Direct Model-Based Inversion for Improved Freehand Optical Ultrasound Imaging

Optical ultrasound imaging uses light to both generate and detect pulse-echo ultrasound. Recently, we presented a fibre-optic optical ultrasound imaging probe comprising 64 sources and a single receiver that allowed for video-rate, freehand imaging. However, its low number of sources limited the image contrast when using Delay-and-Sum reconstruction. Here, we present an alternative image formation paradigm for optical ultrasound based on model-based inversion, where the low number of sources allows for direct (i.e., non-iterative) inversion under modest hardware requirements. The model accurately incorporates the aperture geometry, frequency-dependent source directivity, and performance variation across the aperture, thereby reducing image artefacts associated with these properties. The method achieves a 15 dB gain in image contrast compared to Delay-and-Sum, at a similar image formation time

PDMS composites with photostable NIR dyes for multi-modal ultrasound imaging

All-optical ultrasound (OpUS) imaging has emerged as an imaging paradigm well-suited for minimally invasive surgical procedures. With this modality, ultrasound is generated when pulsed or modulated light is absorbed within a coating material. By engineering wavelength-selective coatings, complementary imaging and therapeutic modalities can be integrated with OpUS. Here, we present a wavelength-selective composite material comprising a near-infrared absorbing dye and polydimethylsiloxane. The optical absorption for this material peaked in the vicinity of 1064 nm, with up to 91% of incident light being absorbed, whilst maintaining lower optical absorption at other wavelengths. This material was used to generate ultrasound, demonstrating ultrasound pressures >1 MPa, consistent with those used for imaging applications. Crucially, long exposure photostability and device performance were found to be stable over a one hour period (peak pressure variation <10%), longer than required for standard clinical imaging applications

Pencil beam all-optical ultrasound imaging

A miniature, directional fibre-optic acoustic source is presented that employs geometrical focussing to generate a nearly-collimated acoustic pencil beam. When paired with a fibre-optic acoustic detector, an all-optical ultrasound probe with an outer diameter of 2.5 mm is obtained that acquires a pulse-echo image line at each probe position without the need for image reconstruction. B-mode images can be acquired by translating the probe and concatenating the image lines, and artefacts resulting from probe positioning uncertainty are shown to be significantly lower than those observed for conventional synthetic aperture scanning of a non-directional acoustic source. The high image quality obtained for excised vascular tissue suggests that the all-optical ultrasound probe is ideally suited for in vivo, interventional applications

Photoacoustic imaging of intracardiac medical devices using internal illumination of carbon nanotube / PDMS composite coatings

Accurate localisation of medical devices is of crucial importance for a wide range of ultrasound-guided interventions. In this study, we investigated visualisation of medical devices by photoacoustic excitation of optically absorbing coatings. Photoacoustic excitation light was provided through optical fibres positioned within a cardiac needle and a steerable-tip catheter. Using a swine heart model, photoacoustic and B-mode ultrasound images were received with a clinical ultrasound scanner in conjunction with a transoesophageal imaging probe. In the photoacoustic images, prominent signals were obtained from the coatings. This study demonstrated that photoacoustic imaging could play a useful role with medical device imaging

White light-activated antimicrobial surfaces: effect of nanoparticles type on activity

Toluidine blue O (TBO) dye together with either silver (Ag) nanoparticles (NPs), gold (Au) NPs, or a mixture of Ag and Au NPs (Mix Ag–Au NPs) were incorporated into polyurethane to make antimicrobial surfaces using a swell-encapsulation-shrink process. Antimicrobial testing against Escherichia coli showed that inclusion of the NPs significantly enhanced the antimicrobial activities of the TBO polyurethane samples. In particular, samples containing Ag NPs exhibited potent antimicrobial activity under white light and surprisingly, also in the dark. The numbers of viable bacteria decreased below the detection limit on the TBO/Ag NPs incorporated samples within 3 h and 24 h under white light and dark conditions. A mechanistic study using furfuryl alcohol indicated that the enhanced photobactericidal activity was most likely due to a type I photochemical reaction. To the best of our knowledge, this is the first report of an antimicrobial surface comprised of a combination of Ag NPs and a light activated agent to provide a dual kill mechanism. These surfaces are promising candidates for use in healthcare environments to reduce the incidence of hospital-acquired infections

A reconfigurable all-optical ultrasound transducer array for 3D endoscopic imaging

A miniature all-optical ultrasound imaging system is presented that generates three-dimensional images using a stationary, real acoustic source aperture. Discrete acoustic sources were sequentially addressed by scanning a focussed optical beam across the proximal end of a coherent fibre bundle; high-frequency ultrasound (156% fractional bandwidth centred around 13.5 MHz) was generated photoacoustically in the corresponding regions of an optically absorbing coating deposited at the distal end. Paired with a single fibre-optic ultrasound detector, the imaging probe (3.5 mm outer diameter) achieved high on-axis resolutions of 97 μm, 179 μm and 110 μm in the x, y and z directions, respectively. Furthermore, the optical scan pattern, and thus the acoustic source array geometry, was readily reconfigured. Implementing four different array geometries revealed a strong dependency of the image quality on the source location pattern. Thus, by employing optical technology, a miniature ultrasound probe was fabricated that allows for arbitrary source array geometries, which is suitable for three-dimensional endoscopic and laparoscopic imaging, as was demonstrated on ex vivo porcine cardiac tissue

Freehand and Video-Rate All-Optical Ultrasound Imaging

All-optical ultrasound (AOUS) imaging, which uses light to both generate and detect ultrasound, is an emerging alternative to conventional electronic ultrasound imaging. To date, AOUS imaging has been performed using paradigms that either resulted in long acquisition times or employed bench-top imaging systems that were impractical for clinical use. In this work, we present a novel AOUS imaging paradigm where scanning optics are used to rapidly synthesise an imaging aperture. This paradigm enabled the first AOUS system with a flexible, handheld imaging probe, which represents a critical step towards clinical translation. This probe, which provides video-rate imaging and a real-time display, is demonstrated with phantoms and in vivo human tissue

Versatile and scalable fabrication method for laser-generated focused ultrasound transducers

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

Modelling and measurement of laser-generated focused ultrasound: Can interventional transducers achieve therapeutic effects?

Laser-generated focused ultrasound (LGFU) transducers used for ultrasound therapy commonly have large diameters (6–15 mm), but smaller lateral dimensions (<4 mm) are required for interventional applications. To address the question of whether miniaturized LGFU transducers could generate sufficient pressure at the focus to enable therapeutic effects, a modelling and measurement study is performed. Measurements are carried out for both linear and nonlinear propagation for various illumination schemes and compared with the model. The model comprises several innovations. First, the model allows for radially varying acoustic input distributions on the surface of the LGFU transducer, which arise from the excitation light impinging on the curved transducer surfaces. This realistic representation of the source prevents the overestimation of the achievable pressures (shown here to be as high as 1.8 times). Second, an alternative inverse Gaussian illumination paradigm is proposed to achieve higher pressures; a 35% increase is observed in the measurements. Simulations show that LGFU transducers as small as 3.5 mm could generate sufficient peak negative pressures at the focus to exceed the cavitation threshold in water and blood. Transducers of this scale could be integrated with interventional devices, thereby opening new opportunities for therapeutic applications from inside the body