Dual contrast microvascular MRI

Department of Biomedical EngineeringThe fundamental magnetic resonance angiography (MRA) has been used for obtaining vascular information, such as vessel size and structure. For many decades, MRA techniques with contrast agent have been developed and implemented in research and the clinical area for in vivo applications. Especially, the longitudinal (T1) and transverse (T2 or T2*) contrasts in MRA provide diverse and different information of same subject???s vasculature. The assessments of vascular structure and function by using two types of contrast are important for monitoring vascular behavior. Generally, the two types of contrast agent are T1- and T2-contrast agents. In recent years, several efforts have been focusing on synthesizing hybrid nanoparticles to achieve T1- and T2-contrast, simultaneously. The MR images with both positively and negatively enhanced contrast over the same anatomical region offer complementary information. The benefits of dual contrast with a single agent for in vivo experiments are obvious. In this study, instead of synthesized hybrid contrast agents or multiple contrast agents, simultaneous acquisitions of in vivo dual contrast with size-controlled superparamagnetic iron oxide nanoparticles (SPION) in MRA were obtained and evaluated. As this method is successful for preclinical investigations, dual contrast has a great potential to directly help to compensate vascular information by positively and negatively enhanced contrast. The results of obtained dual contrast in in vivo images were apparent, the smaller vessels in the head region of rodents were distinctively visible from negatively enhanced contrast MRA, while positively enhanced contrast MRA eliminated false contrasts in regions of airways and bone from negatively enhanced contrast MRA. Based on advantages of dual contrast in in vivo MRA, we systematically compared the strengths and weaknesses of dual contrast-enhanced MRAs with SPION in cerebral micro-vessels of the rodent brain. The vasculatures in rodent brain with positively enhanced contrast were visualized well without any artifact, but smaller vessels than given spatial resolution were hardly detected. On the other hand, negatively enhanced contrast based MRA provided good sensitivity for micro-vessels. However, negatively enhanced vessels and specific regions suffered from susceptibility-induced artifacts. Consequently, dual contrast enhanced MRAs were combined for compensation of those short-comings and visualization of whole-brain micro-MRA. The other subject of this thesis is a feasibility evaluation of newly developed contrast agent for in vivo applications at high magnetic field. From MR perspective, the behavior of higher magnetic field (> 7T) is attractive, as it is expected to drastically increase SNR, resolution and susceptibility contrast, which improves lesion detection and quantifications. Also the reduction of inherent T1 relaxation time of contrast agent at high magnetic field is important to increase positively enhanced contrast with limited MR acquisition parameters. The developed contrast agent used in this study was observed to maintain its favorable positive relaxivity even at 7 T magnetic field without drastic reductions of r1 relaxivity. The developed contrast agent was characterized by this phantom and in vivo experiments. The results of 3D MRA proved the feasibility of vascular imaging within 2 hours after intravenous injection of the contrast agent. And a significant reduction of T1 values was observed in the tumor region 7 hours after contrast agent injection in the tumor mouse model.ope

Accurate geometry reconstruction of vascular structures using implicit splines

3-D visualization of blood vessel from standard medical datasets (e.g. CT or MRI) play an important role in many clinical situations, including the diagnosis of vessel stenosis, virtual angioscopy, vascular surgery planning and computer aided vascular surgery. However, unlike other human organs, the vasculature system is a very complex network of vessel, which makes it a very challenging task to perform its 3-D visualization. Conventional techniques of medical volume data visualization are in general not well-suited for the above-mentioned tasks. This problem can be solved by reconstructing vascular geometry. Although various methods have been proposed for reconstructing vascular structures, most of these approaches are model-based, and are usually too ideal to correctly represent the actual variation presented by the cross-sections of a vascular structure. In addition, the underlying shape is usually expressed as polygonal meshes or in parametric forms, which is very inconvenient for implementing ramification of branching. As a result, the reconstructed geometries are not suitable for computer aided diagnosis and computer guided minimally invasive vascular surgery. In this research, we develop a set of techniques associated with the geometry reconstruction of vasculatures, including segmentation, modelling, reconstruction, exploration and rendering of vascular structures. The reconstructed geometry can not only help to greatly enhance the visual quality of 3-D vascular structures, but also provide an actual geometric representation of vasculatures, which can provide various benefits. The key findings of this research are as follows: 1. A localized hybrid level-set method of segmentation has been developed to extract the vascular structures from 3-D medical datasets. 2. A skeleton-based implicit modelling technique has been proposed and applied to the reconstruction of vasculatures, which can achieve an accurate geometric reconstruction of the vascular structures as implicit surfaces in an analytical form. 3. An accelerating technique using modern GPU (Graphics Processing Unit) is devised and applied to rendering the implicitly represented vasculatures. 4. The implicitly modelled vasculature is investigated for the application of virtual angioscopy

Brain vasculature segmentation from magnetic resonance angiographic image

Master'sMASTER OF ENGINEERIN

Doctor of Philosophy

dissertationMagnetic resonance guided high intensity focused ultrasound (MRgHIFU) is a promising minimal invasive thermal therapy for the treatment of breast cancer. This study develops techniques for determining the tissue parameters - tissue types and perfusion rate - that influence the local temperature during HIFU thermotherapy procedures. For optimal treatment planning for each individual patient, a 3D volumetric breast tissue segmentation scheme based on the hierarchical support vector machine (SVM) algorithm was developed to automatically segment breast tissues into fat, fibroglandular tissue, skin and lesions. Compared with fuzzy c-mean and conventional SVM algorithm, the presented technique offers tissue classification performance with the highest accuracy. The consistency of the segmentation results along both the sagittal and axial orientations indicates the stability of the proposed segmentation routine. Accurate knowledge of the internal anatomy of the breast can be utilized in the ultrasound beam simulation for the treatment planning of MRgHIFU therapy. Completely noninvasive MRI techniques were developed for visualizing blood vessels and determining perfusion rate to assist in the MRgHIFU therapy. Two-point Dixon fat-water separation was achieved using a 3D dual-echo SSFP sequence for breast vessel imaging. The performances of the fat-water separation with various readout gradient designs were evaluated on a water-oil phantom, ex vivo pork sample and in vivo breast imaging. Results suggested that using a dual-echo SSFP readout with bipolar readout gradient polarity, blood vasculature could be successfully visualized through the thin-slab maximum intensity projection SSFP water-only images. For determining the perfusion rate, we presented a novel imaging pulse sequence design consisting of a single arterial spin labeling (ASL) magnetization preparation followed by Look-Locker-like image readouts. This flow quantification technique was examined through simulation, in vitro and in vivo experiments. Experimental results from a hemodialyzer when fitted with a Bloch-equation-based model provide flow measurements that are consistent with ground truth velocities. With these tissue properties, it is possible to compensate for the dissipative effects of the flowing blood and ultimately improve the efficacy of the MRgHIFU therapies. Complete noninvasiveness of these techniques allows multiple measurements before, during and after the treatment, without the limitation of washout of the injected contrast agent

Incorporating the Aortic Valve into Computational Fluid Dynamics Models using Phase-Contrast MRI and Valve Tracking

The American Heart Association states about 2% of the general population have a bicuspid aortic valve (BAV). BAVs exist in 80% of patients with aortic coarctation (CoA) and likely influences flow patterns that contribute to long-term morbidity post-surgically. BAV patients tend to have larger ascending aortic diameters, increased risk of aneurysm formation, and require surgical intervention earlier than patients with a normal aortic valve. Magnetic resonance imaging (MRI) has been used clinically to assess aortic arch morphology and blood flow in these patients. These MRI data have been used in computational fluid dynamics (CFD) studies to investigate potential adverse hemodynamics in these patients, yet few studies have attempted to characterize the impact of the aortic valve on ascending aortic hemodynamics. To address this issue, this research sought to identify the impact of aortic valve morphology on hemodynamics in the ascending aorta and determine the location where the influence is negligible. Novel tools were developed to implement aortic valve morphology into CFD models and compensate for heart motion in MRI flow measurements acquired through the aortic valve. Hemodynamic metrics such as blood flow velocity, time-averaged wall shear stress (TAWSS), and turbulent kinetic energy (TKE) induced by the valve were compared to values obtained using the current plug inflow approach. The influence of heart motion on these metrics was also investigated, resulting in the underestimation of TAWSS and TKE when heart motion was neglected. CFD simulations of CoA patients exhibiting bicuspid and tricuspid aortic valves were performed in models including the aortic sinuses and patient-specific valves. Results indicated the aortic valve impacted hemodynamics primarily in the ascending aorta, with the BAV having the greatest influence along the outer right wall of the vessel. A marked increase in TKE is present in aortic valve simulations, particularly in BAV patients. These findings suggest that future CFD studies investigating altered hemodynamics in the ascending aorta should accurately replicate aortic valve morphology. Further, aortic valve disease impacts hemodynamics in the ascending aorta that may be a predictor of the development or progression of ascending aortic dilation and possible aneurysm formation in this region

Blood

This book examines both the fluid and cellular components of blood. After the introductory section, the second section presents updates on various topics in hemodynamics. Chapters in this section discuss anemia, 4D flow MRI in cardiology, cardiovascular complications of robot-assisted laparoscopic pelvic surgery, altered perfusion in multiple sclerosis, and hemodynamic laminar shear stress in oxidative homeostasis. The third section focuses on thalassemia with chapters on diagnosis and screening for thalassemia, high blood pressure in beta-thalassemia, and hepatitis C infection in thalassemia patients