Flow Quantification with UTE Phase Contrast

Abstract

Aortic Stenosis (AS) is a prevalent heart valvular disease in the senior population which constricts the aortic valve opening and can progress into many severe or even lethal conditions if left untreated. The most effective treatment to AS is the valvular replacement surgery which requires an evaluation of the patient’s symptom and stenosis severity. The severity of AS can be evaluated non-invasively through the use of echocardiogram which measures the blood flow velocity coming through the stenosis. Like echocardiogram, magnetic resonance imaging (MRI) is another non-invasive imaging technique capable of measuring the blood flow velocity; therefore, MRI was proposed as an alternative technique for AS evaluation. However, data from the ZEST trial showed incoherence between the blood flow velocities measured by MRI and echocardiogram. This discrepancy was further investigated by O'Brien in 2009 who identified its cause as the loss of MR signal due to the turbulence in the stenotic jets coming from the constricted aortic valve. Afterwards, O'Brien proposed a new MRI sequence called ultra-short echo time (UTE) phase contrast sequence to image the stenotic jets. In this thesis, the performance of the UTE sequence was thoroughly investigated through healthy volunteer study, static phantom experiment, continuous flow phantom experiment and pulsatile flow phantom experiment. In doing so, several errors were identified from the UTE sequence and resolved. The performance of the modified UTE sequence was then assessed by measuring the stenotic jets generated in continuous and pulsatile flow phantom. In summary, several errors were resolved in the UTE sequence which made the sequence more appropriate for clinical use. However, despite the robustness of the UTE sequence in measuring turbulent flow, further works are required to improve the accuracy of the sequence before it can be used for its intended function of evaluating the severity of AS patients.