Respiratory organ motion in interventional MRI : tracking, guiding and modeling

Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy. In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities. Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well. Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence

Image enhancement in digital X-ray angiography

Anyone who does not look back to the beginning throughout a course of action, does not look forward to the end. Hence it necessarily follows that an intention which looks ahead, depends on a recollection which looks back. | Aurelius Augustinus, De civitate Dei, VII.7 (417 A.D.) Chapter 1 Introduction and Summary D espite the development of imaging techniques based on alternative physical phenomena, such as nuclear magnetic resonance, emission of single photons ( -radiation) by radio-pharmaceuticals and photon pairs by electron-positron annihilations, re ection of ultrasonic waves, and the Doppler eect, X-ray based im- age acquisition is still daily practice in medicine. Perhaps this can be attributed to the fact that, contrary to many other phenomena, X-rays lend themselves naturally for registration by means of materials and methods widely available at the time of their discovery | a fact that gave X-ray based medical imaging an at least 50-year head start over possible alternatives. Immediately after the preliminary communica- tion on the discovery of the \new light" by R¨ ontgen [317], late December 1895, the possible applications of X-rays were investigated intensively. In 1896 alone, almost one 1,000 articles about the new phenomenon appeared in print (Glasser [119] lists all of them). Although most of the basics of the diagnostic as well as the therapeutic uses of X-rays had been worked out by the end of that year [289], research on im- proved acquisition and reduction of potential risks for humans continued steadily in the century to follow. The development of improved X-ray tubes, rapid lm changers, image intensiers, the introduction of television cameras into uoroscopy, and com- puters in digital radiography and computerized tomography, formed a succession of achievements which increased the diagnostic potential of X-ray based imaging. One of the areas in medical imaging where X-rays have always played an im- portant role is angiography,y which concerns the visualization of blood vessels in the human body. As already suggested, research on the possibility of visualization of the human vasculature was initiated shortly after the discovery of X-rays. A photograph of a rst \angiogram" | obtained by injection of a mixture of chalk, red mercury, and petroleum into an amputated hand, followed by almost an hour of exposure to X-rays | was published as early as January 1896, by Hascheck & Lindenthal [139]. Although studies on cadavers led to greatly improved knowledge of the anatomy of the human vascular system, angiography in living man for the purpose of diagnosis and intervention became feasible only after substantial progress in the development yA term originating from the Greek words o (aggeion), meaning \vessel" or \bucket", and -' (graphein), meaning \to write" or \to record". 2 1 Introduction and Summary of relatively safe contrast media and methods of administration, as well as advance- ments in radiological equipment. Of special interest in the context of this thesis is the improvement brought by photographic subtraction, a technique known since the early 1900s and since then used successfully in e.g. astronomy, but rst introduced in X-ray angiography in 1934, by Ziedses des Plantes [425, 426]. This technique al- lowed for a considerable enhancement of vessel visibility by cancellation of unwanted background structures. In the 1960s, the time consuming lm subtraction process was replaced by analog video subtraction techniques [156, 275] which, with the in- troduction of digital computers, gave rise to the development of digital subtraction angiography [194] | a technique still considered by many the \gold standard" for de- tection and quantication of vascular anomalies. Today, research on improved X-ray based imaging techniques for angiography continues, witness the recent developments in three-dimensional rotational angiography [88, 185, 186, 341,373]. The subject of this thesis is enhancement of digital X-ray angiography images. In contrast with the previously mentioned developments, the emphasis is not on the further improvement of image acquisition techniques, but rather on the development and evaluation of digital image processing techniques for retrospective enhancement of images acquired with existing techniques. In the context of this thesis, the term \enhancement" must be regarded in a rather broad sense. It does not only refer to improvement of image quality by reduction of disturbing artifacts and noise, but also to minimization of possible image quality degradation and loss of quantitative information, inevitably introduced by required image processing operations. These two aspects of image enhancement will be claried further in a brief summary of each of the chapters of this thesis. The rst three chapters deal with the problem of patient motion artifacts in digital subtraction angiography (DSA). In DSA imaging, a sequence of 2D digital X-ray projection images is acquired, at a rate of e.g. two per second, following the injection of contrast material into one of the arteries or veins feeding the part of the vasculature to be diagnosed. Acquisition usually starts about one or two seconds prior to arrival of the contrast bolus in the vessels of interest, so that the rst few images included in the sequence do not show opacied vessels. In a subsequent post-processing step, one of these \pre-bolus" images is then subtracted automatically from each of the contrast images so as to mask out background structures such as bone and soft- tissue shadows. However, it is clear that in the resulting digital subtraction images, the unwanted background structures will have been removed completely only when the patient lied perfectly still during acquisition of the original images. Since most patients show at least some physical reaction to the passage of a contrast medium, this proviso is generally not met. As a result, DSA images frequently show patient-motion induced artifacts (see e.g. the bottom-left image in Fig. 1.1), which may in uence the subsequent analysis and diagnosis carried out by radiologists. Since the introduction of DSA, in the early 1980s, many solutions to the problem of patient motion artifacts have been put forward. Chapter 2 presents an overview of the possible types of motion artifacts reported in the literature and the techniques that have been proposed to avoid them. The main purpose of that chapter is to review and discuss the techniques proposed over the past two decades to correct for 1 Introduction and Summary 3 Figure 1.1. Example of creation and reduction of patient motion artifacts in cerebral DSA imaging. Top left: a \pre-bolus" or mask image acquired just prior to the arrival of the contrast medium. Top right: one of the contrast or live images showing opacied vessels. Bottom left: DSA image obtained after subtraction of the mask from the contrast image, followed by contrast enhancement. Due to patient motion, the background structures in the mask and contrast image were not perfectly aligned, as a result of which the DSA image does not only show blood vessels, but also additional undesired structures (in this example primarily in the bottom-left part of the image). Bottom right: DSA image resulting from subtraction of the mask and contast image after application of the automatic registration algorithm described in Chapter 3. 4 1 Introduction and Summary patient motion artifacts retrospectively, by means of digital image processing. The chapter addresses fundamental problems, such as whether it is possible to construct a 2D geometrical transformation that exactly describes the projective eects of an originally 3D transformation, as well as practical problems, such as how to retrieve the correspondence between mask and contrast images by using only the grey-level information contained in the images, and how to align the images according to that correspondence in a computationally ecient manner. The review in Chapter 2 reveals that there exists quite some literature on the topic of (semi-)automatic image alignment, or image registration, for the purpose of motion artifact reduction in DSA images. However, to the best of our knowledge, research in this area has never led to algorithms which are suciently fast and robust to be acceptable for routine use in clinical practice. By drawing upon the suggestions put forward in Chapter 2, a new approach to automatic registration of digital X-ray angiography images is presented in Chapter 3. Apart from describing the functionality of the components of the algorithm, special attention is paid to their computationally optimal implementation. The results of preliminary experiments described in that chapter indicate that the algorithm is eective, very fast, and outperforms alterna- tive approaches, in terms of both image quality and required computation time. It is concluded that the algorithm is most eective in cerebral and peripheral DSA imag- ing. An example of the image quality enhancement obtained after application of the algorithm in the case of a cerebral DSA image is provided in Fig 1.1. Chapter 4 reports on a clinical evaluation of the automatic registration technique. The evaluation involved 104 cerebral DSA images, which were corrected for patient motion artifacts by the automatic technique, as well as by pixel shifting | a manual correction technique currently used in clinical practice. The quality of the DSA images resulting from the two techniques was assessed by four observers, who compared the images both mutually and to the corresponding original images. The results of the evaluation presented in Chapter 4 indicate that the dierence in performance between the two correction techniques is statistically signicant. From the results of the mutual comparisons it is concluded that, on average, the automatic registration technique performs either comparably, better than, or even much better than manual pixel shifting in 95% of all cases. In the other 5% of the cases, the remaining artifacts are located near the borders of the image, which are generally diagnostically non-relevant. In addition, the results show that the automatic technique implies a considerable reduction of post-processing time compared to manual pixel shifting (on average, one second versus 12 seconds per DSA image). The last two chapters deal with somewhat dierent topics. Chapter 5 is concerned with visualization and quantication of vascular anomalies in three-dimensional rota- tional angiography (3DRA). Similar to DSA imaging, 3DRA involves the acquisition of a sequence of 2D digital X-ray projection images, following a single injection of contrast material. Contrary to DSA, however, this sequence is acquired during a 180 rotation of the C-arch on which the X-ray source and detector are mounted antipo- dally, with the object of interest positioned in its iso-center. The rotation is completed in about eight seconds and the resulting image sequence typically contains 100 images, which form the input to a ltered back-projection algorithm for 3D reconstruction. In contrast with most other 3D medical imaging techniques, 3DRA is capable of provid- 1 Introduction and Summary 5 Figure 1.2. Visualizations of a clinical 3DRA dataset, illustrating the qualitative improvement obtained after noise reduction ltering. Left: volume rendering of the original, raw image. Right: volume rendering of the image after application of edge-enhancing anisotropic diusion ltering (see Chapter 5 for a description of this technique). The visualizations were obtained by using the exact same settings for the parameters of the volume rendering algorithm. ing high-resolution isotropic datasets. However, due to the relatively high noise level and the presence of other unwanted background variations caused by surrounding tissue, the use of noise reduction techniques is inevitable in order to obtain smooth visualizations of these datasets (see Fig. 1.2). Chapter 5 presents an inquiry into the eects of several linear and nonlinear noise reduction techniques on the visualization and subsequent quantication of vascular anomalies in 3DRA images. The evalua- tion is focussed on frequently occurring anomalies such as a narrowing (or stenosis) of the internal carotid artery or a circumscribed dilation (or aneurysm) of intracra- nial arteries. Experiments on anthropomorphic vascular phantoms indicate that, of the techniques considered, edge-enhancing anisotropic diusion ltering is most suit- able, although the practical use of this technique may currently be limited due to its memory and computation-time requirements. Finally, Chapter 6 addresses the problem of interpolation of sampled data, which occurs e.g. when applying geometrical transformations to digital medical images for the purpose of registration or visualization. In most practical situations, interpola- tion of a sampled image followed by resampling of the resulting continuous image on a geometrically transformed grid, inevitably implies loss of grey-level information, and hence image degradation, the amount of which is dependent on image content, but also on the employed interpolation scheme (see Fig. 1.3). It follows that the choice for a particular interpolation scheme is important, since it in uences the re- sults of registrations and visualizations, and the outcome of subsequent quantitative analyses which rely on grey-level information contained in transformed images. Al- though many interpolation techniques have been developed over the past decades, 6 1 Introduction and Summary Figure 1.3. Illustration of the fact that the loss of information due to interpola- tion and resampling operations is dependent on the employed interpolation scheme. Left: slice of a 3DRA image after rotation over 5:0, by using linear interpolation. Middle: the same slice, after rotation by using cubic spline interpolation. Right: the dierence between the two rotated images. Although it is not possible with such a comparison to come to conclusions as to which of the two methods yields the smallest loss of grey-level information, this example clearly illustrates the point that dierent interpolation methods usually yield dierent results. thorough quantitative evaluations and comparisons of these techniques for medical image transformation problems are still lacking. Chapter 6 presents such a compar- ative evaluation. The study is limited to convolution-based interpolation techniques, as these are most frequently used for registration and visualization of medical image data. Because of the ubiquitousness of interpolation in medical image processing and analysis, the study is not restricted to XRA and 3DRA images, but also includes datasets from many other modalities. It is concluded that for all modalities, spline interpolation constitutes the best trade-o between accuracy and computational cost, and therefore is to be preferred over all other methods. In summary, this thesis is concerned with the improvement of image quality and the reduction of image quality degradation and loss of quantitative information. The subsequent chapters describe techniques for reduction of patient motion artifacts in DSA images, noise reduction techniques for improved visualization and quantication of vascular anomalies in 3DRA images, and interpolation techniques for the purpose of accurate geometrical transformation of medical image data. The results and con- clusions of the evaluations described in this thesis provide general guidelines for the applicability and practical use of these techniques

Angiography and Monitoring of Hemodynamic Signals in the Brain via Optical Coherence Tomography

The brain is a complex network of interconnected neurons with each cell functioning as a nonlinear processing unit. Neural responses to stimulus can be described by activity in neurons. While blood flow changes have been associated with neural activity and are critical to brain function, this neurovascular coupling is not well understood. This work presents a technique for neurovascular interrogation, combining optogenetics and optical coherence tomography. Optogenetics is a recently developed neuromodulation technique to control activity in the brain using light with precise spatial neuronal control and high temporal resolution. Using this method, cells act as light-gated ion channels and respond to photo stimulation by increasing or decreasing activity. Spectral-domain optical coherence tomography (SD-OCT) is a noninvasive imaging modality that has the ability to image millimeter range depth and with micrometer resolution. SD-OCT has been shown to image rodent cortical microvasculature in-vivo and detect hemodynamic changes in blood vessels. Our proposed system combines optogenetics and SD-OCT to image cortical patches of the brain with the capability of simultaneously stimulating the brain. The combination allows investigation of the hemodynamic changes in response to neural stimulation. Our results detected changes in blood vessel diameter and velocity before, during and after optogenetic stimulation and is presented

Advanced Algorithms for 3D Medical Image Data Fusion in Specific Medical Problems

Fúze obrazu je dnes jednou z nejběžnějších avšak stále velmi diskutovanou oblastí v lékařském zobrazování a hraje důležitou roli ve všech oblastech lékařské péče jako je diagnóza, léčba a chirurgie. V této dizertační práci jsou představeny tři projekty, které jsou velmi úzce spojeny s oblastí fúze medicínských dat. První projekt pojednává o 3D CT subtrakční angiografii dolních končetin. V práci je využito kombinace kontrastních a nekontrastních dat pro získání kompletního cévního stromu. Druhý projekt se zabývá fúzí DTI a T1 váhovaných MRI dat mozku. Cílem tohoto projektu je zkombinovat stukturální a funkční informace, které umožňují zlepšit znalosti konektivity v mozkové tkáni. Třetí projekt se zabývá metastázemi v CT časových datech páteře. Tento projekt je zaměřen na studium vývoje metastáz uvnitř obratlů ve fúzované časové řadě snímků. Tato dizertační práce představuje novou metodologii pro klasifikaci těchto metastáz. Všechny projekty zmíněné v této dizertační práci byly řešeny v rámci pracovní skupiny zabývající se analýzou lékařských dat, kterou vedl pan Prof. Jiří Jan. Tato dizertační práce obsahuje registrační část prvního a klasifikační část třetího projektu. Druhý projekt je představen kompletně. Další část prvního a třetího projektu, obsahující specifické předzpracování dat, jsou obsaženy v disertační práci mého kolegy Ing. Romana Petera.Image fusion is one of today´s most common and still challenging tasks in medical imaging and it plays crucial role in all areas of medical care such as diagnosis, treatment and surgery. Three projects crucially dependent on image fusion are introduced in this thesis. The first project deals with the 3D CT subtraction angiography of lower limbs. It combines pre-contrast and contrast enhanced data to extract the blood vessel tree. The second project fuses the DTI and T1-weighted MRI brain data. The aim of this project is to combine the brain structural and functional information that purvey improved knowledge about intrinsic brain connectivity. The third project deals with the time series of CT spine data where the metastases occur. In this project the progression of metastases within the vertebrae is studied based on fusion of the successive elements of the image series. This thesis introduces new methodology of classifying metastatic tissue. All the projects mentioned in this thesis have been solved by the medical image analysis group led by Prof. Jiří Jan. This dissertation concerns primarily the registration part of the first project and the classification part of the third project. The second project is described completely. The other parts of the first and third project, including the specific preprocessing of the data, are introduced in detail in the dissertation thesis of my colleague Roman Peter, M.Sc.

Application of a High‑Resolution Computed Radiography System in Detecting an Iodinated X‑ray Contrast Agent: Small Animal Phantom Study

Tumor angiogenesis, the creation of new blood vessels, is the characteristic of solid tumors and crucial for their development. Iodinated contrast agents are used to increase the X‑ray detectability of zone of angiogenesis, and thus providing a means for tracking tumor growth. The overall objective of this project was to evaluate the performance of Kodak CR 7400, a high‑resolution compact computed radiography (CR) system in detection of Omnipaque‑240, an iodinated contrast agent, in a phantom mimicking small animal tumor model. The first phase of the project was dedicated to a comprehensive assessment of CR image quality by measuring presampled Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), Relative Standard Deviation of Noise (RSD), Noise Equivalent Quanta (NEQ), and Detective Quantum Efficiency (DQE). Next, dual‑energy and temporal subtraction techniques were implemented to enhance the contrast of iodinated regions and suppress soft tissue background in the phantom. The underlying physics of each technique was discussed, including the design of the phantoms, the simulation and measurement of the Signal‑to‑Noise Ratio (SNR) in the final subtracted iodine image, and dose assessment. In the end, the results of both techniques were compared along with discussions about the advantages and limitations of implementing each technique. Overall, the study supported the potential of low‑cost CR 7400 in small animal study, particularly detecting iodinated contrast agents implementing temporal subtraction technique and provided a background for similar small animal studies using a CR system

Intraoperative Fourier domain optical coherence tomography for microsurgery guidance and assessment

In this dissertation, advanced high-speed Fourier domain optical coherence tomography (FD-OCT)systems were investigated and developed. Several real-time, high resolution functional Spectral-domain OCT (SD-OCT) systems capable of imaging and sensing blood flow and motion were designed and developed. The system were designed particularly for microsurgery guidance and assessment. The systems were tested for their ability to assessing microvascular anastomosis and vulnerable plaque development. An all fiber-optic common-path optical coherence tomography (CP-OCT) system capable of measuring high-resolution optical distances, was built and integrated into di fferent imaging modalities. First, a novel non-contact accurate in-vitro intra-ocular lens power measurement method was proposed and validated based on CP-OCT. Second, CP-OCT was integrated with a ber bundle based confocal microscope to achieve motion-compensated imaging. Distance between the probe and imaged target was monitored by the CP-OCT system in real-time.The distance signal from the CP-OCT system was routed to a high speed, high resolution linear motor to compensate for the axial motion of the sample in a closed-loop control. Finally a motion-compensated hand-held common-path Fourier domain optical coherence tomography probe was developed for image-guided intervention. Both phantom and ex vivo models were used to test and evaluate the probe. As the data acquisition speed of current OCT systems continue to increase, the means to process the data in real-time are in critically needed. Previous graphics processing unit accelerated OCT signal processing methods have shown their potential to achieve real-time imaging. In this dissertation, algorithms to perform real-time reference A-line subtraction and saturation artifact removal were proposed, realized and integrated into previously developed FD-OCT system CPU-GPU heterogeneous structure. Fourier domain phase resolved Doppler OCT (PRDOCT) system capable of real-time simultaneous structure and flow imaging based on dual GPUs was also developed and implemented. Finally, systematic experiments were conducted to validate the system for surgical applications. FD-OCT system was used to detect atherosclerotic plaque and drug effi ciency test in mouse model. Application of PRDOCT for both suture and cu ff based microvascular anastomosis guidance and assessment was extensively stuided in rodent model

3D Visualization of Microvascular Networks Using Magnetic Particles: Application to Magnetic Resonance Navigation

RÉSUMÉ Les différentes modalités d'imagerie médicales fournissent des images cliniques de structures internes du corps humain à des fins diagnostiques et curatives. Leur première application en clinique remonte à trois décennies et depuis, grâce aux découvertes technologiques continues, de nouvelles fonctionnalités ont été intégrées aux systèmes d'imagerie. Aujourd'hui, des informations anatomiques et fonctionnelles précises peuvent être prélevées à partir de ces images dont la dimensionnalité a évolué du bidimensionnel au tridimensionnel incluant la dynamique. Une des modalités d'imagerie qui a largement profité de ces découvertes technologiques est l'imagerie par résonance magnétique (IRM). Par rapport aux autres techniques d'imagerie, l’IRM présente beaucoup d’avantages tels que la haute résolution spatiale et temporelle, le manque d'exposition aux rayonnements X et une pénétration tissulaire illimitée. Ceux-ci ont rendu l'IRM l'une des modalités les plus utilisées en clinique. Malgré des améliorations récentes dans le fonctionnement des bobines de réception d'IRM et aussi des algorithmes de reconstruction, des progrès supplémentaires sont requis afin d’améliorer la visualisation des microstructures en clinique. La visualisation des microvaisseaux avec un diamètre de 200 µm, reste au-delà des capacités des modalités d’imageries cliniques actuelles. Dans le traitement du cancer, une telle capacité pourrait fournir les informations nécessaires pour les nouvelles méthodes de délivrance ciblée de médicaments comme la navigation par résonance magnétique (NRM). Dans cette technique, afin d'améliorer l'indice thérapeutique, les microporteurs, chargés avec des agents thérapeutiques et des particules magnétiques, sont guidés le long d'une trajectoire qui mènerait vers une zone cancéreuse. Notre objectif est de telle trajectoire qui débuterait du bout du cathéter d'injection jusqu'à la destination finale, soit à proximité d’une zone tumorale. Le contraste de susceptibilité magnétique dans l'IRM fournit un moyen pour prononcer l'effet d'une particule magnétique même si sa taille est beaucoup plus petite que la résolution spatiale de l'IRM. En raison de leur susceptibilité magnétique élevée, les matériaux magnétiques provoquent une inhomogénéité dans le champ magnétique local de l'IRM dans une mesure beaucoup plus importante que leur taille réelle. L’inhomogénéité apparaît dans les images de gradient écho pondéré en T2* sous forme d'une perte de signal. Cette approche présente un moyen de visualisation de microstructures en exploitant leur artefact de susceptibilité.----------ABSTRACT Medical imaging modalities strive to provide clinical images of the human body’s internal structures for diagnosis and treatment purposes. Their first application in clinical trial services goes back to three decades and owing to continuous technological inventions, new capabilities have ever since been incorporated into the imaging systems. Today, anatomical and functional data with finer details and larger image sizes can be achieved and dimensionality of the images has been increased from 2D to dynamic 3D fields. One of the imaging modalities that have probably profited the most from technological findings is the magnetic resonance imaging (MRI). Compared to the other imaging techniques, MRI has various advantages such as high spatial and temporal resolution, lack of radiation exposure and unlimited tissue penetration. These have turned the MRI to one of the most available modalities clinically. Despite recent improvements in the MRI’s receiver coils and reconstruction algorithms, further progress is yet sought to improve the visualization of the microstructures using the clinical MR scanners. Visualization of microvessels with an inner overall cross-sectional area of approximately less than 200 µm, remains beyond capabilities of the current clinical imaging modalities. In cancer therapy, such capability would provide the information required for the new delivery methods such as magnetic resonance navigation (MRN). In the MRN, to enhance the therapeutic index, microcarriers loaded with therapeutic agents and magnetic particles are navigated along a planned trajectory in the vicinity of the treatment region. Our objective is to provide such a trajectory map within an area covering the location of the catheter tip for the injection site up to the extremity of the particles’ path i.e. vicinity of the treatment region such as a tumor site. Susceptibility-based negative contrast in the MRI provides a way to enlarge the effect of a magnetic particle whereas its actual size is much smaller than the MRI’s visualization capability. Due to their high magnetic susceptibility, magnetic materials cause an inhomogeneity in the local magnetic field of the MRI to an extent which is much larger than their actual size. The inhomogeneity appears in the T2*-weighed gradient echo images in the form of a signal void. This approach presents a method for visualization of microstructures through the susceptibility artifact

Doctor of Philosophy

dissertationThe gold standard for evaluation of arterial disease using MR continues to be contrast-enhanced MR angiography (MRA) with gadolinium-based contrast agents (Gd-MRA). There has been a recent resurgence in interest in methods that do not rely on gadolinium for enhancement of blood vessels due to associations Gd-MRA has with nephrogenic systemic fibrosis (NSF) in patients with impaired renal function. The risk due to NSF has been shown to be minimized when selecting the appropriate contrast type and dose. Even though the risk of NSF has been shown to be minimized, demand for noncontrast MRA has continued to rise to reduce examination cost, and improve patient comfort and ability to repeat scans. Several methods have been proposed and used to perform angiography of the aorta and peripheral arteries without the use of gadolinium. These techniques have had limitations in transmit radiofrequency field (B1+) inhomogeneities, acquisition time, and specific hardware requirements, which have stunted the utility of noncontrast enhanced MRA. In this work feasibility of noncontrast (NC) MRA at 3T of the femoral arteries using dielectric padding, and using 3D radial stack of stars and compressed sensing to accelerate acquisitions in the abdomen and thorax were tested. Imaging was performed on 13 subjects in the pelvis and thighs using high permittivity padding, and 11 in the abdomen and 19 in the thorax using 3D radial stack of stars with tiny golden angle using gold standards or previously published techniques. Qualitative scores for each study were determined by radiologists who were blinded to acquisition type. Vessel conspicuity in the thigh and pelvis showed significant increase when high permittivity padding was used in the acquisition. No significant difference in image quality was observed in the abdomen and thorax when using undersampling, except for the descending aorta in thoracic imaging. All image quality scores were determined to be of diagnostic quality. In this work it is shown that NC-MRA can be improved through the use of high permittivity dielectric padding and acquisition time can be decreased through the use of 3D radial stack of stars acquisitions