Jérôme Gateau

Three-dimensional optoacoustic tomography using a conventional ultrasound linear detector array: whole-body tomographic system for small animals.

PURPOSE Optoacoustic imaging relies on the detection of ultrasonic waves induced by laser pulse excitations to map optical absorption in biological tissue. A tomographic geometry employing a conventional ultrasound linear detector array for volumetric optoacoustic imaging is reported. The geometry is based on a translate-rotate scanning motion of the detector array, and capitalizes on the geometrical characteristics of the transducer assembly to provide a large solid angular detection aperture. A system for three-dimensional whole-body optoacoustic tomography of small animals is implemented. METHODS The detection geometry was tested using a 128-element linear array (5.0∕7.0 MHz, Acuson L7, Siemens), moved by steps with a rotation∕translation stage assembly. Translation and rotation range of 13.5 mm and 180°, respectively, were implemented. Optoacoustic emissions were induced in tissue-mimicking phantoms and ex vivo mice using a pulsed laser operating in the near-IR spectral range at 760 nm. Volumetric images were formed using a filtered backprojection algorithm. RESULTS The resolution of the optoacoustic tomography system was measured to be better than 130 μm in-plane and 330 μm in elevation (full width half maximum), and to be homogenous along a 15 mm diameter cross section due to the translate-rotate scanning geometry. Whole-body volumetric optoacoustic images of mice were performed ex vivo, and imaged organs and blood vessels through the intact abdominal and head regions were correlated to the mouse anatomy. CONCLUSIONS Overall, the feasibility of three-dimensional and high-resolution whole-body optoacoustic imaging of small animal using a conventional linear array was demonstrated. Furthermore, the scanning geometry may be used for other linear arrays and is therefore expected to be of great interest for optoacoustic tomography at macroscopic and mesoscopic scale. Specifically, conventional detector arrays with higher central frequencies may be investigated.

Raster-scan optoacoustic mesoscopy in the 25-125 MHz range.

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.

Ultrawideband reflection-mode optoacoustic mesoscopy

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.

Improving visibility in photoacoustic imaging using dynamic speckle illumination

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.

Ultra-wideband three-dimensional optoacoustic tomography

Broadband optoacoustic waves generated by biological tissues excited with nanosecond laser pulses carry information corresponding to a wide range of geometrical scales. Typically, the frequency content present in the signals generated during optoacoustic imaging is much larger compared to the frequency band captured by common ultrasonic detectors, the latter typically acting as bandpass filters. To image optical absorption within structures ranging from entire organs to microvasculature in three dimensions, we implemented optoacoustic tomography with two ultrasound linear arrays featuring a center frequency of 6 and 24 MHz, respectively. In the present work, we show that complementary information on anatomical features could be retrieved and provide a better understanding on the localization of structures in the general anatomy by analyzing multi-bandwidth datasets acquired on a freshly excised kidney.

Model-based optoacoustic imaging using focused detector scanning

Optoacoustic (photoacoustic) mesoscopic and microscopic imaging is often implemented by linearly scanning a spherically focused ultrasound transducer. In this case, the resolution and sensitivity along the scan direction are limited by diffraction and therefore degrade rapidly for imaging depths away from the focal point. Partial restoration of the lost resolution can be achieved by using data-processing techniques, such as the virtual detector delay-and-sum method. However, these techniques are based on an approximate description of the detector properties, which limits the improvement in image quality they achieve. Herein we propose a reconstruction method based on an exact model of the optoacoustic generation and propagation that incorporates the spatial response of the sensor. The proposed method shows superior imaging performance over previously considered techniques.

Light focusing and two-dimensional imaging through scattering media using the photoacoustic transmission matrix with an ultrasound array

We implement the photoacoustic transmission matrix approach on a two-dimensional photoacoustic imaging system, using a 15 MHz linear ultrasound array. Using a black leaf skeleton as a complex absorbing structure, we demonstrate that the photoacoustic transmission matrix approach allows to reveal structural features that are invisible in conventional photoacoustic images, as well as to selectively control light focusing on absorbing targets, leading to a local enhancement of the photoacoustic signal.

Influence of nanoscale temperature rises on photoacoustic generation: Discrimination between optical absorbers based on thermal nonlinearity at high frequency.

In this work, we experimentally investigate thermal-based nonlinear photoacoustic generation as a mean to discriminate between different types of absorbing particles. The photoacoustic generation from solutions of dye molecules and gold nanospheres (same optical densities) was detected using a high frequency ultrasound transducer (20 MHz). Photoacoustic emission was observed with gold nanospheres at low fluence for an equilibrium temperature around 4 °C, where the linear photoacoustic effect in water vanishes, highlighting the nonlinear emission from the solution of nanospheres. The photoacoustic amplitude was also studied as a function of the equilibrium temperature from 2 °C to 20 °C. While the photoacoustic amplitude from the dye molecules vanished around 4 °C, the photoacoustic amplitude from the gold nanospheres remained significant over the whole temperature range. Our preliminary results suggest that in the context of high frequency photoacoustic imaging, nanoparticles may be discriminated from molecular absorbers based on nanoscale temperature rises.

High-resolution optoacoustic mesoscopy with a 24 MHz multidetector translate-rotate scanner

Abstract. Optoacoustic (photoacoustic) mesoscopy aims at high-resolution optical imaging of anatomical, functional, and cellular parameters at depths that go well beyond those of optical-resolution optical or optoacoustic microscopy i.e., reaching several millimeters in depth. The approach utilizes tomography to achieve ultrasonic-diffraction resolution and operates at high-ultrasound frequencies (20 to 200 MHz) induced by few-nanosecond laser pulse excitation of tissues. We investigated here the performance of optoacoustic mesoscopy implemented at 24 MHz center frequency and its ability to resolve optical absorption contrast in the mouse kidney ex vivo. The developed system achieved better than 30 μm in-plane resolution and 110 μm elevation resolution over a cylindrical volume of 9-mm diameter and 9-mm height. This unprecedented combination of resolution and depth was achieved by implementing a translate-rotate detection geometry and by tomographic reconstruction. The approach yielded images of optically absorbing structures with a level of detail never-before visualized in an intact mouse kidney and allows insights into their unperturbed architecture. We discuss the ability to offer multispectral acquisitions and enable in vivo imaging.

Super-resolution photoacoustic fluctuation imaging with multiple speckle illumination

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 second-order 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.