Alles, EJ

Colchester, RJ

Desjardins, AE

Noimark, S

UCL Discovery

Optically Modulated Laser Ultrasound using a Fibre-Based Side-Looking SourceErwin J Alles1, Richard J Colchester1, Sacha Noimark1,2 and Adrien E Desjardins11Department of Medical Physics and Biomedical Engineering, University College London, WC1E 6BT, U.K.      E-mail: E.Alles@UCL.ac.uk2Materials Chemistry Research Centre, UCL Department of Chemistry, 20 Gordon St, London WC1H 0AJ, U.K.1. Fibre-based laser ultrasound Recent advances in fibre-based all-optical utrasound imaging have resulted in forward-looking imaging probes that yield high quality images of ex vivo tissue [1]. Due to its small diameter (<0.5 mm), such a probe is ideally suited to interventional biomedical applications, and the low sensitivity of optical fibres to electromagnetic interference allows for long cable leads and concurrent use of different imaging modalities. In this work, a side-viewing probe is presented to be used in, for example, intravascular ultrasound. In order to maximise the acoustic signal-to-noise ratio (SNR), this probe is illuminated with modulated rather than pulsed laser light.3. Modulated optical excitationIn conventional laser ultrasound, light pulses of nanosecond duration are used that can generate ultrasound with large bandwidths (> 100 MHz). However, in practice typically much narrower bandwidths are observed due to source and detector non-idealities and attenuation, and a significant part of the optical energy is lost [4]. In addition, the damage threshold of the optically absorbing coating typically limits the optical fluence. This damage threshold was avoided by using a temporally modulated laser diode and compensating for its lower peak power by emitting temporally extended optical modulations. The large bandwidth of the laser diode (> 200 MHz) allowed for accurate control over the spectral content of the emitted ultrasound.5. ConclusionsBy limiting the spectral content of the optical excitations to the detected acoustic bandwidth, the SNR of images obtained with a side-viewing fibre-based laser ultrasound probe was signifi-cantly increased without sacrificing resolution. In addition, by transmitting temporally extended optical modulations, more optical energy can be transmitted without exceeding the material damage thresholds. These findings are readily extended to other imaging modalities such as photoacoustic imaging, where the spectral content of the detected signals is not known a priori, or to contrast enhanced ultrasound imaging, where tuning the excitation to microbubble resonance will strongly increase image contrast. In addition, by varying the optical modulation the same probe could be used for imaging modalities requiring wide, narrow, or intermediate bandwidths.2. Side-looking ultrasound fibre probeA multimode optical fibre (core/cladding diameter: 200/220 mm) was modified and coated to obtain a source generating an acoustic field perpendicular to the fibre axis. Ultrasound was generated through illumination of a ca. 1 mm thick coating consisting of functionalised multi-walled carbon nanotubes covered by a 20 mm thick layer of PDMS [2]. Acoustic pulse-echo signals were recorded using an omni-directional WiresScan directionSourceReceiverFig. 1 - Schematic of the wire phantom and scan geometrydetector consisting of a Fabry-Pérot etalon built onto a second fibre [3]. A synthetic aperture was mecha-nically scanned to image a phantom con-sisting of two layers of parallel wires spaced approximately 1 mm apart (Fig. 1). Scan parameters: mechanical: 400 steps of 25 mm; Optical: 4 ms chirps, 750 Hz, 1.5 mJ/chirp; Acquisition: 750 averages, +40 dB amplification; pulse compression: matched-filter cross-correlationExcitation laser: Lumics LU1064M450-1006N10A + Picolas BFS-VRM-03-HP; Interrogation laser: Yenista TUNICS T100S-HP; Translation stages: Thorlabs MTS50-Z8; Preamplifier: Femto DHPVA-200; DAQ: Spectrum M4i.4420-x84. ResultsLateral distance [mm]0 4 8Axial depth [mm]0246Lateral distance [mm]0 4 80 dB-10-20Fig. 3 - Images obtained of the wire phantom after pulse compression of linear chirps with bandwidths between 0-50 MHz (left) and 1.5-15 MHz (right).Transmitting linear chirps with different band-widths revealed (Fig. 2) that the detected pulse-echo signals were limited to a bandwidth between 1.5 and 15 MHz. Consequently, limiting the chirp to this bandwidth significantly increased the SNR in phantom images (Fig. 3) while maintaining the resolution.Fig. 2 - Spectrograms of pulse-echo signals obtained using linear chirps with bandwidths between 0-50 MHz (left) and 1.5-15 MHz (right). Two separate reflection events can be observed.Frequency [MHz]0204060Time [ms]2 4 6 8Time [ms]2 4 6 8Spectrogram - 0-50 MHz Spectrogram - 1.5-15 MHzPulse-echoeventsPulse-echoevents0 dB-10-20References[1] Colchester et al, Biomed Opt Express 6(4), 2015[2] Noimark et al, Adv Funct Mater, 2016, to be published[3] Zhang et al, Proc of SPIE Vol. 9323, 2015[4] Alles et al, IEEE Trans Ultrason Ferroelectrics Freq Contr 63(1), 2016This work was supported by ERC Starting Grant 310970 MOPHIM, and by an Innovative Engineering for Health award by the Wellcome Trust [WT101957] & the EPSRC [NS/A000027/1].

Optically Modulated Laser Ultrasound using a Fibre-Based Side-Looking Source

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Optically Modulated Laser Ultrasound using a Fibre-Based Side-Looking Source

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