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.
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.