In Vitro and In Vivo Multispectral Photoacoustic Imaging for the Evaluation of Chromophore Concentration.

Multispectral photoacoustic imaging is a powerful noninvasive medical imaging technique that provides access to functional information. In this study, a set of methods is proposed and validated, with experimental multispectral photoacoustic images used to estimate the concentration of chromophores. The unmixing techniques used in this paper consist of two steps: (1) automatic extraction of the reference spectrum of each pure chromophore; and (2) abundance calculation of each pure chromophore from the estimated reference spectra. The compared strategies bring positivity and sum-to-one constraints, from the hyperspectral remote sensing field to multispectral photoacoustic, to evaluate chromophore concentration. Particularly, the study extracts the endmembers and compares the algorithms from the hyperspectral remote sensing domain and a dedicated algorithm for segmentation of multispectral photoacoustic data to this end. First, these strategies are tested with dilution and mixing of chromophores on colored 4% agar phantom data. Then, some preliminary in vivo experiments are performed. These consist of estimations of the oxygen saturation rate (sO2) in mouse tumors. This article proposes then a proof-of-concept of the interest to bring hyperspectral remote sensing algorithms to multispectral photoacoustic imaging for the estimation of chromophore concentration.

Sparse hand-held probe for optoacoustic ultrasound volumetric imaging: an experimental proof-of-concept study.

We present an experimental proof-of-concept study on the performance of a sparse segmented annular array for optoacoustic imaging. A capacitive micromachined ultrasonic transducer was equipped with a negatively focused acoustic lens and scanned in an annular fashion to exploit the performance of the sparse array geometry proposed in our recent numerical studies [Biomed. Opt. Express10, 1545 (2019)BOEICL2156-708510.1364/BOE.10.001545; J. Biomed. Opt.23, 025004 (2018)JBOPFO1083-366810.1117/1.JBO.23.2.025004]. A dedicated water tank was made using a 3D printer for light delivery and mounting the sample. A phantom experiment was carried out to showcase the possibility of full-field optoacoustic ultrasound (OPUS) imaging and confirm the earlier numerical results. This proof of concept opens the door towards a prototype of OPUS imaging for (pre-) clinical studies.

Liposomes Containing Nickel-Bis(dithiolene) Complexes for Photothermal Theranostics.

New thermosensitive liposomes with a phase transition at 42 °C, containing nickel-bis(dithiolene) complexes as efficient and stable photothermal agents, have been formulated and characterized. These liposomes are highly stable and keep their contents at 37 °C for more than 30 days. On the contrary, the mild hyperthermia generated by the nickel-bis(dithiolene) complex under 940 nm NIR irradiation allows for the fine controlled release of the liposome contents, making such liposomes highly suitable for on-demand drug delivery in the human body under NIR laser irradiation. These liposomes can also be directly used, as shown here, as nanoagents for photothermal therapy. In fact, strong cell death can be generated under laser irradiation in the presence of these photothermally active nanocargos containing less than 10% w/w of metal complex. We also demonstrate, for the first time, that nickel-bis(dithiolene) complexes are good photoacoustic agents, generating easily detectable ultrasonic signals directly proportional to the concentration of complexes and the used laser power.

Analytical model of optical fluence inside multiple cylindrical inhomogeneities embedded in an otherwise homogeneous turbid medium for quantitative photoacoustic imaging.

We present an analytical model of optical fluence for multiple cylindrical inhomogeneities embedded in an otherwise homogeneous turbid medium. The model is based on the diffusion equation and represents the optical fluence distribution inside and outside inhomogeneities as a series of modified Bessel functions. We take into account the interplay between cylindrical inhomogeneities by introducing new boundary conditions on the surface of inhomogeneities. The model is compared with the numerical solution of the diffusion equation with NIRFAST software. The fluences inside the inhomogeneities obtained by the two methods are in close agreement. This permits the use of the model as a forward model for quantitative photoacoustic imaging.

Full-volume three-component intraventricular vector flow mapping by triplane color Doppler.

Objective. Intraventricular vector flow mapping (iVFM) is a velocimetric technique for retrieving two-dimensional velocity vector fields of blood flow in the left ventricular cavity. This method is based on conventional color Doppler imaging, which makesiVFM compatible with the clinical setting. We have generalized theiVFM for a three-dimensional reconstruction (3D-iVFM).Approach.3D-iVFM is able to recover three-component velocity vector fields in a full intraventricular volume by using a clinical echocardiographic triplane mode. The 3D-iVFM problem was written in the spherical (radial, polar, azimuthal) coordinate system associated to the six half-planes produced by the triplane mode. As with the 2D version, the method is based on the mass conservation, and free-slip boundary conditions on the endocardial wall. These mechanical constraints were imposed in a least-squares minimization problem that was solved through the method of Lagrange multipliers. We validated 3D-iVFMin silicoin a patient-specific CFD (computational fluid dynamics) model of cardiac flow and tested its clinical feasibilityin vivoin patients and in one volunteer.Main results.The radial and polar components of the velocity were recovered satisfactorily in the CFD setup (correlation coefficients,r = 0.99 and 0.78). The azimuthal components were estimated with larger errors (r = 0.57) as only six samples were available in this direction. In bothin silicoandin vivoinvestigations, the dynamics of the intraventricular vortex that forms during diastole was deciphered by 3D-iVFM. In particular, the CFD results showed that the mean vorticity can be estimated accurately by 3D-iVFM.Significance. Our results tend to indicate that 3D-iVFM could provide full-volume echocardiographic information on left intraventricular hemodynamics from the clinical modality of triplane color Doppler.

Estimating Arterial Wall Deformations from Automatic Key-Point Detection and Matching.

Assessing arterial-wall motion and deformations may reveal pathologic alterations in biomechanical properties of the parietal tissues and, thus, contribute to the detection of vascular disease onset. Ultrasound image sequences allow the observation of this motion and many methods have been developed to estimate temporal changes in artery diameter and wall thickness and to track 2-D displacements of selected points. Some methods enable the assessment of shearing or stretching within the wall, but none of them can estimate all these deformations simultaneously. The method herein proposed was devised to simultaneously estimate translation, compression, stretching and shearing of the arterial wall in ultrasound B-mode image sequences representing the carotid artery longitudinal section. Salient blob-like patterns, called key points, are automatically detected in each frame and matched between successive frames. A robust estimator based on an affine transformation model is then used to assess frame-to-frame motion explaining at best the key-point matches and to reject outliers. Realistic simulated image sequences were used to evaluate the accuracy and robustness of the method against ground truth. The method was also visually assessed on clinical image sequences, for which true deformations are unknown.

Translation of Simultaneous Vessel Wall Motion and Vectorial Blood Flow Imaging in Healthy and Diseased Carotids to the Clinic: A Pilot Study.

This study aims to investigate the clinical feasibility of simultaneous extraction of vessel wall motion and vectorial blood flow at high frame rates for both extraction of clinical markers and visual inspection. If available in the clinic, such a technique would allow a better estimation of plaque vulnerability and improved evaluation of the overall arterial health of patients. In this study, both healthy volunteers and patients were recruited and scanned using a planewave acquisition scheme that provided a data set of 43 carotid recordings in total. The vessel wall motion was extracted based on the complex autocorrelation of the signals received, while the vector flow was extracted using the transverse oscillation technique. Wall motion and vector flow were extracted at high frame rates, which allowed for a visual appreciation of tissue movement and blood flow simultaneously. Several clinical markers were extracted, and visual inspections of the wall motion and flow were conducted. From all the potential markers, young healthy volunteers had smaller artery diameter (7.72 mm) compared with diseased patients (9.56 mm) ( p -value ≤ 0.001), 66% of diseased patients had backflow compared with less than 10% for the other patients ( p -value ≤ 0.05), a carotid with a pulse wave velocity extracted from the wall velocity greater than 7 m/s was always a diseased vessel, and the peak wall shear rate decreased as the risk increases. Based on both the pathological markers and the visual inspection of tissue motion and vector flow, we conclude that the clinical feasibility of this approach is demonstrated. Larger and more disease-specific studies using such an approach will lead to better understanding and evaluation of vessels, which can translate to future use in the clinic.

Carotid Wall Longitudinal Motion in Ultrasound Imaging: An Expert Consensus Review.

Motion extracted from the carotid artery wall provides unique information for vascular health evaluation. Carotid artery longitudinal wall motion corresponds to the multiphasic arterial wall excursion in the direction parallel to blood flow during the cardiac cycle. While this motion phenomenon has been well characterized, there is a general lack of awareness regarding its implications for vascular health assessment or even basic vascular physiology. In the last decade, novel estimation strategies and clinical investigations have greatly advanced our understanding of the bi-axial behavior of the carotid artery, necessitating an up-to-date review to summarize and classify the published literature in collaboration with technical and clinical experts in the field. Within this review, the state-of-the-art methodologies for carotid wall motion estimation are described, and the observed relationships between longitudinal motion-derived indices and vascular health are reported. The vast number of studies describing the longitudinal motion pattern in plaque-free arteries, with its putative application to cardiovascular disease prediction, point to the need for characterizing the added value and applicability of longitudinal motion beyond established biomarkers. To this aim, the main purpose of this review was to provide a strong base of theoretical knowledge, together with a curated set of practical guidelines and recommendations for longitudinal motion estimation in patients, to foster future discoveries in the field, toward the integration of longitudinal motion in basic science as well as clinical practice.

Investigation of PVA cryogel Young's modulus stability with time, controlled by a simple reliable technique.

We describe a quasistatic method for mechanical characterization of tissue-mimicking material used in elastography. We demonstrate that it is possible to assess the elasticity modulus with a reasonable reproducibility using simple and easy tools and methods. Possessing a simple relevant technique with evaluated relative error to assess Young's modulus of these phantoms could deeply improve the quality of the research in the field of elastography. The method was tested and validated with four samples of polyvinyl alcohol (PVA) cryogel with different elasticity values corresponding to those of stiffer soft biological tissues. Young's moduli, varying from 70 to 180 kPa depending on the number of freeze-thaw cycles (two to five), were measured within strict measurement conditions and found to have a reproducibility varying from 4% to 8%. Relative error, estimated as the ratio between observed and reference values, varied from 16% to 32%. Besides, measurement stability over 4 months was evaluated. The method demonstrated good feasibility and acceptable reproducibility for mechanically characterizing and controlled over time phantoms used for validating new potential ultrasound imaging techniques in the field of elastography. Nevertheless, in this study, investigation was performed on gel possessing young's modulus values ranging from 80 to 215 kPa. Some tissue values of Young'modulus were reported to be lower, ranging from 0.6 to 28 kPa as liver or glandular values. Consequently, further validation of this static method for mechanical characterization of phantom gels should be performed using softer PVA cryogel.

Sparse sampling and reconstruction for an optoacoustic ultrasound volumetric hand-held probe.

Accurate anatomical localization of functional information is the main goal of hybridizing optoacoustic and ultrasound imaging, with the promise of early stage diagnosis and disease pathophysiology. Optoacoustic integration to ultrasound is a relatively mature technique for clinical two-dimensional imaging, however the complexity of biological samples places particular demands for volumetric measurement and reconstruction. This integration is a multi-fold challenge that is mainly associated with the system geometry, the sampling and beam quality. In this study, we evaluated the design geometry for the sparse ultrasonic hand-held probe that is popularly associated with three-dimensional imaging of anatomical deformation, to incorporate the three-dimensional optoacoustic physiological information. We explored the imaging performance of three unconventional annular geometries; namely, segmented, spiral, and circular geometries. To avoid bias evaluation, two classes of analytical and model-based algorithms were used. The superior performance of the segmented annular array for recovery of the true object is demonstrated. Along with the model-based approach, this geometry offers spatial invariant resolution for the optoacoustic mode for the given field of view.The analytical approach, on the other hand, is computationally less expensive and is the method of choice for ultrasound imaging. Our design can potentially evolve into a valuable diagnostic tool, particularly for vascular-related disease.

Video Magnification Applied in Ultrasound.

GOAL: This paper describes a method to enhance, visualize, and reveal subtle motion that can be present in medical images. As proposed in vision applications, the principle is to magnify displacement applied, in this case, to cardiovascular tissues (carotid). METHODS: In the example presented, ultrasound data were acquired at a high frame rate and two-dimensional motion was estimated, amplified, and reapplied in ultrasound carotid sequences. RESULTS: Video magnification makes fast and complex phenomena of human tissue visible. In fact, not only pulse and reflected wave, but also global radial and longitudinal motion in the example presented are visible with video magnification. CONCLUSION: Video magnification can be used in medical imaging for subtle motion visualization. One of the many possible applications is direct visualization of a local modification in terms of stiffness of a tissue (due to local necrosis, for instance) from acquisition. Moreover, video magnification can be executed with any type of imaging modality. SIGNIFICANCE: Video magnification could be a new tool for physicians to highlight new pathology indicators or for long-term disease monitoring.

Theoretical characterization of annular array as a volumetric optoacoustic ultrasound handheld probe.

Optoacoustic ultrasound (OPUS) is a promising hybridized technique for simultaneous acquisition of functional and morphological data. The optical specificity of optoacoustic leverages the diagnostic aptitude of ultrasonography beyond anatomy. However, this integration has been rarely practiced for volumetric imaging. The challenge lies in the effective imaging probes that preserve the functionality of both modalities. The potentials of a sparse annular array for volumetric OPUS imaging are theoretically investigated. In order to evaluate and optimize the performance characteristics of the probe, series of analysis in the framework of system model matrix was carried out. The two criteria of voxel crosstalk and eigenanalysis have been employed to unveil information about the spatial sensitivity, aliasing, and number of definable spatial frequency components. Based on these benchmarks, the optimal parameters for volumetric handheld probe are determined. In particular, the number, size, and the arrangement of the elements and overall aperture dimension were investigated. The result of the numerical simulation suggests that the segmented-annular array of 128 negatively focused elements with 1λ × 20λ size, operating at 5-MHz central frequency showcases a good agreement with the physical requirement of both imaging systems. We hypothesize that these features enable a high-throughput volumetric passive/active ultrasonic imaging system with great potential for clinical applications.

High-Frame-Rate Speckle-Tracking Echocardiography.

Conventional echocardiography is the leading modality for noninvasive cardiac imaging. It has been recently illustrated that high-frame-rate echocardiography using diverging waves could improve cardiac assessment. The spatial resolution and contrast associated with this method are commonly improved by coherent compounding of steered beams. However, owing to fast tissue velocities in the myocardium, the summation process of successive diverging waves can lead to destructive interferences if motion compensation (MoCo) is not considered. Coherent compounding methods based on MoCo have demonstrated their potential to provide high-contrast B-mode cardiac images. Ultrafast speckle-tracking echocardiography (STE) based on common speckle-tracking algorithms could substantially benefit from this original approach. In this paper, we applied STE on high-frame-rate B-mode images obtained with a specific MoCo technique to quantify the 2-D motion and tissue velocities of the left ventricle. The method was first validated in vitro and then evaluated in vivo in the four-chamber view of 10 volunteers. High-contrast high-resolution B-mode images were constructed at 500 frames/s. The sequences were generated with a Verasonics scanner and a 2.5-MHz phased array. The 2-D motion was estimated with standard cross correlation combined with three different subpixel adjustment techniques. The estimated in vitro velocity vectors derived from STE were consistent with the expected values, with normalized errors ranging from 4% to 12% in the radial direction and from 10% to 20% in the cross-range direction. Global longitudinal strain of the left ventricle was also obtained from STE in 10 subjects and compared to the results provided by a clinical scanner: group means were not statistically different ( value = 0.33). The in vitro and in vivo results showed that MoCo enables preservation of the myocardial speckles and in turn allows high-frame-rate STE.

PSF dedicated to estimation of displacement vectors for tissue elasticity imaging with ultrasound.

This paper investigates a new approach devoted to displacement vector estimation in ultrasound imaging. The main idea is to adapt the image formation to a given displacement estimation method to increase the precision of the estimation. The displacement is identified as the zero crossing of the phase of the complex cross-correlation between signals extracted from the lateral direction of the ultrasound RF image. For precise displacement estimation, a linearity of the phase slope is needed as well as a high phase slope. Consequently, a particular point spread function (PSF) dedicated to this estimator is designed. This PSF, showing oscillations in the lateral direction, leads to synthesis of lateral RF signals. The estimation is included in a 2-D displacement vector estimation method. The improvement of this approach is evaluated quantitatively by simulation studies. A comparison with a speckle tracking technique is also presented. The lateral oscillations improve both the speckle tracking estimation and our 2-D estimation method. Using our dedicated images, the precision of the estimation is improved by reducing the standard deviation of the lateral displacement error by a factor of 2 for speckle tracking and more than 3 with our method compared to using conventional images. Our method performs 7 times better than speckle tracking. Experimentally, the improvement in the case of a pure lateral translation reaches a factor of 7. Finally, the experimental feasibility of the 2-D displacement vector estimation is demonstrated on data acquired from a Cryogel phantom.

Quantitative comparison of PZT and CMUT probes for photoacoustic imaging: Experimental validation.

Photoacoustic (PA) signals are short ultrasound (US) pulses typically characterized by a single-cycle shape, often referred to as N-shape. The spectral content of such wideband signals ranges from a few hundred kilohertz to several tens of megahertz. Typical reception frequency responses of classical piezoelectric US imaging transducers, based on PZT technology, are not sufficiently broadband to fully preserve the entire information contained in PA signals, which are then filtered, thus limiting PA imaging performance. Capacitive micromachined ultrasonic transducers (CMUT) are rapidly emerging as a valid alternative to conventional PZT transducers in several medical ultrasound imaging applications. As compared to PZT transducers, CMUTs exhibit both higher sensitivity and significantly broader frequency response in reception, making their use attractive in PA imaging applications. This paper explores the advantages of the CMUT larger bandwidth in PA imaging by carrying out an experimental comparative study using various CMUT and PZT probes from different research laboratories and manufacturers. PA acquisitions are performed on a suture wire and on several home-made bimodal phantoms with both PZT and CMUT probes. Three criteria, based on the evaluation of pure receive impulse response, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) respectively, have been used for a quantitative comparison of imaging results. The measured fractional bandwidths of the CMUT arrays are larger compared to PZT probes. Moreover, both SNR and CNR are enhanced by at least 6 dB with CMUT technology. This work highlights the potential of CMUT technology for PA imaging through qualitative and quantitative parameters.

Hybrid Strategy to Simulate 3-D Nonlinear Radio-Frequency Ultrasound Using a Variant Spatial PSF.

There are several simulators for medical ultrasound (US) applications that can fully compute the nonlinear propagation on the transmitted pulse and the corresponding radio-frequency (RF) images. Creanuis is one recent model used to generate nonlinear RF images; however, the time requirements are long compared with linear models using a convolution strategy. In this paper, we describe an approach using convolution coupled with nonlinear information to create a pseudoacoustic tool that is able to quickly generate realistic US images. Several point-spread functions (PSFs) are computed with Creanuis. These PSFs are extracted at different depths in order to take into account variation in the resolution and apparition of harmonics during propagation. One convolution is then conducted for each PSF to generate a set of nonlinear raw RF images. The final image is obtained by merging these raw images using a PSF-weighting function. This hybrid Creanuis strategy was extended to 2-D, 2-D +t , 3-D, and 3-D +t images for both linear and phased-array geometries. We validated h-Creanuis using the mean deviation between the proposed images and those created using Creanuis and examined their statistical distributions. The mean deviations of Creanuis and h-Creanuis are below 2.5% for fundamental and second-harmonic images. The 3-D +t images obtained demonstrate the correct motion characteristics for speckle in sequences of both fundamental and second-harmonic images.

Full 3-D transverse oscillations: a method for tissue motion estimation.

We present a new method to estimate 4-D (3-D + time) tissue motion. The method used combines 3-D phase based motion estimation with an unconventional beamforming strategy. The beamforming technique allows us to obtain full 3-D RF volumes with axial, lateral, and elevation modulations. Based on these images, we propose a method to estimate 3-D motion that uses phase images instead of amplitude images. First, volumes featuring 3-D oscillations are created using only a single apodization function, and the 3-D displacement between two consecutive volumes is estimated simultaneously by applying this 3-D estimation. The validity of the method is investigated by conducting simulations and phantom experiments. The results are compared with those obtained with two other conventional estimation methods: block matching and optical flow. The results show that the proposed method outperforms the conventional methods, especially in the transverse directions.

Multigrid-based reconstruction algorithm for quantitative photoacoustic tomography.

This paper proposes a multigrid inversion framework for quantitative photoacoustic tomography reconstruction. The forward model of optical fluence distribution and the inverse problem are solved at multiple resolutions. A fixed-point iteration scheme is formulated for each resolution and used as a cost function. The simulated and experimental results for quantitative photoacoustic tomography reconstruction show that the proposed multigrid inversion can dramatically reduce the required number of iterations for the optimization process without loss of reliability in the results.

Progressive attenuation of the longitudinal kinetics in the common carotid artery: preliminary in vivo assessment.

Longitudinal kinetics (LOKI) of the arterial wall consists of the shearing motion of the intima-media complex over the adventitia layer in the direction parallel to the blood flow during the cardiac cycle. The aim of this study was to investigate the local variability of LOKI amplitude along the length of the vessel. By use of a previously validated motion-estimation framework, 35 in vivo longitudinal B-mode ultrasound cine loops of healthy common carotid arteries were analyzed. Results indicated that LOKI amplitude is progressively attenuated along the length of the artery, as it is larger in regions located on the proximal side of the image (i.e., toward the heart) and smaller in regions located on the distal side of the image (i.e., toward the head), with an average attenuation coefficient of -2.5 ± 2.0%/mm. Reported for the first time in this study, this phenomenon is likely to be of great importance in improving understanding of atherosclerosis mechanisms, and has the potential to be a novel index of arterial stiffness.

Real-time ultrasound-tagging to track the 2D motion of the common carotid artery wall in vivo.

PURPOSE: Tracking the motion of biological tissues represents an important issue in the field of medical ultrasound imaging. However, the longitudinal component of the motion (i.e., perpendicular to the beam axis) remains more challenging to extract due to the rather coarse resolution cell of ultrasound scanners along this direction. The aim of this study is to introduce a real-time beamforming strategy dedicated to acquire tagged images featuring a distinct pattern in the objective to ease the tracking. METHODS: Under the conditions of the Fraunhofer approximation, a specific apodization function was applied to the received raw channel data, in real-time during image acquisition, in order to introduce a periodic oscillations pattern along the longitudinal direction of the radio frequency signal. Analytic signals were then extracted from the tagged images, and subpixel motion tracking of the intima-media complex was subsequently performed offline, by means of a previously introduced bidimensional analytic phase-based estimator. RESULTS: The authors' framework was applied in vivo on the common carotid artery from 20 young healthy volunteers and 6 elderly patients with high atherosclerosis risk. Cine-loops of tagged images were acquired during three cardiac cycles. Evaluated against reference trajectories manually generated by three experienced analysts, the mean absolute tracking error was 98 ± 84 µm and 55 ± 44 µm in the longitudinal and axial directions, respectively. These errors corresponded to 28% ± 23% and 13% ± 9% of the longitudinal and axial amplitude of the assessed motion, respectively. CONCLUSIONS: The proposed framework enables tagged ultrasound images of in vivo tissues to be acquired in real-time. Such unconventional beamforming strategy contributes to improve tracking accuracy and could potentially benefit to the interpretation and diagnosis of biomedical images.