Shim Coil Current Interpolation for MRI Field Uniformity
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face challenges in maintaining a uniform static magnetic field, especially when imaging areas wide along the body axis, leading to extended imaging times due to the need for frequent corrections as the subject is moved.
Innovation Solution
An MRI apparatus with a shim coil unit that generates correction magnetic fields, a measuring unit to adjust shim coil current values, and a controller to interpolate shim coil current values for positions where measurements have not been taken, allowing for continuous imaging while maintaining field uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the shaft length of the static magnetic field magnet is shortened to reduce claustrophobia and inhibit dB/dt increase, then the imaging area along the body axis is narrowed, but this requires segmenting and moving the subject which extends imaging time
Solution Approach 1:
The patent performs preliminary shimming measurements at selected positions and uses interpolation to calculate shim coil current values for intermediate positions before imaging begins. This preliminary characterization of the magnetic field allows the system to quickly adjust shim currents during imaging without performing full measurements at every position, thereby reducing overall imaging time while maintaining field uniformity.
Solution Approach 2:
The patent creates a model of the magnetic field uniformity characteristics by measuring at selected positions and interpolating to represent all positions. This copied model allows the system to predict and correct field non-uniformity at positions where direct measurement was not performed, reducing the need for repeated measurements during imaging.
2Reliability
If shim coil current values are measured at every position to maintain field uniformity during subject movement, then imaging quality is maintained, but the time required for imaging increases significantly
Solution Approach 1:
The system performs preliminary shimming measurements at a reduced set of positions before imaging and uses these measurements to establish a model for calculating shim currents at all other positions through interpolation. This preliminary action captures the essential magnetic field characteristics without requiring exhaustive measurements at every imaging position.
Solution Approach 2:
The patent introduces interpolation calculation as an intermediary process between direct measurements and actual imaging. Instead of measuring at every position, the system uses measured values from selected positions as input to interpolation algorithms that generate the necessary shim current values for all positions, acting as a mediator that maintains field uniformity without requiring direct measurement at each location.
3Area of stationary object
If the area to be imaged is segmented and imaged while moving the subject, then imaging of wide areas is possible, but the anatomy changes position causing magnetic field uniformity to change and requiring repeated corrections
Solution Approach 1:
The patent implements dynamic adjustment of shim coil current values based on the position of the subject. As the subject is moved to different positions for imaging wide areas, the system calculates and applies position-specific shim currents using interpolation based on preliminary measurements. This dynamic adaptation maintains field uniformity across different anatomical positions without requiring the subject to be stationary.
Solution Approach 2:
The system changes the parameters of the shim coil current values according to the subject's position and the specific anatomy being imaged. By interpolating between measured positions and calculating appropriate current values for intermediate positions, the system adapts the magnetic field parameters to maintain uniformity across different anatomical regions and subject positions.
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 the time required for imaging by eliminating the need to measure shim coil current values at every position, thereby improving imaging efficiency and reducing subject burden.
Implementation Method 1
the shim coil generates a correction magnetic field for correcting the non-uniformity of a static magnetic field
Implementation Method 2
generates a static magnetic field in an imaging space in which a subject is placed
Data Source
AI summary
In a magnetic resonance imaging apparatus according to an embodiment, a measuring unit moves a couchtop on which a subject is placed to at least one position of a plurality of positions in an imaging space, adjusts shim coil current value supplied to the shim coil, and measures first shim coil current value when a static magnetic field is uniformized. On the basis of the first shim coil current value and at least one of subject information and an imaging condition, a calculator calculates second shim coil current value position of the plurality of positions and at which the measuring unit has measured no shim coil current value.