Orthogonal Propeller EPI for MRI Distortion Reduction
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Solution Overview
Problem
Magnetic resonance imaging (MRI) techniques like single-shot echo-planar imaging (ssEPI) suffer from geometric distortions, Nyquist ghosting, T*2 blurring, and Maxwell effects, especially in diffusion-weighted imaging (DWI) and functional MRI (fMRI), due to rapid k-space traversal, which are exacerbated by patient motion and off-resonance sensitivity.
Innovation Solution
The proposed orthogonal propeller EPI design reduces echo spacing by applying the readout direction along the propeller short-axis and phase encoding along the long-axis, decoupling geometric distortions from target resolution, and employs parallel imaging and reversed gradient polarity methods to correct remaining distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If single-shot echo-planar imaging (ssEPI) is used for rapid acquisition, then imaging speed is improved, but geometric distortions and artifacts increase
Solution Approach 1:
The patent inverts the conventional EPI readout trajectory by using a propeller-shaped sampling pattern that rotates through k-space instead of the traditional linear phase-encoding approach. This inversion of the sampling strategy allows rapid acquisition while the rotating propeller blades continuously oversample the center of k-space, providing reference information for correcting geometric distortions and off-resonance effects that plague conventional ssEPI.
Solution Approach 2:
The patent changes the sampling parameters by using variable density sampling within the propeller blades, with higher sampling density at the center of k-space and lower density at the periphery. This parameter change allows the method to maintain high imaging speed while acquiring sufficient reference information for distortion correction, effectively decoupling the speed-distortion tradeoff present in conventional EPI.
2Reliability
If propeller k-space sampling is used with RF-refocused readout, then motion correction capability is improved, but specific absorption rate (SAR) increases
Solution Approach 1:
The patent extracts the essential motion correction capability from the RF-refocused propeller method by using a non-RF-refocused readout approach. Instead of relying on RF refocusing pulses to achieve motion robustness, the invention uses the oversampled center of k-space acquired during propeller rotation to generate navigation information and correction fields, thereby achieving motion correction without the high SAR penalty of repeated RF refocusing.
Solution Approach 2:
The patent introduces an intermediary approach by using the oversampled center region of k-space as a mediator between the propeller readout and the final image reconstruction. This central region serves as a reference that enables motion detection and correction without requiring additional RF refocusing pulses, thus reducing SAR while maintaining motion correction capability.
3Productivity
If standard ssEPI is used, then acquisition speed is high, but off-resonance sensitivity causes geometric distortions
Solution Approach 1:
The patent applies preliminary action by acquiring multiple propeller blades that oversample the center of k-space before final image reconstruction. This preliminary sampling of the k-space center provides reference information about off-resonance effects and magnetic field inhomogeneities, which are then used to correct geometric distortions in the final image, enabling fast acquisition without sacrificing precision.
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 design increases imaging speed, reduces geometric and radial blurring, and minimizes signal drop-out, providing improved image quality and motion correction while maintaining low SAR levels, especially at higher fields.
Implementation Method 1
applying the readout direction along the propeller short-axis and the phase encoding direction along the long-axis of the propeller blades
Implementation Method 2
Single-shot echo-planar imaging (ssEPI) has a leading role in many MR applications
Data Source
AI summary
Disclosed is a new propeller EPI pulse sequence with reduced sensitivity to field inhomogeneities is proposed. Image artifacts such as blurring due to Nyquist ghosting and susceptibility gradients are investigated and compared with those obtained in previous propeller EPI studies. The proposed propeller EPI sequence uses a readout that is played out along the short axis of the propeller blade, orthogonal to the readout used in previous propeller methods. In contrast to long-axis readout propeller EPI, this causes the echo spacing between two consecutive phase-encoding (PE) lines to decrease, which in turn increases the k-space velocity in this direction and hence the pseudo-bandwidth.