METHOD AND MAGNETIC RESONANCE APPARATUS FOR CORRECTION OF A B0 MAP FOR CHEMICAL SHIFTS

摘要

In a method for correction of a B0 field map measured with a magnetic resonance device, that describes deviations from a nominal field strength in the homogeneity area of the magnetic resonance device by deviations from a nominal frequency for protons bonded to water, the deviations being represented as Larmor frequency values for different picture elements shifted by chemical shifts, the B0 field map is recorded with spins of the fat and water protons not in phase. The B0 field map is segmented by evaluating the differences of the Larmor frequency values of adjacent picture elements of the B0 field map in at least two contiguous clusters. For each cluster, a decision is made on the basis of a smoothness criterion and a compactness criterion as to whether a cluster containing a majority of protons bonded into fat is involved. Clusters identified as containing a majority of protons bonded into fat are corrected by lowering the Larmor frequency values by the difference between the nominal frequency for protons bonded into water and the corresponding nominal frequency for protons bonded to fat.

主权项

1. A method for correction of a B0 field map measured with a magnetic resonance device in which an examination object is situated, describing deviations from a nominal field strength in a homogeneity area of the magnetic resonance device by deviations from a nominal frequency for protons bonded into water, the deviations being represented as Larmor frequency values for different picture elements resulting from chemical shifts of the Larmor frequencies, wherein the B0 field map was recorded at least in part with spins of fat and water protons in the examination object not in phase, said method comprising:
segmenting the B0 field map according to a segmentation criterion that evaluates the differences of the Larmor frequency values of adjacent picture elements of the B0 field map in at least two contiguous clusters, each containing at least two picture elements, that are separated by a jump in the Larmor frequency values; for each cluster, determining, based on a smoothness criterion and a compactness criterion, whether the respective cluster contains a majority of protons bonded into fat; and correcting clusters identified as containing a majority of protons bonded into fat by lowering the Larmor frequency values thereof by a difference between the nominal frequency for protons bonded into water and the corresponding nominal frequency for protons bonded into fat.