MRI Shim Current Calculation via Pixel Shift Map
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Solution Overview
Problem
Current shimming techniques in MRI systems are inefficient due to the time-consuming data acquisition and calculation processes required to determine necessary shim currents, which reduces productivity and fails to adequately correct magnetic field inhomogeneities.
Innovation Solution
A method involving the calculation of a pixel shift map from forward and reverse spatial encoding polarity MR coil images, converted into a magnetic field shift map, which is then used as input for a shim calculation process to determine appropriate shim currents for the shim coils, allowing for quick acquisition and calculation of a field map.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If traditional shimming techniques are used to correct magnetic field inhomogeneities, then the magnetic field homogeneity is improved, but the time required for data acquisition and calculation increases significantly
Solution Approach 1:
The system performs preliminary action by acquiring both forward and reverse spatial encoding polarity images and calculating the pixel shift map before the actual shimming process. This preparation work is done in advance so that when shimming is needed, the field map is already available, significantly reducing the time required during the actual shimming operation.
Solution Approach 2:
The system creates a copy of the spatial encoding process by acquiring images with both forward and reverse polarities. By comparing these two copies and calculating their differences, the system efficiently derives the field map without requiring additional time-consuming measurements during the shimming process itself.
2Measurement precision
If comprehensive field mapping is performed to achieve accurate shimming, then the shimming accuracy is improved, but the calculation complexity and time required increase
Solution Approach 1:
The system extracts only the essential information needed for shimming by calculating the pixel shift map from the difference between forward and reverse images. This extraction approach isolates the relevant field inhomogeneity data from the complete image data, providing accurate shimming information without requiring complex processing of the entire image dataset.
Solution Approach 2:
The shimming process is segmented into distinct steps: acquiring forward image, acquiring reverse image, calculating pixel shift map, converting to field map, and applying shimming. This segmentation allows each step to be optimized independently and enables the system to provide accurate shimming through a manageable sequence of calculations rather than one complex monolithic process.
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 enables rapid and efficient shimming of the MRI magnet, increasing productivity by quickly stabilizing low-order features and determining necessary shim currents, thus improving the homogeneity of the magnetic field.
Implementation Method 1
A series of gradient fields are produced by a set of gradient coils located around the subject. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions.
Implementation Method 2
The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
Implementation Method 3
The shim calculation process includes determining a level of at least one shim current passed through at least one shim coil to shim an MR magnet.
Data Source
AI summary
A method includes receiving a forward spatial encoding polarity magnetic resonance (MR) coil image and a reverse spatial encoding polarity MR coil image generated from data obtained with a magnetic field gradient that is reversed with respect to the magnetic field gradient with which the forward spatial encoding polarity MR coil image is acquired. The method also includes performing an iterative shift map calculation algorithm to determine a pixel shift map corresponding to a minimized difference between the forward and reverse spatial encoding polarity MR coil images, converting the pixel shift map into a magnetic field shift map by determining a magnetic field value corresponding to each pixel in the pixel shift map, and providing the magnetic field shift map as an input to a shim calculation process that includes determining a level of at least one shim current.