APEX MRI Pulse Sequence for Outer Volume Suppression
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Solution Overview
Problem
Current outer volume suppression (OVS) techniques in magnetic resonance imaging (MRI) often result in long durations, high specific absorption rate (SAR), and inflexible selectivity, which hinder the improvement of image quality and reduction of scan time.
Innovation Solution
The APEX (Alternating Pairs of Excitations) sequence combines principles from two OVS paradigms, utilizing spatially selective tip-up pulses for signal suppression and repetition for robustness, achieving a customizable, sharp, and uniform passband with lower SAR and shorter duration, incorporating T2-preparation for enhanced contrast and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional OVS techniques are used, then signal suppression outside ROI is achieved, but scan time increases and SAR increases
Solution Approach 1:
The OVS pulse sequence is divided into multiple individual pulses (e.g., four 90-degree pulses) applied at different locations around the ROI. Each pulse suppresses signal from a specific direction, and their combined effect achieves comprehensive outer volume suppression without requiring a single long-duration pulse, thereby reducing total scan time.
Solution Approach 2:
The sequence uses periodic application of short-duration RF pulses instead of continuous or long-duration pulsing. By applying multiple brief pulses in succession with appropriate timing, the method achieves effective signal suppression while minimizing the total time the RF system is active, thus reducing SAR and scan time.
2Object-affected harmful factors
If conventional OVS techniques are used, then signal suppression outside ROI is achieved, but specific absorption rate increases
Solution Approach 1:
The total suppression task is segmented into multiple low-power pulses rather than one high-power pulse. Each 90-degree pulse delivers less energy individually, and their cumulative effect achieves the same suppression level as conventional methods would require from a single high-SAR pulse, thereby reducing peak and average SAR.
Solution Approach 2:
The sequence allows magnetization to recover between pulses by spacing them appropriately in time. This recovery period reduces the cumulative energy deposition in tissues compared to continuous pulsing, lowering the overall SAR while maintaining suppression effectiveness through the periodic re-application of suppression pulses.
3Area of stationary object
If conventional OVS techniques are used, then FOV reduction is achieved, but selectivity flexibility is reduced
Solution Approach 1:
The pulse sequence is designed to be dynamically configurable, allowing the operator to adjust the number, strength, and spatial positioning of individual suppression pulses. This dynamic adaptability enables the same basic sequence structure to accommodate various ROI shapes, sizes, and locations, providing flexible selectivity regardless of the desired FOV reduction pattern.
Solution Approach 2:
The OVS sequence is designed as a universal platform that can suppress signal from any direction or combination of directions by simply repositioning and reconfiguring the individual pulses. This multi-functional design allows the same sequence to adapt to different anatomical regions and imaging protocols, maintaining selectivity flexibility across diverse applications.
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
APEX reduces scan time, improves image quality by suppressing unwanted signals, and provides robustness to field inhomogeneities, resulting in higher signal-to-noise ratio and reduced patient exposure to radiofrequency energy, while maintaining clinically acceptable images.
Implementation Method 1
magnetic resonance imaging (MRI)
Implementation Method 2
a first tip-down excitation pulse, a first refocusing excitation pulse, a first tip-up excitation pulse
Implementation Method 3
a first refocusing excitation pulse
Data Source
AI summary
A magnetic resonance imaging (MRI) techniques uses a T2-preparation outer volume suppression (OVS) pulse sequence to reduce the longitudinal magnetization outside a region of interest. A region is excited that includes the region of interest, radiofrequency (RF) signals are detected, and MRI images generated from the RF detected signals. The T2-preparation OVS pulse sequence includes, sequentially: a first tip-down excitation pulse, a first refocusing excitation pulse, a first tip-up excitation pulse that is selective spatially and/or spectrally, a second tip-down excitation pulse that is 180° out of phase with respect to the first tip-down excitation pulse, a second refocusing excitation pulse, and a second tip-up excitation pulse that is selective spatially and/or spectrally. Alternatively, the first tip-down excitation pulse is selective spatially and/or spectrally instead of the first tip-up excitation pulse, and the second tip-down excitation pulse is selective spatially and/or spectrally instead of the second tip-up excitation pulse.