The OPUS (Optoacoustic plus Ultrasound) system is a combination of a medical ultrasound scanner with a highrepetition
rate, wavelength-tunable laser system and a suitable triggering interface to synchronize the laser and the
ultrasound system. The pulsed laser generates an optoacoustic (OA), or photoacoustic (PA), signal which is detected by
the ultrasound system. Alternatively, imaging in conventional ultrasound mode can be performed. Both imaging modes
can be superimposed. The laser light is coupled into the tissue laterally, parallel to the ultrasound transducer, which does
not require for any major modification to the transducer or the ultrasound beam forming. This was a basic requirement
on the instrument, as the intention of the project was to establish the optoacoustic imaging modality as add-on to a
conventional standard ultrasound instrument. We believe that this approach may foster the introduction of OA imaging
as routine tool in medical diagnosis.
Another key aspect of the project was to exploit the capabilities of OA imaging for quantitative analysis. The intention of
the presented work is to summarize all steps necessary to extract the significant information from the PA raw data, which
are required for the quantification of local absorber distributions. We show results of spatially resolved absorption
measurements in scattering samples and a comparison of four different image reconstruction algorithms, regarding their
influence on lateral resolution as well as on the signal to noise ratio for different sample depths and absorption values.
The reconstruction algorithms are based on Fourier transformation, on a generalized 2D Hough transformation, on
circular back-projection and the classical delay-and-sum approach which is implemented in most ultrasound scanners.
Furthermore, we discuss the influence of a newly developed laser source, combining diode and flash lamp pumping.
Compared to all-flash-lamp pumped systems it features a significantly higher pulse-to-pulse stability, which is required
for sensitive and precise quantitative analyses.
The migration induced by intensive light is termed photophoresis. We could show that the evaluation of light-induced
velocities of microparticles, bacteria and cells suspended in water is valuable for the prediction of their intrinsic
properties. Two different laser setups were evaluated for photophoretic migration, a He-Ne laser (<i>P</i> = 45 mW, λ = 633
nm) and a diode-pumped cw-Nd:YAG (<i>P</i> = 1.1 W, λ = 532 nm). When analyzing the migration behavior of particles, we
find significant differences depending on both, geometrical size and refractive index. We describe migration of PS
particles of different size as well as with different refractive index but same diameter, SiO<sub>2</sub> and melamine resin. The
potential for the separation of biological matter is shown as velocity distributions of heat killed bacteria of Escherichia
coli, Salmonella enteritidis, and baker's yeast is reported.
The OPUS (OPtoacoustic UltraSound) system combines a conventional ultrasound (US) system with a specially
designed OPO (Optical Parametrical Oscillator) laser system to generate and detect optoacoustical (OA) signals at
multiple wavelengths. The intention of this combination was to demonstrate that a conventional ultrasound system can
be transformed into an optoacoustic module without major modifications. To offer operational ease of use similar to
those of the conventional US instrumentation, i.e. slow moving of the US transducer over the examined tissue area, a
high repetition rate of the laser is required. A repetition rate of 100 Hz of the laser system enables a fast image frame
rate. Different approaches for the presentation of the two types of images to the operator are compared. For an optimum
applicability of the system we found it essential to provide both, the well-known US image and the OA image of the
same tissue section to the user. The operator has now the possibility to overlay both images on one screen and thus to
extract the desired information from each imaging mode.
Besides x-ray imaging, sonography is the most common method for breast cancer screening. The intention of our work is to develop optoacoustical imaging as an add-on to a conventional
system. While ultrasound imaging reveals acoustical properties of tissue, optoacoustics generates an image of the distribution of optical absorption. Hence, it can be a valuable addition to sonography, because acoustical properties of different tissues show only a slight variation whereas the optical properties may differ strongly. Additionally, optoacoustics gives
access to physiological parameters, like oxygen saturation of blood.
For the presented work, we combine a conventional ultrasound system to a 100 Hz laser. The
laser system consists of a Nd:YAG-laser at a wavelength of 532 nm with 7 ns pulse duration,
coupled to a tunable Optical Parametric Oscillator (OPO) with a tuning rage from 680 nm to
2500 nm. The tunable laser source allows the selection of wavelengths which compromising
high spectral information content with high skin transmission. The laser pulse is delivered
fiber-optically to the ultrasound transducer and coupled into the acoustical field of view.
Homogeneous illumination is crucial in order to achieve unblurred images. Furthermore the
maximum allowed pulse intensities in accordance with standards for medical equipment have
to be met to achieve a high signal to noise ration. The ultrasound instrument generates the
trigger signal which controls the laser pulsing in order to apply ultrasound instrument's
imaging procedures without major modifications to generate an optoacoustic image. Detection
of the optoacoustic signal as well as of the classical ultrasound signal is carried out by the
standard medical ultrasound transducer.
The characterization of the system, including quantitative measurements, performed on tissue
phantoms, is presented. These phantoms have been specially designed regarding their acoustical as well as their optical properties.
Photoacoustic imaging is a promising new way to generate unprecedented contrast in ultrasound diagnostic
imaging. It differs from other medical imaging approaches, in that it provides spatially resolved information about
optical absorption of targeted tissue structures. Because the data acquisition process deviates from standard
clinical ultrasound, choice of the proper image reconstruction method is crucial for successful application of
the technique. In the literature, multiple approaches have been advocated, and the purpose of this paper is
to compare four reconstruction techniques. Thereby, we focused on resolution limits, stability, reconstruction
speed, and SNR.
We generated experimental and simulated data and reconstructed images of the pressure distribution using
four different methods: delay-and-sum (DnS), circular backprojection (CBP), generalized 2D Hough transform
(HTA), and Fourier transform (FTA). All methods were able to depict the point sources properly. DnS and CBP
produce blurred images containing typical superposition artifacts. The HTA provides excellent SNR and allows
a good point source separation. The FTA is the fastest and shows the best FWHM.
In our study, we found the FTA to show the best overall performance. It allows a very fast and theoretically
exact reconstruction. Only a hardware-implemented DnS might be faster and enable real-time imaging. A commercial system may also perform several methods to fully utilize the new contrast mechanism and guarantee optimal resolution and fidelity.
We present design and comprehensive characterization of a versatile, small-scale photoacoustic sensor stick. Due to its optimized forward-looking directional characteristic, it is a valuable tool for spatially resolved PA depth scanning and 3D imaging. The pencil-formed, optical fiber-coupled sensor has a diameter of only 6 mm, with a length of 15 cm. For characterization of its fundamental parameters, we applied a pulsed frequency-doubled Nd:YAG laser (532 nm) with a pulse repetition rate of 10 Hz. Different designs of the sensor tip are compared. We present a full characterization of the qualities of the system as imaging tool, i.e. lateral and depth resolution in dependence on light absorption and scattering properties of the samples as well as of the surrounding matrix. Specially tailored phantoms are introduced for these experiments. The phantoms in combination with a xy-scanning stage are applied to produce 2D and 3D images with the sensor. The imaging properties of the endoscope are explored by several methods of characterization. We test the sensitivity to absorbing structures of different size and absorptivity, which can be summarized as contrast. Finally, we present first tomographic images of tissue phantoms resembling the optical properties of human tissue.
Laser-induced plasma spectroscopy (LIPS) was used to study the nature and abundance of heavy metal hydrocolloids with particle diameters between 0.1 micrometer and 1 micrometer in aquifer systems. A miniaturized ultrafiltration system with a 0.1 micrometer membrane filter was employed for on site analysis. For representative heavy metal colloids absolute limits of detection in the ng-range were achieved with good reproducibility. The device is suitable for subsurface sampling under flow conditions, thus minimizing sampling artifacts.