MRI Apparatus Static Field Correction for Image Distortion
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face challenges in achieving complete homogeneity of the static magnetic field, leading to image distortions, particularly in echo planar imaging sequences, which degrade the spatial resolution of MR images due to inhomogeneity.
Innovation Solution
A magnetic resonance imaging apparatus that calculates and applies a static magnetic field correction based on a narrower imaging range, using a shim coil to generate correction magnetic fields for second-order and higher components of inhomogeneity, thereby correcting distortions in MR images by extending or contracting pixels as needed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If static magnetic field shimming is performed to optimize center frequency, then signal quality is improved, but complete homogeneity cannot be achieved leading to image distortion
Solution Approach 1:
The imaging region is divided into multiple slices, and static magnetic field shimming is performed independently for each slice. This segmentation allows optimization of center frequency for each slice while managing the complexity of achieving complete homogeneity across the entire imaging volume, thereby improving signal quality without requiring perfect global homogeneity.
Solution Approach 2:
The patent applies different shimming corrections to different regions (slices) of the imaging volume based on their specific magnetic field characteristics. By tailoring the shimming parameters to local conditions in each slice, the system optimizes signal quality locally while accepting that complete homogeneity across the entire volume is not achievable.
2Productivity
If EPI sequence is used for imaging, then imaging speed is improved, but distortions occur in MR image due to static magnetic field inhomogeneity
Solution Approach 1:
Static magnetic field shimming is performed as a preliminary step before acquiring EPI images. By pre-optimizing the magnetic field homogeneity in each slice through shimming, the system prepares the imaging conditions to minimize distortions that would otherwise occur during the fast EPI acquisition, thereby maintaining both high imaging speed and reduced distortion.
Solution Approach 2:
The shimming process applies preliminary corrective magnetic fields to counteract the static magnetic field inhomogeneity before the EPI imaging sequence is executed. This preliminary anti-action prevents the distortion that would otherwise occur during the fast imaging, allowing the EPI sequence to maintain its speed advantage while producing images with reduced distortion.
3Area of stationary object
If pixels are contracted due to inhomogeneity, then imaging coverage is improved, but spatial resolution is degraded
Solution Approach 1:
The patent modifies the magnetic field parameters through shimming corrections in each slice, changing the resonance frequency distribution to compensate for inhomogeneity. By adjusting these parameters locally, the system can maintain proper pixel representation and spatial resolution while still achieving adequate imaging coverage across the entire volume.
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
The solution effectively corrects image distortions by adjusting the static magnetic field, improving the spatial resolution and quality of MR images by ensuring that pixels are either extended or contracted to facilitate reliable distortion correction.
Implementation Method 1
using a shim coil to generate correction magnetic fields for second-order and higher components of inhomogeneity
Data Source
AI summary
According to one embodiment, a magnetic resonance imaging apparatus includes processing circuitry. The processing circuitry calculates a static magnetic field correction amount based on a static magnetic field distribution of a first imaging range narrower than a second imaging range. The processing circuitry collects a magnetic resonance (MR) image of the second imaging range under a static magnetic field which is corrected based on the static magnetic field correction amount. The processing circuitry corrects distortion of the collected MR image.