Parallel Acoustic Delay Line (PADL) Arrays for Photoacoustic Imaging Applications

Micromachining process, such as laser micromachining and IC microfabrication process, allows production of complex structures in limited space, which reduces both the size and cost of hardware. In this research, using the advantages of micromachining processes, parallel acoustic delay line (PADL) arrays made of optical fibers and single-crystalline silicon (SCS) have been developed to reduce the number of ultrasonic transducers and data acquisition (DAQ) electronics for real-time photoacoustic tomography (PAT). The PADL arrays allow real-time PAT with the significantly reduced number of ultrasonic transducers and DAQs. Handheld optical PADL array enables more practical operation for photoacoustic imaging applications by miniaturizing previously developed optical PADL array. Sixteen channels of optical fiber PADLs were fabricated and assembled with laser micromachined acrylic housing for the compact structure. By conducting ultrasonic transmission testing, acoustic properties of optical fibers have been characterized. PA imaging capability of optical fiber PADL array has been evaluated by PA imaging experiment. Microfabrication process makes it possible to use single-crystalline silicon as a material for acoustic delay lines. Acoustic properties of silicon were characterized by ultrasonic transmission testing. Based on the characterization result, silicon acoustic delay line was designed into a spiral coil shape to minimize the overall size. Silicon PADLs are better than optical fiber PADL for miniaturization due to the advantages of microfabrication process. Silicon PADL array achieved a channel reduction ratio of 16:1, which is twice the ratio of optical fiber PADL. The PA imaging experiment has demonstrated the PA imaging capability of silicon PADL array. For fast imaging speed and good spatial resolution, silicon PADL array has been improved by applying 3D-printed linker structures and tapered input terminal. Linker structure design has been evaluated by both structural and acoustic simulation. The final design of linker structure is 3D-printed polymer linker to securely hold silicon delay lines with minimal contacts. Tapered input terminal was designed to reduce acoustic acceptance angle for better spatial resolution. Tapered input terminal was evaluated by acoustic simulation with different designs. Those designs and techniques are expected to provide new solutions to reduce the cost and complexity of ultrasonic receiving systems for photoacoustic imaging applications

Parallel Acoustic Delay Line (PADL) Arrays for Photoacoustic Imaging Applications

Micromachining process, such as laser micromachining and IC microfabrication process, allows production of complex structures in limited space, which reduces both the size and cost of hardware. In this research, using the advantages of micromachining processes, parallel acoustic delay line (PADL) arrays made of optical fibers and single-crystalline silicon (SCS) have been developed to reduce the number of ultrasonic transducers and data acquisition (DAQ) electronics for real-time photoacoustic tomography (PAT). The PADL arrays allow real-time PAT with the significantly reduced number of ultrasonic transducers and DAQs. Handheld optical PADL array enables more practical operation for photoacoustic imaging applications by miniaturizing previously developed optical PADL array. Sixteen channels of optical fiber PADLs were fabricated and assembled with laser micromachined acrylic housing for the compact structure. By conducting ultrasonic transmission testing, acoustic properties of optical fibers have been characterized. PA imaging capability of optical fiber PADL array has been evaluated by PA imaging experiment. Microfabrication process makes it possible to use single-crystalline silicon as a material for acoustic delay lines. Acoustic properties of silicon were characterized by ultrasonic transmission testing. Based on the characterization result, silicon acoustic delay line was designed into a spiral coil shape to minimize the overall size. Silicon PADLs are better than optical fiber PADL for miniaturization due to the advantages of microfabrication process. Silicon PADL array achieved a channel reduction ratio of 16:1, which is twice the ratio of optical fiber PADL. The PA imaging experiment has demonstrated the PA imaging capability of silicon PADL array. For fast imaging speed and good spatial resolution, silicon PADL array has been improved by applying 3D-printed linker structures and tapered input terminal. Linker structure design has been evaluated by both structural and acoustic simulation. The final design of linker structure is 3D-printed polymer linker to securely hold silicon delay lines with minimal contacts. Tapered input terminal was designed to reduce acoustic acceptance angle for better spatial resolution. Tapered input terminal was evaluated by acoustic simulation with different designs. Those designs and techniques are expected to provide new solutions to reduce the cost and complexity of ultrasonic receiving systems for photoacoustic imaging applications

Functional Connectivity of the Rodent Brain Using Optical Imaging

RÉSUMÉ L'objectif de cette thèse de doctorat est d’appliquer la connectivité fonctionnelle dans une variété de modèles animaux, à l’aide de plusieurs techniques d’imagerie optique. Le cerveau, même au repos, montre une activité métabolique élevée : la corrélation des fluctuations spontanées lentes permet d’identifier des régions cérébrales distantes mais connectées; d’où le terme connectivité fonctionnelle. Les changements dans l’activité spontanée peuvent donner un aperçu des processus neuronaux qui comprennent la majorité de l’activité métabolique du cerveau, et constituent en conséquent une vaste source de changements reliés aux maladies. L’hémodynamique du cerveau peut être modifiée grâce à des affections neurovasculaires et avoir un effet sur l’activité au repos. Cette thèse vise la compréhension des changements de connectivité fonctionnelle induits par des maladies, à l’aide de l’imagerie optique fonctionnelle. Les techniques d’imagerie explorées dans les deux premières contributions de cette thèse sont l’Imagerie Optique Intrinsèque et l’Imagerie par Granularité Laser. Ensemble, elles peuvent estimer les changements de consommation d'oxygène, étroitement liés à l’activité neuronale. Ces techniques possèdent des résolutions temporelles et spatiales adéquates et bien adaptées pour imager la convexité du cortex cérébral. Dans le dernier article, une modalité d’imagerie en profondeur, la Tomographie Photoacoustique a été utilisée chez le rat nouveau-né. La Tomographie par Cohérence Optique et la Tomographie Laminaire Optique font également partie de la gamme des techniques d’imagerie développées et appliquées dans d’autres collaborations. La première partie des résultats mesure les changements de connectivité fonctionnelle dans un modèle d’activité épileptiforme aiguë chez le rongeur. Il y a des augmentations ainsi que des diminutions entre les corrélations homologues, avec une faible dépendance aux crises épileptiques. Ces changements suggèrent un découplage potentiel entre les paramètres hémodynamiques dans les réseaux au repos, en soulignant l’importance d’investiguer les réseaux épileptiques à l’aide de plusieurs mesures hémodynamiques indépendantes. La deuxième partie des travaux étudie un nouveau modèle de rigidité artérielle chez la souris : la calcification unilatérale de la carotide droite. L’analyse de connectivité basé sur les régions d’intérêt montre une tendance décroissante de corrélation homologue dans les cortex moteur et cingulum. L’analyse de graphes montre une randomisation des réseaux corticaux, ce qui suggère une perte de connectivité; plus spécifiquement, dans le cortex moteur ipsilateral à la carotide----------ABSTRACT The aim of this thesis is to apply functional connectivity in a variety of animal models, using several optical imaging modalities. Even at rest, the brain shows high metabolic activity: the correlation in slow spontaneous fluctuations identifies remotely connected areas of the brain; hence the term “functional connectivity”. Ongoing changes in spontaneous activity may provide insight into the neural processing that takes most of the brain metabolic activity, and so may provide a vast source of disease related changes. Brain hemodynamics may be modified during disease and affect resting-state activity. The thesis aims to better understand these changes in functional connectivity due to disease, using functional optical imaging. The optical imaging techniques explored in the first two contributions of this thesis are Optical Imaging of Intrinsic Signals and Laser Speckle Contrast Imaging, together they can estimate the metabolic rate of oxygen consumption, that closely parallels neural activity. They both have adequate spatial and temporal resolution and are well adapted to image the convexity of the mouse cortex. In the last article, a depth-sensitive modality called photoacoustic tomography was used in the newborn rat. Optical coherence tomography and laminar optical tomography were also part of the array of imaging techniques developed and applied in other collaborations. The first article of this work shows the changes in functional connectivity in an acute murine model of epileptiform activity. Homologous correlations are both increased and decreased with a small dependence on seizure duration. These changes suggest a potential decoupling between the hemodynamic parameters in resting-state networks, underlining the importance to investigate epileptic networks with several independent hemodynamic measures. The second study examines a novel murine model of arterial stiffness: the unilateral calcification of the right carotid. Seed-based connectivity analysis showed a decreasing trend of homologous correlation in the motor and cingulate cortices. Graph analyses showed a randomization of the cortex functional networks, suggesting a loss of connectivity, more specifically in the motor cortex ipsilateral to the treated carotid; however these changes are not reflected in differentiated metabolic estimates. Confounds remain due to the fact that carotid rigidification gives rise to neural decline in the hippocampus as well as unilateral alteration of vascular pulsatility; howeve

Photoacoustic Elastography and Next-generation Photoacoustic Tomography Techniques Towards Clinical Translation

Ultrasonically probing optical absorption, photoacoustic tomography (PAT) combines rich optical contrast with high ultrasonic resolution at depths beyond the optical diffusion limit. With consistent optical absorption contrast at different scales and highly scalable spatial resolution and penetration depth, PAT holds great promise as an important tool for both fundamental research and clinical application. Despite tremendous progress, PAT still encounters certain limitations that prevent it from becoming readily adopted in the clinical settings. This dissertation aims to advance both the technical development and application of PAT towards its clinical translation. The first part of this dissertation describes the development of photoacoustic elastography techniques, which complement PAT with the capability to image the elastic properties of biological tissue and detect pathological conditions associated with its alterations. First, I demonstrated vascular-elastic PAT (VE-PAT), capable of quantifying blood vessel compliance changes due to thrombosis and occlusions. Then, I developed photoacoustic elastography to noninvasively map the elasticity distribution in biological tissue. Third, I further enhanced its performance by combing conventional photoacoustic elastography with a stress sensor having known stress–strain behavior to achieve quantitative photoacoustic elastography (QPAE). QPAE can quantify the Young’s modulus of biological tissues on an absolute scale. The second part of this dissertation introduces technical improvements of photoacoustic microscopy (PAM). First, by employing near-infrared (NIR) light for illumination, a greater imaging depth and finer lateral resolution were achieved by near-infrared optical-resolution PAM (NIR-OR-PAM). In addition, NIR-OR-PAM was capable of imaging other tissue components, including lipid and melanin. Second, I upgraded a high-speed functional OR-PAM (HF-OR-PAM) system and applied it to image neurovascular coupling during epileptic seizure propagation in mouse brains in vivo with high spatio-temporal resolution. Last, I developed a single-cell metabolic PAM (SCM-PAM) system, which improves the current single-cell oxygen consumption rate (OCR) measurement throughput from ~30 cells over 15 minutes to ~3000 cells over 15 minutes. This throughput enhancement of two orders of magnitude achieves modeling of single-cell OCR distribution with a statistically meaningful cell count. SCM-PAM enables imaging of intratumoral metabolic heterogeneity with single-cell resolution. The third part of this dissertation introduces the application of linear-array-based PAT (LA-PAT) in label-free high-throughput imaging of melanoma circulating tumor cells (CTCs) in patients in vivo. Taking advantage of the strong optical absorption of melanin and the unique capability of PAT to image optical absorption, with 100% relative sensitivity, at depths with high ultrasonic spatial resolution, LA-PAT is inherently suitable for melanoma CTC imaging. First, with a center ultrasonic frequency of 21 MHz, the LA-PAT system was able to detect melanoma CTCs clusters and quantify their sizes based on the contrast-to-noise ratio (CNR). Second, I developed an LA-PAT system with a center ultrasonic frequency of 40 MHz and imaged melanoma CTCs in patients in vivo with a CNR greater than 12. We successfully imaged 16 melanoma patients and detected melanoma CTCs in 3 of them. Among the CTC-positive patients, 67% had disease progression despite systemic therapy. In contrast, only 23% of the CTC-negative patients showed disease progression. This study lays a solid foundation for translating CTC detection to bedside for clinical care and decision-making

MODELLING AND VERIFICATION OF THERMOACOUSTIC MEDICAL IMAGING FROM NANOSCOPIC TO MACROSCOPIC RESOLUTIONS

In this thesis, three main questions regarding the potential of thermoacoustic imaging are answered: 1) what are the conventional resolution limitations of photoacoustic imaging and how can they be extended to enable high-resolution imaging, 2) Can photoacoustic imaging resolution be brought down to nanoscopic levels, and 3) As laser based photoacoustic imaging has been deployed with great success, is it also possible for other radiation to generate useful ultrasound signals for imaging? Whereas laser-induced photoacoustic tomography has been widely explored for a diverse range of biomedical contexts, there remain some fundamental limits to the resolution levels in which it can operate. Namely, the axial resolution of photoacoustic imaging remains restricted by the fact that ultrasonic transducers are not able to detect high-frequency signals that encode nanoscale resolution information. Therefore, there is a lingering question about how photoacoustic imaging can truly enter the realm of nanoscale imaging, as has been done by other modalities such as STED microscopy, structured illumination microscopy, and STORM microscopy. It is believed that laser-based detection in lieu of a transducer may enable a super-resolution photoacoustic imaging modality. However, there remain important questions about the reach and feasibility of nanoscale photoacoustic imaging. Specifically: will highly focused lasers directed at single cells result in thermal damage of biological samples? Will the axial imaging resolution of laser based detection truly be able to overcome the conventional optical diffraction limit of ~200nm? Will optical detection be sensitive enough to detect photoacoustic signals? Consequently, models are developed for thermoacoustic imaging for nanoscale imaging at super-resolutions exceeding that of the optical diffraction limit (~200nm), that show the potential for thermoacoustic imaging to enable super-resolution imaging of single cells. The models confirm that such imaging is possible while simultaneously ensuring the thermal safety of cells as the laser-induced temperature rise of such imaging is only within mK, potentially allowing for high-resolution imaging in vivo. It is also confirmed that a laser of 7ps duration should generate frequencies high enough to enable super-resolutions. Models are also developed for the estimation of the sensitivity and resolution of these high-resolution imaging, and it is predicted that super-resolution photoacoustic imaging may be able to image at axial resolutions of 10nm at noise equivalent number of molecules of 292 in the case of imaging hemoglobin in red blood cells. A length-scale and time-scale generalizable simulation workflow is developed and deployed to generate simulated images of super-resolution photoacoustic imaging, showing the potential of 3D super-resolution achievable via thermoacoustic imaging. This numerical simulation workflow is generalizable to multiple length scales as well as to other sources of radiation. The model predictions regarding detectable high frequency photoacoustic signal generation is experimentally confirmed via the creation and testing of a pump-probe based preliminary photoacoustic imaging system. The system is shown to be capable of detecting a clear and repeatable signal. Acquired A-lines from this system confirm that GHz frequencies can be detected using pump-probe detection in photoacoustics, thereby opening the door for nanoscale photoacoustic imaging However, the experimental results also demonstrate that feasible and convenient nanoscale imaging will require a more stable laser than is available, as pulse to pulse intensity fluctuations in the laser greatly limit the imaging speed and necessary number of averages for a single A-line scan. The developed models show promise and use towards the development of novel thermoacoustic imaging modalities and can be deployed to assess feasibility of different configurations of thermoacoustic imaging prior to the expenditure of resources on experimental realization. In this way, the developed models have the potential to enable the development of various thermoacoustic imaging modalities via a single generalizable framework through which imaging characteristics can be predicted at multiple length and time scales