MR velocity restoration for consistent myocardial contractility analysis
Creators: name: Huntbatch, Andrew
Publisher: name: Imperial College London
Year: 2019

description: Changes in cardiac structure and contractile performance are key indicators for the early detection of heart disease, the leading cause of premature death in the western world. Accurate assessment of the low-level function of the contractile tissue of the heart, the myocardium, has the potential to provide a valuable tool for the prevention, diagnosis and treatment of these conditions, and key to this process are techniques to accurately measure in vivo tissue motion across the cardiac cycle. While existing techniques for the measurement of cardiac function such as echocardiography provide meaningful indices of cardiac performance, the measurement of small functional parameters such as local tissue contractility remain beyond their scope due to a combination of restrictive viewing windows, low resolution and low signal-to-noise ratio. In addition, higher-resolution approaches such as Computed Tomography expose the patient to ionising radiation and do not provide instantaneous assessment of the internal motion of homogenous tissue. However, Cardiac Magnetic Resonance is rapidly expanding into clinical use as a crucial imaging modality for the measurement of both cardiac structure and dynamics. Through the use of novel image acquisition sequences such as MR tagging, phase contrast velocity encoding and other motion-encoding techniques it has become possible to measure the internal dynamics of homogenous tissue. While MR tagging has been widely reported, it continues to suffer from an inability to measure motion through the imaging plane, a limited resolution and a high post-processing requirement. The use of phase contrast velocity encoding provides instantaneous measurement of tissue velocity on a per-voxel basis and has thus seen increasing interest for introduction to the clinical environment as a tool to assess contractility. However, one of the primary obstacles to the further adoption of this imaging modality is the relatively high level of noise in comparison to other sequence types, resulting from constraints on acquisition times for what is a highly complex imaging protocol. This is a particular problem for the assessment of myocardial contractility, as this involves the use of spatial derivatives that greatly amplify the effects of noise. Through the application of robust restoration algorithms the noise component of the velocity fields obtained through the phase contrast sequence can be reduced, moving the sequence further towards clinical use. Furthermore, the analysis of the resulting patterns of motion and contractility offer an insight into the low-level function of the myocardium that can form a basis for patient-specific evaluation. The primary contributions of this thesis include a robust method for the recovery of low- level myocardial motion from phase contrast velocity imaging, a rigorous comparison of this technique with existing velocity restoration paradigms and methods for the generation of high-resolution 3D contractility maps of the left ventricle. In addition, statistical patterns of myocardial motion are investigated, demonstrating underlying patterns in low-level cardiac function with a view to providing a basis for the future assessment of abnormal variations due to heart disease.