MR imaging using Dixon-type water/fat separation with suppression of flow-induced leakage and/or swapping artifacts
The invention relates to a method of Dixon-type MR imaging. It is an object of the invention to provide a method that enables efficient and reliable Dixon water/fat separation, in particular using a bipolar acquisition strategy, while avoiding flow-induced leakage and swapping artifacts. According to the invention, an imaging sequence is executed which comprises at least one excitation RF pulse and switched magnetic field gradients, wherein pairs of echo signals are generated at two different echo times (TE 1 , TE 2 ) and during two or more different cardiac phases (AW 1 , AW 2 ). The echo signals are acquired and phase images are reconstructed therefrom. A final diagnostic image is reconstructed from the echo signal data using water/fat separation, wherein regions of flow and/or or estimates of flow-induced phase errors are derived from the phase images to suppress or compensate for flow-induced leakage and/or swapping artifacts in the final diagnostic image. Therein, flow-induced phase offsets are determined by voxel-wise comparison of the phase images associated with the different cardiac phases. Moreover, the invention relates to a MR device ( 1 ) and to a computer program to be run on a MR device ( 1 ).
1. A method of magnetic resonance (MR) imaging of a body of a patient placed in an examination volume of a MR device, the method comprising:
subjecting the body to an imaging sequence, which comprises at least one excitation RF pulse and switched magnetic field gradients, wherein a number of pairs of echo signals is generated at two different echo times (TE 1 , TE 2 ) with different phase encodings to sample a region of k-space, and wherein the generation of each pair of echo signals is repeated during two or more different cardiac phases (AW 1 , AW 2 );
acquiring the echo signals from the body;
reconstructing phase images from the acquired echo signal data, wherein a phase image is reconstructed for each echo time and for each cardiac phase; and
reconstructing a final diagnostic image from the echo signal data using water/fat separation, wherein regions of at least one of flow or estimates of flow-induced phase errors are derived from the phase images to suppress or compensate for flow-induced leakage and/or swapping artifacts in the final diagnostic image.
2. The method of claim 1 , wherein the pairs of echo signals are generated after different delays (D 1 , D 2 ) after detection of a cardiac trigger signal.
3. The method of claim 1 , wherein the pairs of echo signals are acquired using bipolar readout magnetic field gradients (M).
4. The method of claim 1 , wherein the regions of the at least one of flow or flow-induced phase errors are determined by voxel-wise comparison of phase images associated with different cardiac phases.
5. The method of claim 4 , wherein the regions of flow are determined by an image processing technique including at least one selected from masking, thresholding or segmentation.
6. The method of claim 1 , wherein the echo signals generated during different cardiac cycles are used for averaging in the step of reconstructing the diagnostic image.
7. The method of claim 1 , wherein a flow velocity map is derived from local phase offsets in the phase images.
8. The method of claim 1 , wherein the water/fat separation involves a two-point Dixon technique.
9. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, static magnetic field B 0 within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing MR images from the received MR signals, wherein the MR device is configured to perform a method including:
subjecting the body to an imaging sequence, which comprises at least one excitation RF pulse and switched magnetic field gradients, wherein a number of pairs of echo signals is generated at two different echo times (TE 1 , TE 2 ) with different phase encodings to sample a region of k-space, and wherein the generation of each pair of echo signals is repeated during two or more different cardiac phases (AW 1 , AW 2 );
acquiring the echo signals from the body;
reconstructing phase images from the acquired echo signal data, wherein a phase image is reconstructed for each echo time and for each cardiac phase; and
reconstructing a final diagnostic image from the echo signal data using water/fat separation, wherein regions of at least one of flow or estimates of flow-induced phase errors are derived from the phase images to suppress or compensate for flow-induced leakage and/or swapping artifacts in the final diagnostic image.
10. A computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions for:
executing an imaging sequence, which comprises at least one excitation RF pulse and switched magnetic field gradients so as to generate a number of pairs of echo signals at two different echo times (TE 1 , TE 2 ) with different phase encodings to sample a region of k-space, with the generation of each pair of echo signals being repeated during two or more different cardiac phases (AW 1 , AW 2 );
acquiring the echo signals;
reconstructing phase images from the acquired echo signal data, wherein a phase image is reconstructed for each echo time and for each cardiac phase; and
reconstructing a final diagnostic image from the echo signal data using water/fat separation, wherein regions of at least one of flow or estimates of flow-induced phase errors are derived from the phase images to suppress or compensate for flow-induced leakage and/or swapping artifacts in the final diagnostic image.