simulation). The difference between the measurements and the model can be attributed to the cleaving quality of the uncoated hydrophone or inaccuracies in the elasto-optic properties of the coating layer. The coated sensor endured pressures over 35 MPa (peak positive), and tests for higher pressures are underway. Moreover, simulations for configurations using multiple layers suggest the sensitivity could be significantly improved further. For instance, a 15-layer structure of alternating TiO2 and SiO2 coatings was predicted to achieve an increase in sensitivity of ca. 73×, while still being mechanically robust for HIFU applications

Improving needle visibility in LED-based photoacoustic imaging using deep learning with semi-synthetic datasets

Photoacoustic imaging has shown great potential for guiding minimally invasive procedures by accurate identification of critical tissue targets and invasive medical devices (such as metallic needles). The use of light emitting diodes (LEDs) as the excitation light sources accelerates its clinical translation owing to its high affordability and portability. However, needle visibility in LED-based photoacoustic imaging is compromised primarily due to its low optical fluence. In this work, we propose a deep learning framework based on U-Net to improve the visibility of clinical metallic needles with a LED-based photoacoustic and ultrasound imaging system. To address the complexity of capturing ground truth for real data and the poor realism of purely simulated data, this framework included the generation of semi-synthetic training datasets combining both simulated data to represent features from the needles and in vivo measurements for tissue background. Evaluation of the trained neural network was performed with needle insertions into blood-vessel-mimicking phantoms, pork joint tissue ex vivo and measurements on human volunteers. This deep learning-based framework substantially improved the needle visibility in photoacoustic imaging in vivo compared to conventional reconstruction by suppressing background noise and image artefacts, achieving 5.8 and 4.5 times improvements in terms of signal-to-noise ratio and the modified Hausdorff distance, respectively. Thus, the proposed framework could be helpful for reducing complications during percutaneous needle insertions by accurate identification of clinical needles in photoacoustic imaging

Dual-modality fibre optic probe for simultaneous ablation and ultrasound imaging

All-optical ultrasound (OpUS) is an emerging high resolution imaging paradigm utilising optical fibres. This allows both therapeutic and imaging modalities to be integrated into devices with dimensions small enough for minimally invasive surgical applications. Here we report a dual-modality fibre optic probe that synchronously performs laser ablation and real-time all-optical ultrasound imaging for ablation monitoring. The device comprises three optical fibres: one each for transmission and reception of ultrasound, and one for the delivery of laser light for ablation. The total device diameter is < 1 mm. Ablation monitoring was carried out on porcine liver and heart tissue ex vivo with ablation depth tracked using all-optical M-mode ultrasound imaging and lesion boundary identification using a segmentation algorithm. Ablation depths up to 2.1 mm were visualised with a good correspondence between the ultrasound depth measurements and visual inspection of the lesions using stereomicroscopy. This work demonstrates the potential for OpUS probes to guide minimally invasive ablation procedures in real time

Miniaturised dual-modality all-optical Laser Interstitial Thermal Therapy (LITT) and ultrasound imaging

Introduction: Laser Interstitial Thermal Therapy (LITT) is a minimally invasive procedure to treat kidney tumours that can be hard to reach with open surgery, with low risks and fast recovery time. This treatment requires robust real-time imaging modalities to track tumour treatment outcomes and avoid damage of healthy tissue during the procedure. All-optical ultrasound (OpUS) is an emerging imaging paradigm that exhibits high imaging resolution, ease of miniaturisation and immunity to electromagnetic interference [1]. With OpUS, the ultrasound wave is generated via photoacoustic effect, with pulsed excitation light delivered to an optically absorbing coating on the end of an optical fibre and received using a plano-concave microresonator on an optical fibre end face. The use of optical fibres allows laser ablation and OpUS imaging to be integrated into a miniaturised device, suitable for the real-time monitoring of tissue damage during LITT procedures. In this work, we developed an optical-fibre-based miniaturised device that combines LITT and OpUS imaging. To assess the capability, the device was used to perform LITT with in situ OpUS lesion imaging on an ex vivo lamb kidney. / Methods: The OpUS device comprised three optical fibres; a multimode optical fibre (200 µm core) coated with Candle soot nanoparticles (CSNPs)-polydimethylsiloxane (PDMS) composite for ultrasound transmission, an single mode optical fibre with a plano-concave microresonator for ultrasound reception and a multimode optical fibre (400 µm core) for delivery of laser ablation light. The distal end face of the ultrasound transmitter and receiver were aligned at their distal tips, whilst the ablation fibre was advanced 10 mm, allowing for the insertion into kidney tissue. Prior to carrying out LITT, a B-mode OpUS image of the kidney tissue was acquired. For LITT experiments, the ablation fibre was inserted into the ex vivo kidney tissue and laser treatment was applied (808 nm, 3.0 W, 60 s) whilst performing concurrent M-mode imaging with the OpUS probe to track the lesion formation. / Results & Discussion: B-mode OpUS imaging provided an imaging depth > 15 mm and high imaging resolution that delineated the structure boundaries inside the kidney (Fig 1. a). M-mode OpUS imaging allowed visualisation of the tissue throughout the procedure. When the ablation laser was switched on the tissue contrast changed immediately, indicating the formation of an ablated lesion (Fig 1. b). Throughout the ablation period, the lesion grew bidirectional from the optical fibre tip in the vertical dimension, which was visible on the M-mode image as increasing brightness and changing contrast. This work demonstrated the feasibility of OpUS imaging to monitor the lesion growth during LITT

Bend-insensitive Fiber Optic Ultrasonic Tracking Probe for Cardiovascular Interventions

Background: Transesophageal echocardiography (TEE) is widely used to guide medical device placement in minimally invasive cardiovascular procedures. However, visualization of the device tip with TEE can be challenging. Ultrasonic tracking, enabled by an integrated fiber optic ultrasound sensor (FOUS) that receives transmissions from the TEE probe, is very well suited to improving device localization in this context. The problem addressed in this study is that tight deflections of devices such as a steerable guide catheter can result in bending of the FOUS beyond its specifications and a corresponding loss of ultrasound sensitivity. Purpose: A bend-insensitive FOUS was developed, and its utility with ultrasonic tracking of a steerable tip during TEE-based image guidance was demonstrated. Methods: Fiberoptic ultrasound sensors were fabricated using both standard and bend insensitive single mode fibers and subjected to static bending at the distal end. The interference transfer function and ultrasound sensitivities were compared for both types of FOUS. The bend-insensitive FOUS was integrated within a steerable guide catheter, which served as an exemplar device; the signal-to-noise ratio (SNR) of tracking signals from the catheter tip with a straight and a fully deflected distal end were measured in a cardiac ultrasound phantom for over 100 frames. Results: With tight bending at the distal end (bend radius < 10 mm), the standard FOUS experienced a complete loss of US sensitivity due to high attenuation in the fiber, whereas the bend-insensitive FOUS had largely unchanged performance, with a SNR of 47.7 for straight fiber and a SNR of 36.8 at a bend radius of 3.0 mm. When integrated into the steerable guide catheter, the mean SNRs of the ultrasonic tracking signals recorded with the catheter in a cardiac phantom were similar for straight and fully deflected distal ends: 195 and 163. Conclusion: The FOUS fabricated from bend-insensitive fiber overcomes the bend restrictions associated with the FOUS fabricated from standard single mode fiber, thereby enabling its use in ultrasonic tracking in a wide range of cardiovascular devices