Rapid Non-Contact Optical Ultrasound for Biomedical Imaging

Biomedical ultrasound imaging is typically performed using electronic transducer technology, which results in imaging probes exhibiting large mechanical footprints that require physical contact with the imaging target. While this mature technology allows for high-quality, versatile free-hand imaging, its applicability is limited in crowded surgical settings and scenarios at risk of infection or trauma. Instead, here a novel system is presented that enables non-contact ultrasound imaging through remote sensing. This is achieved using light rather than electronics to both generate and detect ultrasound, which is delivered in free-space to the surface of the object by weakly-focussed beams. To maximise the signal fidelity, a custom membrane was developed that is deposited to the surface of the imaging target. The combined system and membrane currently achieve real-time and dynamic imaging at a frame rate of up to 22 Hz for highly reflective targets, and requires an acquisition time of ca. 27 s for physiologically relevant phantoms. As such, the system already achieves clinically relevant performance for, e.g., needle or instrument tracking, and various improvements are suggested that in the near future will significantly accelerate image acquisition of soft tissue – ultimately resulting in sub-second biomedical non-contact ultrasound imaging

Developing Real-Time Implementations of Non-Linear Beamformers for Enhanced 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 single fiber-optic detector to achieve video-rate, real-time imaging with a flexible probe that is immune to electromagnetic interference. Due to the use of only a single detector, such probes have limited channel counts, resulting in significant imaging artefacts and limited contrast when imaging is performed with a conventional Delay-and-Sum (DAS) beamformer. Non-linear beamforming can help improve the imaging quality by exploiting cross-channel coherence across the aperture, at the expense of significantly increased computational complexity. In this work, GPU implementations of different non-linear beamformers were implemented and tailored specifically to OpUS array devices and tested on both simulated and experimental data

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

Concurrent Optical Ultrasound and CT Imaging

Optical ultrasound imaging is an emerging paradigm that utilises fiber-optic ultrasound sources and detectors to perform pulse-echo imaging. Using rapid-prototyping techniques, flexible fiber-optic free-hand probes, capable of video-rate imaging can be constructed entirely from glass and plastic. As such, these devices are expected to be inherently compatible with electromagnetic imaging modalities such as magnetic resonance imaging and computed tomography imaging. However, to date, this multimodal capability has not been demonstrated. In this work, a new free-hand optical ultrasound (OpUS) imaging system is introduced, its real-time imaging capability demonstrated on a range of phantoms, and the first concurrent use of OpUS alongside cone-beam CT (CBCT) imaging is presented

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

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

Neural Network Kalman Filtering for 3-D Object Tracking From Linear Array Ultrasound Data

Many interventional surgical procedures rely on medical imaging to visualise and track instruments. Such imaging methods not only need to be real-time capable, but also provide accurate and robust positional information. In ultrasound applications, typically only two-dimensional data from a linear array are available, and as such obtaining accurate positional estimation in three dimensions is non-trivial. In this work, we first train a neural network, using realistic synthetic training data, to estimate the out-of-plane offset of an object with the associated axial aberration in the reconstructed ultrasound image. The obtained estimate is then combined with a Kalman filtering approach that utilises positioning estimates obtained in previous time-frames to improve localisation robustness and reduce the impact of measurement noise. The accuracy of the proposed method is evaluated using simulations, and its practical applicability is demonstrated on experimental data obtained using a novel optical ultrasound imaging setup. Accurate and robust positional information is provided in real-time. Axial and lateral coordinates for out-of-plane objects are estimated with a mean error of 0.1mm for simulated data and a mean error of 0.2mm for experimental data. Three-dimensional localisation is most accurate for elevational distances larger than 1mm, with a maximum distance of 6mm considered for a 25mm aperture

Enhancement of instrumented ultrasonic tracking images using deep learning

PURPOSE: Instrumented ultrasonic tracking provides needle localisation during ultrasound-guided minimally invasive percutaneous procedures. Here, a post-processing framework based on a convolutional neural network (CNN) is proposed to improve the spatial resolution of ultrasonic tracking images. METHODS: The custom ultrasonic tracking system comprised a needle with an integrated fibre-optic ultrasound (US) transmitter and a clinical US probe for receiving those transmissions and for acquiring B-mode US images. For post-processing of tracking images reconstructed from the received fibre-optic US transmissions, a recently-developed framework based on ResNet architecture, trained with a purely synthetic dataset, was employed. A preliminary evaluation of this framework was performed with data acquired from needle insertions in the heart of a fetal sheep in vivo. The axial and lateral spatial resolution of the tracking images were used as performance metrics of the trained network. RESULTS: Application of the CNN yielded improvements in the spatial resolution of the tracking images. In three needle insertions, in which the tip depth ranged from 23.9 to 38.4 mm, the lateral resolution improved from 2.11 to 1.58 mm, and the axial resolution improved from 1.29 to 0.46 mm. CONCLUSION: The results provide strong indications of the potential of CNNs to improve the spatial resolution of ultrasonic tracking images and thereby to increase the accuracy of needle tip localisation. These improvements could have broad applicability and impact across multiple clinical fields, which could lead to improvements in procedural efficiency and reductions in risk of complications

Low‐Field Actuating Magnetic Elastomer Membranes Characterized using Fibre‐Optic Interferometry

Smart robotic devices remotely powered by magnetic field have emerged as versatile tools for wide biomedical applications. Soft magnetic elastomer (ME) composite membranes with high flexibility and responsiveness are frequently incorporated to enable local actuation for wireless sensing or cargo delivery. However, the fabrication of thin ME membranes with good control in geometry and uniformity remains challenging, as well as the optimization of their actuating performances under low fields (milli‐Tesla). In this work, the development of ME membranes comprising of low‐cost magnetic powder and highly soft elastomer through a simple template‐assisted doctor blading approach, is reported. The fabricated ME membranes are controllable in size (up to centimetre‐scale), thickness (tens of microns) and high particle loading (up to 70 wt.%). Conflicting trade‐off effects of particle concentration upon magnetic responsiveness and mechanical stiffness are investigated and found to be balanced off as it exceeds 60 wt.%. A highly sensitive fibre‐optic interferometric sensing system and a customized fibre‐ferrule‐membrane probe are first proposed to enable dynamic actuation and real‐time displacement characterization. Free‐standing ME membranes are magnetically excited under low field down to 2 mT, and optically monitored with nanometer accuracy. The fast and consistent responses of ME membranes showcase their promising biomedical applications in nanoscale actuation and sensing

Comparison of Fabrication Methods for Fiber‐Optic Ultrasound Transmitters Using Candle‐Soot Nanoparticles

Candle-soot nanoparticles (CSNPs) have shown great promise for fabricating optical ultrasound (OpUS) transmitters. They have a facile, inexpensive synthesis whilst their unique, porous structure enables a fast heat diffusion rate which aids high-frequency ultrasound generation necessary for high-resolution clinical imaging. These composites have demonstrated high ultrasound generation performance showing clinically relevant detail, when applied as macroscale OpUS transmitters comprising both concave and planar surfaces, however, less research has been invested into the translation of this material's technology to fabricate fiber-optic transmitters for image guidance of minimally invasive interventions. Here, are reported two fabrication methods of nanocomposites composed of CSNPs embedded within polydimethylsiloxane (PDMS) deposited onto fiber-optic end-faces using two different optimized fabrication methods: “All-in-One” and “Direct Deposition.” Both types of nanocomposite exhibit a smooth, black domed structure with a maximum dome thickness of 50 µm, broadband optical absorption (>98% between 500 and 1400 nm) and both nanocomposites generated high peak-to-peak ultrasound pressures (>3 MPa) and wide bandwidths (>29 MHz). Further, high-resolution (<40 µm axial resolution) B-mode ultrasound imaging of ex vivo lamb brain tissue demonstrating how CSNP-PDMS OpUS transmitters can allow for high fidelity minimally invasive imaging of biological tissues is demonstrated