Slice Multiplexing EPI Navigator Signal Correction
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Solution Overview
Problem
Slice multiplexing methods in magnetic resonance imaging (MRI) face challenges in efficiently correcting phase errors and N/2 ghosts without increasing measurement time or SAR load, particularly when recording multiple slices simultaneously, as existing methods require additional reference measurements and navigator signals that can introduce artifacts and reduce temporal resolution.
Innovation Solution
A method for slice-multiplexing EPI that records an odd number of navigator signals after an RF excitation pulse, allowing subvolume-specific correction of phase errors without additional reference data, using a slice-GRAPPA approach and Fourier transforms to separate collapsed data, thereby reducing measurement time and maintaining high temporal resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If additional reference measurements and navigator signals are used for phase error correction, then correction accuracy is improved, but measurement time increases
Solution Approach 1:
The patent combines the acquisition of navigator signals with the acquisition of imaging data by recording navigator signals during the same measurement process, thereby eliminating separate reference measurements and reducing total measurement time while maintaining correction accuracy
Solution Approach 2:
The navigator signals serve multiple functions simultaneously: they are used for both phase error correction and for tracking temporal variations in the base magnetic field, reducing the need for separate dedicated reference measurements
2Measurement precision
If additional reference measurements are performed for each slice, then subvolume-specific correction is improved, but SAR load increases
Solution Approach 1:
The patent combines the acquisition of navigator signals with the acquisition of imaging data by recording navigator signals during the same measurement process, thereby eliminating separate reference measurements and reducing total measurement time while maintaining correction accuracy
Solution Approach 2:
The navigator signals are acquired as a byproduct of the imaging process itself, using the same RF excitation pulses and gradient fields, so no additional energy is required beyond what is already applied for imaging
3Manufacturing precision
If conventional phase correction methods are used, then N/2 ghosts are corrected, but temporal resolution is reduced
Solution Approach 1:
The patent performs phase error correction on individual navigator signals before combining them, allowing for more efficient correction of N/2 ghosts and temporal variations without requiring extensive post-processing of the entire dataset
Solution Approach 2:
The patent applies different phase corrections to navigator signals recorded at different times to account for temporal variations in the base magnetic field, enabling high temporal resolution correction without sacrificing image quality
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 method enables efficient, subvolume-specific correction of phase errors and N/2 ghosts in MRI data, reducing measurement time and SAR load while maintaining high temporal resolution, without the need for additional reference measurements, thus improving image quality and reducing artifacts.
Implementation Method 1
The magnetic resonance technique (hereinafter the abbreviation MR stands for magnetic resonance) is a known technique with which images of the inside of an examination object can be generated. In simple terms, the examination object is positioned in a magnetic resonance device in a comparatively strong static, homogeneous base magnetic field, also called a B0 field
Implementation Method 2
For spatial encoding of the measurement data, fast-switched magnetic gradient fields are superimposed on the base magnetic field
Implementation Method 3
One of the fastest known MR recording techniques is echo planar imaging (EPI), in which, after an RF excitation pulse, an oscillating, in other words bipolar, readout gradient is used, which refocuses the transverse magnetization every time the direction of polarization of the gradient is changed until it allows the T2* decay and thereby generates a gradient echo
Implementation Method 4
For the correction of such N/2-ghosts it is known, for example from the U.S. Pat. No. 6,043,651, to record three navigator signals by switching a bipolar readout gradient with which a correction of phase shifts of the zero and first orders between gradient echoes recorded with different polarity can be carried out in the readout direction
Implementation Method 5
From the k-space matrix filled with values, an associated MR image can be reconstructed, for example by means of a multi-dimensional Fourier transform
Data Source
AI summary
In a method for simultaneous generation of measurement data of at least two subvolumes of an examination object by means of a slice multiplexing EPI-method, after an RF excitation pulse, at least three navigator signals, but a total of at least one navigator signal per possible polarity and per subvolume to be simultaneously recorded, are recorded in the absence of phase encoding gradients. From the recorded navigator signals, subvolume-specific correction data is determined, which can be used in a reconstruction of image data from acquired raw data for correcting shifts caused by phase errors in the MR raw data.