We demonstrate experimentally an optical system for under-sampling several bandwidth-limited signals with carrier frequencies that are not known apriori and can be located anywhere within a very broad frequency region between 0–18GHz. The system is based on under-sampling asynchronously at three different sampling rates. The optical pulses required for the under-sampling are generated by a combination of an electrical comb generator and an electro-absorption optical modulator. To reduce loss and improve performance the implementation of the optical system is based on a wavelength division multiplexing technique. An accurate reconstruction of both the phase and the amplitude was obtained when two chirped signals each with a bandwidth of about 150 MHz were sampled. [Read more…]
In this letter we experimentally demonstrate the sensitivity and overall performance of iterative correction for light attenuation in optoacoustic tomography as a function of number of iterations and accuracy of the tissue optical properties estimations. Experimental optoacoustic data were obtained by circularly illuminating a tissue-mimicking phantom with a high intensity pulsed near infrared laser and measuring the subsequent acoustic waves using a broadband acoustic hydrophone. We showcase an improvement in image fidelity and quantification due to the iterative inversion but find the method sensitive to the background optical properties and of a diverging behavior when increasing the number of iterations. Â [Read more…]
Fig. 2 The reconstruction error of the algorithm as a function of iteration for different assumed reduced scattering coefficient values. In all cases μa=20 cm−1 and σ=0.001. The quality measures showed assumed (a) the standard deviation within the insertion and (b) a root-mean-square error estimate given in Eq. (4).
We demonstrate a new method based on inverse scattering theory for designing the refractive index profile of single-mode planar waveguides in order to obtain a desired TE-mode profile. The method enables a direct design of the waveguide profile without the need for iterative optimization algorithms. The design is based on a first order solution to the Gel’fand-Levitan-MarCcaronenko integral equation that gives a simple linear connection between a small change in the scattering data and the corresponding change in the kernel function. This connection reduces the design problem to a simple linear constrained minimization problem which has an explicit solution. Our design method allows adding additional constraints on the refractive index profile such as the waveguide width. The method presented in this paper can be expanded to analyze TM modes and for designing multi-mode planar waveguides. Â [Read more…]
Fig. 2 Comparison between the original and the extracted refractive index profiles that were reconstructed from the linearized GLM equation. Both profiles were obtained by changing the refractive index profile of an hyperbolic secant waveguide that corresponds to a reflectionless waveguide. An excellent agreement between the reconstructed (dotted line) and the original (solid line) profiles was obtained for (a) a truncated hyperbolic secant profile and (b) a sinusoidal perturbation.
Recent advances in electro-optical systems make them ideal for undersampling multiband signals with very high carrier frequencies. In this paper, we propose a new scheme for sampling and reconstructing of a multiband sparse signals that occupy a small part of a given broad frequency range under the constraint of a small number of sampling channels. The locations of the signal bands are not known a priori. The scheme, which we call synchronous multirate sampling (SMRS), entails gathering samples synchronously at few different rates whose sum is significantly lower than the Nyquist sampling rate. The signals are reconstructed by finding a solution of an underdetermined system of linear equations by applying a pursuit algorithm and assuming that the solution is composed of a minimum number of bands. The empirical reconstruction success rate is higher than obtained using previously published multicoset scheme when the number of sampling channels is small and the conditions for a perfect reconstruction in the multicoset scheme are not fulfilled. The practical sampling system which is simulated in our work consists of three sampling channels. Our simulation results show that a very high empirical success rate is obtained when the total sampling rate is five times higher than the total signal support of a complex signal with four bands. By comparison, a multicoset sampling scheme obtains a very high empirical success rate with a total sampling rate which is three times higher than the total signal support. However, the multicoset scheme requires 14 channels. Â [Read more…]
Fig. 2 Empirical success percentages for four-band complex signals calculated by using the SMRS reconstruction scheme (circles) and by using SBR4 in 7 as a function of the spectral support (BW) for FNyquist= 20 GHz and a total sampling rate Ftotal= 3 GHz. The number of sampling channels is equal to 3 in the SMRS scheme and is equal to p=58 in the equivalent multicoset sampling scheme.
Quantification of tissue morphology and biomarker distribution by means of optoacoustic tomography is an important and longstanding challenge, mainly caused by the complex heterogeneous structure of biological tissues as well as the lack of accurate and robust reconstruction algorithms. The recently introduced model-based inversion approaches were shown to mitigate some of reconstruction artifacts associated with the commonly used back-projection schemes, while providing an excellent platform for obtaining quantified maps of optical energy deposition in experimental configurations of various complexity. In this work, we introduce a weighted model-based approach, capable of overcoming reconstruction challenges caused by per-projection variations of object’s illumination and other partial illumination effects. The universal weighting procedure is equally shown to reduce reconstruction artifacts associated with other experimental imperfections, such as non-uniform transducer sensitivity fields. Significant improvements in image fidelity and quantification are showcased both numerically and experimentally on tissue phantoms and mice.  [Read more…]
Fig. 1 Optoacoustic imaging configurations with partial or variable tomographic data. (a) Circular scanning with narrow laser beam and a rotating object. Illumination and detector are static; (b) circular scanning with ultrasonic detector having limited angular view. The imaged object and illumination are static; (c) optoacoustic microscopy (B-mode) imaging with confocal illumination-detection geometry and linear translation.
New imaging methods are urgently needed to identify high-risk atherosclerotic lesions prior to the onset of myocardial infarction, stroke, and ischemic limbs. Molecular imaging offers a new approach to visualize key biological features that characterize high-risk plaques associated with cardiovascular events. While substantial progress has been realized in clinical molecular imaging of plaques in larger arterial vessels (carotid, aorta, iliac), there remains a compelling, unmet need to develop molecular imaging strategies targeted to high-risk plaques in human coronary arteries. We present recent developments in intravascular near-IR fluorescence catheter-based strategies for in vivo detection of plaque inflammation in coronary-sized arteries. In particular, the biological, light transmission, imaging agent, and engineering principles that underlie a new intravascular near-IR fluorescence sensing method are discussed. Intravascular near-IR fluorescence catheters appear highly translatable to the cardiac catheterization laboratory, and thus may offer a new in vivo method to detect high-risk coronary plaques and to assess novel atherosclerosis biologics. Â [Read more…]
Fig 1. Catheter prototype for intravascular sensing of NIR fluorescence signals. (a) The NIRF catheter consists of a 0.36-mm∕0.014-in. floppy radiopaque tip with a maximum outer diameter of 0.48mm∕0.019in.. The arrow highlights the focal spot (40±15μm) for the 90-deg arc-sensing catheter at a distance of 2±1mm (arrow). (b) Phantom experiment to measure NIR light attenuation in the presence of whole blood. Plaque (P) consists of 1% Intralipid plus India ink 50ppm plus AF750 (an NIR fluorochrome, concentration 300nmol∕L); tissue (T: fibrous cap) consists of polyester casting resin plus titanium dioxide plus India ink; a container (gray shaded area) was filled with fresh rabbit blood or saline. The catheter was immersed in fresh rabbit blood and positioned at variable distance (D) from a fluorescent phantom representing the plaque (P). To mimic the presence of a fibrous cap, a solid tissue phantom of thickness T was interposed between the plaque and the lumen. (c) Plot of detected NIRF signal as a function of distance D in presence of blood compared to saline, showing only modest attenuation by blood. Inset, fluorescence signal decay in saline at distance of up to 10mm. (d) Plot of the detected NIRF signal in blood in the presence of a tissue phantom (T) of thickness 500μm shows modest NIRF signal attenuation (<35%) vs the case in (c) where T=0. Reproduced by permission from Ref. 22.
Neuroscience investigations may significantly benefit from the availability of accurate imaging methods of brain parameters in small animals. In this letter, we investigate the imaging performance of the recently introduced interpolated model-matrix inversion (IMMI), in quantitative optoacoustic imaging of the mouse head. We compare the findings of the method against back-projection inversion methods that have more commonly been considered. We find that cross-sectional images of the mouse head accurately match anatomical structures seen on cryosliced head images serving as the gold standard. Moreover, superior imaging performance is found for IMMI compared to previously reported optoacoustic imaging of the mouse head. Â [Read more…]
Fig 2.Several cross-sectional slices of the head region of a mouse back projection based reconstructions of (a) head region, (b) lower part of the head, (c) and (d) corresponding IMMI reconstructions, (e) and (f) corresponding IMMI high-pass filtered images, and (g) and (h) cryoslices. Anatomical structure: 1, 3, 4, 6—eye sockets 2, 5, 7, 8—blood vessels.
Purpose:
Optoacoustic imaging enables mapping the optical absorption of biological tissue using optical excitation and acoustic detection. Although most imageâ€reconstruction algorithms are based on the assumption of a detector with an isotropic sensitivity, the geometry of the detector often leads to a response with spatially dependent magnitude and bandwidth. This effect may lead to attenuation or distortion in the recorded signal and, consequently, in the reconstructed image.
Methods: Herein, an accurate numerical method for simulating the spatially dependent response of an arbitraryâ€shape acoustic transducer is presented. The method is based on an analytical solution obtained for a twoâ€dimensional line detector. The calculated response is incorporated in the forward model matrix of an optoacoustic imaging setup using temporal convolution, and image reconstruction is performed by inverting the matrix relation.  [Read more…]
Fig. 8 Experimental reconstructions of a point optoacoustic source detected by a flat detector with a width of 1.3 cm obtained using (a) the backâ€projection algorithm (b) IMMI modeled with a point detector (c) IMMI modeled with a 1.3â€mm flat detector using spatial convolution and (d) IMMI modeled with a 1.3â€mm flat detector using temporal convolution. The point source was obtained by applying planeâ€selective illuminating on a black hair embedded in a clear agar phantom, as shown in Fig. 6(a). Although both the spatial†and temporalâ€convolution methods managed enhancing the reconstruction resolution, the temporalâ€convolution method yielded a more accurate reconstruction with less background texture.
Results:
The method was numerically and experimentally demonstrated in two dimensions for both flat and focused transducers and compared to the spatialâ€convolution method. In forward simulations, the developed method did not suffer from the numerical errors exhibited by the spatialâ€convolution method. In reconstruction simulations and experiments, the use of both temporalâ€convolution and spatialâ€convolution methods lead to an enhancement in resolution compared to a reconstruction with a point detector model. However, because of its higher modeling accuracy, the temporalâ€convolution method achieved a noise figure approximated three times lower than the spatialâ€convolution method.
Conclusions:
The demonstrated performance of the spatialâ€convolution method shows it is a powerful tool for reducing reconstruction artifacts originating from the detector finite size and improving the quality of optoacoustic reconstructions. Furthermore, the method may be used for assessing new system designs. Specifically, detectors with nonstandard shapes may be investigated.
Quantification of biomarkers using multispectral optoacoustic tomography can be challenging due to photon fluence variations with depth and spatially heterogeneous tissue optical properties. Herein we introduce a spectral ratio approach that accounts for photon fluence variations. The performance and imaging improvement achieved with the proposed method is showcased both numerically and experimentally in phantoms and mice.  [Read more…]
Fig. 3 (a) Optoacoustic image of a mouse with ICG filled tubes at 800 nm . (b) Sketch of the mouse and the implanted tubes. (c) Spectral difference. (d) Spectral ratio. (e) Profiles along the dashed lines of (b) for spectral ratio and spectral difference. (f) Superimposed image of (d) on (a) after application of a threshold at ð¼=0.5 .
Advances in fabrication of high-finesse optical resonators hold promise for the development of miniaturized, ultra-sensitive, wide-band optical sensors, based on resonance-shift detection. Many potential applications are foreseen for such sensors, among them highly sensitive detection in ultrasound and optoacoustic imaging. Traditionally, sensor interrogation is performed by tuning a narrow linewidth laser to the resonance wavelength. Despite the ubiquity of this method, its use has been mostly limited to lab conditions due to its vulnerability to environmental factors and the difficulty of multiplexing – a key factor in imaging applications. In this paper, we develop a new optical-resonator interrogation scheme based on wideband pulse interferometry, potentially capable of achieving high stability against environmental conditions without compromising sensitivity. Additionally, the method can enable multiplexing several sensors. The unique properties of the pulse-interferometry interrogation approach are studied theoretically and experimentally. Methods for noise reduction in the proposed scheme are presented and experimentally demonstrated, while the overall performance is validated for broadband optical detection of ultrasonic fields. The achieved sensitivity is equivalent to the theoretical limit of a 6 MHz narrow-line width laser, which is 40 times higher than what can be usually achieved by incoherent interferometry for the same optical resonator.  [Read more…]
Fig. 4 (a) The schematic of the system used to evaluate the effect of ASE on the noise in the detection scheme. The visibility and noise level were measured for OPDs varying from 0 to 15 mm (b) The noise at the differential amplifier for wideband pulsed (blue square markers) and CW (red circle markers) sources as function of OPD obtained when the source is filtered to a bandwidth of 0.3 nm. The noise at OPD = 0 was mostly a result of electronic noise and was similar for both optical sources. (c) the noise data of Fig. 4(b) scaled to the same level for better visualization displayed with the measured fringe visibility (dashed curve). The similar dependency of noise for both cases indicates that the ASE noise is dominant in the pulse interferometry scheme. (d) The system used to test the effect of ASE rejection on noise reduction in the pulse interferometry setup. The saturable absorber (SA) added to the system had a transmission approximately 2.8 higher for the pulses compared to CW. (e) The noise recorded with the SA (solid-blue curve) and with an attenuator replacing the SA to ensure the same signal level (dashed-red curve). A reduction of 2.3 in the noise was observed, in correspondence with the SA rejection ratio.
