Optical ultrasound (OpUS): a novel concept for intravascular imaging

Aims

To evaluate whether Optical Ultrasound (OpUS), a novel method for performing ultrasound imaging, could provide compelling, real-time visualizations of coronary vasculature.

Methods and results

With current commercial intravascular ultrasound (IVUS) devices, piezoelectric transducers are used to electrically generate and receive Ultrasound (US). With this paradigm, there are several challenges that limit further improvement in image resolution. Firstly, with increasing miniaturization of these piezoelectric transducers it can be difficult to achieve adequate sensitivity and bandwidth for high resolution imaging. Secondly, the complexities associated with fabricating and electrically connectorising broadband piezocomposite transducers can result in high manufacturing costs. Lastly, with increasing interest in identifying the molecular composition of atherosclerotic plaque, it has been challenging to achieve high resolution and high imaging depths, whilst also allowing for hybrid imaging with photoacoustics (PA) or near-infrared spectroscopy (NIRS).

With OpUS, US is generated at the surface of a fibre optic transducer via the photoacoustic effect. Here, pulsed or modulated light from a laser source is transmitted along the fibre, absorbed in a coating on the fibre surface and converted to thermal energy. The subsequent heat rise leads to a corresponding pressure rise within the coating which propagates as ultrasound. This process is facilitated through the use of custom, engineered nanocomposite materials comprising an optical absorber with an elastomeric host. US reflections from tissue are received with optical interferometry in a method similar to optical coherence tomography (OCT) signal interrogation. For this study we included these elements into a probe and imaged ex-vivo coronary artery tissue. A novel, optically-selective nanocomposite coating enabled concurrent OpUS and PA imaging for molecular contrast using the same imaging probe.

Using OpUS we demonstrated high resolution imaging (<40 microns axial), large imaging depths (>2 cm) of coronary tissue and performed a comparison with histology. Numerous features of atherosclerotic plaque were identifiable, including a lipid pool, a calcified nodule, and the different layers comprising the vessel wall. The fiber-optic transducer generated ultra-high pressures and bandwidths: 21.5 MPa and 39.8 MHz respectively. Hybrid imaging using OpUS and PA was also demonstrated, highlighting regions with high lipid content.

Conclusion

This new platform for intravascular imaging offers high resolution equivalent to 60 Mhz high-definition IVUS whilst maintaining deep tissue penetration. Hybrid imaging with PA can be used for directly visualizing lipid plaque. OpUS transducers are highly flexible, with small diameters (<400 microns) and have low fabrication costs, making them well suited for incorporation into interventional devices.

Funding Acknowledgement

Type of funding source: Private grant(s) and/or Sponsorship. Main funding source(s): Wellcome Trust/EPSRC, National Institute for Health Research Biomedical Research Centre - University College London

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.

Dynamic physiological temperature and pressure sensing with phase-resolved low-coherence interferometry

We report the development and characterisation of highly miniaturised fibre-optic sensors for simultaneous pressure and temperature measurement, and a compact interrogation system with a high sampling rate. The sensors, which have a maximum diameter of 250 µm, are based on multiple low-finesse optical cavities formed from polydimethylsiloxane (PDMS), positioned at the distal ends of optical fibres, and interrogated using phase-resolved low-coherence interferometry. At acquisition rates of 250 Hz, temperature and pressure changes of 0.0021 °C and 0.22 mmHg are detectable. An in vivo experiment demonstrated that the sensors had sufficient speed and sensitivity for monitoring dynamic physiological pressure waveforms. These sensors are ideally suited to various applications in minimally invasive surgery, where diminutive lateral dimensions, high sensitivity and low manufacturing complexities are particularly valuable.

Photosensitisation studies of silicone polymer doped with methylene blue and nanogold for antimicrobial applications

2 nm gold nanoparticle (AuNP) and methylene blue (MB) incorporated into medical-grade silicone polymer for antimicrobial applications.

Advanced analysis of nanoparticle composites – a means toward increasing the efficiency of functional materials

Direct visualisation of embedded nanoparticles allows for quantification of their concentration, at the surface and the bulk of host matrix.