MR Imaging Coil Calibration for Gradient Null Artifact Reduction
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Solution Overview
Problem
Magnetic resonance (MR) imaging is affected by gradient nulls, which cause artifacts and ghosting in reconstructed images, particularly in fast spin echo imaging, due to nearly zero slope magnetic field gradients, leading to adverse effects like FSE cusp artifacts, especially in spine imaging. Existing solutions such as improved gradient coil design, RF blankets, and phase cycling are either impractical or ineffective.
Innovation Solution
Measuring coil sensitivity at gradient null locations and using this data to weight MR data from multiple receive coils, allowing for the reconstruction of images that are substantially free of gradient null artifacts by combining weighted data from the coils.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If gradient coils are designed to place gradient nulls outside the imaging volume, then gradient null artifacts are reduced, but the imaging volume or field-of-view must be reduced
Solution Approach 1:
The patent segments the field-of-view into multiple regions, identifying and separating the gradient null region from the desired imaging volume. By treating the gradient null artifact as a distinct segment that can be independently processed and removed, the system maintains the full imaging volume while eliminating artifacts from specific regions.
Solution Approach 2:
The patent extracts and removes the gradient null signal from the composite MR signal. By identifying the specific frequency range where gradient null artifacts appear and selectively removing only that portion of the signal, the system eliminates artifacts while preserving the full imaging volume and desired anatomical information.
2Object-affected harmful factors
If a larger field-of-view is used to prevent aliasing of gradient null signals, then artifact aliasing is reduced, but scan time and data processing complexity increase
Solution Approach 1:
The patent changes the frequency domain parameters of the acquired signal by applying frequency shifts and filtering operations. By transforming the signal in the frequency domain to separate and remove artifact frequencies, the system eliminates aliasing without requiring a larger field-of-view, thus maintaining efficient scan times.
3Object-affected harmful factors
If gradient null locations are moved outside the imaging volume through coil design, then artifact generation is reduced, but hardware complexity and cost increase
Solution Approach 1:
The patent creates a frequency-domain representation (copy) of the spatial signal distribution and processes artifacts in this copied domain. By working with frequency-domain copies of the signal rather than modifying the physical gradient coil geometry, the system eliminates artifacts through software processing while maintaining simple, standard hardware design.
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 method effectively reduces gradient null artifacts in MR images without requiring hardware retrofitting, improving image quality by isolating and canceling artifact magnetization from true object magnetization, thus enhancing the accuracy of MR imaging.
Implementation Method 1
magnetic field gradients (Gx, Gy, and Gz) are employed
Implementation Method 2
the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency
Implementation Method 3
Measuring coil sensitivity at gradient null locations and using this data to weight MR data from multiple receive coils
Data Source
AI summary
Coil sensitivity of a receive coil to a gradient null location is measured and, from the measurements, a coil calibration value is determined and used to modify the MR data acquired with that receive coil to reduce the adverse effects of gradient nulling on MR images. Coil sensitivity values are determined for each coil of a coil array and the data for each coil is respectively weighted. An image that is substantially free of gradient null artifacts or ghosting is then reconstructed from the weighted data.