Complex B1+ Field Estimation for MRI Transmit Coils
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Solution Overview
Problem
Current MRI systems face challenges in accurately estimating the transmit field (B1+) of RF coils, leading to non-uniform signal reception and image quality issues due to inhomogeneities caused by coil configuration, subject loading effects, and calibration instability, especially at higher magnetic fields.
Innovation Solution
The method involves encoding both phase and magnitude of the RF pulses in the acquisition sequence, acquiring 2D or 3D k-space data, and applying algorithms to estimate a complex B1+ map in the k-space domain, which can be used for B1+ mapping, RF shimming, and image quality improvement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional B1+ mapping methods are used, then the transmit field can be estimated, but the process is time-consuming and accuracy is limited due to inhomogeneities
Solution Approach 1:
The patent applies preliminary action by performing B1+ mapping measurements before actual MRI scanning. The complex B1+ map is estimated in advance using a calibration scan with a phantom, allowing the system to pre-determine transmit field inhomogeneities. This preliminary measurement enables subsequent correction during actual scanning without adding time to the diagnostic scan itself.
Solution Approach 2:
The patent introduces an intermediary approach by using a phantom object as a mediator between the transmit coil and the patient. The phantom provides a known, stable reference that allows accurate measurement of B1+ field distribution without requiring direct measurement in the patient's body. This intermediary measurement can then be applied to correct images acquired from actual patients.
2Area of stationary object
If RF coils are configured to cover larger areas, then imaging coverage is improved, but field inhomogeneity increases leading to poorer image quality
Solution Approach 1:
The patent applies local quality by measuring and correcting B1+ field inhomogeneities on a pixel-by-pixel basis. Instead of attempting to create a perfectly uniform field across the entire imaging area, the system characterizes the actual field distribution locally at each position and applies position-specific correction factors to achieve uniform image quality throughout the coverage area.
Solution Approach 2:
The patent implements feedback by using the measured complex B1+ map to correct image intensities. The system measures the actual transmit field distribution, compares it to the desired uniform distribution, and applies correction algorithms that feed back into the image reconstruction process to compensate for the measured inhomogeneities.
3Measurement precision
If calibration is performed frequently to maintain accuracy, then measurement precision is improved, but productivity decreases due to repeated calibration time
Solution Approach 1:
The patent applies partial action by performing calibration only on a subset of the imaging volume using a phantom that covers the most critical or variable regions. This partial calibration approach captures the essential B1+ field characteristics without requiring exhaustive calibration of the entire imaging space, reducing calibration time while maintaining sufficient accuracy for clinical scanning.
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 significantly reduces the time and improves the accuracy of B1+ mapping, enabling better RF coil design, image quality enhancement, and RF safety evaluation by providing a comprehensive understanding of the transmit field distribution.
Implementation Method 1
Magnetic resonance imaging (MRI) is one of the most important modern medical imaging modalities... the object being imaged must be exposed to a static basic magnetic field... radio-frequency pulses with a defined field strength are radiated into the objected being imaged... the nuclear spins of the atoms in the object being imaged are excited such that the atoms are deflected by a so-called 'excitation flip angle' from their equilibrium position parallel to the basic magnetic field B0
Implementation Method 2
the object being imaged must be exposed to a static basic magnetic field (usually designated as the B0 field) which is as homogeneous as possible. The basic magnetic field can be generated by a basic field magnet of the MRI system
Implementation Method 3
While the magnetic resonance images are being recorded, the basic magnetic field has fast-switched gradient fields superimposed on it for spatial encoding, which are generated by gradient coils
Implementation Method 4
The magnetic resonance signals generated in this manner are recorded by RF receiver coil. The receiver coil can be either the same coil which was used to generate the RF pulses (i.e., a transceiver coil) or a separate receive-only coil. The MR signal is picked up by a receive coil, amplified and processed
Data Source
AI summary
Systems and methods for estimating complex transmit field B1+ a transmit coil of a magnetic resonance imaging (MRI) system in both k-space and image domains are described herein. Estimating complex RF field B1+ in the k-space domain includes acquiring complex data in a k-space domain, estimating a complex B1+ map in the k-space domain of a transmit coil and storing the complex B1+ map. The complex B1+ map can be estimated based on the complex images. Estimating complex transmit field B1+ in the image domain includes acquiring at least two complex images in a k-space domain, transforming the complex images into an image domain, estimating a complex B1+ map in the image domain of a transmit coil, and storing the complex B1+ map.