Sibylle Gratt

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.

Photoacoustic section imaging with an integrating cylindrical detector

A piezoelectric detector with a cylindrical shape is investigated for photoacoustic section imaging. Images are acquired by rotating a sample in front of the cylindrical detector. With its length exceeding the size of the imaging object, it works as an integrating sensor and therefore allows reconstructing section images with the inverse Radon transform. Prior to the reconstruction the Abel transform is applied to the measured signals to improve the accuracy of the image. A resolution of about 100 µm within a section and of 500 µm between sections is obtained. Additionally, a series of images of a zebra fish is shown.

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.

Downstream Fabry–Perot interferometer for acoustic wave monitoring in photoacoustic tomography

An optical detection setup consisting of a focused laser beam fed into a downstream Fabry-Perot interferometer (FPI) for demodulation of acoustically generated optical phase variations is investigated for its applicability in photoacoustic tomography. The device measures the time derivative of acoustic signals integrated along the beam. Compared to a setup where the detection beam is part of a Mach-Zehnder interferometer, the signal-to-noise ratio of the FPI is lower, but the image quality of the two devices is similar. Using the FPI in a photoacoustic tomograph allows scanning the probe beam around the imaging object without moving the latter.

Comparison of optical and piezoelectric integrating line detectors

Currently two different types of integrating line sensors are used in photoacoustic tomography (PAT). Thin film piezoelectric polymer sensors (PVDF) are characterized by compactness, easy handling and the possibility to manufacture sensing areas with different shape. However, they are vulnerable to electrical disturbance and to scattered light from the illuminated sample. Also optical sensors are used as integrating line sensors in combination with some kind of interferometric setup. For example, one arm of a Mach-Zehnder interferometer or the cavity of a Fabry-Perot interferometer can be used as line detector. In both cases, the light wave either propagates freely in the liquid or is guided in an optical fiber. Such sensors are quite immune against noise sources described above and suitable for high bandwidth detection. One drawback is the limited mobility due to the complex arrangement of the setup. This study is focused on the comparison of the different implementations of line detectors, mainly on directivity and sensitivity. Shape and amplitude of signals generated by defined sources are compared among the various sensor types. While the shape of the signals recorded with the optical free beam detector matches quite well to the simulation the signals detected with the PVDF detector are affected by directivity effects. This causes a strong distortion of the signal shape depending on the incident angle of the acoustic wave. How these effects influence the reconstructed projection image is discussed.

Photoacoustic section imaging using an elliptical acoustic mirror and optical detection

A method is proposed that utilizes the advantages of optical ultrasound detection in two-dimensional photoacoustic section imaging, combining an optical interferometer with an acoustic mirror. The concave mirror has the shape of an elliptical cylinder and concentrates the acoustic wave generated around one focal line in the other one, where an optical beam probes the temporal evolution of acoustic pressure. This yields line projections of the acoustic sources at distances corresponding to the time of flight, which, after rotating the sample about an axis perpendicular to the optical detector, allows reconstruction of a section using the inverse Radon transform. A resolution of 120 [micro sign]m within and 1.5 mm between the sections can be obtained with the setup. Compared to a bare optical probe beam, the signal-to-noise ratio (SNR) is seven times higher with the mirror. Furthermore, the imaging system is tested on a biological sample.

Piezoelectric annular array for large depth of field photoacoustic imaging

A piezoelectric detection system consisting of an annular array is investigated for large depth of field photoacoustic imaging. In comparison to a single ring detection system, X-shaped imaging artifacts are suppressed. Sensitivity and image resolution studies are performed in simulations and in experiments and compared to a simulated spherical detector. In experiment an eight ring detection systems offers an extended depth of field over a range of 16 mm with almost constant lateral resolution.

Laser-generation of ultrasonic X-waves using axicon transducers

Photoacoustic and classical ultrasound imaging are both based upon ultrasonic waves but use different contrast mechanisms. For the development of a scanning acoustic microscope that uses both contrasts, an axicon transducer generating nondiffracting ultrasound, also called X-waves, by illumination with short laser pulses is investigated. Such a transducer provides simultaneously high depth of field and high lateral resolution. In this work, the spatial and temporal characteristics of the laser-generated X-waves are investigated using experiments and theoretical simulations. The experimental results reveal a characteristic spatial pulse width of 50 μm and a focal depth that complies well with the theoretical predictions.

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.