Magnetic Resonance Tomography B0 Field Correction via Phase Difference
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Solution Overview
Problem
Magnetic resonance tomography (MRT) imaging methods, particularly fast imaging techniques like EPI and SSEPI, face challenges in correcting image artifacts caused by B0 field changes, especially in functional and diffusion imaging, due to temporal fluctuations and variations in the B0 field, leading to image distortions and noise issues.
Innovation Solution
A method involving three phase correction scans to calculate a one-dimensional B0 field map, allowing for point-by-point phase difference evaluation and frequency offset calculation, enabling correction of measurement data to account for B0 field changes, thereby improving image accuracy and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If fast MRT imaging methods (EPI, SSEPI) are used to improve acquisition speed, then productivity is improved, but B0 field fluctuations cause image artifacts and displacement errors
Solution Approach 1:
The patent applies preliminary action by performing phase correction scans at specific echo times before the main imaging sequence to measure and correct B0 field offsets. By proactively measuring the B0 field characteristics and calculating correction factors in advance, the method compensates for field fluctuations that would otherwise cause image artifacts and displacement errors in fast EPI imaging.
2Measurement precision
If multiple phase correction scans are performed to improve B0 field measurement accuracy, then measurement precision is improved, but measurement time increases
Solution Approach 1:
The patent merges the phase correction measurement process with the main imaging sequence by integrating phase correction scans at specific echo times within the EPI sequence structure. This combination allows simultaneous acquisition of both B0 field correction data and imaging data without requiring separate dedicated measurement time, thus improving measurement precision without proportionally increasing total measurement time.
Solution Approach 2:
The patent uses a limited number of phase correction scans (typically 2-4 scans at different echo times) rather than continuous monitoring. This partial action approach provides sufficient B0 field characterization for correction purposes while avoiding the time penalty of excessive or continuous measurements, achieving an optimal balance between precision and efficiency.
3Reliability
If B0 field correction is applied to improve image quality, then image artifacts are reduced, but calculation complexity increases
Solution Approach 1:
The patent replaces complex iterative image post-processing correction methods with a simpler mathematical approach based on phase difference calculations from multi-echo measurements. By substituting the correction mechanism with direct frequency offset calculations and phase compensation formulas, the method reduces computational complexity while maintaining effective artifact correction and image quality improvement.
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 accurate correction of B0 field-induced artifacts, ensuring precise image overlay with anatomical high-resolution images, differentiating apparent from real translations, and maintaining image stability, even in applications with low signal-to-noise ratios, without requiring additional scans or complex calculations.
Implementation Method 1
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method in medicine and biophysics for over 15 years. In this examination method, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms of the subject, which were previously oriented randomly, thereby align. Radio-frequency energy can now excite these 'ordered' nuclear spins to a specific oscillation (resonant frequency).
Implementation Method 2
Using non-homogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions, which is generally designated as a 'spatial coding'.
Implementation Method 3
calculating the frequency offset between the actual resonant frequency relative to the adjusted resonant frequency based on the average phase difference and the echo time difference between the phase correction scans used, calculating a B0 field map dependent on this frequency offset, correcting the measurement data for the slice using the calculated B0 field map.
Data Source
AI summary
In the examination of a subject with a magnetic resonance tomography apparatus, data for a slice of the subject to be examined are obtained with a sequence of a fast MRT imaging method that includes at least three phase correction scans and measurement signals of the respective phase correction scans as well as of the slice are obtained. The phase difference of corresponding data points of two phase correction scans are calculated point-by-point, the average phase difference between the phase correction scans is evaluated, and the frequency offset between the actual resonance frequency relative to the adjusted resonance frequency is calculated based on the average phase difference and the echo time difference between the phase correction scans used. A B0 field map is calculated dependent on the frequency offset and, the measurement data for the slice are corrected using the calculated B0 field map.