MRI B0 Inhomogeneity Correction via Volumetric Navigators
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Solution Overview
Problem
MRI scanners face challenges in maintaining homogeneous main magnetic fields during scanning due to internal and external factors, leading to image distortions and inaccuracies, especially in long scanning sessions like fMRI and DTI, where existing technologies lack effective real-time compensation for changes in the magnetic field.
Innovation Solution
The method involves acquiring successive volumes using three-dimensional volumetric navigators with different echo times to create a magnetic field map, allowing for real-time adjustment of the MRI scanner's central frequency and shim coils to correct for main magnetic field inhomogeneity and motion, ensuring accurate image acquisition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If active shimming is performed once before scanning begins, then the initial magnetic field homogeneity is established, but the field becomes compromised during long scanning periods due to temporal changes
Solution Approach 1:
The patent implements continuous monitoring and adjustment of magnetic field homogeneity throughout the scanning process. Field maps are acquired repeatedly during the scan, and shim parameters are continuously updated to maintain optimal field homogeneity, transforming the single-point adjustment into a continuous correction process that persists throughout the entire scanning duration.
Solution Approach 2:
The system acquires field maps during scanning to monitor changes in magnetic field homogeneity, uses this information to calculate updated shim parameters, and applies corrections in real-time. This closed-loop feedback mechanism detects field drift caused by patient motion or respiration and automatically compensates for it, ensuring reliable field homogeneity throughout long scanning sessions.
2Loss of information
If the scanning sequence acquires successive volumes repeatedly for fMRI or DTI, then comprehensive imaging data is obtained, but image accuracy deteriorates due to drift in central frequency and distortion in shim gradients
Solution Approach 1:
The patent performs preliminary acquisition of field maps and calculation of shim parameters before the main imaging sequence begins. This pre-characterization of the magnetic field allows the system to establish baseline correction parameters that can be applied throughout the subsequent volumetric scanning, proactively compensating for expected field drift rather than reacting to it after image degradation occurs.
Solution Approach 2:
The system dynamically adjusts shim parameters (zero-order for central frequency, first-order for gradient distortion) based on continuously acquired field maps. By changing these parameters in real-time according to measured field conditions, the patent maintains image accuracy throughout prolonged scanning sessions despite temporal drift in the magnetic field.
3Measurement precision
If shim parameters are adjusted frequently to maintain field homogeneity, then image accuracy is maintained, but scanning time increases due to repeated field map acquisitions
Solution Approach 1:
The patent acquires field maps at selected intervals during the scanning sequence rather than continuously after every single volume. This partial monitoring approach provides sufficient information to track field drift trends and update shim parameters at appropriate frequencies, maintaining image accuracy without the excessive time penalty of acquiring full field maps after every volume.
Solution Approach 2:
The system implements periodic acquisition of field maps at predetermined intervals during the scanning sequence. This periodic sampling of field conditions allows for timely updates of shim parameters to correct drift while maintaining an efficient scanning rhythm, balancing the need for accuracy with the constraint of total scanning 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 effectively corrects for magnetic field inhomogeneity and motion, improving image quality by adjusting the system's central frequency and shim coils in real-time, addressing the limitations of existing technologies in maintaining image accuracy during prolonged scanning sessions.
Implementation Method 1
The fields required to be generated by the active shim coils must be determined by first acquiring a map of the main magnetic field, which includes the offset phase in the B0 filed due to filed inhomogeneity (ΔB0)
Implementation Method 2
Magnetic Resonance Imaging (MRI) involves applying different types of electromagnetic fields and radiofrequency (RF) excitations to a subject. The aim in doing so is to generate spatial RF signals from a specific region of the subject
Implementation Method 3
a main superconductive coil which produces a powerful main magnetic field (called 'B0') which polarizes an object to be scanned
Data Source
AI summary
A method for correcting for main magnetic field (B0) inhomogeneity in a Magnetic Resonance Imaging (MRI) scanner is disclosed. The method includes applying a first and a second three-dimensional volumetric navigator after an acquisition of a volume in a scanning sequence and before the next volume is acquired. From a resultant pair of navigator images, a magnetic field map is obtained by complex division of the pair of navigator images, and the field map is used to determine parameters to adjust the MRI scanner to compensate for B0 inhomogeneity. The navigators may excite only a portion of an entire object to be imaged, so that adjustment of the MR scanner can be done slice-by-slice or slab-by-slab. Motion correction can also be implemented by comparing the first navigator to a stored reference image and updating for motion before acquisition of the next volume in the scanning sequence.