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.
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.
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.