Robert Nuster

Three-dimensional photoacoustic imaging using fiber-based line detectors

For photoacoustic imaging, usually point-like detectors are used. As a special sensing technology for photoacoustic imaging, integrating detectors have been investigated that integrate the acoustic pressure over an area or line that is larger than the imaged object. Different kinds of optical fiber-based detectors are compared regarding their sensitivity and resolution in three-dimensional photoacoustic tomography. In the same type of interferometer, polymer optical fibers yielded much higher sensitivity than glass fibers. Fabry-Perot glass-fiber interferometers in turn gave higher sensitivity than Mach-Zehnder-type interferometers. Regarding imaging resolution, the single-mode glass fiber showed the best performance. Last, three-dimensional images of phantoms and insects using a glass-fiber-based Fabry-Perot interferometer as integrating line detector are presented.

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 imaging with integrating line detectors

Photoacoustic Tomography is an emerging imaging technology mainly for medical and biological applications. A sample is illuminated by a short laser pulse. Depending on the optical properties the electromagnetic radiation is distributed and absorbed. Thereby local temperature increase generates thermal expansion and broadband ultrasonic signals, also called photoacoustic signals. Unlike conventional ultrasound in photoacoustic imaging the contrast depends on the optical properties of the sample which provides not only morphologic information but also functional information. This way photoacoustic imaging combines the advantages of optical imaging (high contrast) and ultrasonic imaging (high spatial resolution) and is particularly suited for medical applications like mammography or skin cancer detection. Our group uses integrating line detectors instead of ultrasonic point receivers. Line detectors integrate the pressure along one dimension whereby the 3D problem is reduced to a 2D problem and enables a tomography setup that requires only a single axis of rotation. Implementations of line detectors use optical interferometers, e.g. a Fabry-Perot interferometer or a Mach-Zehnder interferometer. We use free-beam interferometers as well as fiber-based interferometers for collecting photoacoustic signals. The latter are somewhat easier to handle because they require fewer optical components. Finally, the advantages of optical detection methods over piezoelectric detection methods are the better frequency response and the resistance against electrical interference from the environment. First measurements on phantoms and image reconstruction using a time reversal method demonstrated the capability of integrating line detectors for collecting broadband ultrasonic signals for photoacoustic tomography.

Photoacoustic tomography using integrating line detectors

Photoacoustic Imaging (also known as thermoacoustic or optoacoustic imaging) is a novel imaging method which combines the advantages of Diffuse Optical Imaging (high contrast) and Ultrasonic Imaging (high spatial resolution). In photoacoustic imaging, a short laser pulse excites the sample. The absorbed energy causes a thermoelastic expansion and thereby launches a broadband ultrasonic wave (photoacoustic signal). This way one can measure the optical contrast of a sample with ultrasonic resolution. For collecting photoacoustic signals our group introduced so called integrating detectors a few years ago. Such integrating detectors integrate the pressure in one or two dimensions (line or plane detectors). Thereby the three dimensional imaging problem is reduced to a two or a one dimensional problem for the pressure projections for line or plane detectors, respectively. Several reconstruction methods like Fourier or F-SAFT reconstruction or back projection are used for the two dimensional first step, but the model-based time reversal method shows a significant advantage: acoustical heterogeneity and attenuation, which both cause blurring of reconstructions, can be directly implemented in the reconstruction method. The integrating detectors are mainly optical detectors and thus can provide a high bandwidth up to several 100 MHz. Using these detectors the resolution is often limited by the acoustic attenuation in the sample itself, because attenuation increases with higher frequencies. For thin layers, small cylinders, and small spherical inclusions the effect of attenuation in human fat is simulated and the influence of dispersion on image reconstruction is shown.

Polymer fiber detectors for photoacoustic imaging

Photoacoustic imaging is a novel imaging method for medical and biological applications, combining the advantages of Diffuse Optical Imaging (high contrast) and Ultrasonic Imaging (high spatial resolution). A short laser pulse hits the sample. The absorbed energy causes a thermoelastic expansion and thereby launches a broadband ultrasonic wave (photoacoustic signal). The distribution of absorbed energy density is reconstructed from measurements of the photoacoustic signals around the sample. For collecting photoacoustic signals either point like or extended, integrating detectors can be used. The latter integrate the pressure at least in one dimension, e.g. along a line. Thereby, the three dimensional imaging problem is reduced to a two dimensional problem. For a tomography device consisting of a scanning line detector and a rotating sample, fiber-based detectors made of polymer have been recently introduced. Fiber-based detectors are easy to use and possess a constant, high spatial resolution over their entire active length. Polymer fibers provide a better impedance matching and a better handling compared with glass fibers which were our first approach. First measurement results using polymer fiber detectors and some approaches for improving the performance are presented.

Three dimensional photoacoustic imaging using fiber-based line detectors

Unlike other research groups we use so called integrating detectors which integrate the pressure along the detector. Such integrating line detectors can be realised by interferometers. We developed fiber-based integrating detectors for photoacoustic imaging which are easy to handle and are more convenient for clinical use. Now we present first 3D photoacoustic measurements on simple objects using a glass fiber Fabry-Perot detector as integrating line detector

Photoacoustic Generation of X-waves and their Application in a Dual Mode Scanning Acoustic Microscope

Photoacoustic imaging is based on the excitation of ultrasound waves by irradiating objects with short laser pulses. Absorbing laser energy causes thermal expansion, which leads to broadband ultrasonic waves, carrying information about size, location and optical properties of the observed target. Images reveal purely optical contrast, yet the technique is acoustic. Classical ultrasonic imaging generates images with purely acoustical contrast based on the impedance differences of structures in observed samples. For developing a dual mode scanning acoustic microscope, which uses simultaneously both contrast mechanism (acoustic pulse-echo and photoacoustic image contrast) ultrasonic pulses with a large depth of field are advantageous. By illuminating special conically shaped transducers, so called axicons, with short laser pulses, broadband ultrasonic pulses with a large depth of field at small lateral extension can be excited. These special pulses, so called X-waves and their use in a microscope are investigated.

Photoacoustic imaging with limited diffraction beam transducers

Photoacoustic imaging with a scanning, fixed focus receiver gives images with high resolution, without the need for reconstruction algorithms. However, the usually employed spherical ultrasound lenses have a limited focal depth that decreases with increasing lateral resolution due to the inverse relation between numerical aperture and Rayleigh length. In this study the use of an axicon detector is proposed, consisting of a conical surface onto which a piezoelectric polymer film is attached. The detector is characterized in simulations and in experiments, demonstrating the expected high resolution over an extended depth of focus. Simulated and experimental images reveal X-shaped artifacts that are due to the conical detector surface. Since the point spread function (PSF) of the detector is spatially invariant over the depth of field, a frequency domain deconvolution can be applied to the images. Although this clearly improves the image quality in simulations, the reduction of artifacts was not so efficient in experiments. However, the detector is able to produce images with accurate position and shape of objects. Moreover, the axicon transducer rejects signals from planar surfaces (e.g. the skin surface) and favors signals from small, isolated sources.

Photoacoustic imaging using a conical axicon detector

Photoacoustic imaging with a scanning, fixed focus receiver gives images with high resolution, without the need for image reconstruction. For achieving high depth of field, a conically shaped piezoelectric ultrasound detector, the so called axicon-detector, is investigated. It is characterized by a sustained line of focus with a length that depends only on the geometry of the detector but not on the wavelength. Simulated and experimentally taken images of various objects reveal X-shaped artifacts due to the conical surface of the detector. To improve the image quality a frequency domain deconvolution can be applied, as the point spread function (PSF) of the detector is spatially invariant over the depth of field. The reduction of the artifacts works well for simulated images but is not functional for experimental data yet. Nevertheless, the detector gives images with precise shape and position of the investigated samples.

Using a phase contrast imaging method in photoacoustic tomography

To speed up the data acquisition in photoacoustic tomography full field detection can be used to avoid the time consuming scanning around the object. The full field detection is realized using a phase contrast method like commonly used in optical microscopy. An expanded light beam considerably larger than the object size illuminates the sample placed in the middle of the propagating light beam. Images obtained with a CCD-camera at a certain time show a projection of the instantaneous pressure field (phase object) in a given direction. The reconstruction method is related to imaging with integrating line detectors, but has to be matched to the specific information in the recorded images, which is now purely spatially resolved as opposed to spatiotemporally for a single scanning detector. The reconstruction of the projection images of the initial pressure distribution is done by back propagating the observed wave pattern in Radon space. Numerical simulations and experiments are performed to show the overall adaptability of this technique in photoacoustic tomography.