Contrast enhanced photoacoustic detection of fibrillar collagen in the near infrared region-I.

Fibrillar collagen accumulation emerges as a promising biomarker in several diseases, such as desmoplastic tumors and unstable atherosclerotic plaque. Gold nanorods (GNRs) hold great potential as contrast agents in high-resolution, biomedically safe, and non-invasive photoacoustic imaging (PAI). This study presents the design and characterization of a specialized imaging tool which exploits GNR assisted targeted photoacoustic imaging that is tailored for the identification of fibrillar collagen. In addition to the photoacoustic characterization of collagen in the NIR 1 and 2 regions, we demonstrate the detailed steps of conjugating a decoy to GNRs. This study serves as a proof of concept, that demonstrates that conjugated collagenase-1 (MMP-1) generates a distinct and collagen-specific photoacoustic signal, facilitating real-time visualization in the wavelength range of 700-970 nm (NIR I). As most of the reported studies utilized the endogenous contrast of collagen in the NIR II wavelength that has major limitations to perform in vivo deep tissue imaging, the approach that we are proposing is unique and it highlights the promise of MMP-1 decoy-functionalized GNRs as novel contrast agents for photoacoustic imaging of collagen in the NIR 1 region. To our knowledge this is the first time functionalized GNRs are optimized for the detection of fibrillar collagen and utilized in the field of non-invasive photoacoustic imaging that can facilitate a better prognosis of desmoplastic tumors and broken atherosclerotic plaques.

Fast Marching based Tissue Adaptive Delay Estimation for Aberration Corrected Delay and Sum Beamforming in Ultrasound Imaging.

Conventional ultrasound (US) imaging employs the delay and sum (DAS) receive beamforming with dynamic receive focus for image reconstruction due to its simplicity and robustness. However, the DAS beamforming follows a geometrical method of delay estimation with a spatially constant speed-of-sound (SoS) of 1540 m/s throughout the medium irrespective of the tissue in-homogeneity. This approximation leads to errors in delay estimations that accumulate with depth and degrades the resolution, contrast and overall accuracy of the US image. In this work, we propose a fast marching based DAS for focused transmissions which leverages the approximate SoS map to estimate the refraction corrected propagation delays for each pixel in the medium. The proposed approach is validated qualitatively and quantitatively for imaging depths of upto ∼ 11 cm through simulations, where fat layer-induced aberration is employed to alter the SoS in the medium. To the best of the authors' knowledge, this is the first work considering the effect of SoS on image quality for deeper imaging.Clinical relevance- The proposed approach when employed with an approximate SoS estimation technique can aid in overcoming the fat-induced signal aberrations and thereby in the accurate imaging of various pathologies of liver and abdomen.

Enhanced Needle Visualization With Reflection Tuned Apodization Based on the Radon Transform for Ultrasound Imaging.

In ultrasound (US)-guided interventions, accurately tracking and visualizing needles during in-plane insertions are significant challenges due to strong directional specular reflections. These reflections violate the geometrical delay and apodization estimations in the conventional delay and sum beamforming (DASB) degrading the visualization of needles. This study proposes a novel reflection tuned apodization (RTA) to address this issue and facilitate needle enhancement through DASB. The method leverages both temporal and angular information derived from the Radon transforms of the radio frequency (RF) data from plane-wave imaging to filter the specular reflections from the needle and their directivity. The directivity information is translated into apodization center maps through time-to-space mapping in the Radon domain, which is subsequently integrated into DASB. We assess the influence of needle angulations, projection angles in the Radon transform, needle gauge sizes, and the presence of multiple specular interfaces on the approach. The analysis shows that the method surpasses conventional DASB in enhancing the image quality of needle interfaces while preserving the diffuse scattering from the surrounding tissues without significant computational overhead. The work offers promising prospects for improved outcomes in US-guided interventions and better insights into characterizing US reflections with Radon transforms.

On the physics of ultrasound transmission for in-plane needle tracking in guided interventions.

Objective.In ultrasound (US) guided interventions, the accurate visualization and tracking of needles is a critical challenge, particularly during in-plane insertions. An inaccurate identification and localization of needles lead to severe inadvertent complications and increased procedure times. This is due to the inherent specular reflections from the needle with directivity depending on the angle of incidence of the US beam, and the needle inclination.Approach.Though several methods have been proposed for improved needle visualization, a detailed study emphasizing the physics of specular reflections resulting from the interaction of transmitted US beam with the needle remains to be explored. In this work, we discuss the properties of specular reflections from planar and spherical wave US transmissions respectively through multi-angle plane wave (PW) and synthetic transmit aperture (STA) techniques for in-plane needle insertion angles between 15°-50°.Main Results.The qualitative and quantitative results from simulations and experiments reveal that the spherical waves enable better visualization and characterization of needles than planar wavefronts. The needle visibility in PW transmissions is severely degraded by the receive aperture weighting during image reconstruction than STA due to greater deviation in reflection directivity. It is also observed that the spherical wave characteristics starts to alter to planar characteristics due to wave divergence at large needle insertion depths.Significance.The study highlights that synergistic transmit-receive imaging schemes addressing the physical properties of reflections from the transmit wavefronts are imperative for the precise imaging of needle interfaces and hence have strong potential in elevating the quality of outcomes from US guided interventional practices.

Pixel Intensity Vector Field: An Inside Out Approach of Looking at Ultrasound Reflections from the Lung at High Frame Rates.

Ultrasound (US) imaging is becoming the routine modality for the diagnosis and prognosis of lung pathologies. Lung US imaging relies on artifacts from acoustic impedance (Z) mismatches to distinguish and interpret the normal and pathological lung conditions. The air-pleura interface of the normal lung displays specularity due to the huge Z mismatches. However, in the presence of pathologies, the interface alters exhibiting a diffuse behavior due to increased density and reduced spatial distribution of air in the sub-pleural space. The specular or the diffuse behavior influences the reflected acoustic intensity distribution. This study aims to understand the reflection pattern in a normal and pathological lung through a novel approach of determining pixel-level acoustic intensity vector field (IVF) at high frame rates. Detailed lung modeling procedures using k-Wave US toolbox under normal, edematous, and consolidated conditions are illustrated. The analysis of the IVF maps on the three lung models clearly shows the drifting of the air-pleura interface from specular to diffuse with the severity of the pathology.Clinical Relevance- The presented acoustic simulation lung models are an aid to teaching and research by providing a quick visual and intuitive understanding of lung ultrasound physics. The proposed intensity vector field maps are supplementary information to aid diagnostics and characterization of any tissue composed of specular and diffuse components.

Towards A Pixel-Level Reconfigurable Digital Beamforming Core for Ultrasound Imaging.

Ultrasound (US) imaging systems typically employ a single beamforming scheme which is the delay and sum (DAS) beamforming due to its reduced complexity. However, DAS results in images with limited resolution and contrast. The limitations of DAS have been overcome by, delay multiply and sum (DMAS) beamforming, making it especially preferable in cases where finer image details are required in larger depth of scans for an accurate diagnosis. But, DMAS is confined to transducer frequencies where the generated harmonics also fall in the processable frequency range of the US system. However, if US systems could provide the flexibility to reconfigure beamforming considering the restrictions of each beamforming scheme, it is possible to select the best beamforming according to the clinical requirement and system constraints. This work is a fundamental step towards enabling reconfigurable beamforming for on-the-fly selection among the DAS and DMAS beamforming schemes, with low reconfiguration overhead, specifically for each imaging scenario to aid better diagnosis. Two novel architectures are proposed, that reconfigures between DAS and DMAS beamforming as a function of transducer's center frequency with minimum additional computational overhead. The implementation results of the proposed architectures on xc7z010clg400-1 FPGA are reported. The possibilities of pixel-level beamforming reconfigurability, where the different tissue regions are beamformed with either DAS or DMAS are also shown through simulations.