Dixon-type water/fat separation MR imaging
The invention relates to a method of Dixon-type MR imaging. The object ( 10 ) is subjected to a dual- or multi-acquisition imaging sequence comprising a series of temporally equidistant RF pulses. An echo signal is generated in the presence of a readout magnetic field gradient in each time interval (TR) between successive RF pulses, with the echo time varying between at least a first value (TE 1 ) associated with a first acquisition (ACQ 1 ) and a second value (TE 2 ) associated with a second acquisition (ACQ 2 ). The invention proposes that at least one of the magnetic field gradients preceding and/or succeeding the readout magnetic field gradient in each time interval (TR) is temporally shifted, varied in duration and/or varied in amplitude between time intervals (TR). In this way, a reduction of the acoustic noise generated by the multi-acquisition Dixon sequence and, thus, of the discomfort for patients undergoing a corresponding examination is achieved. The echo signals are recorded and an MR image is reconstructed with separating signal contributions from water and fat based on the recorded echo signals of the at least two acquisitions (ACQ 1 , ACQ 2 ). Moreover the invention relates to an MR device ( 1 ) and to a computer program to be run on an MR device ( 1 ).
1. A method of magnetic resonance (MR) imaging of an object placed in an examination volume of an MR device, the method comprising:
subjecting the object to a dual-or multi-acquisition imaging sequence comprising a series of temporally equidistant RF pulses, wherein an echo signal is generated in the presence of a readout magnetic field gradient in each time interval (TR) between successive RF pulses, with the echo time varying between at least a first value (TE 1 ) associated with a first acquisition (ACQ 1 ) and a second value (TE 2 ) associated with a second acquisition (ACQ 2 ), and wherein on a basis of a relationship between (i) arrangement of the magnetic field gradients preceding and/or succeeding the readout gradient in each time interval (TR) and (ii) an acoustic noise level caused by the switching of the magnetic field gradients at least one of the magnetic field gradients preceding and/or succeeding the readout magnetic field gradient in each time interval (TR) is temporally shifted, varied in at least one of duration or varied in amplitude between time intervals (TR), so as to reduce the acoustic noise level
recording the echo signals, and
arranging for reconstructing an MR image with separating signal contributions from water and fat based on the recorded echo signals of the at least two acquisitions (ACQ 1 , ACQ 2 ).
2. The method of claim 1 , wherein the magnetic field gradients at least one of preceding or succeeding the readout magnetic field gradient in each time interval (TR) are temporally shifted, varied in at least one of duration or varied in amplitude between time intervals (TR) in such a fashion that acoustic noise caused by the switching of the magnetic field gradients is minimized.
3. The method of claim 2 , wherein the minimization is based on the modelling of an acoustic spectrum, involving an MR device-specific acoustic transfer function relating the frequency components of the temporal course of the switched magnetic field gradients of the imaging sequence to acoustic frequency components, wherein the frequency components could be frequency of the magnetic field gradients that generate acoustic noise.
4. The method of claim 1 , wherein the sequence of echo signals associated with the first respectively with the second acquisition (ACQ 1 , ACQ 2 ) is chosen in such a fashion that acoustic noise caused by the switching of the magnetic field gradients is minimized.
5. The method of claim 1 , wherein the reconstruction of the MR image involves estimating and correcting for eddy current-induced phase errors in the recorded echo signals.
6. The method of claim 1 wherein the imaging sequence is a turbo spin echo sequence comprising an RF excitation pulse succeeded by a number of RF refocusing pulses, with each of the echo signals being generated in the time interval between successive RF refocusing pulses.
7. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, static magnetic field Bo 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 an object positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and access to a reconstruction unit for reconstructing MR images from the received MR signals, wherein the MR device is configured to perform the method of claim 1 .
8. A computer program comprising executable instructions stored on a non-transitory computer readable medium, which when executed by a processor causes a magnetic resonance (MR) device, to perform the method of claim 1 .