Timothy N. Mills

Extrinsic optical-fiber ultrasound sensor using a thin polymer film as a low-finesse Fabry-Perot interferometer

Theoretical and experimental aspects of an extrinsic optical-fiber ultrasound sensor are described. The sensor is based on a thin transparent polymer film acting as a low-finesse Fabry-Perot cavity that is mounted at the end of a multimode optical fiber. Performance was found to be comparable with that of a piezoelectric polyvinylidene dinuoride-membrane (PVDP) hydrophone with a sensitivity of 61 mV/MPa, an acoustic noise floor of 2.3 KPa over a 25-MHz bandwidth, and a frequency response to 25 MHz. The wideband-sensitive response and design flexibility of the concept suggests that it may find application as an alternative to piezoelectric devices for the detection and measurement of ultrasound.

A through-the-scope device for suturing and tissue approximation under EUS control.

BACKGROUND The ability to place sutures under EUS control might allow development of a new type of transluminal endosurgery. The aim of this study was to develop endoscopic methods for suturing to variable predetermined depths in the wall of the GI tract and to allow fixation of adjacent hollow organs under EUS control. METHODS A suturing device was constructed for suturing under EUS control to any desired depth. Sutures can also be placed into hollow or solid organs within 5 cm of the endoscope tip. The device allows multiple sutures to be placed without withdrawing the endoscope. Stitching, knot-tying, and thread-cutting are achieved through a 2.8-mm accessory channel. RESULTS Multiple (>100) sutures were placed in predetermined gut wall layers in pigs. Sutures were placed in the gallbladder (n = 7) and small intestine (n = 8) to fix the gallbladder/small intestine to the stomach and allow traction for the insertion of stents and other devices through the 2 lumens. CONCLUSION A new method for stitching under flexible EUS control is described. This technology was used to place sutures at precise depths in the GI tract. It allowed fixation of other organs to the accessible GI tract for various purposes including delivery of stents and devices for creating anastomoses.

Characterization of post mortem arterial tissue using time-resolved photoacoustic spectroscopy at 436, 461 and 532 nm

Time-resolved photoacoustic spectroscopy has been used to characterize post mortem arterial tissue for the purpose of discriminating between normal and atheromatous areas of tissue. Ultrasonic thermoelastic waves were generated in post mortem human aorta by the absorption of nanosecond laser pulses at 436, 461 and 532 nm produced by a frequency doubled Q-switched Nd:YAG laser in conjunction with a gas filled Raman cell. A PVDF membrane hydrophone was used to detect the thermoelastic waves. At 436 nm, differences in the photoacoustic signatures of normal tissue and atherorma were found to be highly variable. At 461 nm, there was a clear and reproducible difference between the photacoustic response of atheroma and normal tissue as a result of increased optical attenuation in atheroma. At 532 nm, the generation of subsurface thermoelastic waves provided a means of determining the structure and thickness of the tissue sample. It is suggested that pulsed photoacoustic spectroscopy at 461 and 532 nm may find application in characterizing arterial tissue in situ by providing information about both the composition and thickness of the vessel wall.

Optical fiber photoacoustic–photothermal probe

We describe the operation of an all-optical probe that provides an alternative means of implementing photoacoustic and photothermal investigative techniques, particularly those used in biomedical applications. The probe is based on a transparent, acoustically and thermally sensitive Fabry-Perot polymer film sensor mounted at the end of an optical fiber. We demonstrate the ability of the system to make photoacoustic and photothermal measurements simultaneously and evaluate its photothermal response, using a nonscattering liquid target of known and adjustable absorption coefficient. The acoustic and thermal noise floors were 2 kPa and 6 x 10(-3) degrees C , respectively, obtained over a 25-MHz measurement bandwidth and 30 signal averages.

Monte Carlo simulations of coherent backscatter for identification of the optical coefficients of biological tissues in vivo.

A Monte Carlo model of light backscattered from turbid media has been used to simulate the effects of weak localization in biological tissues. A validation technique is used that implies that for the scatteringand absorption coefficients and for refractive index mismatches found in tissues, the Monte Carlo method is likely to provide more accurate results than the methods previously used. The model also has theability to simulate the effects of various illumination profiles and other laboratory-imposed conditions. A curve-fitting routine has been developed that might be used to extract the optical coefficients from theangular intensity profiles seen in experiments on turbid biological tissues, data that could be obtained in vivo.

Interstitial laser photocoagulation: Evaluation of a 1320 nm Nd-YAG and an 805 nm diode laser: the significance of charring and the value of precharring the fibre tip

Interstitial laser photocoagulation (ILP) is a new percutaneous technique of thermal destruction (necrosis) of deep-seated tumours, using low power laser energy. Our purpose was to investigate: (i) the effects of different laser wavelengths on the extent of thermal damage produced; and (ii) the role of charring around the fibre tip during ILP. Forty-five normal Wistar rats (250–300 g) had ILP to their liver (exposed at laparotomy) by inserting a 400 μm optical fibre into the liver, and activating the laser at 1, 2 or 3W. This was performed at three laser wavelengths (1064 nm Nd-YAG, 1320 nm Nd-YAG, 805 nm diode) using a clean plane-cleaved fibre, and at two wavelengths (1064 nm and 1320 nm Nd-YAG) using a fibre with pre-charring at its tip. The 805 nm and 1320 nm laser wavelengths produced significantly greater necrosis than the 1064 nm, using a clean fibre tip (mean diameters at 2 W were 21.7 mm, 18.3 mm, 8 mm respectively). Pre-charring the fibre significantly increased the necrotic lesion size at 1064 nm (mean diameter at 2 W was 14.7 mm). Using more strongly absorbed wavelengths (805 nm and 1320 nm) and pre-charring the fibre tip give greater thermal damage during ILP, contrary to previously held views that the optimal wavelength for ILP was 1064 nm in the absence of charring.

An optical detection system for biomedical photoacoustic imaging

An all-optical system for the detection of photoacoustic transients is under development for photoacoustic imaging applications. The sensing mechanism is based upon the detection of acoustically-induced variations in the optical thickness of a Fabry-Perot polymer film interferometer and provides an alternative to piezoelectric based detection methods. A key advantage is that the sensing geometry is defined by the area of the polymer sensing film that is optically addressed. This offers the prospect of obtaining sufficiently small element sizes and interelement spacing to e ply the synthetic focusing techniques of phased arrays for image reconstruction. The optical nature of detection also allows for a transparent sensor head through which the excitation laser pulses can be transmitted for backward-mode photoacoustic imaging. Preliminary work has shown that the detection sensitivity and bandwidth are comparable to wideband piezoelectric PVDF ultrasound transducers with the prospect of achieving substantially smaller element sizes.

Electrophysical factors influencing endoscopic sphincterotomy

BACKGROUND Analog computer techniques were used to measure electrosurgical power during sphincterotomy in experimental models and patients. METHODS Total energy and transient changes in power were measured during sphincterotomy of bile ducts in the livers of pigs, ampullae of humans post mortem, and during clinical sphincterotomy. The effect of waveform on hemostasis was studied in experiments on canine mesenteric arteries. RESULTS Electrosurgical waveforms (CUT, COAG, BLEND) were measured. Halving wire contact length halved energy needed to initiate cutting. The CUT waveform rarely initiated cutting at lower power settings than the BLEND waveform. With CUT, BLEND, and COAG waveforms, approximately the same energy initiated cutting. Efficiency of cutting increased linearly with power. The COAG waveform required higher power settings than BLEND or CUT to initiate cutting (p < 0.05). Force and wire diameter influenced cutting. BLEND was more effectively hemostatic than CUT (p < 0.05). COAG was significantly more hemostatic than BLEND and CUT. Cutting efficiency during clinical sphincterotomy was poor. CONCLUSIONS This work has practical implications. Shortening wire contact length was effective in starting a cut at suboptimal settings, whereas changing from BLEND to CUT made little difference. Increasing power setting may help if cutting does not start. BLEND stops bleeding better than CUT. COAG stops bleeding better than BLEND but cuts poorly. Cutting during clinical sphincterotomy is inefficient and can be improved.

Comparison of a miniature, ultrasonic, optical fibre hydrophone with PVDF hydrophone technology

A miniature optical fibre hydrophone has been developed for the measurement of ultrasound in the range 1-30 MHz. The acoustically sensitive element comprises a 23 /spl mu/m thick polymer film mounted at the end of an optical fibre. When illuminated by laser light launched into the fibre, the polymer film acts as a Fabry Perot interferometer. An incident acoustic wave modulates the optical thickness of the interferometer thereby producing a corresponding intensity modulation in the light reflected from the film. The system was characterised in terms of sensitivity, frequency response and directivity using a broadband (1-30 MHz) ultrasonic field produced by nonlinear propagation obtained by driving a 1 MHz PZT source with a high amplitude 1 MHz toneburst. PVDF needle and membrane reference hydrophones were used as comparisons. The minimum detectable acoustic pressure of the optical fibre hydrophone was found to be 10 kPa in a 25 MHz measurement bandwidth with a wideband response to 30 MHz. The -3dB beamwidth at 10 MHz was 60/spl deg/. Such performance is comparable to that achieved with PVDF hydrophone technology, with additional advantages of immunity to EMI, small physical size, a flexible probe-type configuration, robustness and potentially low cost. Among the applications that might benefit from these advantages are single-use applications such as the measurement of industrial CW fields in hostile environments and in vivo measurements of medical ultrasound exposure.

Comparison of the photothermal sensitivity of an interferometric optical fiber probe with pulsed photothermal radiometry

An interferometric optical fiber probe for making photothermal measurements of tissue optical and thermal properties is compared to pulsed photothermal radiometry in terms of its overall thermal sensitivity, linearity, and response time. The principles of operation of the probe are described and its performance as a low finesse Fabry–Perot interferometer is discussed. A probe with a 12 μm sensing film is characterized by a thermal noise floor of 50 mK and a response time of 850 μs. The sensitivities to the optical and thermal coefficients of the two techniques have been analyzed. As a result of the different source geometries, the optical fiber probe was found to be more sensitive to the thermal coefficients of tissue than the optical coefficients while pulsed photothermal radiometry provided maximum sensitivity to the optical coefficients.