MRI Slice Position Correction for Chemical Shift Artifacts
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face challenges in generating accurate slice exposures due to chemical shifts between different tissue types, such as fat and water, which become more pronounced at higher magnetic field strengths, leading to artifacts and anatomical distortions.
Innovation Solution
A method involving a series of slice measurement sequences that spatially shift measurement slices to compensate for chemical shifts, allowing precise alignment of image data from different materials, enabling artifact-free slice exposures even with low radio-frequency bandwidths, particularly in high-field MRI systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the magnetic field strength is increased to improve image quality and signal-to-noise ratio, then the diagnostic capability is improved, but the chemical shift artifacts between fat and water increase proportionally, leading to slice position errors and anatomical distortions
Solution Approach 1:
The patent applies preliminary action by performing a B0 field map measurement before the actual slice acquisition. The measured field inhomogeneities are used to calculate and apply slice position corrections in advance, compensating for chemical shift effects before they manifest as artifacts in the final images.
Solution Approach 2:
The patent changes the slice position parameter dynamically based on the measured B0 field strength at each location. By adjusting the slice positions according to the actual field conditions (which vary with strength and inhomogeneity), the method compensates for chemical shift artifacts that would otherwise increase with field strength.
2Manufacturing precision
If a high bandwidth radio-frequency pulse is used to reduce chemical shift artifacts, then the slice position accuracy is improved, but the radio-frequency exposure (SAR) to the patient increases
Solution Approach 1:
The patent changes the approach from modifying the RF pulse bandwidth to modifying the slice position parameter. By correcting slice positions based on B0 field measurements, the method achieves accurate slice positioning without requiring high bandwidth RF pulses, thereby maintaining low SAR levels.
Solution Approach 2:
The patent replaces the RF pulse bandwidth adjustment mechanism with a computational slice position correction mechanism. Instead of using higher energy RF pulses to reduce artifacts, the system uses measured field data to calculate and apply position corrections, substituting a low-energy computational approach for a high-energy physical approach.
3Object-generated harmful factors
If fat suppression techniques are applied to eliminate chemical shift artifacts, then the image artifact level is reduced, but the sequence complexity and scan time increase
Solution Approach 1:
The patent extracts and measures the B0 field inhomogeneity component separately using a field map measurement. By isolating and characterizing the field inhomogeneity, the method can correct for its effects without requiring complex fat suppression pulse sequences, thereby reducing overall sequence complexity.
Solution Approach 2:
The patent replaces complex RF-based fat suppression mechanisms with a simpler computational correction approach. By using B0 field measurements to calculate slice position adjustments, the system eliminates the need for additional fat suppression pulses and complex sequence design, reducing both complexity and scan time.
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 reduces or eliminates artifacts caused by chemical shifts, allowing for accurate diagnostic imaging without the need for fat suppression, even at high magnetic field strengths, while minimizing radio-frequency exposure to the patient.
Implementation Method 1
the body or the body part to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field, which is most often designated as a B0 field. The nuclear spins in the body are thereby aligned parallel to the direction of the B0 field
Implementation Method 2
radio-frequency pulses (RF pulses) having a frequency in the range of the resonance frequency (known as the Larmor frequency) of the nuclei to be excited in the present B0 field, are radiated into the examination subject with suitable radio-frequency antennas. The spins of the nuclei to be excited are excited by these radio-frequency pulses
Implementation Method 3
Spatial coding takes place with the use of rapidly switched gradient magnetic fields that are superimposed on the basic magnetic field B0 during the emission of the magnetic resonance radio-frequency pulses and/or the acquisition of the raw data
Data Source
AI summary
In a method processor and magnetic resonance (MR) system to generate MR slice exposures of an examination subject, measurement data for a stack of measurement slices through the examination subject are initially acquired using a series of slice measurement sequences. The series of slice measurement sequences is designed to allow a separation of a first material from a second material that has a defined chemical shift relative to said first material, and the position of a measurement slice with measurement data for the first material is spatially shifted relative to the position of a measurement slice with measurement data for the second material. Combination slice exposures are then formed by combining measurement data of the first material from at least a first slice measurement sequence with measurement data of the second material from at least one second slice measurement sequence, such that the image data of the first and second materials are spatially arranged with precise positioning relative to one another, at least within a predetermined degree of tolerance in the combination slice exposures.