Novel Approaches for Nondestructive Testing and Evaluation

Nondestructive testing and evaluation (NDT&E) is one of the most important techniques for determining the quality and safety of materials, components, devices, and structures. NDT&E technologies include ultrasonic testing (UT), magnetic particle testing (MT), magnetic flux leakage testing (MFLT), eddy current testing (ECT), radiation testing (RT), penetrant testing (PT), and visual testing (VT), and these are widely used throughout the modern industry. However, some NDT processes, such as those for cleaning specimens and removing paint, cause environmental pollution and must only be considered in limited environments (time, space, and sensor selection). Thus, NDT&E is classified as a typical 3D (dirty, dangerous, and difficult) job. In addition, NDT operators judge the presence of damage based on experience and subjective judgment, so in some cases, a flaw may not be detected during the test. Therefore, to obtain clearer test results, a means for the operator to determine flaws more easily should be provided. In addition, the test results should be organized systemically in order to identify the cause of the abnormality in the test specimen and to identify the progress of the damage quantitatively

Photoacoustic Reporter Gene Imaging And Optical Coherence Computed Tomography

Advances in imaging technologies have always been the major driving forces for the evolution of biomedical research. Compared with other modalities, optical imaging possesses several prominent merits. Because light interacts with tissue at the microscopic level through many distinct physical mechanisms, optical methods allow sensitive exploration of various aspects of the life down to the single-molecule level. From the technical perspective, optical systems utilize safe non-ionizing radiation, could be implemented at relatively low cost, also have the potential to be miniaturized for portable or endoscopic applications. As a result, optical imaging tools are playing an increasingly important role in both laboratorial research and clinical practice. Among them, photoacoustic imaging: PAI) and optical coherence tomography: OCT) are the two fastest growing branches. PAI measures the laser-induced acoustic wave, and produces high-resolution images of the optically absorbing features of tissue at multiple length-scales. OCT detects singly backscattered photons, and enables real-time high-resolution in vivo biopsy of tissue up to an optical transport mean-free-path. My doctoral research is focused on developing three novel optical imaging techniques based on the spirits of PAI and OCT. In the first part of this study, we established a new paradigm to visualize gene expression in vivo based on optical absorption. In the post-genomic era, we are now being challenged to develop novel molecular imaging methods to identify the functions of genes. PAI can detect specific molecules according to their characteristic absorption spectra, thus is a promising candidate for molecular imaging of gene expression. The full potential of photoacoustic molecular imaging still remains to be explored. For the first time, we demonstrated imaging gene expression by PAI in living mice and rats, using a chromogenic lacZ/X-gal reporter gene system. We demonstrated the expression of the lacZ reporter gene can be detected by PAI as deep as 5 cm inside tissue. In addition, we showcased that PAI could follow gene expression from the microscopic to the macroscopic level. This work represents one of the pioneering efforts to extend photoacoustic methods for molecular imaging. In the second part of this study, we developed a novel multimodal microscope, called the integrated photoacoustic and optical coherence microscope: iPOM), which combines PAI and OCT in a single imaging platform. PAI is predominantly sensitive to optical absorption, while OCT exploits optical scattering. By combining their naturally complementary imaging contrasts, iPOM can provide comprehensive information about biological tissue. We designed and built a reflection-mode prototype of iPOM, which fuses optical-resolution photoacoustic microscopy with spectral-domain optical coherence tomography. The potential applications of iPOM in studying cutaneous and ocular microcirculation, and tissue engineering were demonstrated. Finally, we invented a new optical tomography, named optical coherence computed tomography: optical CCT), which overcomes several major limitations of OCT. OCT relies on singly backscattered photons to obtain high-resolution images. Its image quality degrades fast with the increase of the depth, because the multiply scattered photons quickly become dominant at a penetration larger than 500 &mum. As a result, OCT can only effectively penetrate ~1 mm into highly scattering tissue like skin. In addition, OCT is mainly sensitive to optical scattering, which does not reflect the molecular content of tissue directly. Optical CCT measures both singly and multiply scattered photons using a low-coherence interferometer. We make use of both types of photons by adopting a model-based reconstruction algorithm. The light-tissue interaction model was established using the time-resolved Monte Carlo method. The optical properties of the tissue were reconstructed from measurements by solving the inverse radiative transport problem under the first Born approximation. As a result, optical CCT could image deeper than OCT, and provide extra molecule-specific contrasts, such as optical absorption. We designed and built the first optical CCT system. In experiments, absorbing inclusions of 100 &mum diameter were imaged with consistent quality through a 2.6-mm-thick: equivalent to ~3 transport mean-free-paths) tissue-mimicking phantom

Imaging Sensors and Applications

In past decades, various sensor technologies have been used in all areas of our lives, thus improving our quality of life. In particular, imaging sensors have been widely applied in the development of various imaging approaches such as optical imaging, ultrasound imaging, X-ray imaging, and nuclear imaging, and contributed to achieve high sensitivity, miniaturization, and real-time imaging. These advanced image sensing technologies play an important role not only in the medical field but also in the industrial field. This Special Issue covers broad topics on imaging sensors and applications. The scope range of imaging sensors can be extended to novel imaging sensors and diverse imaging systems, including hardware and software advancements. Additionally, biomedical and nondestructive sensing applications are welcome

Photoacoustic Image Analysis for Cancer Detection and Building a Novel Ultrasound Imaging System

Photoacoustic (PA) imaging is a rapidly emerging non-invasive soft tissue imaging modality which has the potential to detect tissue abnormality at early stage. Photoacoustic images map the spatially varying optical absorption property of tissue. In multiwavelength photoacoustic imaging, the soft tissue is imaged with different wavelengths, tuned to the absorption peaks of the specific light absorbing tissue constituents or chromophores to obtain images with different contrasts of the same tissue sample. From those images, spatially varying concentration of the chromophores can be recovered. As multiwavelength PA images can provide important physiological information related to function and molecular composition of the tissue, so they can be used for diagnosis of cancer lesions and differentiation of malignant tumors from benign tumors. In this research, a number of parameters have been extracted from multiwavelength 3D PA images of freshly excised human prostate and thyroid specimens, imaged at five different wavelengths. Using marked histology slides as ground truths, region of interests (ROI) corresponding to cancer, benign and normal regions have been identified in the PA images. The extracted parameters belong to different categories namely chromophore concentration, frequency parameters and PA image pixels and they represent different physiological and optical properties of the tissue specimens. Statistical analysis has been performed to test whether the extracted parameters are significantly different between cancer, benign and normal regions. A multidimensional [29 dimensional] feature set, built with the extracted parameters from the 3D PA images, has been divided randomly into training and testing sets. The training set has been used to train support vector machine (SVM) and neural network (NN) classifiers while the performance of the classifiers in differentiating different tissue pathologies have been determined by the testing dataset. Using the NN classifier, performance of parameters belonging to different categories in differentiating malignant tissue from nonmalignant tissue has been determined. It has been found that, among different categories, the frequency parameters performed best in differentiating malignant from nonmalignant tissue [sensitivity and specificity with testing dataset are 85% and 84%] while performance of all the categories combined was better than that [sensitivity and specificity with testing dataset are 93% and 91%]. However, PA imaging cannot be used to provide the anatomical cues required to determine the position of the detected or suspected malignant tumor region relative to familiar organ landmarks. On the other hand, although accuracy of Ultrasound (US) imaging in detecting cancer lesions is low, major anatomical cues like organ boundaries or presence of nearby major organs are visible in US images. A dual mode PA and US imaging system can potentially detect as well as localize cancer lesions with high accuracy. In this study, we have developed a novel pulse echo US imaging system which can be easily integrated with our existing ex-vivo PA imaging system to produce the dual mode imaging system. Here a Polyvinylidene fluoride (PVDF) film has been used as US transmitter. To improve the anticipated low signal to noise ratio (SNR) of the received US signal due to the low electromechanical coupling coefficient of the PVDF film, we implemented pulse compression technique using chirp signals. Comparisons among the different SNR values obtained with short pulse and after pulse compression with chirp signal show a clear improvement of the SNR for the compressed pulse. The axial resolution of the imaging system improved with increasing sweep bandwidth of input chirp signals, whereas the lateral resolution remained almost constant. This work demonstrates the feasibility of using a PVDF film transducer as an US transmitter and implementing pulse compression technique in an acoustic lens focusing based imaging system

Photoacoustic Technologies beyond Medical Imaging-PASA and LGFU

The Photoacoustic (PA) effect has been extensively studied as a direct and efficient light-to-sound conversion process. The majority of previous applications of the PA effect are focused on photoacoustic imaging, where the combination of optics and acoustics ensures both optical level resolution and large penetration depth into bio-samples. This thesis aims to explore the possibilities to applying the photoacoustic effects on other fields including non-imaging PA signal spectrum analysis, therapeutic treatment and nozzle-free jetting for printing. First, in the case of fast and quantitative analysis of bio-samples, a non-imaging approach is preferable to analyze the spectrum of the PA signals. This is referred to as photoacoustic spectrum analysis. We have found a solid relationship between the morphological characteristics of the objects generating PA signals, and the polynomial fit of the measured spectrum of these PA signals. By measuring the signal with an ultra-broad-bandwidth ultrasound detector, we are able to characterize the size and shape of bio-samples in single cell level, paving the way for applications such as flow cytometry, cell counting, disease detection including blood diseases and blood freshness detection. Second, with the development of highly-efficient photoacoustic generation materials, PA transmitters that generate large amplitude, broadband and complex ultrasounds waveforms have been fabricated, extending the PA applications from pure imaging to therapeutic treatment and other areas. Our PA transmitters are fabricated with simple and inexpensive ways compared with piezoelectric ceramic film fabrication used in conventional transducers or arrays. In this thesis, a self-focusing PA lens, made from candle soot (CS)/polydimethylsiloxane (PDMS) composite has been developed and applied in laser-generated-focused-ultrasound (LGFU). A Pulsed laser illuminates the PA lens to generate a tightly focused PA wave within an ellipsoid of 90μm (minor axis) * 200μm (major axis). Compared with previously developed PA generation layers made from carbon nanotubes (CNT) and metals, candle soot can be deposited with much lower cost and simplicity. Within the focal region, a negative pressure over 27MPa is achieved, which ensures steady bubble cavitation. We demonstrate one way to enhance this cavitation with the help of a superimposed low-frequency, low-amplitude ultrasound field, and get ~30% generation rate enhancement and ~60% cavitation bubble size enhancement. The energy released from bubbles collapse are used for ablation of bio-tissues or generation of thin and high-speed streams. Taking advantages of these properties, LGFU equipped with CS/PDMS PA lens has been applied in two major areas: 1) Selective treatment, where the LGFU is used for direct ablation and dysfunction of the soft tissues less than 1mm. This treatment enjoys both selectiveness and accuracy and can treat each individual with his/her own need; 2) Nozzle-free jet printing, where the LGFU is used to generate ~10μm jets of materials with various viscosity and density. It has been demonstrated that the 2D materials like graphene and MoS2 can be printed with resolution of ~200μm. This can be applied as a potential nozzle-free high-resolution patterning modality in flexible electronics.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149789/1/qiaochul_1.pd

Real-time Intravascular Photoacoustics

The rupture of vulnerable atherosclerotic plaque is the most frequent cause of acute cardiovascular events and sudden cardiac deaths. The identification of the vulnerable plaque, which is believed to be related to the structure and composition of the plaque, can greatly benefit the management of the cardiovascular disease in clinics. Intravascular photoacoustic (IVPA) imaging can characterize the composition of the plaque based on the optical contrast between different tissue types, which can be easily applied by performing IVPA imaging at different wavelengths for different imaging targets. Combined IVPA/US imaging shows great potentials to image the vulnerable atherosclerotic plaque, morphologically (co-registered IVUS image) and compositionally (especially lipid-rich plaque), and is becoming a powerful tool to guide the assessment and treatment of the atherosclerotic plaque lesions. The aim of this thesis is to develop a prototype of fast IVPA/US imaging system capable of performing in vivo experiments on swine model, accelerating the translation of IVPA/US imaging toward clinical application

Acoustic Waves

The concept of acoustic wave is a pervasive one, which emerges in any type of medium, from solids to plasmas, at length and time scales ranging from sub-micrometric layers in microdevices to seismic waves in the Sun's interior. This book presents several aspects of the active research ongoing in this field. Theoretical efforts are leading to a deeper understanding of phenomena, also in complicated environments like the solar surface boundary. Acoustic waves are a flexible probe to investigate the properties of very different systems, from thin inorganic layers to ripening cheese to biological systems. Acoustic waves are also a tool to manipulate matter, from the delicate evaporation of biomolecules to be analysed, to the phase transitions induced by intense shock waves. And a whole class of widespread microdevices, including filters and sensors, is based on the behaviour of acoustic waves propagating in thin layers. The search for better performances is driving to new materials for these devices, and to more refined tools for their analysis