MRI Shimming via Disjoint Sub-Volume Segmentation
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Solution Overview
Problem
Magnetic resonance imaging (MRI) techniques face challenges in achieving homogeneous static field homogeneity, particularly in larger imaging volumes, which limits the effectiveness of fat saturation techniques and image quality due to field inhomogeneities and the spatial limitations of local shimming.
Innovation Solution
The method involves subdividing the imaging volume into disjointed sub-volumes and defining subgroups for each sub-volume, allowing for localized shimming within the smallest possible bounding box of the subgroup, with shim settings dynamically adjusted for each concatenation to improve field homogeneity and image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If local shimming is applied to a large imaging volume, then field homogeneity is improved across the entire volume, but the effectiveness of shimming decreases due to the large spatial extent
Solution Approach 1:
The imaging volume is divided into multiple disjointed sub-volumes, and slices are distributed across different sub-volumes and concatenations. This segmentation allows local shimming to be applied to smaller sub-volumes where it is more effective, rather than attempting to shim the entire large imaging volume at once.
Solution Approach 2:
The shimming parameters are dynamically adjusted for each concatenation based on the specific sub-volume being imaged at that time. This dynamic adaptation allows the shim box to be optimized for each local region, maintaining high field homogeneity throughout the entire imaging volume despite its large overall size.
2Object-generated harmful factors
If frequency selective pulses are used for fat saturation, then fat signal suppression is achieved, but the technique becomes sensitive to local Larmor frequency offsets from field inhomogeneities
Solution Approach 1:
Local shimming is performed as a preliminary step before applying frequency selective fat saturation pulses. By pre-optimizing the field homogeneity in each sub-volume through shimming, the Larmor frequency offsets are minimized, which ensures that subsequent frequency selective fat saturation pulses work effectively without being compromised by field inhomogeneities.
3Loss of time
If all slices are acquired in one concatenation, then scan time is reduced, but field homogeneity cannot be optimized for each local region
Solution Approach 1:
The acquisition is segmented into multiple concatenations, each dedicated to imaging a specific sub-volume. This segmentation enables local shimming to be optimized for each sub-volume without significantly increasing the total scan time, as each concatenation focuses on a localized region where shimming is more effective.
Solution Approach 2:
The shimming parameters are changed and optimized for each concatenation based on the specific sub-volume being imaged. This parameter adaptation allows each local region to have optimal field homogeneity settings, while the overall scan time remains manageable through efficient concatenation 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
This approach enhances image quality by reducing the size of the shim box, improving static magnetic field homogeneity, and optimizing fat saturation techniques, even in larger imaging volumes like the abdomen or thorax, by restricting shimming to the smallest necessary volume during each concatenation.
Implementation Method 1
shim coils are controlled to produce magnetic shim fields which, at least in part, compensate inhomogeneities in a certain shim box
Implementation Method 2
These imaging techniques rely on the chemical shift between the Larmor frequencies of differently bound protons
Implementation Method 3
high frequency excitation pulses are used to excite the spins, which are aligned in a main magnetic field of the magnetic resonance apparatus
Data Source
AI summary
Techniques are disclosed for acquiring magnetic resonance data of an object with a magnetic resonance imaging apparatus. A slice group is imaged whose slices define a contiguous imaging volume and which contains a first number of slices. In a number of concatenations, the magnetic resonance data for subgroups of the slices, each containing a respective second number of slices depending on the first number of concatenations, are acquired, and shimming is performed to increase field homogeneity in the imaging volume. To define the subgroups, the imaging volume is subdivided into at least two disjoint contiguous sub-volumes, and at least two subgroups are defined for each sub-volume, each subgroup only containing non-adjacent slices in the sub-volume. During acquisition of the magnetic resonance data of each subgroup, shimming is at least restricted to the respective sub-volume.