Dual-mode photoacoustic and ultrasound imaging system based on a Fabry-Pérot scanner

The planar Fabry-Pérot (FP) scanner is an ultrasound detector that simultaneously provides high sensitivity, a high density of small (sub-100 μm) acoustic elements, and a broad bandwidth (> 30 MHz). These features enable the FP scanner to acquire high-resolution 3D in vivo photoacoustic images of biological tissues up to depths of approximately 10 mm. The aim was to add complementary morphological ultrasound contrast to photoacoustic images to extend their clinical applicability. This was achieved by developing a dual-mode photoacoustic and ultrasound imaging system based on the FP scanner, which was modified to transmit optically generated ultrasound. The FP sensor head was coated with an optically absorbing polydimethylsiloxane(PDMS) composite layer, which was excited with nanosecond laser pulses to generate broadband planar ultrasound waves for pulse-echo imaging. First, an all-optical ultrasound system was developed using a highly absorbing carbon nanotube-PDMS composite coating. The system was characterised with a series of experiments, and its imaging performance was tested on tissue mimicking phantoms and ex vivo tissue samples. Second, the effect of the frequency content of the detected signals and the effect of spatial aliasing on the image quality were investigated in simulation. A broadband system was found to reduce the effect of spatial undersampling of high frequencies which results in a reduction of contrast due to the formation of grating lobe artefacts. Third, to improve the image quality, frequency and angle compounding were explored in simulations and experimentally. Coherent and incoherent compounding were considered, as well as the effect of the filter bandwidth on frequency compounded images, and the influence of the number and spread of angles used in angle compounded images. Finally, a dual- mode photoacoustic and ultrasound imaging system was demonstrated with a gold nanoparticle-PDMS composite which enabled wavelength-selective absorption of light. The system was shown to obtain high-resolution 3D dual-mode images providing complementary contrast from optically absorbing and acoustically scattering structures

A multi-angle plane wave imaging approach for high frequency 2D flow visualization in small animals: simulation study in the murine arterial system

To preclinically investigate the role of hemodynamics in atherogenesis, mouse models are particularly useful due to the rapid disease development. As such, murine blood flow visualization has become an important tool, with current US systems equipped with traditional 1D flow imaging techniques, lacking spatial and/or temporal resolution to accurately resolve in-vivo flow fields. Hence, we investigated multi-angle plane wave imaging for ultrafast, 2D vector flow visualization and compared this approach with conventional pulsed Doppler in the setting of a mouse aorta with abdominal aortic aneurysm. For this purpose, we used a multiphysics model which allowed direct comparison of synthetic US images with the true flow field behind the image. In case of the abdominal aorta, we showed the mean flow estimation improved 9 % when using 2D vector Doppler compared to conventional Doppler, but still underestimated the true flow because the full spatial velocity distribution remained unknown. We also evaluated a more challenging measurement location, the mesenteric artery (aortic side branch), often assessed in a short-axis view close to the origin of the branch to avoid the smaller dimensions downstream. Even so, complex out-ofplane flow dynamics hampered a reliable flow assessment for both techniques. Hence, both cases illustrated the need for 3D vascular imaging, allowing acquisition of the full 3D spatial velocity profile

Imaging with therapeutic acoustic wavelets–short pulses enable acoustic localization when time of arrival is combined with delay and sum

—Passive acoustic mapping (PAM) is an algorithm that reconstructs the location of acoustic sources using an array of receivers. This technique can monitor therapeutic ultrasound procedures to confirm the spatial distribution and amount of microbubble activity induced. Current PAM algorithms have an excellentlateral resolution but have a poor axial resolution, making it difficult to distinguish acoustic sources within the ultrasound beams. With recent studies demonstrating that short-length and low-pressure pulses—acoustic wavelets—have the therapeutic function, we hypothesizedthat the axial resolution could be improved with a quasi-pulse-echo approach and that the resolution improvement would depend on the wavelet’s pulse length. This article describes an algorithm that resolves acoustic sources axially using time of flight and laterally using delayand-sum beamforming, which we named axial temporal position PAM (ATP-PAM). The algorithm accommodates a rapid short pulse (RaSP) sequence that can safely deliver drugs across the blood–brain barrier. We developed our algorithm with simulations (k-wave) and in vitro experiments for one-, two-, and five-cycle pulses, comparing our resolution against that of two current PAM algorithms. We then tested ATP-PAM in vivo and evaluated whether the reconstructed acoustic sources mapped to drug deliver

Ultrasound Aberration Correction based on Local Speed-of-Sound Map Estimation

For beamforming ultrasound (US) signals, typically a spatially constant speed-of-sound (SoS) is assumed to calculate delays. As SoS in tissue may vary relatively largely, this approximation may cause wavefront aberrations, thus degrading effective imaging resolution. In the literature, corrections have been proposed based on unidirectional SoS estimation or computationally-expensive a posteriori phase rectification. In this paper we demonstrate a direct delay correction approach for US beamforming, by leveraging 2D spatial SoS distribution estimates from plane-wave imaging. We show both in simulations and with ex vivo measurements that resolutions close to the wavelength limit can be achieved using our proposed local SoS-adaptive beamforming, yielding a lateral resolution improvement of 22% to 29% on tissue samples with up to 3% SoS-contrast (45m/s). We verify that our method accurately images absolute positions of tissue structures down to sub-pixel resolution of a tenth of a wavelength, whereas a global SoS assumption leads to artifactual localizations.Comment: will be published in the proceedings of the IEEE International Ultrasonics Symposium (IUS) 201

Forward model for quantitative pulse-echo speed-of-sound imaging

Computed ultrasound tomography in echo mode (CUTE) allows determining the spatial distribution of speed-of-sound (SoS) inside tissue using handheld pulse-echo ultrasound (US). This technique is based on measuring the changing phase of beamformed echoes obtained under varying transmit (Tx) and/or receive (Rx) steering angles. The SoS is reconstructed by inverting a forward model describing how the spatial distribution of SoS is related to the spatial distribution of the echo phase shift. CUTE holds promise as a novel diagnostic modality that complements conventional US in a single, real-time handheld system. Here we demonstrate that, in order to obtain robust quantitative results, the forward model must contain two features that were not taken into account so far: a) the phase shift must be detected between pairs of Tx and Rx angles that are centred around a set of common mid-angles, and b) it must account for an additional phase shift induced by the error of the reconstructed position of echoes. In a phantom study mimicking liver imaging, this new model leads to a substantially improved quantitative SoS reconstruction compared to the model that has been used so far. The importance of the new model as a prerequisite for an accurate diagnosis is corroborated in preliminary volunteer results