Heinz Schmidt-Kloiber

Measurement of laser-induced acoustic waves with a calibrated optical transducer

Irradiation of an absorbing material with a short laser pulse generates a thermoelastic stress wave, from which the distribution of absorbed energy can be derived. This method is ideal to measure the light penetration in biological tissue. Especially for in vivo applications, we developed an optical stress transducer that can be positioned directly in front of the irradiated surface, inside the laser beam, in order to avoid distortion of the stress wave due to acoustic diffraction. The detector is based on stress wave-induced changes of optical reflectance of a glass-water interface, probed with a continuous laser beam that is incident at an angle close to the critical angle of total internal reflection to achieve maximum sensitivity. In this study, we describe the theory for the calibration of the transducer and compare the measured with the theoretically predicted signals. In the experiments, an aqueous dye solution is irradiated with pulses from either a Q-switched, frequency-doubled Nd:yttrium aluminu...

Time‐resolved investigations of laser‐induced shock waves in water by use of polyvinylidenefluoride hydrophones

Laser light from a Q‐switched Nd:yttrium‐aluminum‐garnet laser (λ=1064 nm; pulse duration=20 ns; pulse energies up to 150 mJ) focused into water creates shock waves by rapidly expanding microplasmas. Using piezoelectric, thin‐film polyvinylidenefluoride (PVDF) as a transducer, a broadband hydrophone (100‐MHz bandwidth) was developed to investigate underwater shock waves. The electrical signal is analyzed with respect to reflections of the shock wave within the transducer and the input impedance of the measuring device. The shock waveform is determined, its peak pressure ranging to kbars (108 Pa), decreasing with r−1.12 and increases by the square root of the laser pulse energy. The time resolution of the hydrophone (4 ns) is sufficient to determine the plasma dimensions and the number of shock waves generated by a single laserpulse. Both vary statistically, primarily because of contaminations in the fluid. Because of the length of the region containing plasmas, different peak pressures are found in the di...

LIGHT DISTRIBUTION MEASUREMENTS IN ABSORBING MATERIALS BY OPTICAL DETECTION OF LASER-INDUCED STRESS WAVES

A method for optimized generation and detection of thermoelastic stress waves for the measurement of tissue optical properties and structure is investigated. The stress waves are formed by short pulsed irradiation of an absorbing dye solution with a Q‐switched Nd:YAG laser at 532 nm. An optical transducer based on pressure‐induced reflectivity changes of a continuous laser beam at a glass‐water interface detects the stress wave in front of the irradiated sample surface. It is shown theoretically and experimentally that this kind of detector, where the active area is a small spot close to the irradiated surface, minimizes signal distortion due to acoustic diffraction. Comparisons of absorption coefficients measured acoustically and from optical transmission show a good agreement between the two methods. The high sensitivity of the detector (1.5 mV/bar) makes it possible to keep the temperature and pressure rise in the investigated target low, which enables in vivo applications of the optical transducer.

Optical method for two-dimensional ultrasonic detection

Two-dimensional detection of ultrasonic waves is based on pressure-induced changes of optical reflectance at a glass–liquid interface, imaged with a time-gated video camera. The method is used to record optoacoustic waves generated after irradiation of optically absorbing targets with 6 ns long laser pulses. Measurements of absolute pressure values with high temporal and spatial resolution (in the range of 10 ns and 10 μm, respectively) is demonstrated. The sensitivity is varied between 0.19% and 0.81% gray level modulation per bar. The detector plane is optically transparent, making it possible to irradiate the sample through the detector without disturbing the acoustic measurement. Two-dimensional recording of ultrasonic waves is ideally suited for the analysis of acoustic emission from small sources and for optoacoustic imaging of optical absorption differences in an opaque material.

Photoacoustic waves excited in liquids by fiber-transmitted laser pulses

The acoustic wave field generated in front of a submerged fiber tip by short laser pulses is theoretically and experimentally studied by fast imaging and optical pressure measurements. It is shown that the finite size of the fiber causes strong tensile stress leading to cavitation. Depending on the absorption coefficient of the laser radiation, cavitation-induced bubble formation occurs inside (low absorption) or outside (high absorption) the volume of heat deposition. The results are used to characterize the cavitation bubble formation mechanisms and to predict possible consequences for applications of fiber-guided short-pulsed laser sources in medicine.

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.

Pulsed optoacoustic characterization of layered media

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6–8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelectric film was used. Generating the optoacousti...

First Clinical Experience with a Q-Switched Neodymium: YAG Laser for Urinary Calculi

Animal studies using a high intensity nanosecond pulsed neodymium:YAG laser did not reveal any serious tissue damage. Following these investigations patient treatment was begun in June 1987. Laser energy of a neodymium:YAG laser with an 8 nsec. pulse duration and a repetition rate of up to 50 Hz. was coupled into a flexible 600 resp. 400 micron. quartz fiber. Laser-induced breakdown was created with 35 to 50 mJ. at the fiber tip, resulting in a shock wave that disintegrated the calculus into tiny fragments. A total of 56 patients with 58 calculi (54 ureteral and 4 kidney stones) was treated from June 1987 to March 1988. Of the calculi 48 could be fragmented completely, while 6 others were reduced to a size small enough to be removed with forceps. Four stones composed of calcium oxalate monohydrate could not be disintegrated. The combination of laser stone disintegration with flexible ureterorenoscopy implies the possibility of an atraumatic, 1-step procedure for fragmentation of ureteral and kidney calculi.

Optoacoustic infrared spectroscopy of soft tissue

Optical properties of soft tissue in the near infrared are determined using optoacoustic spectroscopy. The acoustic signals are generated with an optical parametric oscillator (OPO) having a tuning range from 1500 to 3500 nm. In order to record the acoustic wave on the same side as the exciting laser pulse (backward mode), an infrared transparent pressure transducer was developed. The effective attenuation coefficients of cartilage and chicken breast were determined in a range between 1860 and 1940 nm. The minimum absorption or effective attenuation coefficient that could be measured with the presented method was 10 cm−1, limited by the detector sensitivity of 1.68% signal change per bar. The maximum measurable coefficient was about 1000 cm−1, limited by temporal broadening of the acoustic signals due to the finite pulse duration (6 ns) of the OPO. Optoacoustic infrared spectroscopy is shown to be suitable for on-line noninvasive, in vivo tissue characterization.

Study of different ablation models by use of high-speed-sampling photography

In our study we investigated the ablation characteristics of an aqueous dye solution with a defined absorption coefficient, irradiated by short (8 ns) and long (100 microsecond(s) ) pulses from a Nd:YAG laser (wavelength: 1064 nm). The experimental technique was schlieren photography with a second Nd:YAG laser at 532 nm as a light source and with a variable delay between the two laser pulses. With a special arrangement of the laser beams and the sample effects below and above the surface of the liquid could be simultaneously observed. We could distinguish three ablation mechanisms, depending on the pulse duration and the incident fluence. With short pulses and a fluence below the vaporization threshold the tensile pulse from the bipolar thermoelastic wave, propagating from the liquid-air interface into the sample, caused rupture and spallation of the liquid. At fluences generating a surface temperature in excess of 100 degree(s)C the short pulses caused explosive vaporization, characterized by shock wave emission both in air and in liquid. At the same fluence the long pulses caused slow vaporization, meaning that vapor and liquid ejection started during the laser pulse and was less violent than with the 8 ns pulses.