EPI Slice Selection Gradient Concurrent Phase Encoding
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Solution Overview
Problem
Echo planar imaging (EPI) sequences in MRI suffer from image distortions due to magnetic field nonuniformity, which existing methods fail to fully address, particularly increasing scanning time and inaccuracy in pixel displacement correction.
Innovation Solution
Applying additional slice selection gradients simultaneously with phase encoding gradients during EPI, allowing voxel projection angles to incline and thereby correcting phase differences caused by magnetic field nonuniformity, while also increasing the effective bandwidth of the phase encoding gradient.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional EPI sequence is used for fast imaging, then scanning speed is improved, but image distortions occur due to magnetic field nonuniformity
Solution Approach 1:
The patent combines slice selection gradient application with phase encoding gradient application into a simultaneous operation. The slice selection gradient is applied during the phase encoding period rather than separately before it, merging two gradient operations into one time period. This reduces the total echo train length and minimizes the time during which magnetic field nonuniformity can cause phase errors, thereby reducing image distortions while maintaining fast imaging capability.
Solution Approach 2:
The patent applies the slice selection gradient in advance during the phase encoding period before the readout begins. By performing the slice selection operation concurrently with phase encoding rather than after, the gradient is applied during an earlier time period when less time has elapsed for magnetic field nonuniformity effects to accumulate, thereby preventing phase errors before they significantly impact image quality.
2Manufacturing precision
If re-scanning with bi-gradient echo sequence is performed to correct nonuniform field patterns, then image distortion correction is improved, but scanning time increases
Solution Approach 1:
The patent merges the slice selection gradient operation with the phase encoding gradient operation into a single concurrent operation. By applying both gradients simultaneously during the phase encoding period, the total number of gradient operations is reduced, eliminating the need for separate re-scanning operations and thereby reducing total scanning time while still achieving distortion correction.
Solution Approach 2:
The patent maintains continuous imaging operation by applying the slice selection gradient concurrently with phase encoding rather than requiring separate re-scanning steps. This continuous approach keeps the imaging process flowing without interruption for correction scans, thereby maintaining high productivity while achieving distortion correction through the modified gradient timing.
3Manufacturing precision
If effective bandwidth of phase encoding gradient is increased to reduce image distortion, then image accuracy is improved, but hardware limitations are reached
Solution Approach 1:
The patent changes the timing parameters of gradient application rather than increasing gradient amplitude or bandwidth to its maximum. By altering when the slice selection gradient is applied (concurrently with phase encoding rather than separately), the patent achieves distortion reduction through temporal parameter optimization rather than pushing hardware to its bandwidth limits, thereby avoiding hardware saturation.
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 effectively reduces image distortions by aligning voxel phase differences with ideal conditions, improving image accuracy and reducing scanning time, and allows for increased effective bandwidth within hardware limitations.
Implementation Method 1
the external magnetic field includes a main (basic) magnetic field and three orthogonal gradient magnetic fields... the gradient magnetic field along the direction of the Z axis is referred to as a slice selection (SS) gradient... the gradient magnetic field along the direction of the Y axis is referred to as the phase encoding (PE) gradient
Implementation Method 2
radiofrequency (RF) pulses of a specific frequency are employed to excite the protons in a tissue under examination, the protons absorbing certain energy to resonate; after the RF pulse emission is stopped, the excited protons gradually release the absorbed energy in the form of scanning signals
Data Source
AI summary
In a method and system for echo planar imaging, after having applied a radiofrequency pulse and a slice selection gradient, continuous readout gradients alternating between positive and negative are applied and a phase encoding gradient is applied before starting each readout gradient. A slice selection gradient is applied at the same time as applying the phase encoding gradient. Scanning signals are collected during the duration of the readout gradients. Image reconstruction is implemented based on the scanning signals to obtain a scanned image.