Gerhild Wurzinger

Simultaneous three-dimensional photoacoustic and laser-ultrasound tomography

A tomographic setup that provides the co-registration of photoacoustic (PA) and ultrasound (US) images is presented. For pulse-echo US-tomography laser-induced broadband plane ultrasonic waves are produced by illuminating an optically absorbing target with a short near-infrared laser pulse. Part of the same pulse is frequency doubled and used for the generation of PA waves within the object of interest. The laser-generated plane waves are scattered at the imaging object and measured with the same interferometric detector that also acquires the photoacoustic signals. After collection and separation of the data image reconstruction is done using back-projection resulting in three-dimensional, co-registered PA and US images. The setup is characterized and the resolution in PA and US mode is estimated to be about 85 µm and 40 µm, respectively. Besides measurements on phantoms the performance is also tested on a biological sample.

Hybrid photoacoustic and ultrasound section imaging with optical ultrasound detection.

A setup is proposed that provides perfectly co-registered photoacoustic (PA) and ultrasound (US) section images. Photoacoustic and ultrasound backscatter signals are generated by laser pulses coming from the same laser system, the latter by absorption of some of the laser energy on an optically absorbing target near the imaged object. By measuring both signals with the same optical detector, which is focused into the selected section by use of a cylindrical acoustic mirror, the information for both images is acquired simultaneously. Co-registered PA and US images are obtained after applying the inverse Radon transform to the data, which are gathered while rotating the object relative to the detector. Phantom experiments demonstrate a resolution of 1.1 mm between the sections of both imaging modalities and a in-plane resolution of about 60 µm and 120 µm for the US and PA modes, respectively. The complementary contrast mechanisms of the two modalities are shown by images of a zebrafish.

Combined photoacoustic, pulse-echo laser ultrasound, and speed-of-sound imaging using integrating optical detection.

Abstract. A purely optical setup for the coregistration of photoacoustic (PA), ultrasound (US), and speed-of-sound (SOS) section images is presented. It extends a previously developed method for simultaneous PA and laser-US (LUS) pulse-echo imaging with a LUS transmission imaging setup providing two-dimensional (2-D) SOS maps. For transmission imaging, the sound waves traversing the investigated object are generated instantaneously by illuminating optically absorbing targets that are arranged at various distances in front of the sample. All signals are recorded by an optical beam which is part of a Mach–Zehnder interferometer that integrates the acoustic field along its path. Due to the cascaded arrangement of LUS sources, a single-recorded signal yields information for a projection of the SOS distribution. After collection of data from all directions, an inverse Radon transform is applied to this set of projections to obtain a 2-D SOS image. The setup is characterized and its performance is tested on phantom experiments. In addition to providing additional contrast, it is also shown that the resolution of the coregistered PA and LUS images can be improved by implementing the knowledge of the SOS distribution in the reconstruction.

64-line-sensor array: fast imaging system for photoacoustic tomography

Three-dimensional photoacoustic tomography with line sensors, which integrate the pressure along their length, has shown to produce accurate images of small animals. To reduce the scanning time and to enable in vivo applications, a detection array is built consisting of 64 piezoelectric line sensors which are arranged on a semi-cylinder. When measuring line integrated pressure signals around the imaging object, the three-dimensional photoacoustic imaging problem is reduced to a set of two-dimensional reconstructions and the measurement setup requires only a single axis of rotation. The shape and size of the array were adapted to the given problem of biomedical imaging and small animal imaging in particular. The length and width of individual line elements had to be chosen in order to take advantage of the favorable line integrating properties, maintaining the requested resolution of the image. For data acquisition the signals from the 64 elements are amplified and multiplexed into a 32 channel digitizer. Single projection images are recorded with two laser pulses within 0.2 seconds, as determined by the laser pulse repetition rate of 10 Hz. Phantom experiments are used for characterization of the line-array. Compared to previous implementations with a single line sensor scanning around an object, with the developed array the data acquisition time can be reduced from about one hour to about one minute.

Combined photoacoustic and speed-of-sound imaging using integrating optical detection

A setup that allows for the co-registration of photoacoustic (PA) and speed-of-sound (SOS) section images is presented. By means of ultrasound (US) transmission imaging the distribution of the acoustic speed within an object can be obtained. Our method uses the PA effect for the generation of the traversing US waves. Short near-infrared (NIR) laser pulses emitted by a Nd:YAG laser system are used to illuminate external optical absorbing targets at various distances in front of the sample. At the same time the object under investigation is illuminated by a part of the same frequency-doubled laser pulse. A free laser beam, which is part of a Mach-Zehnder interferometer, is used for the detection of the US signals coming from and passing through the sample. Due to a cascaded arrangement of absorbing targets for laser ultrasound (LUS) generation a single laser pulse yields information for a projection of the SOS distribution. The resolution is determined by the number and width of LUS sources. Separation of the signals arriving at the integrating detector is possible because of their different times of flight. After collection of the data reconstruction of a two-dimensional SOS map is accomplished by applying an inverse Radon transform to the projections. For PA section imaging a cylindrical acoustic reflector behind the detector yields an acoustic focus in the observed slice. To the data gathered by detecting the reflected PA signals also the inverse Radon transform is applied to obtain a reconstructed image of the illuminated section. In this paper a detailed description of the setup is given and the results of experiments on two- and three-dimensional phantoms are presented.

Speed-of-sound correction for photoacoustic and laser-ultrasound imaging with an integrating cylindrical detector

A method based on a modified inverse Radon transform is presented to correct photoacoustic and laser-ultrasound section images taken with a cylindrical integrating detector for heterogeneity in the speed of sound. Data for the correction are obtained with a laser ultrasound transmission method, providing two-dimensional maps of the speed of sound distribution. Photoacoustic, laser ultrasound and speed of sound tomography are combined in a single setup using illumination of the sample and several external absorbers with short pulses from a single laser source. The performance of the corrected reconstruction is compared in a simulation and a phantom experiment with a reconstruction using an average, constant sound speed. In this comparison, the reconstruction using the modified inverse Radon transform shows a small but distinguishable improvement.

Dual-modality section imaging system with optical ultrasound detection for photoacoustic and ultrasound imaging

We propose the further development of the optical detection setup towards photoacoustic (PA) and ultrasound (US) dual-modality section imaging. Both imaging modalities use optical generation and detection of ultrasound waves. A onesided chrome coated concave cylindrical optical lens is used as target to induce acoustic signals for US imaging and as acoustic mirror that forms acoustic images. By probing the temporal evolution of the acoustic images with an optical beam perpendicular to the acoustic axis and simultaneously rotating the object, data for reconstruction of PA and US slice images are acquired. All acoustic signals are excited optically via the thermoelastic effect using laser pulses coming from the same laser system.

Free Beam Fabry-Perot-Interferometer as Detector for Photoacoustic Tomography

Acoustic line detectors have been shown to be capable of providing accurate signals for three-dimensional photoacoustic tomography. Free and guided beam optical Mach-Zehnder interferometers (MZI) have been used as well as a waveguide Fabry-Perot interferometer (FPI). The ultimate sensitivity is expected from a FPI where the optical field in the resonator propagates in the acoustic coupling medium (water) surrounding the imaged object. Such a free-beam FPI is completely optically and acoustically transparent, while providing a higher sensitivity compared to the MZI due to the multiple beam interference. In this work the performance of a FPI for measurement of ultrasound waves is compared to a MZI. It is shown that an at least 4.5-fold higher signal to noise ratio is achieved compared to a MZI. The resolution of the FPI is simulated and measured, showing a constant diameter of the interferometer beam. Verification of the stability of the free beam FPI over longer time periods is demonstrated by acquiring a two-dimensional tomography image of a phantom. The sensitivity and stability of the setup makes it suitable for tomographic imaging.

First steps towards dual-modality 3D photoacoustic and speed of sound imaging with optical ultrasound detection

CCD camera based optical ultrasound detection is a promising alternative approach for high resolution 3D photoacoustic imaging (PAI). To fully exploit its potential and to achieve an image resolution <50 μm, it is necessary to incorporate variations of the speed of sound (SOS) in the image reconstruction algorithm. Hence, in the proposed work the idea and a first implementation are shown how speed of sound imaging can be added to a previously developed camera based PAI setup. The current setup provides SOS-maps with a spatial resolution of 2 mm and an accuracy of the obtained absolute SOS values of about 1%. The proposed dual-modality setup has the potential to provide highly resolved and perfectly co-registered 3D photoacoustic and SOS images.

Simultaneous three-dimensional laser-ultrasound and photoacoustic imaging

A purely optical setup for simultaneous photoacoustic (PA) and laser-ultrasound (US) tomography is presented. It is shown that combined imaging can be achieved by using the same laser pulse for photoacoustic generation and for launching a broadband ultrasound pulse from an optically absorbing target. Detection of the laser-generated plane waves that have been scattered at the imaging object and of the photoacoustic signals emitted from the sample is done interferometrically. This way data for PA and US imaging are acquired within one single measurement. Distinction between the signals is possible due to their different times of flight. After data separation, image reconstruction is done using standard back-projection algorithms. The resolution of the setup was estimated and images of a zebra fish are shown, demonstrating the complementary information of the two imaging modalities.