US-12618925-B2 - MR imaging with spiral acquisition

US12618925B2US 12618925 B2US12618925 B2US 12618925B2US-12618925-B2

Abstract

The invention relates to a method of MR imaging of an object ( 10 ) positioned in an examination volume of a MR device ( 1 ). It is an object of the invention to enable efficient spiral MR imaging without blurring artefacts, even in situations of strong B 0 inhomogeneity. The method of the invention comprises the following steps: —subjecting the object ( 10 ) to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, —acquiring MR signals along two or more planar spiral k-space trajectories ( 31, 32, 33 ), wherein the radial k-space speed, i.e. the rate of variation of the radial distance from the spiral origin is essentially constant along each planar spiral k-space trajectory, and wherein the two or more k-space trajectories ( 31, 32, 33 ) are offset in-plane from each other, and—reconstructing an MR image from the acquired MR signals. Moreover, the invention relates to a MR device ( 1 ) and to a computer program for a MR device ( 1 ).

Inventors

Miha Fuderer

Assignees

KONINKLIJKE PHILIPS N.V.

Dates

Publication Date: 20260505
Application Date: 20211102
Priority Date: 20201110

Claims (10)

1 . A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of an MR device, the method comprising: subjecting the object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed outward from a predetermined current initial k-space position until a current final k-space position such that at the current final k-space position a radial k-space sampling density decreases below a predetermined current threshold value, and a next spiral trajectory is traversed outward from a next initial k-space position until a next final k-space position such that the radial k-space sampling density decreases below a predetermined next threshold value at the next final k-space position and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space.
2 . A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of an MR device, the method comprising: subjecting the object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed inward from a predetermined start k-space position such that a radial k-space sampling density is at least a current threshold value until a current end k-space position at which the radial k-space sampling density increases to a current ceiling value, and a next spiral trajectory is traversed inward from a next start k-space position after the current end k-space position such that the radial k-space sampling density is at least a next threshold value until a next end k-space position at which the radial k-space sampling density reaches a next ceiling value and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space.
3 . The method of claim 1 , wherein the MR signals are acquired along the multiple planar spiral k-space trajectories after a single RF excitation pulse.
4 . The method of claim 1 , wherein origins of the multiple planar spiral k-space trajectories are offset in-plane from each other.
5 . The method of claim 1 , wherein the multiple planar spiral k-space trajectories are rotated relative to each other around their spiral axes.
6 . The method of claim 1 , wherein a distance between windings of the multiple planar spiral k-space trajectories increases with increasing distance from an origin of the spiral.
7 . The method of claim 1 , wherein a B 0 map is derived by comparing the MR signals acquired along different of the multiple planar spiral k-space trajectories.
8 . The method of claim 7 , wherein an MR image is reconstructed with correction of B 0 inhomogeneity based on the derived B 0 map.
9 . A magnetic resonance (MR) device including at least one main magnet coil for generating a uniform, static magnetic field 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 a temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing a MR image from the received MR signals, wherein the MR device is arranged to perform either subjecting the object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed outward from a predetermined current initial k-space position until a current final k-space position such that at the current final k-space position a radial k-space sampling density decreases below a predetermined current threshold value, and a next spiral trajectory is traversed outward from a next initial k-space position until a next final k-space position such that the radial k-space sampling density decreases below a predetermined next threshold value at the next final k-space position and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space, or, subjecting the object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along the multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed inward from a predetermined start k-space position such that a radial k-space sampling density is at least a current threshold value until a current end k-space position at which the radial k-space sampling density increases to a current ceiling value, and a next spiral trajectory is traversed inward from a next start k-space position after the current end k-space position such that the radial k-space sampling density is at least a next threshold value until a next end k-space position at which the radial k-space sampling density reaches a next ceiling value and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space.
10 . A non-transitory computer-readable medium storing a computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions to: subjecting an object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed outward from a predetermined current initial k-space position until a current final k-space position such that at the current final k-space position the radial k-space sampling density decreases below a predetermined current threshold value, and a next spiral trajectory is traversed outward from a next initial k-space position until a next final k-space position such that the radial k-space sampling density decreases below a predetermined next threshold value at the next final k-space position and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space, or, subjecting the object to an imaging sequence comprising at least one RF excitation pulse and modulated magnetic field gradients, acquiring MR signals by sampling k-space along multiple planar spiral k-space trajectories in k-space traversed with a constant radial k-space speed, wherein a current spiral trajectory is traversed inward from a predetermined start k-space position such that the radial k-space sampling density is at least a current threshold value until a current end k-space position at which the radial k-space sampling density increases to a current ceiling value, and a next spiral trajectory is traversed inward from a next start k-space position after the current end k-space position such that the radial k-space sampling density is at least a next threshold value until a next end k-space position at which the radial k-space sampling density reaches a next ceiling value and wherein the multiple planar spiral k-space trajectories are offset in-plane from an origin of k-space so as to each start at a different origin for enhancing the sampling density in a periphery of the k-space.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a U.S. national phase application of International Application No. PCT/EP2021/080301 filed on Nov. 2, 2021, which claims the benefit of EP Application No. 20206585.0 filed on Nov. 10, 2020 and is incorporated herein by reference. FIELD OF THE INVENTION The invention relates to the field of magnetic resonance (MR) imaging. It concerns a method of MR imaging of an object. The invention also relates to a MR device and to a computer program to be run on a MR device. BACKGROUND OF THE INVENTION Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive. According to the MR method in general, the object, for example the body of the patient to be examined, is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse), so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the 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 (flip angle 90°). After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated, e.g., by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils. To realize spatial resolution in the body, magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. A set of k-space data is converted to a MR image by means of an image reconstruction algorithm. Spiral imaging is a fast MR imaging technique that benefits from an efficient k-space coverage and a low sensitivity to motion and flow artifacts. Spiral k-space trajectories allow for an efficient and temporally flexible sampling of k-space as shorter pathways are required to cover a desired k-space region, and the signal acquisition may start in the center of k-space. Spiral imaging techniques are, however, vulnerable to inhomogeneities in the amplitude of the main magnetic field B0, which causes blurring and degrades the image quality. E.g., images obtained by spiral MR imaging of the brain are typically contaminated by off-resonance signal contributions from the sagittal sinus and nasal cavities. The strong magnetic field inhomogeneities may result in the shape of the true spiral k-space trajectories to deviate to such a large extent from the theoretical spiral shape that practically no usable signal data is sampled from certain regions of k-space. Magnetic field inhomogeneities induced by the patient anatomy may lead to local magnetic