Image Based Correction of Phasewraps in 4d Pc-mri Data Using Fast Reference Scans

INTRODUCTION: Recent studies have shown the feasibility of 4D PC-MRI to measure four dimensional (3D-spatial plus temporal domain) blood flow and to give a qualitative and quantitative analysis of the hemodynamics e.g. in the aorta or the carotids. This technique requires the conversion of the measured phase angle (-π to +π) to velocity vectors by the usage of a parameter that specifies the velocity sensitivity (venc, velocity encoding) [1] [2]. If the actual flow velocities exceed this parameter, velocity phase wraps will occur, which lead to errors in the flow data. Using a lower velocity sensitivity (a higher venc) leads to a lower velocity to noise ratio (VNR), which is also undesirable. Manual correction of the phase wraps is possible but unpractical considering the large quantity of the data. Automatic image based post processing techniques that focus on finding and correcting large jumps between neighboring voxels in the phase data have been developed and extended to 4D. [3] [4]. An alternative approach to solve this problem is to acquire the data using more complex velocity encoding schemes [5]. A simple method is to acquire the data twice. Once with the lower venc and the higher VNR. Then with a higher venc to correct the phase wraps in the first low venc dataset [6]. This requires additional scanning time for the reference dataset. The aim of this study is, to systematically analyze in phantom scans to what extent a reduction of the additional scan time can be achieved by reducing the spatial and temporal resolution of the high venc reference scan.