Reconstruction of Quantitative Acoustic Microscopy Images from RF Signals Sampled at Innovation Rate

The principle of quantitative acoustic microscopy (QAM) is to form two-dimensional acoustic parameter maps from a collection of radiofrequency (RF) signals acquired by raster scanning a biological sample. Despite their relatively simple structure, i.e. two main reflections, QAM RF signals are currently sampled at very high frequencies, e.g., at 2.5 GHz for QAM system employing a single-element transducer with a center frequency of 250-MHz. The use of such high sampling frequencies is challenging because of the potentially large amount of acquired data and the cost of the necessary analog to digital converters. In this work, we propose a sampling scheme based on the finite rate of innovation theory that exploits the limited numbers of degrees of freedom of QAM RF signals and allows the reconstruction of accurate acoustic maps from a very limited number of samples

Approximate message passing reconstruction of quantitative acoustic microscopy images.

A novel framework for compressive sensing (CS) data acquisition and reconstruction in quantitative acoustic microscopy (QAM) is presented. Three different CS patterns, adapted to the specifics of QAM systems, were investigated as an alternative to the current raster-scanning approach. They consist of diagonal sampling, a row random, and a spiral scanning pattern and can all significantly reduce both the acquisition time and the amount of sampled data. For subsequent image reconstruction, we design and implement an innovative technique, whereby a recently proposed approximate message passing method is adapted to account for the underlying data statistics. A Cauchy maximum a posteriori image denoising algorithm is thus employed to account for the non-Gaussianity of QAM wavelet coefficients. The proposed methods were tested retrospectively on experimental data acquired with a 250- or 500-MHz QAM system. The experimental data were obtained from a human lymph node sample (250 MHz) and human cornea (500 MHz). Reconstruction results showed that the best performance is obtained using a spiral sensing pattern combined with the Cauchy denoiser in the wavelet domain. The spiral sensing matrix reduced the number of spatial samples by a factor of 2 and led to an excellent peak signal-to-noise ratio of 43.21 dB when reconstructing QAM speed-of-sound images of a human lymph node. These results demonstrate that the CS approach could significantly improve scanning time, while reducing costs of future QAM systems

A comprehensive review on photoacoustic-based devices for biomedical applications

The photoacoustic effect is an emerging technology that has sparked significant interest in the research field since an acoustic wave can be produced simply by the incidence of light on a material or tissue. This phenomenon has been extensively investigated, not only to perform photoacoustic imaging but also to develop highly miniaturized ultrasound probes that can provide biologically meaningful information. Therefore, this review aims to outline the materials and their fabrication process that can be employed as photoacoustic targets, both biological and non-biological, and report the main components’ features to achieve a certain performance. When designing a device, it is of utmost importance to model it at an early stage for a deeper understanding and to ease the optimization process. As such, throughout this article, the different methods already implemented to model the photoacoustic effect are introduced, as well as the advantages and drawbacks inherent in each approach. However, some remaining challenges are still faced when developing such a system regarding its fabrication, modeling, and characterization, which are also discussed.This work was supported by Fundação para a Ciência e Tecnologia national funds, under the national support to R&D units grant, through the reference project UIDB/04436/2020 and UIDP/04436/2020

Optical Coherence Photoacoustic Microscopy (OC-PAM) for Multimodal Imaging

Optical coherence tomography (OCT) and Photoacoustic microscopy (PAM) are two noninvasive, high-resolution, three-dimensional, biomedical imaging modalities based on different contrast mechanisms. OCT detects the light backscattered from a biological sample either in the time or spectral domain using an interferometer to form an image. PAM is sensitive to optical absorption by detecting the light-induced acoustic waves to form an image. Due to their complementary contrast mechanisms, OCT and PAM are suitable for being combined to achieve multimodal imaging. In this dissertation, an optical coherence photoacoustic microscopy (OC-PAM) system was developed for in vivo multimodal retinal imaging with a pulsed broadband NIR light source. To test the capabilities of the system on multimodal ophthalmic imaging, the retina of pigmented rats was imaged. The OCT images showed the retinal structures with quality similar to conventional OCT, while the PAM images revealed the distribution of melanin in the retina since the NIR PAM signals are generated mainly from melanin in the posterior segment of the eye. By using the pulsed broadband light source, the OCT image quality highly depends on the pulse-to-pulse stability of the light source without averaging. In addition, laser safety is always a concern for in vivo applications, especially for eye imaging with a pulsed light source. Therefore, a continuous wave (CW) light source is desired for OC-PAM applications. An OC-PAM system using an intensity-modulated CW superluminescent diode was then developed. The system was tested for multimodal imaging the vasculature of a mouse ear in vivo by using Gold Nanorods (GNRs) as contrast agent for PAM, as well as excised porcine eyes ex vivo. Since the quantitative information of the optical properties extracted from the proposed NIR OC-PAM system is potentially able to provide a unique technique to evaluate the existence of melanin and lipofuscin specifically, a phantom study has been conducted and the relationship between image intensity of OCT and PAM was interpreted to represent the relationship between the optical scattering property and optical absorption property. It will be strong evidence for practical application of the proposed NIR OC-PAM system

Medical Imaging of Microrobots: Toward In Vivo Applications

Medical microrobots (MRs) have been demonstrated for a variety of non-invasive biomedical applications, such as tissue engineering, drug delivery, and assisted fertilization, among others. However, most of these demonstrations have been carried out in in vitro settings and under optical microscopy, being significantly different from the clinical practice. Thus, medical imaging techniques are required for localizing and tracking such tiny therapeutic machines when used in medical-relevant applications. This review aims at analyzing the state of the art of microrobots imaging by critically discussing the potentialities and limitations of the techniques employed in this field. Moreover, the physics and the working principle behind each analyzed imaging strategy, the spatiotemporal resolution, and the penetration depth are thoroughly discussed. The paper deals with the suitability of each imaging technique for tracking single or swarms of MRs and discusses the scenarios where contrast or imaging agent's inclusion is required, either to absorb, emit, or reflect a determined physical signal detected by an external system. Finally, the review highlights the existing challenges and perspective solutions which could be promising for future in vivo applications

Ex-vivo and In-vivo Characterization of Human Accommodation

A completely satisfying approach to restoring accommodation still needs to be developed. Besides, there are considerable discrepancies between objective and subjective trials to evaluate the therapeutic success. A substantial biomechanical understanding of all structures and processes involved in accommodation as well as presbyopia are needed to develop promising new strategies. This contribution focuses on developing advanced imaging techniques to create a basic understanding of accommodation and presbyopia and to evaluate existing concepts for restoring accommodation. Besides, the emphasis is also on replacing stiff presbyopic lenses by a material that imitates the young crystalline lens

Penta-Modal Imaging Platform with OCT- Guided Dynamic Focusing for Simultaneous Multimodal Imaging

Complex diseases, such as Alzheimer’s disease, are associated with sequences of changes in multiple disease-specific biomarkers. These biomarkers may show dynamic changes at specific stages of disease progression. Thus, testing/monitoring each biomarker may provide insight into specific disease-related processes, which can result in early diagnosis or even development of preventive measures. Obtaining a comprehensive information of biological tissues requires imaging of multiple optical contrasts, which is not typically offered by a single imaging modality. Thus, combining different contrast mechanisms to achieve simultaneous multimodal imaging is desirable. However, this process is highly challenging due to specific optical and hardware requirements for each optical imaging system. The objective of this dissertation is to develop a novel Penta-modal optical imaging system integrating photoacoustic microscopy (PAM), optical coherence tomography (OCT), optical Doppler tomography (ODT), OCT angiography (OCTA) and confocal fluorescence microscopy (CFM) in one platform providing comprehensive structural, functional, and molecular information of living biological tissues. The system can simultaneously image different biomarkers with a large field-of-view (FOV) and high-speed imaging. The large FOV and the high imaging speed is achieved by combining optical and mechanical scanning mechanisms. To compensate for an uneven surface of biological samples, which result in images with non-uniform resolution and low signal to noise ratio (SNR), we further develop a novel OCT-guided surface contour scanning methodology, a technique for adjusting objective lens focus to follow the contour of the sample surface, to provide a uniform spatial resolution and SNR across the region of interest (ROI). The imaging system was tested by imaging phantoms, ex vivo biological samples, and in vivo. The OCT-guided surface contour scanning methodology was utilized for imaging a leaf of purple queen plant, which resulted in a significant contrast improvement of 41% and 38% across a large imaging area for CFM and PAM, respectively. The nuclei and cells walls were also clearly observed in both images. In an in vivo imaging of the Swiss Webster mouse ear, our multimodal imaging system was able to provide images with uniform resolution in an FOV of 10 mm x 10 mm with an imaging time of around 5 minutes. In addition to measuring the blood flow in the mouse ear, the system also successfully imaged mouse ear blood vessels, sebaceous glands, as well as several tissue structures. We further conducted a comparative study of OCTA for rodent retinal imaging by evaluating the performance of three OCTA algorithms, namely the phase variance (PV), improved speckle contrast (ISC), and optical microangiography (OMAG). It was concluded that the OMAG algorithm provided statistically significant higher mean values of BVD and VPI compared to the ISC algorithm (0.27±0.07 vs. 0.24±0.05 for BVD; 0.09±0.04 and 0.08±0.04 for VPI), while no statistically significant difference was observed for VDI and VCI among the algorithms. Results showed that both the ISC and OMAG algorithms are more robust than PV, and they can reveal similar vasculature features. Lastly, we utilized the proposed imaging system to monitor, for the first time, the invasion process of malaria parasites in the mosquito midgut. The system shows a promising potential to detect parasite motion as well as structural changes inside the mosquito midgut. The multimodal imaging system outlined in this dissertation can be useful in a variety of applications thanks to the specific optical contrast offered by each employed modality, including retinal and brain imaging

Ultrasound-driven microbubble dynamics in microvessels

Ultrasound and microbubble induced blood-brain barrier opening has shown success in clinical trials as a promising method to deliver drugs to the brain. Shelled gas bubbles, a few micrometres in diameter, are administered intravenously, and distribute throughout the cardiovascular system. When ultrasound is applied to the brain, the microbubbles expand and contract within the vasculature, temporarily disrupting the blood-brain barrier, and allowing drugs to pass through. While this technique has been shown to be effective at delivering drugs, its mechanisms remain relatively poorly understood. Better understanding how microbubbles interact with tissues could enable refinement of therapies. This thesis investigates the fundamental physical interactions between microbubbles and soft tissues using two distinct but related experimental platforms that utilise high-speed microscopy. Firstly, microbubbles within soft tissue-mimicking hydrogel channels are observed during exposure to typical therapeutic ultrasound pulses. The primary radiation force is shown to be significant, and can cause bubbles to deform the soft gels by several micrometres. Microbubbles are also investigated in brain tissue, using acute cortical slices from the brains of juvenile rats, transcardially perfused post-mortem with a concentrated solution of SonoVue®. This technique is shown to be an effective method of observing microbubbles using optical microscopy within the microvasculature of live brain tissue. Radial oscillations of bubbles within brain microvessels can deform surrounding tissue at both microsecond and millisecond time scales. Extravasation of microbubbles due to the primary radiation force can occur during typical ultrasound pulses, and is common at higher ultrasound pressures (mechanical index of 0.6 and above). These results demonstrate the significance of both radial oscillations and the primary radiation force as ways in which microbubbles can physically impact their surroundings. Additionally, acute brain slices are shown to be a valuable tool to investigate microbubble behaviours and mechanisms of drug delivery in a physiologically relevant environment.Open Acces