G. Paltauf

Thermoacoustic computed tomography with large planar receivers

Thermoacoustic imaging is a promising new modality for nondestructive evaluation. So far point measurement data for thermoacoustic imaging are used. In this paper we propose a novel measurement set-up with relatively large piezo foils (planar receivers) and an according real time imaging algorithm based on the Radon transform. We present numerical simulations for simulated and real world data.

Temporal back-projection algorithms for photoacoustic tomography with integrating line detectors

Line detectors integrate the measured acoustic pressure over a straight line and can be realized by a thin line of a piezoelectric film or by a laser beam as part of an interferometer. Photoacoustic imaging with integrating line detectors is performed by rotating a sample or the detectors around an axis perpendicular to the line detectors. The subsequent reconstruction is a two-step procedure: first, two-dimensional (2D) projections parallel to the line detector are reconstructed, then the three-dimensional (3D) initial pressure distribution is obtained by applying the 2D inverse Radon transform. The first step involves an inverse problem for the 2D wave equation. Wave propagation in two dimensions is significantly different from 3D wave propagation and reconstruction algorithms from 3D photoacoustic imaging cannot be used directly. By integrating recently established 3D formulae in the direction parallel to the line detector we obtain novel back-projection formulae in two dimensions. Numerical simulations demonstrate the capability of the derived reconstruction algorithms, also for noisy measurement data, limited angle problems and 3D reconstruction with integrating line detectors.

Photoacoustic microtomography using optical interferometric detection

A device for three-dimensional (3-D) photoacoustic tomography with resolution in the range of tens of micrometers is presented that uses a light beam for interferometric detection of acoustic waves. Reconstruction of the 3-D initial pressure distribution from the signals representing line integrals of the acoustic field is a two-step process. It uses an inversion of 2-D wave propagation to obtain line projections of the initial pressure distribution and the inverse Radon transform. The light beam, propagating freely in a water bath, is scanned either in an arc- or box-shaped curve around the object. Simulations are performed to compare the two scanning procedures. The projection images are obtained either using the filtered back projection algorithm for the pi-arc scanning mode or the frequency domain algorithm for the box scanning mode. While the former algorithm provides slightly better image quality, the latter is about 20 times faster. The ability of the photoacoustic tomography device to create 3-D images with constant resolution throughout the reconstruction volume is demonstrated experimentally using a human hair phantom. These measurements revealed a 3-D resolution below 100 mum. In a second experiment, 3-D imaging of an isolated mouse heart is demonstrated to show the applicability for preclinical and biological research.

Characterization of integrating ultrasound detectors for photoacoustic tomography

Photoacoustic tomography is based on generation of sound waves in a semitransparent medium by illumination with short light pulses. In standard methods, measurements of the acoustic waves around the sample with point like ultrasound detectors are used for reconstruction of the distribution of absorbed energy, which contains information on light-absorbing structures such as blood vessels in tissue. Integrating ultrasound detectors are planes or lines larger than the imaged object and measure temporal signals that are given by spatial integrals over the sound field. It can be shown that such integrated signals give exact reconstructions with constant, high resolution throughout the imaging zone. The goal of the present study was to investigate with the help of simulations and experiments how far real implementations of integrating detectors based on piezoelectric films or optical interferometry have characteristics approximating those of ideal planes or lines. It is shown that the directive sensitivity of p...

Photoacoustic tomography using a fiber based Fabry-Perot interferometer as an integrating line detector and image reconstruction by model-based time reversal method

Photoacoustic imaging is based on the generation of acoustic waves in a semitransparent sample (e.g. soft tissue) after illumination with short pulses of light or radio waves. The goal is to recover the spatial distribution of absorbed energy density inside the sample from acoustic pressure signals measured outside the sample (photoacoustic inverse problem). If the acoustic pressure outside the illuminated sample is measured with a large-aperture detector, the signal at a certain time is given by an integral of the generated acoustic pressure distribution over an area that is determined by the shape of the detector. For example a planar detector measures the projections of the initial pressure distribution over planes parallel to the detector plane, which is the Radon transform of the initial pressure distribution. Stable and exact three-dimensional imaging with planar integrating detector requires measurements in all directions of space and so the receiver plane has to be rotated to cover the entire detection surface. We have recently presented a simpler set-up for exact imaging which requires only a single rotation axis and therefor the fragmentation of the area detector into line detectors perpendicular to the rotation axis. Using a two-dimensional reconstruction method and applying the inverse two-dimensional Radon transform afterwards gives an exact reconstruction of the three-dimensional sample with this set-up. In order to achieve high resolution, a fiber based Fabry-Perot interferometer is used. It is a single mode fiber with two fiber bragg gratings on both ends of the line detector. Thermal shifts and vibrations are compensated by frequency locking of the laser. The high resolution and the good performance of this integrating line detector has been demonstrated by photoacoustic measurements with line grid samples and phantoms using a model-based time reversal method for image reconstruction. The time reversed pressure field can be calculated directly by retransmitting the measured pressure on the detector positions in a reversed temporal order.

Thermoacoustic tomography using a fiber-based Fabry-Perot interferometer as an integrating line detector

Thermoacoustic tomography is based on the generation of acoustic waves by a bulk illumination of a sample with a short electromagnetic pulse. The thermal expansion of the sample generates acoustic waves. The absorption density inside the sample is reconstructed from the acoustic pressure signals measured outside of the sample. So far signals have been collected with small detectors that approximate point-detectors. In the present study we report on a novel measurement setup applying integrating detectors. With these detectors the pressure is integrated along one or two dimensions. This enables the use of numerically efficient algorithms, such as the inverse Radon transformation, for thermoacoustic tomography. To reconstruct a three-dimensional image, either a two-dimensional integrating detector has to be moved tangentially around a sphere enclosing the object or an array of line-detectors has to be rotated around a single axis. Implementations of line-detectors are demonstrated that are adaptations of a Fabry--Perot interferometer. As a novel approach for the implementation a Fabry-Perot interferometer consisting of a single mode fiber is demonstrated.

Comparison of surface plasmon resonance devices for acoustic wave detection in liquid

Surface plasmon resonance (SPR) sensors represent a suitable method for broadband acoustic pulse detection. The reflectivity and phase of a p-polarized laser beam incident on an optical device under SPR conditions are strongly dependent on ambient conditions that are changed by an acoustic wave. Depending on the order of layers, SPR sensors can be arranged in the Kretschmann or in the Otto configuration acting as a pressure or as a displacement sensor. The aim of this study was to compare both configurations and to find linear and sensitive conditions for the application. Numerical calculations were carried out varying the layer dimensions and the angle of incidence. The results of the experimental investigation on both configurations confirm the working principle.

Three-dimensional photoacoustic tomography using acoustic line detectors

A method for phoatoacoustic tomography (PAT) is presented that uses line integrals over the acoustic wave field from a photoacoustic source for the reconstruction of a three-dimensional image. The line integrals are acquired with an optical line sensor based on a Mach-Zehnder interferometer. Image reconstruction is a two-step process. In the first step data from a scan of the line outside the object is used to reconstruct a linear projection of the source distribution. In the second step the inverse linear Radon transform is applied to multiple projections taken at different directions. This study focuses on the optimization of the first step using a frequency-domain algorithm and input data from a scan of the line detector in an L-shaped curve around the object. Simulations and a phantom experiment demonstrate that equally high resolution in all directions in the projection plane can be achieved with this method.

On the use of frequency-domain reconstruction algorithms for photoacoustic imaging

We investigate the use of a frequency-domain reconstruction algorithm based on the nonuniform fast Fourier transform (NUFFT) for photoacoustic imaging (PAI). Standard algorithms based on the fast Fourier transform (FFT) are computationally efficient, but compromise the image quality by artifacts. In our previous work we have developed an algorithm for PAI based on the NUFFT which is computationally efficient and can reconstruct images with the quality known from temporal backprojection algorithms. In this paper we review imaging qualities, such as resolution, signal-to-noise ratio, and the effects of artifacts in real-world situations. Reconstruction examples show that artifacts are reduced significantly. In particular, image details with a larger distance from the detectors can be resolved more accurately than with standard FFT algorithms.

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.