Nyquist Ghost Reduction in MRI Echo-Planar Imaging
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Solution Overview
Problem
Existing magnetic resonance tomography (MRT) techniques using echo-planar imaging sequences suffer from Nyquist ghosts, which degrade image quality due to asymmetries in readout gradient pulse trains, and current phase correction methods either extend measurement time or fail to detect time-dynamic effects.
Innovation Solution
A method that determines progression curves for echo characteristics based on pulse shape, amplitude, and duration, allowing for phase-coded data correction and Fourier transformation to reduce Nyquist ghosts, eliminating the need for time-consuming phase correction scans and accounting for dynamic effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If phase correction scans are performed to compensate for asymmetries in readout gradient pulse trains, then Nyquist ghosts are reduced, but measurement time is extended
Solution Approach 1:
The patent applies preliminary action by determining progression curves for echo characteristics (phase, amplitude, position) as a function of echo number before actual image acquisition. These curves are stored and reused for correcting multiple subsequent EPI measurements, eliminating the need to perform time-consuming phase correction scans for each measurement while maintaining effective Nyquist ghost reduction.
Solution Approach 2:
The patent uses copying by creating a model (progression curves) of echo characteristics from initial measurements and then copying/applying this correction model to multiple subsequent measurements. Instead of re-measuring correction parameters for each scan, the system copies the previously determined progression curves and applies them to correct new data, significantly reducing total measurement time.
2Manufacturing precision
If conventional phase correction methods are used, then some Nyquist ghosts are reduced, but time-dynamic effects during longer acquisition series are not detected
Solution Approach 1:
The patent applies dynamics by making the correction system adaptive to time-varying conditions. Progression curves are determined separately for different time points or conditions, and the system can detect and respond to changes in echo characteristics that occur during longer acquisition series. This allows the correction to dynamically adapt rather than using static correction parameters throughout the entire measurement.
Solution Approach 2:
The patent performs preliminary determination of progression curves under different conditions (e.g., different time points, different gradient amplitudes) to capture time-dynamic effects. By pre-determining multiple sets of progression curves corresponding to different states during the acquisition series, the system can select or interpolate between appropriate curves to correct for dynamic changes without extending measurement time significantly.
3Productivity
If alternating readout gradient pulse trains are used for EPI sequences, then measurement speed is increased, but asymmetries cause Nyquist ghosts that degrade image quality
Solution Approach 1:
The patent applies feedback by using the determined progression curves (which characterize the asymmetries in the alternating gradient pulse train) to correct the phase-coded data set. The correction process uses the echo number-dependent progression curves to calculate and apply compensating phase corrections, effectively feeding back information about the system's asymmetric behavior to eliminate the resulting Nyquist ghosts while maintaining the fast EPI measurement speed.
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 effectively reduces Nyquist ghosts in MRT images without extending measurement time, dynamically adapting to changes during longer acquisition series and avoiding dephasing effects, thereby improving image quality.
Implementation Method 1
MRT is based on the physical phenomenon of nuclear magnetic resonance
Implementation Method 2
The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align
Implementation Method 3
The measurement subject can be spatially coded in all three spatial directions by non-inhomogeneous magnetic fields generated by gradient coils
Implementation Method 4
multiple gradient echoes are generated via a sinusoidally-oscillating frequency coding gradient in the readout direction
Implementation Method 5
The basic idea of this technology is to generate a series of echoes in the readout gradient (Gx) after an individual (selective) RF excitation, the echoes being associated with various lines in k-space matrix by a suitable modulation of the phase coding gradients (Gy)
Implementation Method 6
An MR image of the slice in question with a resolution of N×M pixels can be directly reconstructed from this raw data set by a two-dimensional Fourier transformation
Data Source
AI summary
In a method and MRT apparatus for reduction of Nyquist ghosts in medical magnetic resonance imaging given the application of EPI sequences of one or more progression curves with regard to one or more characterizing quantities of the EPI echo, is determined dependent on the echo number of a readout gradient pulse train defined by pulse shape, pulse amplitude and pulse duration, an EPI measurement is implemented with a readout gradient pulse train, composed of alternating positive and negative amplitudes with under which alternating positive and negative echoes and respectively readout, and a phase coding gradient, so a phase-coded data set is acquired, the phase-coded data set is corrected on the basis of the progression curves, and is Fourier-transformed the corrected data set, to obtain an artifact-reduced (with regard to the Nyquist ghosts) image data set.