MRI Imaging in Inhomogeneous Magnetic Fields
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Solution Overview
Problem
Magnetic resonance imaging (MRI) in inhomogeneous fields poses challenges due to variations in the background magnetic field, making it difficult to achieve homogeneous imaging, especially with permanent gradients, which require complex coil designs and high power consumption.
Innovation Solution
The method involves using non-homogeneous background fields and non-linear gradient fields to enable imaging by selectively exciting nuclear spins and spatially encoding phases, allowing for reconstruction of the spin density function, even in the presence of permanent gradients, through techniques like 3D encoding and refocusing pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a strong homogeneous background field is used for MRI, then imaging quality is improved, but device complexity and power consumption increase due to requiring complex coil arrangements
Solution Approach 1:
The patent converts the harmful effect of permanent magnet inhomogeneity into a beneficial feature by using the inherent field variations for spatial encoding. Instead of attempting to correct the inhomogeneity, the method utilizes the permanent gradient to encode spatial information directly, transforming what was previously a problem into the basis for image formation.
Solution Approach 2:
The patent replaces the complex mechanical coil system required to generate homogeneous fields with a simpler permanent magnet arrangement. By substituting the electromagnetic coil system with a permanent magnet that naturally produces the required field gradients, the device complexity is significantly reduced while maintaining imaging capability.
2Measurement precision
If pulsed gradients are used for spatial encoding in inhomogeneous fields, then spatial encoding is achieved, but acquisition time increases
Solution Approach 1:
The patent applies preliminary action by using the permanent magnet to establish the background field gradients before the imaging sequence begins. The spatial encoding gradients are pre-configured through the permanent magnet arrangement, eliminating the need for time-consuming pulsed gradient applications during the acquisition phase.
Solution Approach 2:
The patent maintains continuity of useful action by using continuous permanent magnet fields for spatial encoding rather than intermittent pulsed gradients. The permanent gradients continuously provide the necessary spatial encoding information throughout the imaging process, improving efficiency and reducing acquisition time.
3Stability of the object's composition
If refocusing pulses are applied repeatedly to correct phase accumulation, then phase coherence is maintained, but signal-to-noise ratio decreases
Solution Approach 1:
The patent extracts the phase coherence maintenance function from the refocusing pulse sequence and integrates it into the permanent magnet field design. By incorporating the gradient information directly into the background field, the system maintains phase coherence without requiring additional refocusing pulses that would degrade the signal.
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 allows for practical, non-invasive measurements and image reconstruction in inhomogeneous fields, providing high signal-to-noise ratio (SNR) and efficient imaging protocols, even with substantial permanent gradients, without significant sacrifice in resolution or acquisition time.
Implementation Method 1
the local Larmor frequency, which equals gamma times the magnitude of B0
Implementation Method 2
spatially encoding phases of the excited nuclear spins using gradient fields G1, G2
Data Source
AI summary
Methods for providing practical magnetic resonance imaging systems that utilize non-homogeneous background fields, B0, as well as, possibly non-linear, gradient fields G1, G2 to make non-invasive measurements to determine, among other things, a spin density function. Two types of non-homogeneous background fields are considered: background fields B0 in which the function |B0| does not have a critical point within the field of view, and background fields B0 such that the function |B0| has a single critical point within the field of view. In the first case, an MR-imaging device may be constructed by using the permanent gradient in the background field, B0, as a slice select gradient, so long as particular criteria are met. In the second case, magnets may be constructed so that |B0| has an isolated non-zero local minimum. Using selective excitation, one can excite only the spins lying in a small neighborhood of this local minimum.