Wideband Fiber-Interferometer Stabilization With Variable Phase

source: © 2012 IEEE Photonics Technology Letters

We report on a robust scheme for wideband variable-phase interferometer stabilization based on active modulation. In contrast to previous schemes, the correction signal is generated without using second harmonics, whose low amplitude often requires employing narrowband lock-in amplifiers. Resonances in the element modulating the phase are attenuated to enable high gain without high-frequency oscillations. Operation over a 3-kHz bandwidth is demonstrated.  [Read more…]

Fig. 2 (a) Schematic description of the analog feedback circuit. PS: phase shifter. BSF: band-stop filter. HPF: high-pass filter. FB: feedback. (b) Measured combined response of the electronic filter and optical system. The figure shows a decrease in the strength of the PZT resonance.

Amir Rosenthal , Stephan Kellnberger , George Sergiadis , Vasilis Ntziachristos,”Wideband Fiber-Interferometer Stabilization With Variable Phase,” IEEE Photonics Technology Letters ( Volume: 24 , Issue: 17 , Sept.1, 2012 )

Optoacoustic determination of spatio-temporal responses of ultrasound sensors

source: © 2013 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control

The characterization of the spatial and frequency response of acoustic detectors is important for enabling accurate optoacoustic imaging. In this work, we developed a hybrid method for the characterization of the spatially dependent response of ultrasound detectors. The method is based on the experimental determination of the receive-mode electrical impulse response (EIR) of the sensor, which is subsequently convolved with the corresponding spatial impulse response (SIR), computed numerically. The hybrid method is shown to have superior performance over purely experimental techniques in terms of accurate determination of the spatial and temporal responses of ultrasonic detectors, in high as well as low sensitivity regions of the sensor.  [Read more…]

Fig. 1 Effect of the spatial impulse response (SIR) on an optoacoustic signal. (a) Optoacoustic waves emanating from the source at r ′ reach the different points of the transducer, r d1 and r d2 , at different times t1 and t2(c represents the speed of sound). (b) Geometry for the numerical example: the sensor is 1.8 mm along the y direction, 15 mm along the z direction (here the sensor is shown from the side) and it is cylindrically focused to 40 mm. The source is located at 33 mm from the sensor along its median axis x . The relative dimensions have been exaggerated for ease of representation. (c) Simulated optoacoustic signal (solid blue curve) and the distorted signal (dashed red curve) that results after convolution with the SIR at a point out of focus. Inset: SIR used for convolution. (d) Frequency spectra of the simulated signal (solid blue curve) and the signal convolved with the SIR (dashed red curve). Inset: spectrum of the SIR.

Miguel Angel Araque Caballero , Amir Rosenthal , Andreas Buehler , Daniel Razansky , Vasilis Ntziachristos,”Optoacoustic determination of spatio-temporal responses of ultrasound sensors,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ( Volume: 60 , Issue: 6 , June 2013 )

Modeling the shape of cylindrically focused transducers in three-dimensional optoacoustic tomography

source: © 2013 Journal of Biomedical Optics

Cross sectional tomographic systems based on cylindrically focused transducers are widely used in optoacoustic (photoacoustic) imaging due to important advantages they provide such as high-cross sectional resolution, real-time imaging capacity, and high-throughput performance. Tomographic images in such systems are commonly obtained by means of two-dimensional (2-D) reconstruction procedures assuming point-like detectors, and volumetric (whole-body) imaging is performed by superimposing the cross sectional images for different positions along the scanning direction. Such reconstruction strategy generally leads to in-plane and out-of-plane artifacts as well as significant quantification errors. Herein, we introduce two equivalent full three-dimensional (3-D) models capable of accounting for the shape of cylindrically focused transducers. The performance of these models in 3-D reconstructions considering several scanning positions is analyzed in this work. Improvements of the results rendered with the introduced reconstruction procedure as compared with the 2-D-based approach are described and discussed for simulations and experiments with phantoms and biological tissues.  [Read more…]

Fig. 2 Full-view tomographic geometry for the simulations and experiments. The ROI is depicted by the red cuboid. The blue points represent the positions of the centers of the cylindrically focused detectors. All the transducer positions lie on the surface of a cylinder with radius 2.54 cm.

Daniel Queirós, Xose Luis Dean-Ben, Andreas Buehler, Daniel Razansky, Amir Rosenthal, Vasilis Ntziachristos,”Modeling the shape of cylindrically focused transducers in three-dimensional optoacoustic tomography,” J. of Biomedical Optics, 18(7), 076014 (2013)

Weighted model-based optoacoustic reconstruction in acoustic scattering media

source: © 2013 Physics in Medicine & Biology

Model-based optoacoustic inversion methods are capable of eliminating image artefacts associated with the widely adopted back-projection reconstruction algorithms. Yet, significant image artefacts might also occur due to reflections and scattering of optoacoustically-induced waves from strongly acoustically-mismatched areas in tissues. Herein, we modify the model-based reconstruction methodology to incorporate statistically-based weighting in order to minimize these artefacts. The method is compared with another weighting procedure termed half-image reconstruction, yielding generally better results. The statistically-based weighting is subsequently verified experimentally, attaining quality improvement of the optoacoustic image reconstructions in the presence of acoustic mismatches in tissue phantoms and small animals ex-vivo.  [Read more…]

Fig. 5 Tomographic reconstructions of the zebrafish obtained with the IMMI algorithm (a)–(c), with the statistically-based weighted IMMI algorithm (d)–(f) and with the half-time weighted IMMI algorithm (g)–(i). The reconstructions are done by considering all the measuring locations in a full-view scenario (a), (d), (g), or for a limited-view case by taking measuring locations along an arc covering an angle of 270° (b), (e), (h) or 180° (c), (f), (i). For the limited-view case, the centre of the detection arc is located above the images. (j) and (k) show a comparison of the reconstructions obtained with the IMMI algorithm and the statistically-based IMMI algorithm for several slices. The area A is taken as the as the area inside the dashed circumferences and the weighting parameter ω = 1 for all cases.

X Luís Deán-Ben, Rui Ma, Amir Rosenthal, Vasilis Ntziachristos and Daniel Razansky,”Weighted model-based optoacoustic reconstruction in acoustic scattering media,” Physics in Medicine & Biology, Volume 58, Number 16 (2013)

Characterization of the Spatio-temporal Response of Optical Fiber Sensors to Incident Spherical Waves

source: © 2014 The Journal of the Acoustical Society of America

In this study a theoretical framework for calculating the acoustic response of optical fiber-based ultrasound sensors is presented. The acoustic response is evaluated for optical fibers with several layers of coating assuming a harmonic point source with arbitrary position and frequency. First, the fiber is acoustically modeled by a layered cylinder on which spherical waves are impinged. The scattering of the acoustic waves is calculated analytically and used to find the normal components of the strains on the fiber axis. Then, a strain-optic model is used to calculate the phase shift experienced by the guided mode in the fiber owing to the induced strains. The framework is showcased for a silica fiber with two layers of coating for frequencies in the megahertz regime, commonly used in medical imaging applications. The theoretical results are compared to experimental data obtained with a sensing element based on a pi-phase-shifted fiber Bragg grating and with photoacoustically generated ultrasonic signals.  [Read more…]

Fig. 1 Problem statement: Spherical waves generated from a point source scatter from the optical fiber which is located at a distance of d from the source. The radii of the glass fiber and the coatings are 62.5, 110, and 130 μm, respectively.

István A. Veres, Peter Burgholzer, Thomas Berer, Amir Rosenthal, Georg Wissmeyer, and Vasilis Ntziachristos,”Characterization of the spatio-temporal response of optical fiber sensors to incident spherical waves,” The Journal of the Acoustical Society of America 135, 1853 (2014)

Multiscale Multispectral Optoacoustic Tomography by a Stationary Wavelet Transform Prior to Unmixing

Multiscale view of perfusion imaging in the extremities.

source: © 2014  IEEE Transactions on Medical Imaging. 

Multispectral optoacoustic tomography (MSOT) utilizes broadband ultrasound detection for imaging biologically-relevant optical absorption features at a range of scales. Due to the multiscale and multispectral features of the technology, MSOT comes with distinct requirements in implementation and data analysis. In this work, we investigate the interplay between scale, which depends on ultrasonic detection frequency, and optical multispectral spectral analysis, two dimensions that are unique to MSOT and represent a previously unexplored challenge. We show that ultrasound frequency-dependent artifacts suppress multispectral features and complicate spectral analysis. In response, we employ a wavelet decomposition to perform spectral unmixing on a per-scale basis (or per ultrasound frequency band) and showcase imaging of fine-scale features otherwise hidden by low frequency components. We explain the proposed algorithm by means of simple simulations and demonstrate improved performance in imaging data of blood vessels in human subjects.  [Read more…]

Fig. 5 A simulated example. Each of the single wavelength images (a), which contain negative values, is decomposed by SWT and individually reconstructed (inverse SWT=ISWT ), as shown in (b). It can be observed in this particular case that the fine scale features are mainly captured in the detail coefficients, while the coarse scale features are mainly captured in the level three approximation. NNLS is applied per scale and the resulting oxyhemoglobin and deoxyhemoglobin images are shown (c). These images compare favorably with the “ground truth” images (d), which are the HbO2 and Hb distributions used to generate the data, but with negative values truncated. The images obtained by the conventional approach (e), NNLS without SWT, do not resemble the “ground truth” because of the effect of negative value artifacts. The example applies the Daubechies wavelet with two vanishing moments and a three level decomposition.

Adrian Taruttis , Amir Rosenthal , Marcin Kacprowicz , Neal C. Burton , Vasilis Ntziachristos, “Multiscale multispectral optoacoustic tomography by a stationary wavelet transform prior to unmixing,” IEEE transactions on medical imaging, Volume 33, Issue 5, Pages 1194-1202