4D ultra-short TE (UTE) phase-contrast MRI for assessing stenotic flow and hemodynamics.

Phase-contrast (PC) MRI is a non-invasive technique to assess cardiovascular blood flow. However, this technique is not accurate in the case of atherosclerotic disease and vascular and valvular stenosis due to intravoxel dephasing secondary to disturbed blood flow, flow recirculation, and turbulence distal to the narrowing, resulting in flow-related artifacts. Previous studies have shown that reducing the echo time (TE) decreases the errors associated with phase incoherence due to random motions as observed in unsteady and turbulent flows. As part of this dissertation, a novel 3-D cine Ultra-Short (UTE)-PC imaging method has been developed, and implemented to measure the blood velocity using a UTE center-out radial k-space trajectory with short TE time compared to standard PC MRI sequences. 3D UTE characterizes flow in one direction in a 3D volume, resulting in a single component of the flow velocities. In order to obtain a comprehensive flow assessment in three directions, the 3D UTE sequence needs to be repeated three times, which can be inefficient and time consuming. 4-D flow MRI has been recently used for quantitative flow assessment and visualization of complex flow patterns resulting in more anatomical information and comprehensive assessment of blood flow. With 4D flow MRI method, all the flow information in three direction in a 3D volume though the time can be achieved as part of a single scan. In this dissertation, a novel 4D UTE flow MRI technique has also been designed and implemented which is capable of deriving the three orthogonal components of the velocity field in the flow in a single scan, while achieving very short echo times. In flow phantom studies, comprehensive investigation of several different flow rates revealed significant improvement in flow quantification and reduction of flow artifacts when compared to conventional 4D flow. Furthermore, a reduced TE 4D Spiral flow MRI method has also been implemented which reduces scan times when compared to conventional 4D flow MRI (as well as 4D UTE flow). Despite reduction of scan time as well as TE relative to conventional 4D flow, the achieved TE with the 4D spiral technique is indeed longer than 4D UTE flow. In order to assess clinical feasibility and in order to perform further validation of 4D UTE flow, in an IRB-approved study, twelve aortic stenosis (AS) patients underwent Doppler Ultrasound, conventional 4D flow, and 4D UTE flow scans for a 3 way comparison. 4D UTE flow displayed good correlation with Doppler Ultrasound in patients with moderately severe aortic stenosis, though with the added benefit of not having confounding factors encountered in Doppler Ultrasound (e.g., angle dependence, 2D measurement, and difficulty in locating a proper acoustic window). The proposed 4D UTE flow permits 4D visualization of flow and true 3D measurement of all flow quantities, not possible with Doppler. Further investigations will be required to test the technique in patients with severe or critical aortic stenosis wherein conventional 4D flow will be less accurate due to intravoxel dephasing and spin incoherence

Patient-specific analysis of the hemodynamic performance of surgical and transcatheter aortic valve replacements

Aortic valve (AV) diseases are life-threatening conditions which affect millions of people worldwide and, if left untreated, can lead to death a few years after symptom onset. Patients affected by AV diseases are commonly referred to surgical AV replacement (SAVR). However, more than 30% of patients are not suitable for SAVR. For this reason, transcatheter aortic valve implantation (TAVI) has been attracting growing interest. Several clinical studies compared the outcomes of these techniques, showing that TAVI could be a valid alternative to SAVR. However, there is a lack of detailed knowledge about changes in the aortic hemodynamic conditions following these procedures. The main aim of this thesis is to develop efficient and robust methodologies to study and compare the influences of different AV replacement procedures on aortic hemodynamics. An image-based patient-specific computational model has been developed, which uses magnetic resonance images (MRI) acquired from patients to obtain realistic geometry and boundary conditions (BCs) for computational fluid dynamics (CFD) analysis. The implemented physiological BCs were compared with the most commonly used inlet and outlet BCs, and showed the best agreement with in vivo data. The model was then applied to study and compare SAVR, TAVI and aortic root replacement using a variety of prostheses. In addition, an experimental set-up was designed to further study TAVI hemodynamics by combining 3D-printing, 4D flow MRI and CFD. Finally, a preliminary analysis of valve leaflet thrombosis was conducted. It has been shown that both TAVI and SAVR are able to greatly improve the aortic hemodynamics, but this often deviates from conditions in healthy volunteers, with the extent of abnormalities strongly dependent on the type of prostheses or valve disease. The work also demonstrated the feasibility of predicting valve leaflet thrombosis using a shear-driven model for thrombus formation and growth.Open Acces