The spatial resolution in photoacoustic imaging is essentially limited by acoustic attenuation, which can be numerically compensated only up to a theoretical limit. The physical background for this “ill-posedness” is the second law of thermodynamics: the loss of information is equal to the entropy production, which is the energy decay of the attenuated wave divided by the temperature. As acoustic attenuation increases with higher frequencies, a cut-off frequency can be determined, where the information content for that frequency gets so low that it cannot be distinguished from equilibrium distribution within a certain statistical significance. This cut-off frequency can be determined also by setting the amplitude of the attenuated signal in frequency domain equal to the noise-level. Compensating for acoustic attenuation requires to solve an ill-posed inverse problem, where an adequate regularization parameter is the cut-off frequency, when the acoustic wave amplitude is damped just below the noise level. If additional information, such as positivity or sparsity is used, this theoretical resolution limit can be overcome. This is experimentally demonstrated for the propagation of planar acoustic waves in fat tissue, which are induced by short laser pulses and measured by piezoelectric transducers. For fatty porcine tissue the frequency dependent acoustic attenuation was measured. This was used to invert the problem and by using additional information, in the form of positivity and sparsity (Douglas-Rachford splitting algorithm) the resolution could be enhanced significantly compared to the limit given by the cut-off frequency from attenuation through 20 mm of porcine fat tissue.
We discuss several methods for image reconstruction in compressed sensing photoacoustic tomography (CS-PAT). In particular, we apply the deep learning method of [H. Li, J. Schwab, S. Antholzer, and M. Haltmeier. NETT: Solving Inverse Problems with Deep Neural Networks (2018), arXiv:1803.00092], which is based on a learned regularizer, for the first time to the CS-PAT problem. We propose a network architecture and training strategy for the NETT that we expect to be useful for other inverse problems as well. All algorithms are compared and evaluated on simulated data, and validated using experimental data for two different types of phantoms. The results one the hand indicate great potential of deep learning methods, and on the other hand show that significant future work is required to improve their performance on real-word data.
Several annular detector arrays are compared for scanning photoacoustic imaging. Compared to single, spherically focused detectors, the arrays offer similar sensitivity, but have an extended depth of field due to their dynamic focusing capability. The investigated arrays consist either of piezoelectric polymer film (PVDF) with a large sensing area for optimized sensitivity or of fiber optic rings, where the small width of elements gives rise to high bandwidth and resolution. Simulations demonstrate the superior resolution of the fiber-optic rings over a flat piezo array. However, with inclined sensing elements also the piezo-detector reaches a similar resolution as the optical array. In phantom experiments with the PVDF array the extended depth of field and the capability of imaging complex objects are demonstrated.
We present a prototype for all-optical photoacoustic projection imaging. By generating projection images, photoacoustic information of large volumes can be retrieved with less effort compared to common photoacoustic computed tomography where many detectors and/or multiple measurements are required. In our approach, an array of 60 integrating line detectors is used to acquire photoacoustic waves. The line detector array consists of fiber-optic MachZehnder interferometers, distributed on a cylindrical surface. From the measured variation of the optical path lengths of the interferometers, induced by photoacoustic waves, a photoacoustic projection image can be reconstructed. The resulting images represent the projection of the three-dimensional spatial light absorbance within the imaged object onto a two-dimensional plane, perpendicular to the line detector array. The fiber-optic detectors achieve a noise-equivalent pressure of 24 Pascal at a 10 MHz bandwidth. We present the operational principle, the structure of the array, and resulting images. The system can acquire high-resolution projection images of large volumes within a short period of time. Imaging large volumes at high frame rates facilitates monitoring of dynamic processes.
We present a system for large depth-of-field photoacoustic scanning macroscopy (PASMac). Photoacoustic waves are detected optically with a fiber-optic annular detector array. Pressure changes, induced by the photoacoustic waves, modulate the refractive index in the fibers. The resulting variation of the optical path is measured by interferometric means. The ring-shape of the fibers results in a higher sensitivity to pressure waves stemming from the ring symmetry axis compared to off-axis signals. Hence, the fiber-optic ring-shaped detector array embodies a focused ultrasound transducer, however with a greatly extended depth-of-field. To reduce off-axis sensitivity, the signals from multiple rings with varying diameters are summed up using coherence weighting. By raster scanning of the sensor array, threedimensional or cross-sectional images are formed. We report on the design of an enhanced detector with 8 concentric rings for targeted imaging depths ranging from 5 mm to 50 mm.
We present a multimodal optical setup, allowing non-contact photoacoustic imaging (PAI) and optical coherence tomography (OCT). Optical coherence tomography is sensitive to changes in the specimen’s refractive index, thereby offering complementary information to photoacoustic signals which are induced by light absorption. A multimodal setup, allowing OCT and photoacoustic measurements, should ideally not rely on any physical contact to a specimen and, thus, commonly used transducers for photoacoustic signal detection which require acoustic coupling to the specimen should be avoided. In this work photoacoustic signals are acquired by measuring the surface displacement of a specimen using a fiber-optic Mach-Zehnder interferometer. Photoacoustic signals are excited with a Nd:YAG pulse laser. The interferometer for non-contact photoacoustic detection and the OCT system are realized in the same fiber-optic network. Light from the PAI detection laser and the OCT source are multiplexed into a single optical fiber and the same objective is used for both imaging modalities. Light reflected from specimens is demultiplexed and guided to the respective imaging systems. To allow fast non-contact PAI and OCT imaging the detection spot is scanned across the specimens’ surface using a galvanometer scanner. As the same fiber-network and optical components are used for photoacoustic and OCT imaging the obtained, images are co-registered intrinsically. Imaging is demonstrated on a tissue mimicking sample.
We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.
In this work we present photoacoustic projection imaging with a 64-channel integrating line detector array, which average the pressure over cylindrical surfaces. For imaging, the line detectors are arranged parallel to each other on a cylindrical surface surrounding a specimen. Thereby, the three-dimensional imaging problem is reduced to a twodimensional problem, facilitating projection imaging. After acquisition of a dataset of pressure signals, a twodimensional photoacoustic projection image is reconstructed. The 64 channel line detector array is realized using optical fibers being part of interferometers. The parts of the interferometers used to detect the ultrasonic pressure waves consist of graded-index polymer-optical fibers (POFs), which exhibit better sensitivity than standard glass-optical fibers. Ultrasonic waves impinging on the POFs change the phase of light in the fiber-core due to the strain-optic effect. This phase shifts, representing the pressure signals, are demodulated using high-bandwidth balanced photo-detectors. The 64 detectors are optically multiplexed to 16 detection channels, thereby allowing fast imaging. Results are shown on a Rhodamine B dyed microsphere.
In this paper we present multimodal non-contact photoacoustic and optical coherence tomography (OCT) imaging using a galvanometer scanner. Photoacoustic signals are acquired without contact on the surface of a specimen using an interferometric technique. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as source. In the same fiber-optic network a spectral-domain OCT system is realized, using a broadband light source at 1300 nm. Light from the fiber laser and the OCT source are multiplexed into the same fiber and the same objective is used for both imaging modalities. Fast non-contact photoacoustic and OCT imaging is demonstrated by scanning the detection spot utilizing a galvanometer scanner. Multimodal photoacoustic and OCT imaging is shown on agarose phantoms. As the same fiber network and optical components are used for non-contact photoacoustic and OCT imaging the obtained images are co-registered intrinsically.
We present a remote photoacoustic imaging system without the need of a physical contact to the specimen. The setup is based on a Mach-Zehnder interferometer using optical wave guide technology as usually used in telecommunication industries, thus guaranteeing long life times and relatively low costs. A detection beam is transmitted through an optical fiber to a lens system which focuses the beam to the surface of a specimen. The back reflected light is than collected by the same lens system and coupled into the same optical fiber. As the collected light intensity is less than 0.1% of the transmitted intensity in forward direction an optical amplifier is used for amplifying the collected light. After amplification the light is brought to interference with a reference beam for demodulation of the ultrasound signals. The modulated light intensity is converted into electrical signals by a self-built balanced photo detector. We present noncontact photoacoustic imaging of a tissue-mimicking phantom and on chicken skin.
In this paper we present multimodal non-contact photoacoustic and OCT imaging. Photoacoustic signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiberoptic network using a fiber laser at 1550nm as source. In the same fiber-optic network a spectral-domain OCT system is realized. The OCT system utilizes a superluminescent diode at 1325nm as light source; imaging data are acquired using a spectrometer with an InGaAs line array. Light from the fiber laser and the superluminescent diode are multiplexed into one fiber and the same objective is used for both imaging modalities. Reflected light is demultiplexed and guided to the respective imaging systems. We demonstrate the photoacoustic and OCT imaging modalities on different phantom samples. Finally, we show multimodal imaging with both modalities simultaneously. The resulting photoacoustic and OCT images match perfectly.
We introduce a multichannel optical fiber based detector for photoacoustic imaging. By using in-house produced
photodetectors and relative low-cost components from telecommunication industries we were able to reduce the costs for
one channel significantly compared to previous setups. The estimated cost for one channel (without sampling device) is
below 800 €. The self-made balanced photodetector for 1550 nm achieves a gain of 100 dB, a -3dB bandwidth of 45
MHz and a maximum signal-to-noise-ratio of 48 dB. We present a four channel annular detector array based on optical
fiber Mach-Zehnder interferometers. Photoacoustic imaging is demonstrated by measuring photoacoustic signals of a
black polyethylene microsphere.
Ultrasonic attenuation in biomaterials limits the quality and resolution of ultrasonic imaging. This work presents a simple
and reliable method to investigate acoustic attenuation of biological tissue samples and liquids in order to improve
reconstruction algorithms for photoacoustic imaging. For this purpose broadband high-frequency single transmission
measurements were performed. The spectra of the acquired signals were compared to reference measurements in
distilled water. Unfocused broadband piezoelectric transducers were used as ultrasound source and detector. Moreover,
laser generated ultrasound, which provides more intensity and signals with higher bandwidth, was used to measure
acoustic attenuation. Only few studies concerned with attenuation of fat tissue performed broadband high frequency
measurements and to our knowledge none of those used the simple and reliable single transmission approach with
unfocused ultrasound. Our results for acoustic attenuation in olive oil show good agreement with literature. Many studies
indicate linear frequency increase of attenuation of fat tissue. However, we observed significant non-linear frequency
behaviour of porcine subcutaneous fat tissue at room temperature with a power-law exponent of around 1.45.
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). 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 -a line or a plane detector,
respectively. Thereby the three dimensional imaging problem is reduced to a two or a one dimensional problem for the
projections and a two or three dimensional inverse radon transform as a second step to get the three dimensional initial
pressure distribution. 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.
Stoke's equation describes the attenuation of photoacoustic generated waves in liquids very well, which results in an
increase of the acoustic attenuation with the square of the frequency. For fat tissue an absorption coefficient which is
approximately linear proportional to frequency is reported. Presented measurements give an exponential power law
dependency with an exponent between 1.31 and 1.36 in fat tissue near the skin of a pig.
These equations describing frequency dependent acoustic attenuation have been solved in the past by decomposing the
pressure wave into plane waves damped in space, described by the complex part of the wave number equal to the
attenuation coefficient. One main result of this paper is that for Photoacoustic Tomography another description seems to
be very useful: like for a standing wave in a resonator the wave number is real but the frequency is complex. The
complex part of the frequency is the damping in time. Both descriptions are equivalent, but with the complex frequency
description acoustic attenuation can be included in all "k-space" methods well known in Photoacoustic Tomography just
by introducing a factor describing the exponential decay in time.
In this work measurements of the acoustic attenuation coefficient for water in the frequency range of 20-40 Mhz have
been performed. These measurements have been conducted with a photoacoustic setup, containing a nano second pulse
laser and an integrating line detector. The nanosecond pulse laser is used to generate ultra sound waves in a target. Those
waves are transmitted into water and are then detected by the line sensor. In this way the resulting ultra sound field was
scanned and the absorption coefficient was evaluated. Different pulse energies, beam diameters and target materials were
used. The resulting coefficients are in the range of the reported values found in literature.
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