We developed a reflection-mode optoacoustic mesoscopy system, based on raster-scanning of a custom designed spherically focused ultrasound detector, enabling seamless epi-illumination of the volume imaged. We study the performance of acoustic-resolution mesoscopy operating at an ultrawideband bandwidth of 20-180 MHz. i.e., a frequency band spreading over virtually an order of magnitude. Using tomographic reconstruction we showcase previously unreported, to our knowledge, axial resolutions of 4 μm and transverse resolutions of 18 μm reaching depths of up to 5 mm. We further investigate the frequency-dependence of features seen on the images to understand the implications of ultrawideband measurements. We show the overall imaging performance and the frequency ranges that contribute to observable resolution improvements from phantoms and animals.

The activation of natural gas nuclei to induce larger bubbles is possible using short ultrasonic excitations of high amplitude, and is required for ultrasound cavitation therapies. However, little is known about the distribution of nuclei in tissues. Therefore, the acoustic pressure level necessary to generate bubbles in a targeted zone and their exact location are currently difficult to predict. To monitor the initiation of cavitation activity, a novel all-ultrasound technique sensitive to single nucleation events is presented here. It is based on combined passive detection and ultrafast active imaging over a large volume using the same multi-element probe. Bubble nucleation was induced using a focused transducer (660 kHz, f-number = 1) driven by a high-power electric burst (up to 300 W) of one to two cycles. Detection was performed with a linear array (4 to 7 MHz) aligned with the single-element focal point. In vitro experiments in gelatin gel and muscular tissue are presented. The synchronized passive detection enabled radio-frequency data to be recorded, comprising high-frequency coherent wave fronts as signatures of the acoustic emissions linked to the activation of the nuclei. Active change detection images were obtained by subtracting echoes collected in the unnucleated medium. These indicated the appearance of stable cavitating regions. Because of the ultrafast frame rate, active detection occurred as quickly as 330 μs after the high-amplitude excitation and the dynamics of the induced regions were studied individually.

We developed a raster-scan acoustic resolution broadband optoacoustic mesoscopy system and investigated the imaging performance using ultrasonic frequencies up to 125 MHz. The developed system achieves 7 μm axial resolution and transverse resolution of 30 μm reaching depths of at least 5 mm. This unprecedented performance is achieved by operating at out-of-focus ultrasonic detection and tomographic reconstruction. We demonstrate the limits reached due to the width of the laser pulse employed and showcase the technique on drosophila fly and drosophila pupae ex vivo.

Ntziachristos. Threedimensional optoacoustic tomography using a conventional ultrasound linear detector array: wholebody tomographic system for small animals.. Medical Physics, American Association of Physicists in Medicine, 2012, 40, pp.013302 Purpose: Optoacoustic imaging relies on the detection of ultrasonic waves induced by laser pulse excitations to map 10 optical absorption in biological tissue. A tomographic geometry employing a conventional ultrasound linear detector 11 array for volumetric optoacoustic imaging is reported. The geometry is based on a translate-rotate scanning motion 12 of the detector array, and capitalizes on the geometrical characteristics of the transducer assembly to provide a large 13 solid angular detection aperture. A system for three-dimensional whole-body optoacoustic tomography of small 14 animals is implemented. 15Methods: The detection geometry was tested using a 128-element linear array (5.0/7.0MHz, Acuson L7, Siemens), 16 moved by steps with a rotation/translation stage assembly. Translationand rotation range of 13.5 mm and 180° 17 respectively were implemented. Optoacoustic emissions were induced in tissue-mimicking phantoms and ex-vivo 18 mice using a pulsed laser operating in the near-IR spectral range at 760nm. Volumetric images were formed using a 19 filtered back-projection algorithm. 20Results: The resolution of the optoacoustic tomography system was measured to be better than 130µmin-plane and 21 330µm in elevation (full width half maximum), and to be homogenous along a 15 mm diameter cross-section due to 22 the translate-rotate scanning geometry. Whole-body volumetric optoacoustic images of mice were performed ex-23 vivo, and imaged organs and blood vessels through the intact abdominal and head regions were correlated to the 24 mouse anatomy. 25Conclusions: Overall, the feasibility of three-dimensional and high-resolution whole-body optoacoustic imaging of 26 small animal using a conventional linear array was demonstrated. Furthermore, the scanning geometry may be used 27 for other linear arrays and is therefore expected to be of great interest for optoacoustic tomography at macroscopic

In high-frequency photoacoustic imaging with uniform illumination, homogeneous photoabsorbing structures may be invisible because of their large size or limited-view issues. Here we show that, by exploiting dynamic speckle illumination, it is possible to reveal features that are normally invisible with a photoacoustic system comprised of a 20 MHz linear ultrasound array. We demonstrate imaging of a ∅5 mm absorbing cylinder and a 30 μm black thread arranged in a complex shape. The hidden structures are directly retrieved from photoacoustic images recorded for different random speckle illuminations of the phantoms by assessing the variation in the value of each pixel over the illumination patterns.

In deep tissue photoacoustic imaging, the spatial resolution is inherently limited by acoustic diffraction. Moreover, as the ultrasound attenuation increases with frequency, resolution is often traded-off for penetration depth. Here we report on super-resolution photoacoustic imaging by use of multiple speckle illumination. Specifically, we show that the analysis of secondorder fluctuations of the photoacoustic images combined with image deconvolution enables resolving optically absorbing structures beyond the acoustic diffraction limit. A resolution increase of almost a factor 2 is demonstrated experimentally. Our method introduces a new framework that could potentially lead to deep tissue photoacoustic imaging with sub-acoustic resolution.Light scattering prevents standard optical microscopes to obtain well-resolved images deep inside biological tissues. In the past twenty years, photoacoustic (PA) imaging has been developed to overcome this limitation, by imaging optical absorption deep inside strongly scattering tissue with the resolution of ultrasound [1]. PA imaging relies on the unscattered ultrasonic waves emitted by absorbing structures under pulsed illumination via thermo-elastic stress generation. It therefore provides images at depth in tissue with a spatial resolution limited by acoustic diffraction. Ultimately, the ultrasound resolution for biological soft tissue is limited by the attenuation of ultrasound, which typically increases linearly with frequency. As a result, the depth-to-resolution ratio of PA imaging at depth is around 200 in practice [1,2]. As an illustration, axial resolution down to 5 µm and lateral resolution down to 10 µm have been reached with high frequency acoustic detectors at depth up to 5 mm [3].In this letter, we demonstrate that the conventional acousticdiffraction limit in PA imaging may be overcome by exploiting PA signal fluctuations, building on the super-resolution optical fluctuation imaging (SOFI) technique developed for fluorescence microscopy [4]. SOFI is based on the idea that a higher-order statistical analysis of temporal fluctuations caused by fluorescence blinking provides a way to resolve uncorrelated fluorophores within a same diffraction spot. In this work, we introduce multiple optical speckle illumination as a source of fluctuations for super-resolution PA imaging, inspired by the principle introduced in optics with SOFI [4] or from derived approaches using speckle illumination [5]. In PA imaging, multiple speckle illumination was initially introduced by our group as a mean to palliate limited-view or highpass-filtering artefacts [6]. Here, we demonstrate that a second-order analysis of optical speckleinduced PA fluctuations also provides super-resolved PA images beyond the acoustic diffraction limit.In this work, we consider PA images reconstructed from a set of PA signals measured with an ultrasound array. A conventional backprojection algorithm is used to reconstruct the images, and it is assumed that the reconstructed PA quantity A(r) may be written...

Brain treatment through the skull with high-intensity focused ultrasound can be achieved with multichannel arrays and adaptive focusing techniques such as time reversal. This method requires a reference signal to be either emitted by a real source embedded in brain tissues or computed from a virtual source, using the acoustic properties of the skull derived from computed tomography images. This noninvasive computational method focuses with precision, but suffers from modeling and repositioning errors that reduce the accessible acoustic pressure at the focus in comparison with fully experimental time reversal using an implanted hydrophone. In this paper, this simulation-based targeting has been used experimentally as a first step for focusing through an ex vivo human skull at a single location. It has enabled the creation of a cavitation bubble at focus that spontaneously emitted an ultrasonic wave received by the array. This active source signal has allowed 97 +/- 1.1% of the reference pressure (hydrophone-based) to be restored at the geometrical focus. To target points around the focus with an optimal pressure level, conventional electronic steering from the initial focus has been combined with bubble generation. Thanks to step-by-step bubble generation, the electronic steering capabilities of the array through the skull were improved.

Gas nuclei exist naturally in living bodies. Their activation initiates cavitation activity, and is possible using short ultrasonic excitations of high amplitude. However, little is known about the nuclei population in vivo, and therefore about the rarefaction pressure required to form bubbles in tissue. A novel method dedicated to in vivo investigations was used here that combines passive and active cavitation detection with a multi-element linear ultrasound probe (4-7 MHz). Experiments were performed in vivo on the brain of trepanated sheep. Bubble nucleation was induced using a focused single-element transducer (central frequency 660 kHz, f-number = 1) driven by a high power (up to 5 kW) electric burst of two cycles. Successive passive recording and ultrafast active imaging were shown to allow detection of a single nucleation event in brain tissue in vivo. Experiments carried out on eight sheep allowed statistical studies of the bubble nucleation process. The nucleation probability was evaluated as a function of the peak negative pressure. No nucleation event could be detected with a peak negative pressure weaker than -12.7 MPa, i.e. one order of magnitude higher than the recommendations based on the mechanical index. Below this threshold, bubble nucleation in vivo in brain tissues is a random phenomenon.