We have developed a clinically compatible, real-time ultrasound needle tracking system (UNT) that can be appended to a clinical ultrasound system, superimposing a crosshair onto the ultrasound image at the needle tip position. The UNT was developed under the ISO 13485 Medical Devices quality standard for deployment in the clinic. During handheld ultrasound guidance, the location of the needle tip within the imaging plane is determined from the acoustic signals received by an embedded fibre-topic hydrophone. Assessment of tracking accuracy found that the mean distance between tracked and true positions was 0.7 ± 0.4 mm with a repeatability of 0.3 ± 0.2 mm.
Many percutaneous needle-based procedures such as foetal interventions, tumor biopsies, nerve blocks, and central venous catheterizations are guided by ultrasound (US) imaging to identify the procedural target and to visualize the needle. A key challenge associated with ultrasound-guided needle insertions is accurate and efficient identification of the needle tip, as thin needles can readily stray from the imaging plane and can have poor visibility at large insertion angles. Ultrasonic tracking is a method for localising the needle tip relative to the imaging plane in real-time, using an ultrasonic transmitter or receiver integrated into the needle that is in communication with an external ultrasound imaging probe. This study had two foci. The first was to increase the sensitivity with which ultrasound reception was performed, using a custom fiber optic hydrophone with a high-finesse Fabry-Pérot cavity based ultrasound sensor. This sensor, which comprised of a polymer layer sandwiched between dielectric mirrors, was interrogated continuously during insertions into tissue. The second focus of the study was to develop a custom needle stylet into which the fiber optic hydrophone was integrated, which was fully compatible with clinical practice and which could be adapted to different needles. We tested the sensitivity of the sensorized stylet across a wide range of needle angulations, depths and insertion angles in different biological tissues. We demonstrated, for the first time, needle tip localization in ex-vivo tissues at depths beyond 6 cm and insertion angles steeper than 80°. We conclude that ultrasonic tracking with high-finesse Fabry-Pérot fiber optic hydrophone is very promising for use in clinical practice.
Intravascular (IV) imaging in percutaneous coronary interventions can be invaluable to treat coronary artery disease, to facilitate decision making and to guide stent placement. Intravascular ultrasound (IVUS) and optical coherence tomography (OCT) are both established IV imaging modalities. However, achieving contrast for specific structures such as lipid plaques can be challenging; with OCT, visualisation is typically limited to tissue depths less than 2 mm. Photoacoustic (PA) imaging provides contrast that is complementary to those of IVUS and OCT, and with previous demonstrations, visualisation of lipid plaques at depths greater than 4 mm has been achieved. In this study, we developed an intravascular PA probe that comprises a commercial OCT catheter and a high sensitivity miniature fibre optic ultrasound sensor with a Fabry-Pérot cavity. This probe, which can provide both PA imaging and OCT, had a maximum width of 1.2 mm. The PA excitation sources included both pulsed and modulated lasers at different wavelengths. The omni-directionality of the US sensor allowed for three-dimensional PA images. The PA-OCT probe was characterised using a series of resolution phantoms, including fine carbon fibres. It was found that with PA imaging, the probe can provide a lateral resolution better than 25 µm and an axial resolution better than 100 µm at the optical focus. Co-registered PA and OCT images of blood vessels ex-vivo with stents and lipid injections were acquired. We conclude that PA imaging with OCT catheters is viable and that it has strong potential to guide clinical interventions.
A miniature (175 μm) all-optical photoacoustic probe has been developed for minimally invasive sensing and imaging applications. The probe comprises a single optical fibre which delivers the excitation light and a broadband 50 MHz Fabry-Pérot (F-P) ultrasound sensor at the distal end for detecting the photoacoustic waves. A graded index lens proximal to the F-P sensor is used to reduce beam walk-off and thus increase sensitivity as well as confine the excitation beam in order to increase lateral spatial resolution. The probe was evaluated in non-scattering media and found to provide lateral and axial resolutions of < 100 μm and < 150 μm respectively for distances up to 1 cm from the tip of the probe. The ability of the probe to detect a blood vessel mimicking phantom at distances up to 7 mm from the tip was demonstrated in order to illustrate its potential suitability for needle guidance applications.
There is considerable interest in the development of photoacoustic endoscopy probes (PAE) for applications in foetal medicine, interventional surgery and gastroenterology. However, most previous PAE probes employ a combination of mechanical scanning and piezoelectric transducers at the distal end which can be technically complex and pose challenges in achieving the required level of miniaturisation and acoustic performance. To overcome these limitations, we present two novel all-optical forward-viewing endoscopic probes that use coherent fibre bundles to address a Fabry-Perot polymer film ultrasound sensor.
A feasibility study on using polymer fiber Bragg sensors (PFBG) for providing force feedback to minimally invasive surgical devices is carried out. For this purpose a 3 mm long PFBG is fabricated and characterized for strain and temperature sensitivities. The PFBG sensor is then integrated onto a commercial laparoscopic clip applicator which is used as a proof of concept device. The force characterization of the clip applicator is carried out, with a replica setup which simulates the clip forming process of the device. An original clip is then formed without and with synthetic tissue samples of different hardness. The replica device force profile and original clip forming force profile follows the same pattern and thus the calibration data can be used to calculate the original force exerting on the tissues which can help in optimizing the clip formation process or can be used for providing force feedback capability to the device.
KEYWORDS: Liquid crystals, Microsoft Foundation Class Library, Polymers, Temperature metrology, Sensors, Optical fibers, Telecommunications, Overlay metrology, Tunable filters, Ultraviolet radiation
The paper presents the results of experimental studies of the temperature dependence of a microfibre coupler (MFC) with a waist diameter of ~4 μm covered with a layer of liquid crystal (LC) material. The microfiber coupler is fabricated by fusing together and tapering of two standard telecom fibers using a microheater brushing technique, followed by partially embedding the structure in a low-refractive index UV curable polymer (Efiron PC-363) for stability and later by placing a thin heated LC layer over the polymer-free uniform taper waist region. The temperature dependence of the embedded in polymer MFC sensor before the application of the LC layer demonstrates a redshift of the coupler’s spectrum with an average sensitivity of ~0.5 nm/°C in the temperature range of 14-70 °C. The application of the LC overlay increases the average temperature sensitivity to ~0.7 nm/°C. The demonstrated device offers several advantages such as ease of fabrication and light coupling, the potential for better stability and the possibility of electric field tuning for realizing temperature, electric field, bio-, chemical sensors and tunable add-drop filters for fiber communication systems. Further work is ongoing to explore various tuning mechanisms of the MFC spectrum.
KEYWORDS: Liquid crystals, Microsoft Foundation Class Library, Temperature metrology, Optical fibers, Fabrication, Silica, Waveguides, Liquids, Motion controllers, Control systems
This paper demonstrates temperature-induced tuning of the optical spectrum of a microfiber coupler covered with a low-refractive
index liquid crystal (LC) layer. The microfiber coupler with a minimum waist diameter of ~6 μm is fabricated
by fusing together and tapering of two standard telecom fibers using a microheater brushing technique and by placing a
thin heated LC layer over the uniform taper waist region. Repeatable and reversible tuning of the optical spectrum
(~4 nm) of the microfiber coupler is demonstrated for the temperature range of 50-78 °C.
The preliminary results of the characterization of nematic liquid crystal coated photonic crystal fiber (PCF)
interferometers are presented in this paper. Two types of interferometer were fabricated; one by sandwiching a small
section of PCF between standard single mode fibers and the second one is formed by tapering a small section of a PCF
by collapsing the air holes, and thinning down the air hole collapsed region to a micron size. The interferometers are
fabricated from different types of the photonic crystal fiber and temperature studies are carried out to select an
interferometer to be coated with the liquid crystal material. The requirement for a suitable liquid crystal materials to be
used for PCF interferometers are also discussed. The behaviour of liquid crystal (LC) coated interferometers with
temperature and applied electric field is studied to determine the feasibility of using such interferometers in sensing
applications.
The usage of Nematic Liquid Crystal (NLC) infiltrated Photonic Crystal Fibers (PCFs) for a low cost, all-fiber, high
voltage sensing device is experimentally demonstrated. The infiltrated PCF which transmit light based on the photonic
bandgap mechanism is employed in an intensity based all-fiber voltage measurement scheme, by applying the voltage
perpendicular to the fiber axis. The output transmittance of the PCF varies linearly with increasing voltage. The PCF
voltage sensor is capable of measuring high voltages in the range from 100 V to 850 V. The voltage sensing device is
compact and exhibits accurate voltage measurement owing to its intrinsic nature with the NLC material infiltrated within
the air-holes of the PCF microstructure.
A discretely tunable Surface-Stabilized Ferroelectric Liquid Crystal based Lyot Filter, with tuning speeds in the order of
microseconds, is demonstrated experimentally as a channel dropper for the demodulation of multiple Fibre Bragg
Grating sensors. The 3-stage Lyot Filter designed and experimentally verified can be used together with the high-speed
ratiometric wavelength measurement system employing a fibre bend loss edge filter. Such systems can be used for the
demodulation of distributed Fibre Bragg Grating sensors employed in applications such as structural monitoring,
industrial sensing and haptic telerobotic surgical systems.
The electronic tunability of ferroelectric liquid crystal filled photonic crystal fibers is experimentally demonstrated in the
wavelength range of 1500 nm - 1600 nm. The tunability is achieved by applying electric field onto the ferroelectric
liquid crystal infiltrated photonic crystal fiber. Tuning of the fiber propagation properties is achieved due to re-orientation
of ferroelectric liquid crystal molecules on the application of the applied electric field. Such fibers could find
applications in the fabrication of fast, low loss, cost effective and highly efficient in-fiber tunable devices to be used in
the telecom wavelength range.
A ratiometric wavelength measurement system based on a fiber bend loss edge filter has been proposed and
demonstrated previously. Such a system offers the advantage of a high resolution, simplicity and a high measurement
speed. The applications of such a system for the demodulation of the outputs of multiple Fiber Bragg grating (FBG)
sensors requires the use of a high speed tunable filter to separate responses from multiple sensors. Here we present the
results of modelling and analysis for a discretely tunable ferroelectric liquid crystal filter, which could be used as a
channel dropper in a WDM-based demodulation system containing multiple FBG sensors.
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