Method and apparatus for carrying out a phase correction in mr

Abstract

An MR angiography method in which an angiogram is formed from the difference between amplitude and/or phase information contained in first and second complex images produced by different gradient waveforms in respective first and second types of excitation sequences. The gradient waveforms are chosen so that in the angiogram obtained from the first and second complex images, ideally, signals orginating from excited stationary spin nuclei are not present while the signals orginating from moving spin nuclei, due to flow in blood vessels, are present. However, stationary spin nuclei can be somewhat differently excited in the first and second sequence due to the different gradient waveforms used which cause different eddy currents to flow in metal parts of the apparatus. In the absence of correction, these eddy currents would exert their influence magnetically by causing a smoothly varying phase error in the difference between the first and second complex images resulting in erroneous signal contribution to the angiogram from stationary spin nuclei. This erroneous signal is eliminated by estimating the smoothly varying phase difference before forming the angiogram from the first and second complex images and by thus carrying out a non-linear phase correction.