Offset Spiral Trajectories for MR Imaging
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Solution Overview
Problem
Spiral MR imaging is vulnerable to magnetic field inhomogeneities, leading to blurring and incomplete k-space data sampling, which degrades image quality, especially in strong B0 inhomogeneous fields.
Innovation Solution
The method involves using multiple sinusoidally modulated magnetic field gradients to acquire MR signals along offset spiral k-space trajectories, ensuring optimal coverage of the central k-space region even in the presence of B0 inhomogeneities, and reconstructing images with correction using derived B0 maps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If spiral MR imaging is used, then imaging speed is improved, but image quality deteriorates due to vulnerability to magnetic field inhomogeneities
Solution Approach 1:
The patent divides the single spiral k-space trajectory into multiple offset spiral trajectories. By segmenting the sampling path into several concentric spirals with different centers, the method ensures comprehensive coverage of the central k-space region even when B0 inhomogeneities cause distortion of individual spiral paths. This segmentation approach maintains the fast imaging speed of spiral acquisition while improving robustness to field inhomogeneities.
2Loss of time
If single spiral k-space trajectory is used, then acquisition time is reduced, but central k-space coverage becomes incomplete in presence of B0 inhomogeneities
Solution Approach 1:
The patent introduces an additional dimension to the k-space sampling by using multiple offset spiral trajectories instead of a single trajectory. This dimensional expansion ensures that the central k-space region is sampled from multiple angular positions and offsets, creating redundant and complementary information that compensates for distortions caused by B0 inhomogeneities, thereby preventing loss of central k-space data.
3Productivity
If spiral imaging is used, then scanning efficiency is improved, but sensitivity to flow artifacts decreases
Solution Approach 1:
The patent applies local quality by using multiple offset spiral trajectories that are specifically optimized for sampling the central k-space region. Each spiral trajectory is locally adapted to compensate for B0 inhomogeneities in different regions, thereby maintaining scanning efficiency while reducing sensitivity to flow artifacts through improved local k-space coverage and redundancy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach ensures efficient spiral MR imaging with improved k-space coverage and image quality by preventing missing data in central k-space regions, even in areas with significant B0 non-uniformity or gradient imperfections.
Implementation Method 1
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).
Implementation Method 2
To realize spatial resolution in the body, constant 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.
Implementation Method 3
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.
Implementation Method 4
The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle.
Implementation Method 5
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.
Implementation Method 6
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).
Data Source
AI summary
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 even in situations of strong Bo inhomogeneity. The method of the invention comprises: subjecting the object (10) to an imaging sequence comprising at least one RF excitation pulse and sinusoidally modulated magnetic field gradients, acquiring MR signals along two or more spiral k-space trajectories (31, 32, 33) as determined by the sinusoidal modulation of the magnetic field gradients, wherein the origins of the spiral k-space trajectories are offset 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).