A medical image processing apparatus for receiving a target phase data set and a reference phase data set at different times, obtaining a segmentation of an elongated structure in the target phase data and the reference phase data set, determining a target phase line along the segmentation of the elongated structure in the target phase data set and a reference phase line along the segmentation of the elongated structure in the reference phase data set, and determining a synthesized target phase line by combining the target phase line and reference phase line.