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Optical ultrasound, where ultrasound is both generated and received using light, can be integrated in very small diameter instruments making it ideally suited to minimally invasive interventions. One-dimensional information can be obtained using a single pair of optical fibres comprising of a source and detector but this can be difficult to interpret clinically. In this paper, we present a robotic-assisted scanning solution where a concentric tube robot manipulates an optical ultrasound probe along a consistent trajectory. A torque coil is utilized as a buffer between the curved nitinol tube and the probe to prevent torsion on the probe and maintain the axial orientation of the probe while the tube is rotating. The design and control of the scanning mechanism are presented along with the integration of the mechanism with a fibre-based imaging probe. Trajectory repeatability is assessed using electromagnetic tracking and a technique to calibrate the transformation between imaging and robot coordinates using a known model is presented. Finally, we show example images of 3D printed phantoms generated by collecting multiple OpUS A-scans within the same 3D scene to illustrate how robot-assisted scanning can expand the field of view.

Laser-generated focused ultrasound (LGFU) transducers used for ultrasound therapy commonly have large diameters (6–15 mm), but smaller lateral dimensions (

Optical ultrasound is an emerging imaging paradigm that utilises optical fibres to emit and receive ultrasound. Their small lateral dimensions enable highly miniaturised imaging probes, for a range of interventional applications. This work presents a numerical study to determine the beam width, divergence angle and pressure retention for a range of probe geometries. Additionally, pulse-echo imaging of point scatterer targets is presented, and the impact of bandwidth filtering investigated. The results from this study provide the basis for developing a wide range of single sensor ultrasound devices.

A versatile and scalable fabrication method for laser-generated focused ultrasound transducers is proposed. The method is based on stamping a coated negative mold onto polydimethylsiloxane, and it can be adapted to include different optical absorbers that are directly transferred or synthesized in situ. Transducers with a range of sizes down to 3 mm in diameter are presented, incorporating two carbonaceous (multiwalled carbon nanoparticles and candle soot nanoparticles) and one plasmonic (gold nanoparticles) optically absorbing component. The fabricated transducers operate at central frequencies in the vicinity of 10 MHz with bandwidths in the range of 15-20 MHz. A transducer with a diameter of 5 mm was found to generate a positive peak pressure greater than 35 MPa in the focal zone with a tight focal spot of 150 μm in lateral width. Ultrasound cavitation on the tip of an optical fiber was demonstrated in water for a transducer with a diameter as small as 3 mm.

All-optical ultrasound transducers are well-suited for use in imaging during minimally invasive surgical procedures. This requires highly miniaturised and flexible devices. Here we present optical ultrasound transmitters for imaging applications based on modified optical fibre distal tips which allow for larger transmitter element sizes, whilst maintaining small diameter proximal optical fibre. Three optical ultrasound transmitter configurations were compared; a 400 µm core optical fibre, a 200 µm core optical fibre with a 400 µm core optical fibre distal tip, and a 200 µm core optical fibre with a 400 µm core capillary distal tip. All the transmitters used a polydimethylsiloxane-dye composite material for ultrasound generation. The material comprised a photostable infra-red absorbing dye to provide optical absorption for the ultrasound transduction. The generated ultrasound beam profile for the three transmitters was compared, demonstrating similar results, with lateral beam widths ca. 30 MHz. These highly flexible ultrasound transmitters can be readily incorporated into medical devices with small lateral dimensions.

n this study we report the integration of an all-optical ultrasound probe and robotic manipulator. The alloptical ultrasound probe comprised two optical fibres, a MWCNT/PDMS composite coated multimode fibre for ultrasound generation, and a concave Fabry-Perot fibre optic hydrophone for ultrasound reception. The ultrasound probe generated pressures in excess of 2 MPa at 1:5 mm, with a corresponding -6 dB bandwidth of ca. 30 MHz. The probe was built into a rigid endoscope (outer diameter: 5 mm, length: 300 mm) and mounted on a robotic manipulator. Ultrasound A-lines were acquired during robot manipulation in order to reconstruct a 3D image which was displayed as a point cloud. Large area images (80 × 80 mm) of a tissue mimicking gel wax phantom where acquired and displayed in real-time. This work demonstrates the potential for integrating miniature fibre optic ultrasound devices with robotics.

Radiofrequency ablation is widely used in cardiology as an effective minimally invasive treatment for atrial fibrillation. However, radiofrequency noise, electronic interference, low resolution and poor tissue contrast complicate real-time lesion monitoring using conventional imaging modalities such as magnetic resonance imaging or ultrasound imaging based on electronic transducers. Recently, a bench-top all-optical ultrasound imaging system, where ultrasound is both generated and detected using light, was presented (doi:10.1364/BOE.9.003481) that achieved high-resolution, video-rate 2D images. In this system, pulsed excitation light was focussed onto a nanocomposite membrane, where it was converted into ultrasound via the photoacoustic effect. Using scanning optics, the resulting optical ultrasound source was translated to synthesise a 1D source aperture comprising irregularly spaced ultrasound sources. Back-scattered ultrasound was detected using a single fibre-optic Fabry-Perot cavity. Here, this system (which is inherently insensitive to electromagnetic interference) was used to achieve the first video-rate, depth-resolved 2D images acquired during RF ablation using an all-optical ultrasound imaging setup. We used this system to monitor the formation of radiofrequency ablation lesions (max 30 W, 65°C, 60 s) in ex vivo chicken breast samples, at a frame rate of 9 Hz, resolution of 100 µm, an imaging depth >15 mm, and a contrast of up to 30 dB. With its high miniaturisation potential, all-optical ultrasound imaging shows great promise for guiding interventional procedures, where real-time ablation lesion visualisation could improve lesion delivery and patient outcome.

Percutaneous coronary interventions are widely performed minimally invasive procedures used to treat narrowing (stenosis) of arteries in the heart. Differential blood pressure measurements across a stenosis are invaluable to estimate the prognostic benefit of performing angioplasty and stenting via calculation of the fractional flow reserve. Achieving stable measurements from within pressure microcatheters and guidewires that are compatible with stenosed vessels, and which can be fabricated with low cost manufacturing methods, remains an important challenge. We have developed all-optical pressure and temperature sensors with a single optical fibre and sensing element. This approach provides simultaneous temperature and pressure measurements in a highly miniaturised device, with a simple construction method using low cost materials. Polymeric structures including membranes and domes are applied to the distal ends of single mode optical fibres. Temperature and pressure changes induce time-varying displacements of these structures, which are monitored using phase-resolved low-coherence interferometry. Phase measurements are acquired at 250 Hz with a sensitivity of approximately 0.2 rad/°C for temperature measurements between 20 and 45°C, and approximately 0.08 rad/mmHg for pressure between 760 and 1060 mmHg. In vivo studies in arteries and hearts of sheep and swine indicate that the sensors have sufficient sensitivity and speed for measurement of physiological pressure waveforms in clinical settings. We will discuss the integration of these sensors within medical devices, and the potential for providing additional physiological parameters with the same devices.

All-optical ultrasound imaging, where ultrasound is generated photoacoustically and back-scattered fields are detected with optically resonant structures, has distinct advantages over conventional piezoelectric or capacitive transducers. An absence of electrical connections facilitates probe miniaturisation and confers MRI compatibility, while similar or better sensitivities and bandwidths can be achieved. Previously reported all-optical systems used mechanical translation of a single source and receiver to scan a synthetic aperture, resulting in image acquisition times ranging from seconds to minutes. This frame-rate can be improved by reducing the number of A-scans per image; however, this increases the noise and artefact levels of the image. For conventional probes, typically employing unalterable periodic arrays, side- and grating lobes can give rise to substantial artefacts that are commonly suppressed using amplitude-weighted apodisation. In this work, however, the spatial density of the acoustic sources rather than their amplitudes is weighted to avoid array periodicity, in order to achieve a better image quality using the same aperture and number of A-scans.

An optical ultrasound generator was used to perform broadband (2–35 MHz) acoustical characterisation measurements of a nanocomposite comprising carbon nanotubes (CNT) and polydimethylsiloxane (PDMS), a composite that is commonly used as optical ultrasound generator. Samples consisting of either pure PDMS or CNT-loaded PDMS were characterised to determine the influence of CNTs on the speed of sound and power-law acoustic attenuation parameters. A small weight fraction (< 1.8%) of added CNTs was found to yield a prominent increase in the exponent of the power law, resulting in a significant increase in acoustic attenuation at higher frequencies. The speed of sound was found to be nearly identical, however. These results could prove useful in the numerical modelling and design of future optical ultrasound sources based on CNT-loaded PDMS.

Strongly directional ultrasound sources are desirable for many minimally invasive applications, as they enable high-quality imaging in the presence of positioning uncertainty. All-optical ultrasound is an emerging paradigm that exhibits high frequencies, large bandwidths, and a strong miniaturisation potential. Here, we report the design, modelling, and fabrication of a highly directional fibre-optic ultrasound transmitter that uses a composite of reduced graphene oxide and polydimethylsiloxane as the optical ultrasound generator. The ultrasound transmitter, which had an outer diameter of just 630 μm, generated ultrasound with a pressure exceeding 0.4 MPa for axial distances up to 16 mm, at a large bandwidth of 24.3 MHz. The ultrasound beam exhibited low divergence, with a beam diameter ranging between 0.6 and 2.1 mm for distances between 0 and 16 mm. The presented directional optical ultrasound source is hence well-suited to high-resolution interventional imaging.

Ultrasound arrays used for medical imaging consist of many elements placed closely together. Ideally, each element vibrates independently. However, because of mechanical coupling, crosstalk between neighboring elements may occur. To quantify the amount of crosstalk, the transducer velocity distribution should be measured. In this work, a method is presented to reconstruct the velocity distribution from far-field pressure field measurements acquired over an arbitrary surface. The distribution is retrieved from the measurements by solving an integral equation, derived from the Rayleigh integral of the first kind, using a conjugate gradient inversion scheme. This approach has the advantages that it allows for arbitrary transducer and pressure field measurement geometries, as well as the application of regularization techniques. Numerical experiments show that measuring the pressure field along a hemisphere enclosing the transducer yields significantly more accurate reconstructions than measuring along a parallel plane. In addition, it is shown that an increase in accuracy is achieved when the assumption is made that all points on the transducer surface vibrate in phase. Finally, the method has been tested on an actual transducer with an active element of 700 × 200 μm which operates at a center frequency of 12.2 MHz. For this transducer, the velocity distribution has been reconstructed accurately to within 50 μm precision from pressure measurements at a distance of 1.98 mm (=16λ0) using a 200-μm-diameter needle hydrophone.

In biomedical all-optical pulse-echo ultrasound systems, ultrasound is generated with the photoacoustic effect by illuminating an optically absorbing structure with a temporally modulated light source. Nanosecond range laser pulses are typically used, which can yield bandwidths exceeding 100 MHz. However, acoustical attenuation within tissue or nonuniformities in the detector or source power spectra result in energy loss at the affected frequencies and in a reduced overall system efficiency. In this work, a laser diode is used to generate linear and nonlinear chirp optical modulations that are extended to microsecond time scales, with bandwidths constrained to the system sensitivity. Compared to those obtained using a 2-ns pulsed laser, pulse-echo images of a phantom obtained using linear chirp excitation exhibit similar axial resolution (99 versus 92 $\upmu \text{m}$ , respectively) and signal-to-noise ratios (SNRs) (10.3 versus 9.6 dB). In addition, the axial point spread function (PSF) exhibits lower sidelobe levels in the case of chirp modulation. Using nonlinear (time-stretched) chirp excitations, where the nonlinearity is computed from measurements of the spectral sensitivity of the system, the power spectrum of the imaging system was flattened and its bandwidth broadened. Consequently, the PSF has a narrower axial extent and still lower sidelobe levels. Pulse-echo images acquired with time-stretched chirps as optical modulation have higher axial resolution (64 $\upmu \text{m}$ ) than those obtained with linear chirps, at the expense of a lower SNR (6.8 dB). Using a linear or time-stretched chirp, the conversion efficiency from optical power to acoustical pressure improved by a factor of 70 or 61, respectively, compared to that obtained with pulsed excitation.

As recently demonstrated, all-optical B-mode ultrasound imaging can be performed with a pair of coated optical fibres for transmission and reception that is translated to create a virtual array of elements. However, with translation in the in-plane dimension, the small lateral dimensions of the fibres results in out-of-plane acoustic divergence, which leads to image clutter and loss of sensitivity.

A clinical spectroscopic eiphotoacoustic scanner is presented, evaluated and optimised. The scanner combines a clinical ultrasound scanner with commercially available lasers to generate photocoustic images that are inherently coregistered with the ultrasound images and can be acquired by freehand scanning. Arrays of graphite rods are used to determine the point spread function (PSF), signal-to-noise (SNR) and signal-to-clutter ratios (SCR) of the system and optimise the image quality by varying the illumination geometry. At a distance of 26 mm, mall spatial PSF extents are found (lateral 399 ± 80 μm, axial 340 ± 20 μm, elevational 1.43 ± 0.26 mm) and hence a high solution is obtained. Due to the strong optical scattering in biological tissue, the illumination geometry has virtually no effect on the image intensity, SNR and SCR, and hence practicality is the main concern when designing the clinical scan-head geometry.

Novel ultrasound arrays used in medical imaging contain many small elements (down to 100 μm) operating at high frequencies (upto tens of MHz). In such arrays, each element requires an individual electrical connection. Typically, these connections are made to soldering pads located some distance away from the transducer, which in turn are connected to the element by a pathway on a printed circuit board. However, these pathways are unshielded and therefore sensitive to electromagnetic interference and cross-talk. In this work, a casting-based fabrication process is presented where the room required for electrical connections is created vertically, i.e., in the direction perpendicular to the transducer surface. Using this process, chambers are created into which the cores of coaxial cables are inserted, and on top of these chambers the elements are built. As only the part of the cables inserted into the chambers is unshielded, the sensitivity to electromagnetic interference is strongly reduced. In addition, the elements share a single ground electrode which acts as a Faraday cage and further reduces interference. Using this fabrication process, a linear array was constructed containing eight elements of dimensions 100 μm by 350 μm operating at a center frequency of 21 MHz. The array is validated both acoustically and mechanically, and is shown to consist of eight virtually identical elements exhibiting high fractional bandwidths of more than 80%. Virtually no mechanical crosstalk is observed.

Optimising the design of medical ultrasound transducers requires precise knowledge of the motion of the transmitting elements, especially if cross-talk between neighboring elements plays a significant role. Direct measurement of the motion of the elements is complicated. Therefore we propose to compute these velocity profiles from a pressure field measured in a plane parallel to the surface of the transducer. The method employs an inverse extrapolation of the measurements using an analytical deconvolution based on Weyl's representation of the Green's function. The method is tested for transducers operating in the MHz-range and with spatial dimensions in the order of 100 µm. Both measured and simulated pressure fields are used to investigate its performance. It is shown that relevant results, in terms of energy localisation and temporal behaviour, can be obtained for realistic distances between the transducer and measurement planes and for coarse sampling grids. In addition, the results show that sharp reconstructions of the velocity profiles are only possible if fine measurement grids are used and if the measurement distance is less than a wavelength of the ultrasound wave field.

Polydimethylsiloxane (PDMS) is widely used in biomedical science and can form composites that have broad applicability. One promising application where PDMS composites offer several advantages is optical ultrasound generation via the photoacoustic effect. Here, methods to create these PDMS composites are reviewed and classified. It is highlighted how the composites can be applied to a range of substrates, from micrometer‐scale, temperature‐sensitive optical fibers to centimeter‐scale curved and planar surfaces. The resulting composites have enabled all‐optical ultrasound imaging of biological tissues both ex vivo and in vivo, with high spatial resolution and with clinically relevant contrast. In addition, the first 3D all‐optical pulse‐echo ultrasound imaging of ex vivo human tissue, using a PDMS‐multiwalled carbon nanotube composite and a fiber‐optic ultrasound receiver, is presented. Gold nanoparticle‐PDMS and crystal violet‐PDMS composites with prominent absorption at one wavelength range for pulse‐echo ultrasound imaging and transmission at a second wavelength range for photoacoustic imaging are also presented. Using these devices, images of diseased human vascular tissue with both structural and molecular contrast are obtained. With a broader perspective, literature on recent advances in PDMS microfabrication from different fields is highlighted, and methods for incorporating them into new generations of optical ultrasound generators are suggested.