Piezoelectric annular array for large depth of field photoacoustic imaging.

A piezoelectric detection system consisting of an annular array is investigated for large depth of field photoacoustic imaging. In comparison to a single ring detection system, X-shaped imaging artifacts are suppressed. Sensitivity and image resolution studies are performed in simulations and in experiments and compared to a simulated spherical detector. In experiment an eight ring detection systems offers an extended depth of field over a range of 16 mm with almost constant lateral resolution.

Scanning acoustic-photoacoustic microscopy using axicon transducers.

A dual mode scanning acoustic microscope is investigated, yielding simultaneously images with optical and acoustical contrast. Short laser pulses are used to excite acoustic waves in a sample for the photoacoustic imaging mode. At the same time the pulses irradiate a conical target generating limited diffraction acoustic waves (X-waves) for large depth of field ultrasound imaging. For photoacoustic as well as for ultrasound imaging a focusing, ring shaped detector is applied. First phantom experiments demonstrate the possibility to acquire data for both imaging modes in a single scan, by separating images due to their different time of flight.

Weight factors for limited angle photoacoustic tomography.

Photoacoustic tomography (PAT) is based on the generation of ultrasound waves by heating an object with short light pulses. A three-dimensional image of the distribution of absorbed energy within the object is reconstructed from signals measured around the object with either point-like or extended, linear sensors. Limited angle artefacts arise when the curve or surface connecting neighbouring detectors is not closed around the object. For this case, there exists a 'detection region' in which all boundaries of an object are visible in the reconstruction. All straight lines passing through each point in this region intersect the detection curve or surface at least once. Although for these points an accurate reconstruction is possible, direct back projection leads to artefacts when some of the straight lines intersect the detection surface twice and others just once. In this work, special weight functions for direct, non-iterative back projection are presented that reduce these kinds of artefacts. A clear improvement in image quality is shown in simulations for three-dimensional (3D) imaging with point detectors and for two-dimensional (2D) imaging using line detectors compared to reconstruction without weight factors. For the 2D case also an experiment is shown. The presented weight factors make commonly used back projection formulae suitable for a more accurate reconstruction of the initial pressure distribution in cases where the detection aperture only covers a limited angle, and the region of interest lies within the detection region.

Comparison of surface plasmon resonance devices for acoustic wave detection in liquid.

Surface plasmon resonance (SPR) sensors represent a suitable method for broadband acoustic pulse detection. The reflectivity and phase of a p-polarized laser beam incident on an optical device under SPR conditions are strongly dependent on ambient conditions that are changed by an acoustic wave. Depending on the order of layers, SPR sensors can be arranged in the Kretschmann or in the Otto configuration acting as a pressure or as a displacement sensor. The aim of this study was to compare both configurations and to find linear and sensitive conditions for the application. Numerical calculations were carried out varying the layer dimensions and the angle of incidence. The results of the experimental investigation on both configurations confirm the working principle.

Iterative reconstruction algorithm for optoacoustic imaging.

Optoacoustic imaging is based on the generation of thermoelastic stress waves by heating an object in an optically heterogeneous medium with a short laser pulse. The stress waves contain information about the distribution of structures with preferential optical absorption. Detection of the waves with an array of broadband ultrasound detectors at the surface of the medium and applying a backprojection algorithm is used to create a map of absorbed energy inside the medium. With conventional reconstruction methods a large number of detector elements and filtering of the signals are necessary to reduce backprojection artifacts. As an alternative this study proposes an iterative procedure. The algorithm is designed to minimize the error between measured signals and signals calculated from the reconstructed image. In experiments using broadband optical ultrasound detectors and in simulations the algorithm was used to obtain three-dimensional images of multiple optoacoustic sources. With signals from a planar array of 3x3 detector elements a significant improvement was observed after about 10 iterations compared to the simple radial backprojection. Compared to conventional methods using filtered backprojection, the iterative method is computationally more intensive but requires less time and instrumentation for signal acquisition.

Optoacoustic tomography: time-gated measurement of pressure distributions and image reconstruction.

Optoacoustic imaging is a potential novel medical imaging technology to image structures in turbid media to depths of several millimeters with a resolution of some tens of micrometers. Thereby short laser pulses generate thermoelastic pressure waves inside a tissue, which are detected on the surface with a wideband ultrasonic transducer. Image reconstruction has the goal of calculating the distribution of the absorbing structures in the tissue. We present a method in which the acoustic field distribution is captured as a two-dimensional snapshot at the sample surface, using an optical-reflectance-based detection principle with a detection resolution of 20 mum. A new image reconstruction is accomplished by backprojection of the detected two-dimensional pressure distributions into the sample volume by use of the delay between the laser pulse and the time the snapshot was taken. Two-dimensional pressure-wave distribution and image reconstruction are demonstrated by simulations and experiments, in which small objects are irradiated with laser pulses of 6-ns duration. The method opens the possibility to irradiate the sample hidden in a light-scattering medium directly through the detector plane, thus enabling front-surface detection of the optoacoustic signals, which is especially important if structures close to the tissue surface have to be imaged. Reconstructed tomography images with a depth resolution of 20 mum and a lateral resolution of 200 mum are presented.

Stealth ryanodine-sensitive Ca2+ release contributes to activity of capacitative Ca2+ entry and nitric oxide synthase in bovine endothelial cells.

1. The involvement of ryanodine-sensitive Ca2+ release (RsCR) in bradykinin (Bk)-induced Ca2+ release, capacitative Ca2+ entry (CCE) and nitric oxide synthase (NOS) activation was assessed in freshly isolated bovine coronary artery endothelial cells. 2. Using deconvolution microscopy fura-2 was found throughout the whole cytosol, while the cell membrane impermeable dye FFP-18 was exclusively in the cell membrane. Thus, perinuclear ([Ca2+]pn) and subplasmalemmal Ca2+ concentration ([Ca2+]sp) were monitored using fura-2 and FFP-18. 3. Inhibition of Na+-Ca2+ exchange by lowering extracellular Na+ concentration augmented the Bk-induced [Ca2+]pn signal in Ca2+-free solution. This effect was abolished when RsCR was prevented with 25 micromol l-1 ryanodine, while inhibition of RsCR had no effect on Bk-induced increase in [Ca2+]pn without inhibition of Na+-Ca2+ exchange. 4. Initiating RsCR by 200 nmol l-1 ryanodine increased [Ca2+]sp, while [Ca2+]pn remained constant. However, when Na+-Ca2+ exchange was prevented, ryanodine was also able to elevate [Ca2+]pn. 5. Blockage of RsCR diminished Ca2+ extrusion in response to stimulation with Bk in normal Na+-containing solution. 6. Inhibition of RsCR blunted Bk-activated CCE, while inhibition of Na+-Ca2+ exchange during stimulation enhanced CCE. 7. Although direct activation of RsCR failed to activate NOS, inhibition of RsCR diminished the effect of ATP and Bk on NOS, while the effect of thapsigargin remained unchanged. 8. These data suggest that during stimulation subplasmalemmal RsCR occurs, which contributes to the activities of CCE and NOS. Thus, the function of the subplasmalemmal Ca2+ control unit must be extended as a regulator for CCE and NOS.

Model study to investigate the contribution of spallation to pulsed laser ablation of tissue.

BACKGROUND AND OBJECTIVE: Absorption of a short laser pulse produces high thermoelastic stress in the irradiated volume. The relaxation of this stress at a free (tissue-air) surface leads to tensile loading, resulting in mechanical spallation. Using model substances, we investigated the role of this effect in tissue ablation. STUDY DESIGN/MATERIALS AND METHODS: Stained water and gelatine were irradiated with short pulses (8 ns duration) from a Nd:YAG laser at 1,064 nm wavelength. The dynamics of the induced effects were observed with laser-flash photography and stress wave detection. RESULTS: Spallation is indicated by the formation of cavitation bubbles below the irradiated surface and is strongly influenced by impurities serving as nucleation sites. Material ejection due to spallation was observed in the liquid sample at a fluence leading to a temperature below the boiling point but needed a temperature in excess of 100 degrees C in gelatine, owing to the small mechanical energy available for this process, estimated to be < 1%. CONCLUSION: The mechanical action of thermoelastic stress waves is characterized by high stress amplitudes but low energetic efficiency. A model combining spallation and vaporization is therefore proposed for efficient tissue ablation.

[Demonstration of biophysical effects of pulsed laser irradiation with histological tissue sections]. / Darstellung biophysikalischer Effekte gepulster Laserstrahlung anhand histologischer Gewebeschnitte.

For therapeutic application of laser light it is necessary to minimize defects in the nonirradiated tissue. These defects depend on the primary mechanism of interaction between tissue and laser light. Three experiments were performed to distinguish between mechanical and thermal effects of nano- and microsecond laser pulses in skeletal muscle of the rat. The light, transmission and scanning electron microscopes were In the ns-experiments the mechanical action of a single ns pulse (8 ns) produced a crater. Only zones I and IV developed. With 50 to 100 pulses all zones can be identified. These results show that a single ns pulse suffices to form a tissue crater by mechanical action. A higher number of ns pulses leads to heat accumulation and produces thermal lesions similar to those seen after application of microseconds-pulses.

A special irrigation liquid to increase the reliability of laser-induced shockwave lithotripsy.

For the laser-induced shockwave lithotripsy (LISL) the laser-pulses of a Q-switched Nd:YAG laser produce an optical breakdown in the irrigation liquid surrounding the urinary stone. Subsequently high-pressure shockwaves are emitted causing stone fragmentation. Since the LISL is an endoscopic technique, problems arise from the transmission of the laser pulses through optical fibers. The intensity threshold for an optical breakdown in commonly used saline solution amounts to 21 GW/cm2, in optical silica fibers, to about 3 GW/cm2. Therefore bare fibers cannot be used without being destroyed by a breakdown. So we have developed an irrigation liquid by adding small quantities of metal ions to saline solution to lower the threshold intensity. The most suitable ion was Fe3+ in a concentration of 0.02 mmol/l, which shows a lowering to 5 GW/cm2. In combination with a spherically shaped fiber exit the intensities that have to be transmitted are below the threshold of the fiber material. Using this irrigation liquid the overall reliability of the method could be significantly increased and several stone fragmentations can be performed with a single optical fiber.