RESUMEN

In conventional photoacoustic tomography, several effects contribute to the loss of resolution, such as the limited bandwidth and the finite size of the transducer, or the space-dependent speed of sound. They can all be compensated (in principle) technically or numerically. Frequency-dependent acoustic attenuation also limits spatial resolution by reducing the bandwidth of the photoacoustic signal, which can be numerically compensated only up to a theoretical limit given by thermodynamics. The entropy production, which is the dissipated energy of the acoustic wave divided by the temperature, turns out to be equal to the information loss, which cannot be compensated for by any reconstruction method. This is demonstrated for the propagation of planar acoustic waves in water, which are induced by short laser pulses and measured by piezoelectric acoustical transducers. It turns out that for water, where the acoustic attenuation is proportional to the squared frequency, the resolution limit is proportional to the square root of the distance and inversely proportional to the square root of the logarithm of the signal-to-noise ratio. The proposed method could be used in future work for media other than water, such as biological tissue, where acoustic attenuation has a different power-law frequency dependence.

RESUMEN

Compressed sensing (CS) is a promising approach to reduce the number of measurements in photoacoustic tomography (PAT) while preserving high spatial resolution. This allows to increase the measurement speed and reduce system costs. Instead of collecting point-wise measurements, in CS one uses various combinations of pressure values at different sensor locations. Sparsity is the main condition allowing to recover the photoacoustic (PA) source from compressive measurements. In this paper, a different concept enabling sparse recovery in CS PAT is introduced. This approach is based on the fact that the second time derivative applied to the measured pressure data corresponds to the application of the Laplacian to the original PA source. As typical PA sources consist of smooth parts and singularities along interfaces, the Laplacian of the source is sparse (or at least compressible). To efficiently exploit the induced sparsity, a reconstruction framework is developed to jointly recover the initial and modified sparse sources. Reconstruction results with simulated as well as experimental data are given.

RESUMEN

We introduce all-optical photoacoustic projection imaging. An array of fiber-optic interferometers is used to measure photoacoustic signals. The obtained images represent the projection of the three-dimensional spatial light absorbance within a sample onto a two-dimensional plane. We assess the performance of the system by phantom measurements and show that the fiber-optic detectors achieve a noise-equivalent pressure of 24 Pascal at a 10 MHz bandwidth. Furthermore, we demonstrate the ability to acquire high-resolution projection images of large volumes within a short period of time.

RESUMEN

We report on a novel imaging system for large depth of field photoacoustic scanning macroscopy. Instead of commonly used piezoelectric transducers, fiber-optic based ultrasound detection is applied. The optical fibers are shaped into rings and mainly receive ultrasonic signals stemming from the ring symmetry axes. Four concentric fiber-optic rings with varying diameters are used in order to increase the image quality. Imaging artifacts, originating from the off-axis sensitivity of the rings, are reduced by coherence weighting. We discuss the working principle of the system and present experimental results on tissue mimicking phantoms. The lateral resolution is estimated to be below 200 µm at a depth of 1.5 cm and below 230 µm at a depth of 4.5 cm. The minimum detectable pressure is in the order of 3 Pa. The introduced method has the potential to provide larger imaging depths than acoustic resolution photoacoustic microscopy and an imaging resolution similar to that of photoacoustic computed tomography.

RESUMEN

We developed a multimodal imaging system, combining noncontact photoacoustic imaging and optical coherence tomography (OCT). Photoacoustic signals are recorded without contact to the specimens' surface by using an interferometric technique. The interferometer is realized within a fiber-optic network using a fiber laser at 1550 nm as source. The fiber-optic network allows the integration of a fiber-based OCT system operating at a wavelength region around 1310 nm. Light from the fiber laser and the OCT source are multiplexed into one fiber using wavelength-division multiplexing. The same focusing optics is used for both modalities. Back-reflected light from the sample is demultiplexed and guided to the respective imaging systems. As the same optical components are used for OCT and photoacoustic imaging, the obtained images are co-registered intrinsically in lateral direction. Three-dimensional imaging is implemented by hybrid galvanometer and mechanical scanning. To allow fast B-scan measurements, scanning of the interrogation beam along one dimension is executed by a galvanometer scanner. Slow-axis scanning, perpendicular to the fast axis, is performed utilizing a linear translational stage. We demonstrate two-dimensional and three-dimensional imaging on agarose phantoms.

Asunto(s)

RESUMEN

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.

Asunto(s)

RESUMEN

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications contact with the tissue should be avoided. As alternatives to contacting transducers interferometric means can be used to acquire photoacoustic signals remotely. In this paper we report on non-contact three and two dimensional photoacoustic imaging using an optical fiber-based Mach-Zehnder interferometer. A detection beam is transmitted through an optical fiber network onto the surface of the specimen. Back reflected light is collected and coupled into the same optical fiber. To achieve a high signal/noise ratio the reflected light is amplified by means of optical amplification with an erbium doped fiber amplifier before demodulation. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain reconstruction algorithm. We present remote photoacoustic imaging of a tissue mimicking phantom and on chicken skin.

RESUMEN

The ongoing expansion of the frequency range used for ultrasonic imaging requires increasing attention to the acoustic attenuation of biomaterials. This work presents a novel method for measuring the attenuation of tissue and liquids in vitro on the basis of single transmission measurements. Ultrasound was generated by short laser pulses directed onto a silicon wafer. In addition, unfocused piezoelectric transducers with a center frequency of 50 MHz were used to detect and emit ultrasound. The laser ultrasound method produces signals with a peak frequency of 30 MHz. In comparison to piezoelectric generation, pulse laser excitation provides approximately 4 times higher amplitudes and 20% larger bandwidth. By using two excitation methods in succession, the attenuation parameters of porcine fat samples with thicknesses in the range of 1.5 to 20 mm could be determined quantitatively within a total frequency range of 5 to 45 MHz. The setup for liquid measurements was tested on samples of human blood and olive oil. Our results are in good agreement with reports in literature.

Asunto(s)

RESUMEN

The frequency response of fiber optic line detectors is investigated in the presented paper. An analytical model based on oblique scattering of elastic waves is used to calculate the frequency dependent acousto-optical transfer functions of bare glass optical and polymer optical fibers. From the transfer functions the transient response of fibers detectors to photoacoustically excited spherical sources is derived. Photoacoustic tomography is simulated by calculating the temporal response of arrays of fiber optic line detectors and subsequent image reconstruction. The results show that the choice of the fiber material is of significant importance and influences the quality of imaging.

Asunto(s)