Method and apparatus for carrying out a phase correction in MR angiography.

摘要

A method of obtaining with MR angiography an angiogram of a body is based on the formation of a difference image of two images produced by different gradient wave forms of the body. The gradient wave forms are chosen so that essentially in a difference image to be obtained from the two images the signals originating from excited stationary spin nuclei are not present and the signals originating from moving spin nuclei - due to flow in blood vessels - form the angiogram. The problem then arising is that due to the different gradient wave forms when forming signals for the two images different eddy currents, which flow in metal parts of the apparatus, exert their influence. The two images of the body contain a phase error induced by the eddy currents. This phase error varies smoothly over the image of the body. This is the case because the body is located at a comparatively large distance from the fields produced by the eddy currents in the metal parts of the apparatus. Due to the different eddy currents, signals originating from stationary spin nuclei are not fully eliminated in the difference image, as a result of which the angiogram contains signals originating from stationary tissue. The signals originating from flowing blood yield a deviating phase at the area of a vessel. A method is proposed of eliminating the signals in the angiogram originating from stationary tissue. By estimating the smoothly varying phase difference before forming the difference image from the two images and by thus carrying out a non-linear phase correction, the smoothly varying phase difference is eliminated. The difference image no longer contains substantially any signals originating from stationary tissue.