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

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

Comparison of noise reduction methods in photoacoustic microscopy

Photoacoustic microscopy (PAM) is classified as a hybrid imaging technique based on the photoacoustic effect and has been frequently studied in recent years. Photoacoustic (PA) signals are inherently recorded in a noisy environment and are also exposed to noise by system components. Therefore, it is essential to reduce the noise in PA signals to reconstruct images with less error. In this study, an image reconstruction algorithm for PAM system was implemented and different filtering approaches for denoising were compared. Studies were carried out in three steps: simulation, experimental phantom and blood cell studies. FIR low-pass and band-pass filters and Discrete Wavelet Transform (DWT) based filters (mother wavelets: “bior3.5″, “bior3.7″, “sym7″) with four different thresholding techniques were examined. For the evaluation purposes, Root Mean Square Error (RMSE), Signal to Noise Ratio (SNR) and Contrast to Noise Ratio (CNR) metrics were calculated. In the simulation studies, the most effective methods were obtained as: sym7/heursure/hard thresh. combination (low and medium level noise) and bior3.7/sqtwolog/soft thresh. combination (high-level noise). In experimental phantom studies, noise was classified into five levels. Different filtering approaches perform better depending on the SNR of PA images. For the blood cell study, based on the standard deviation in the background, sym7/sqtwolog/soft thresh. combination provided the best improvement and this result supported the experimental phantom results

Thermal Modelling of Fibre-Optic Laser Generated Ultrasound Transmitters - Data.zip

Optical generation of ultrasound has broad applicability in diagnostic and therapeutic clinical applications. With fibre-optic ultrasound transmitters, ultrasound waves are generated photoacoustically by laser pulses incident on an optically-absorbing coating at the distal end of an optical fibre. Energy from the laser pulses that is not converted to ultrasound raises the temperature of the transmitter coating and surrounding medium. Limiting the maximum temperature is important for tissue safety and the integrity of the transmitter. In this study, we used a finite element thermal model of a fibre-optic ultrasound transmitter to study the influence of three parameters on the temperature rises in the transmitter and the surrounding medium: the laser pulse energy, the laser pulse repetition frequency, and the coating absorption coefficient. To evaluate the validity of the model, the simulation results were compared with thermal imaging experiments of a carbon-polydimethylsiloxane composite-based fibre-optic ultrasound transmitter. Of the studied parameters, the pulse repetition frequency (PRF) has the greatest impact on the temperature rise in the surrounding medium, with a six-fold rise in temperature change resulting from an increase in PRF from 100 Hz to 1 kHz. Our findings have direct applicability to optimising the performance of fibre optic transmitters.</p

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

Abstract 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