Miniaturized fiber-optic ultrasound probes for endoscopic tissue analysis by micro-opto-mechanical technology

All-optical ultrasound catheter for rapid B-mode oesophageal imaging

All-optical ultrasound (OpUS) is an imaging paradigm that uses light to both generate and receive ultrasound, and has progressed from benchtop to in vivo studies in recent years, demonstrating promise for minimally invasive surgical applications. In this work, we present a rapid pullback imaging catheter for side-viewing B-mode ultrasound imaging within the upper gastrointestinal tract. The device comprised an ultrasound transmitter configured to generate ultrasound laterally from the catheter and a plano-concave microresonator for ultrasound reception. This imaging probe was capable of generating ultrasound pressures in excess of 1 MPa with corresponding -6 dB bandwidths > 20 MHz. This enabled imaging resolutions as low as 45 µm and 120 µm in the axial and lateral extent respectively, with a corresponding signal-to-noise ratio (SNR) of 42 dB. To demonstrate the potential of the device for clinical imaging, an ex vivo swine oesophagus was imaged using the working channel of a mock endoscope for device delivery. The full thickness of the oesophagus was resolved and several tissue layers were present in the resulting ultrasound images. This work demonstrates the promise for OpUS to provide rapid diagnostics and guidance alongside conventional endoscopy.

High-resolution silicon photonics focused ultrasound transducer with a sub-millimeter aperture

We present an all-optical focused ultrasound transducer with a sub-millimeter aperture and demonstrate its capability for high-resolution imaging of tissue ex vivo. The transducer is composed of a wideband silicon photonics ultrasound detector and a miniature acoustic lens coated with a thin optically absorbing metallic layer used to produce laser-generated ultrasound. The demonstrated device achieves axial resolution and lateral resolutions of 12 μm and 60 μm, respectively, well below typical values achieved by conventional piezoelectric intravascular ultrasound. The size and resolution of the developed transducer may enable its use for intravascular imaging of thin fibrous cap atheroma.

A Comprehensive Review on Photoacoustic-Based Devices for Biomedical Applications

The photoacoustic effect is an emerging technology that has sparked significant interest in the research field since an acoustic wave can be produced simply by the incidence of light on a material or tissue. This phenomenon has been extensively investigated, not only to perform photoacoustic imaging but also to develop highly miniaturized ultrasound probes that can provide biologically meaningful information. Therefore, this review aims to outline the materials and their fabrication process that can be employed as photoacoustic targets, both biological and non-biological, and report the main components' features to achieve a certain performance. When designing a device, it is of utmost importance to model it at an early stage for a deeper understanding and to ease the optimization process. As such, throughout this article, the different methods already implemented to model the photoacoustic effect are introduced, as well as the advantages and drawbacks inherent in each approach. However, some remaining challenges are still faced when developing such a system regarding its fabrication, modeling, and characterization, which are also discussed.

Photoacoustic imaging with fiber optic technology: A review

Photoacoustic imaging (PAI) has achieved remarkable growth in the past few decades since it takes advantage of both optical and ultrasound (US) imaging. In order to better promote the wide clinical applications of PAI, many miniaturized and portable PAI systems have recently been proposed. Most of these systems utilize fiber optic technologies. Here, we overview the fiber optic technologies used in PAI. This paper discusses three different fiber optic technologies: fiber optic light transmission, fiber optic US transmission, and fiber optic US detection. These fiber optic technologies are analyzed in different PAI modalities including photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and minimally invasive photoacoustic imaging (MIPAI).

All-Optical Rotational Ultrasound Imaging

Miniaturised high-resolution imaging devices are valuable for guiding minimally invasive procedures such as vascular stent placements. Here, we present all-optical rotational B-mode pulse-echo ultrasound imaging. With this device, ultrasound transmission and reception are performed with light. The all-optical transducer in the probe comprised an optical fibre that delivered pulsed excitation light to an optical head at the distal end with a multi-walled carbon nanotube and polydimethylsiloxane composite coating. This coating was photoacoustically excited to generate a highly directional ultrasound beam perpendicular to the optical fibre axis. A concave Fabry-Pérot cavity at the distal end of an optical fibre, which was interrogated with a tuneable continuous-wave laser, served as an omnidirectional ultrasound receiver. The transmitted ultrasound had a -6 dB bandwidth of 31.3 MHz and a peak-to-peak pressure of 1.87 MPa, as measured at 1.5 mm from the probe. The receiver had a noise equivalent pressure <100 Pa over a 20 MHz bandwidth. With a maximum outer probe diameter of 1.25 mm, the probe provided imaging with an axial resolution better than 50 µm, and a real-time imaging rate of 5 frames per second. To investigate the capabilities of the probe, intraluminal imaging was performed in healthy swine carotid arteries. The results demonstrate that the all-optical probe is viable for clinical rotational ultrasound imaging.

Laser generated ultrasound sources using carbon-polymer nanocomposites for high frequency metrology

The characterization of ultrasound fields generated by diagnostic and therapeutic equipment is an essential requirement for performance validation and to demonstrate compliance against established safety limits. This requires hydrophones calibrated to a traceable standard. Currently, the upper calibration frequency range available to the user community is limited to 60 MHz. However, high frequencies are increasingly being used for both imaging and therapy necessitating calibration frequencies up to 100 MHz. The precise calibration of hydrophones requires a source of high amplitude, broadband, quasi-planar, and stable ultrasound fields. There are challenges to using conventional piezoelectric sources, and laser generated ultrasound sources offer a promising solution. In this study, various nanocomposites consisting of a bulk polymer matrix and multi-walled carbon nanotubes were fabricated and tested using pulsed laser of a few nanoseconds for their suitability as a source for high frequency calibration of hydrophones. The pressure amplitude and bandwidths were measured using a broadband hydrophone from 27 different nanocomposite sources. The effect of nonlinear propagation of high amplitude laser generated ultrasound on bandwidth and the effect of bandlimited sensitivity response on the deconvolved pressure waveform were numerically investigated. The stability of the nanocomposite sources under sustained laser pulse excitation was also examined.

Video-rate all-optical ultrasound imaging

All-optical ultrasound imaging, where ultrasound is generated and detected using light, has recently been demonstrated as a viable modality that is inherently insensitive to electromagnetic interference and exhibits wide bandwidths. High-quality 2D and 3D all-optical ultrasound images of tissues have previously been presented; however, to date, long acquisition times (ranging from minutes to hours) have hindered clinical application. Here, we present the first all-optical ultrasound imaging system capable of video-rate, real-time two-dimensional imaging of biological tissue. This was achieved using a spatially extended nano-composite optical ultrasound generator, a highly sensitive fibre-optic acoustic receiver, and eccentric illumination resulting in an acoustic source exhibiting optimal directivity. This source was scanned across a one-dimensional source aperture using a fast galvo mirror, thus enabling the dynamic synthesis of source arrays comprising spatially overlapping sources at non-uniform source separation distances. The resulting system achieved a sustained frame rate of 15 Hz, a dynamic range of 30 dB, a penetration depth of at least 6 mm, a resolution of 75 µm (axial) by 100 µm (lateral), and enabled the dynamics of a pulsating ex vivo carotid artery to be captured.

Biodistribution and toxicity assessment of intratumorally injected arginine-glycine-aspartic acid peptide conjugated to CdSe/ZnS quantum dots in mice bearing pancreatic neoplasm

Quantum dots (QDs) conjugated with arginine-glycine-aspartic acid (RGD) peptides (which are integrin antagonists) are novel nanomaterials with the unique optical property of high molar extinction coefficient, and they have potential utility as photosensitizers in photodynamic therapy (PDT). Our group previously demonstrated significant benefits of using PDT with QD-RGD on pancreatic tumor cells. This study aimed to evaluate the biodistribution and toxicity of QD-RGD in mice prior to in vivo application. Mice with pancreatic neoplasms were intratumorally injected with varying doses of QD-RGD, and the biodistribution 0-24 h post injection was compared to that in control mice (intravenously injected with unconjugated QD). Various tissue samples were collected for toxicity analyses, which included inductively coupled plasma mass spectrometry (ICP-MS) to assess Cd²⁺ concentrations and hematoxylin-eosin staining for histopathological examination. Fluorescent imaging revealed relatively sufficient radiant efficiency in mice under specific conditions. The ICP-MS and HE data showed no significant signs of necrosis due to Cd²⁺ release by QDs. The mice survived well and had no apparent weakness or weight loss during the 4 weeks post injection. These findings provide novel insights into the biodistribution of QD-RGD and encourage profound in vivo studies regardless of safety concerns. These findings alleviate safety concerns and provide novel insights into the biodistribution of QD-RGD, offering a solid foundation for comprehensive in vivo studies.

Optical fiber ultrasound transmitter with electrospun carbon nanotube-polymer composite

All-optical ultrasound transducers are promising for imaging applications in minimally invasive surgery. In these devices, ultrasound is transmitted and received through laser modulation, and they can be readily miniaturized using optical fibers for light delivery. Here, we report optical ultrasound transmitters fabricated by electrospinning an absorbing polymer composite directly onto the end-face of optical fibers. The composite coating consisting of an aqueous dispersion of multi-walled carbon nanotubes (MWCNTs) in polyvinyl alcohol was directly electrospun onto the cleaved surface of a multimode optical fiber and subsequently dip-coated with polydimethylsiloxane (PDMS). This formed a uniform nanofibrous absorbing mesh over the optical fiber end-face wherein the constituent MWCNTs were aligned preferentially along individual nanofibers. Infiltration of the PDMS through this nanofibrous mesh onto the underlying substrate was observed and the resulting composites exhibited high optical absorption (>97%). Thickness control from 2.3 μm to 41.4 μm was obtained by varying the electrospinning time. Under laser excitation with 11 μJ pulse energy, ultrasound pressures of 1.59 MPa were achieved at 1.5 mm from the coatings. On comparing the electrospun ultrasound transmitters with a dip-coated reference fabricated using the same constituent materials and possessing identical optical absorption, a five-fold increase in the generated pressure and wider bandwidth was observed. The electrospun transmitters exhibited high optical absorption, good elastomer infiltration, and ultrasound generation capability in the range of pressures used for clinical pulse-echo imaging. All-optical ultrasound probes with such transmitters fabricated by electrospinning could be well-suited for incorporation into catheters and needles for diagnostics and therapeutic applications.

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.

Wide bandwidth fiber-optic ultrasound probe in MOMS technology: Preliminary signal processing results

An ultrasonic probe consisting of two optical fiber-based miniaturized transducers for wideband ultrasound emission and detection is employed for the characterization of in vitro biological tissues. In the probe, ultrasound generation is obtained by thermoelastic emission from patterned carbon films in Micro-Opto-Mechanical-System (MOMS) devices mounted on the tip of an optical fiber, whereas acousto-optical detection is performed in a similar way by a miniaturized polymeric interferometer. The microprobe presents a wide, flat bandwidth that is a very attractive feature for ultrasonic investigation, especially for tissue characterization. Thanks to the very high ultrasonic frequencies obtained, the probe is able to reveal different details of the object under investigation by analyzing the ultrasonic signal within different frequencies ranges, as shown by specific experiments performed on a patterned cornstarch flour sample in vitro. This is confirmed by measurements executed to determine the lateral resolution of the microprobe at different frequencies of about 70μm at 120MHz. Moreover, measurements performed with the wideband probe in pulsed-echo mode on a histological finding of porcine kidney are presented, on which two different spectral signal processing algorithms are applied. After processing, the ultrasonic spectral features show a peculiar spatial distribution on the sample, which is expected to depend on different ultrasonic backscattering properties of the analyzed tissues.

Adaptive Light Modulation for Improved Resolution and Efficiency in All-Optical Pulse-Echo Ultrasound

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 μm, 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 μm) 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.