B0-Mapping Accuracy in Low Proton Density MRI Regions
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Solution Overview
Problem
Existing B0-mapping methods for determining the spatial distribution of a static magnetic field in MRI systems are not accurate enough, especially in regions with low proton density where signal-to-noise ratio is low, leading to unreliable measurements.
Innovation Solution
The method computes the spatial distribution of the static magnetic field by combining the spatial distribution of spin-phase accruals between magnetic resonance echo signals and an estimate of the proton density distribution, using an iterative approach to fit a magnetic susceptibility distribution that reconciles both distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing B0-mapping methods are used to determine spatial distribution of magnetic field, then the measurement can be obtained, but the accuracy is insufficient especially in low proton density regions
Solution Approach 1:
The patent combines two different measurement approaches: phase difference method (using echo signals at different echo times) and proton density estimation method. By merging these two independent measurements and reconciling them through iterative optimization, the system achieves accurate B0-mapping in all regions including low proton density areas where individual methods would fail.
Solution Approach 2:
The patent implements an iterative feedback mechanism where the B0-mapping result is continuously refined by comparing phase-based estimates with proton density-based estimates. The system uses feedback loops to adjust and reconcile the two distributions until they converge, ensuring high accuracy and reliability in the final B0-mapping.
2Device complexity
If only phase difference method is used for B0-mapping, then the process is simple, but the results are unreliable in low signal-to-noise ratio regions
Solution Approach 1:
The patent creates a composite measurement approach by integrating two different measurement paradigms: phase difference analysis and proton density estimation. This composite method leverages the strengths of both approaches while compensating for their individual weaknesses, particularly the phase-wrapping issues and low signal-to-noise ratio problems.
3Measurement precision
If proton density estimation is incorporated into B0-mapping, then accuracy in low proton density regions improves, but computational complexity increases
Solution Approach 1:
The patent performs preliminary proton density estimation and segmentation before the final B0-mapping calculation. By pre-processing the data to identify tissue types and estimate proton density distributions in advance, the system reduces the computational burden during the iterative reconciliation phase while maintaining high accuracy.
Solution Approach 2:
The patent segments the imaged volume into different tissue types (e.g., soft tissue, air, bone) based on proton density characteristics. This segmentation allows the system to apply appropriate processing strategies to different regions and reduces overall computational complexity by handling homogeneous regions separately.
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 provides a more accurate and stable B0-mapping, reducing errors due to low proton density regions and improving the reliability of the magnetic field distribution estimation, including in cavities and outside tissue.
Implementation Method 1
The magnetic field B 0 causes 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
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) while the corresponding magnetic field B1 of this RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession motion about the z-axis.
Implementation Method 3
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 and shorter time constant T2 (spin-spin or transverse relaxation time).
Implementation Method 4
The transverse magnetization and its variation can be detected by means of receiving RF antennae (coil arrays) which are arranged and oriented within an examination volume of the magnetic resonance examination system in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
Implementation Method 5
In order to realize spatial resolution in the subject being imaged, such as a patient to be examined , constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B 0 , leading to a linear spatial dependency of the spin resonance frequency.
Implementation Method 6
The dephasing can be compensated by means of a refocusing RF pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
Implementation Method 7
The transverse magnetization dephases also in presence of constant magnetic field gradients. This process can be reversed, similar to the formation of RF induced (spin) echoes, by appropriate gradient reversal forming a so-called gradient echo.
Data Source
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AI summary
A B 0 -mapping method determines the spatial distribution of a static magnetic field in a pre-selected imaging zone comprising computation of the spatial distribution of a static magnetic field from a spatial distribution of spin-phase accruals between magnetic resonance echo signals from the imaging zone and an estimate of the proton density distribution in the imaging zone. The invention provides the field estimate also in cavities and outside tissue. Also the field estimate of the invention suffers less from so-called phase-wraps.