Search

JP-7857297-B2 - Dixon water/fat separation MR imaging

JP7857297B2JP 7857297 B2JP7857297 B2JP 7857297B2JP-7857297-B2

Inventors

  • エガース ホルガー
  • ボルネート ペーター

Assignees

  • コーニンクレッカ フィリップス エヌ ヴェ

Dates

Publication Date
20260512
Application Date
20211202
Priority Date
20201208

Claims (9)

  1. A method for MR imaging of an object placed within the inspection volume of an MR device, wherein the method is A step of performing an imaging sequence of at least two shots on the object, wherein each shot includes an excitation RF pulse and a subsequent series of refocusing RF pulses, and at least one pair of phase-encoded echoes, i.e., a first echo at a first echo time and a second echo at a second echo time, are generated at each time interval between two consecutive refocusing RF pulses. The steps include: obtaining a first set of echo signal pairs from the object in a first shot of the imaging sequence using a bipolar pair of readout magnetic field gradients at each repetition interval; A step of acquiring a second set of echo signal pairs from the object in a second shot of the imaging sequence, using a bipolar pair of readout magnetic field gradients at each iteration interval , The acquisition of the first and second sets is an acquisition step in which the gradient area of the magnetic field gradient in the reading direction preceding and succeeding the bipolar pair of the read magnetic field gradient differs from that of the other, A method comprising the steps of configuring to reconstruct an MR image from the first and second sets of acquired echo signal pairs, wherein the signal contributions from water protons and lipid protons are separated, and the reconstruction includes suppressing or removing artifacts arising from bipolar acquisition.
  2. The method according to claim 1, wherein some or all of the echo signals of the first set and/or the second set are acquired only partially.
  3. The method according to claim 1 or 2, wherein the reconstruction of the MR image includes the reconstruction of single echo images from the acquired echo signal pairs for each of the first and second sets, i.e., a first single echo image belonging to the first echo time and a second single echo image belonging to the second echo time.
  4. The method according to claim 3, wherein phase errors induced by eddy currents are eliminated by aligning the phases of the first single-echo images of the first and second sets on a pixel-by-pixel or voxel-by-voxel basis, and by aligning the phases of the second single-echo images of the first and second sets on a pixel-by-pixel or voxel-by-voxel basis.
  5. The method according to claim 3 or 4, wherein the reconstruction of the MR image includes a first water/fat separation based on the first single-echo image of the first set and one single-echo image of the second set, resulting in a first water image and a first fat image; and a second water/fat separation based on the second single-echo image of the first set and the other single-echo image of the second set, resulting in a second water image and a second fat image.
  6. The method according to claim 5, wherein fat shift and/or B0 distortion are corrected.
  7. The method according to claim 5 or 6, wherein the first and second water images are combined to form a final water image, and/or the first and second fat images are combined to form a final fat image.
  8. An MR apparatus comprising: at least one main magnet coil for generating a uniform static magnetic field B0 within an inspection volume; several gradient coils for generating a switching magnetic field gradient in different spatial directions within the inspection volume; at least one RF coil for generating RF pulses within the inspection volume and/or receiving an MR signal from an object located within the inspection volume; a control unit for controlling the temporal transitions of the RF pulses and the switching magnetic field gradient; and a reconstruction unit for reconstructing an MR image from the received MR signal, wherein the MR apparatus is configured to perform steps of the method according to any one of claims 1 to 7.
  9. A computer program executed on an MR device, wherein the computer program includes instructions necessary to perform the method described in any one of claims 1 to 7.

Description

This invention relates to the field of magnetic resonance (MR) imaging. It relates to MR imaging of an object placed within the examination volume of an MR apparatus. This invention also relates to an MR apparatus and a computer program executed on the MR apparatus. MRI (Magnetic Resonance) imaging, which utilizes the interaction between magnetic fields and nuclear spins to form two-dimensional or three-dimensional images, is superior to other imaging methods in many respects for soft tissue imaging. Because it does not require ionizing radiation and is generally non-invasive, it is now widely used, particularly in the field of medical diagnosis. In a typical MRI (Magnetic Resonance) procedure, the patient's body is placed within a strong, uniform magnetic field B0 . The direction of the magnetic field B0 simultaneously defines the axis of the coordinate system (usually the z-axis), and measurements are performed based on this coordinate system. The magnetic field B0 generates different energy levels for individual nuclear spins, depending on its strength, which can be excited (spin resonated) by applying an electromagnetic field (RF field) with a frequency defined in the radio frequency domain (Larmor frequency). From a macroscopic perspective, the distribution of individual nuclear spins creates an overall magnetization, which can be deflected from equilibrium by applying an electromagnetic pulse (RF pulse) of an appropriate frequency perpendicular to the z-axis, causing the magnetization to precess around the z-axis. This precession traces the surface of a cone, and the angle of this precession is called the flip angle. The magnitude of the flip angle depends on the intensity and duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z-axis to the transverse plane (90° flip angle). After the RF pulse ends, the magnetization relaxes and returns to its original equilibrium state. The magnetization in the z-direction is rebuilt with a first time constant T1 (spin-lattice relaxation time or longitudinal relaxation time), and the magnetization perpendicular to the z-direction is relaxed with a second time constant T2 (spin-spin relaxation time or transverse relaxation time). The magnetization fluctuations are measured in a direction perpendicular to the z-axis and can be detected by an oriented receiving RF coil placed within the inspection volume of the MR apparatus. The decay of transverse magnetization results in, for example, after applying a 90° pulse, a transition (diffusing) of nuclear spins (induced by local magnetic field inhomogeneity) from an ordered state with the same phase to a state with uniformly dispersed phases. Diffusing can be compensated for by a refocusing pulse (e.g., a 180° pulse). This generates an echo signal in the receiving coil. To achieve spatial resolution within the body, constant magnetic field gradients extending along three principal axes are superimposed on a uniform magnetic field B0 , thereby introducing a linear spatial dependence of the Larmor frequency. Therefore, the signal picked up by the receiving coil contains components of different frequencies that can be associated with different locations within the body. The signal data acquired via the receiving coil corresponds to the spatial frequency domain and is called k-space data. k-space data typically includes data from multiple lines in k-space acquired with different phase encodings. Each k-space line is digitized by collecting several samples. A set of samples from multiple lines in k-space is then transformed into an MR image, for example, by a Fourier transform. In MR imaging, it is often desirable to acquire information about the relative contributions of water and fat to the overall signal, either to suppress the contribution of one or the other, or to analyze the contributions of both water and fat separately or together. These contributions can be calculated by combining information from two or more corresponding echoes acquired at different echo times (with respect to excitation or spin-echo reconvergence). This is considered chemical shift encoding, where an additional dimension, the chemical shift dimension, is defined and encoded by acquiring two or more MR images at slightly different echo times. For water/fat separation, these types of experiments are often called Dixon measurements. Water/fat separation is achieved by calculating the contributions of water and fat from two or more corresponding echoes acquired at different echo times using Dixon MR imaging or Dixon water/fat MR imaging. Generally, such separation is possible because there are known differences in the precessional frequencies of hydrogen in water and fat. In its simplest form, images of water and fat are generated by either adding or subtracting in-phase and non-in-phase datasets. In recent years, several Dixon-type MR imaging methods have been proposed. Besides various st