Dynamic shimming method based on three-dimensional gradient dual echos

Abstract

The invention discloses a dynamic shimming method based on three-dimensional gradient dual echoes. The method comprises the steps that 1, after a target scanning portion is positioned, a dual-echo 3DGRE sequence is excited for scanning and collecting phase data of a needed shimming region; 2, due to the two continuously-collected echo data, two K space images are obtained; 3, a phase difference image of the two K space images is obtained through calculation; 4, an image field pattern is further obtained through the phase difference image; 5, current in a shimming coil is calculated through spherical harmonic analysis; 6, current of the same magnitude and opposite directions is applied to a gradient coil for performing shimming. The phase unwinding operation can be avoided, meanwhile, dueto the fact that the sequence collecting speed is high, it is ensured that the interval time between the two echoes is short, the probability that the phase offset changes is lowered, the shimming precision is improved, and the influence caused by motion artifact is lowered for the portion with multiple motions.

Description

technical field [0001] The invention relates to a dynamic shimming method based on three-dimensional gradient double echo. Background technique [0002] As we all know, shimming adjusts the current in the shimming coil to offset the inhomogeneity of the field, and there will be mutual influence between multiple shimming fields. Shimming includes passive shimming and active shimming. Passive shimming generally passes through the Placing iron pieces at a specific position is the first measure to improve the uniformity of the magnetic field during the installation of the magnet. Active shimming is achieved by using a specific shim coil, and the uniformity of the magnetic field is optimized by adjusting the current in the shim coil. The uniformity of the main magnetic field of the magnet plays a key role in the various indicators of the magnetic resonance image and whether some sequences that are highly sensitive to the field uniformity can be realized. However, in the productio...

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Problems solved by technology

like figure 2 Shown is the original dynamic shimming sequence, which requires two excitations, that is, two sequential scans, with different TE parameter settings, to collect two images with different echo times, and then obtain a 3D field map. The two TE times should be As short as possible, if the phase of the two acquisitions exceeds 180°, phase unwrapping is required. After obtaining the field diagram, calculate the reversely applied current in the coil through spherical harmonic analysis, so as to achieve the purpose of shimming , usually the scan and calculation need to be iterated multiple times, but because the acquisition time of the 3D gradient shimming is relatively long, the phase offset of the two acquisitions at the same position will change, and the shimming accuracy will decrease. Many parts, easily affected by breathing movements

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