| 摘要 |
An apparatus and method are provided for performing phase unwrapping for an acquired magnetic resonance (MR) image. The method includes modelling the MR phase in the MR image using a Markov random field (MRF) in which the true phase &phgr;(t) and the wrapped phase &phgr;(w) are modelled as random variables such that at voxel i of said MR image &phgr;(t)(i)=&phgr;(w)(i)+2&pgr;n(i), where n(i) is an unknown integer that needs to be estimated for each voxel i. The method further includes constructing a graph consisting of a set of vertices V and edges E and two special terminal vertices representing a source s and sink t, where there is a one-to-one correspondence between cuts on the graph and configurations of the MRF, a cut representing a partition of the vertices V into disjoint sets S and T such that s ∈ S and t ∈ T. The method further includes finding the minimum energy configuration, E(n(i)| &phgr;(w)) of the MRF on the basis that the total cost of a given cut represents the energy of the corresponding MRF configuration, where the cost of a cut is the sum of all edges going from S to T across the cut boundary. The method further includes using the values of n(i) in the minimum energy configuration to perform the phase unwrapping from &phgr;(w) to &phgr;(t) for the MR image. A confidence may be computed for each voxel using dynamic graph cuts. The unwrapped phase from two MR images acquired at different times may be used to estimate a field map from the phase difference between the two MR images. The field map may be converted into a deformation field which is then used to initialise a non-rigid image registration of the acquired MR image against a reference image. The deformation field of the non-rigid registration is controlled to be smoother where the confidence is high. |
