Multi-Echo MRI Gradient Moment Nulling for Signal Loss
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Solution Overview
Problem
Fast spin echo sequences in MRI systems face challenges with eddy current-induced field perturbations and concomitant fields, leading to signal loss due to destructive interference, which existing methods struggle to fully mitigate without prolonging echo spacing.
Innovation Solution
A method using a bipolar gradient pulse with one negative and one positive lobe in the readout direction between the RF excitation and first refocusing pulse, combined with zeroth and first moment nulled gradients after each refocusing pulse, to enhance the integral of squared gradient amplitudes, thereby minimizing signal loss while maintaining short echo spacing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If gradient polarity switching is employed to suppress eddy currents, then eddy current-induced field perturbations are reduced, but signal loss from destructive interference remains due to concomitant fields
Solution Approach 1:
The patent changes the parameter of gradient moment nulling by applying different gradient moment conditions to different parts of the sequence. Specifically, zeroth and first gradient moments are nulled for gradients after refocusing pulses, while the excitation gradient uses a bipolar pulse with specific moment properties to optimize the overall effect and minimize signal loss.
Solution Approach 2:
The patent segments the gradient application into distinct phases: an excitation gradient with specific bipolar properties, and subsequent refocusing gradients with zeroth and first moment nulling. This segmentation allows each gradient to be optimized for its specific function in the sequence.
2Reliability
If echo spacing is prolonged to enhance gradient moment nulling effects, then signal loss is reduced, but scan time is significantly increased
Solution Approach 1:
The patent optimizes the parameter of gradient moment nulling by using a bipolar excitation gradient with specific moment properties, allowing effective signal loss reduction without requiring prolonged echo spacing. This parameter optimization enables short echo spacing while maintaining gradient moment nulling effectiveness.
3Productivity
If conventional gradient pulses are used in fast spin echo sequences, then scan time is kept short, but signal loss occurs due to unbalanced gradient moments
Solution Approach 1:
The patent changes the parameter of gradient moment balancing by applying zeroth and first gradient moment nulling to refocusing gradients. This allows conventional fast spin echo sequencing to be maintained with improved signal quality, as the moment nulling compensates for unbalanced gradient effects without requiring prolonged echo spacing.
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 signal loss from eddy currents and concomitant fields, improving image quality with a higher signal-to-noise ratio and contrast, while allowing for slightly increased echo spacing without significantly prolonging the scan time.
Implementation Method 1
a first magnetic field gradient is applied in the readout direction between the RF excitation pulse and the first of the at least two RF refocusing pulses
Implementation Method 2
As each gradient ramp can create eddy currents in conducting components of an MRI scanner, this can lead to build-up of eddy current-induced field perturbations
Implementation Method 3
an RF excitation pulse is applied to the object under examination in order to generate a transverse magnetization
Data Source
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AI summary
The present invention relates to a method for acquiring MR signals of an object under examination (13) in an MR system using a multi echo imaging sequence, wherein the method comprises the steps of applying an RF excitation pulse (30) to the object under examination in order to generate a transverse magnetization, applying at least two RF refocusing pulses (31, 32) for refocusing the transverse magnetization in order to generate at least two MR spin echoes for the RF excitation pulse, applying a first magnetic field gradient (100) in a read out direction between the RF excitation pulse (30) and the first (31) of the at least two RF refocusing pulses, wherein the first magnetic field gradient (100) is a bipolar gradient having only one negative gradient lobe (100b) and only one positive gradient lobe (100c), applying a second magnetic field gradient (101) in the read out direction after each of the at least two RF refocusing pulses such that the zeroth and first gradient moment is substantially zero for the second magnetic field gradient, and acquiring the at least two MR spin echoes during the at least two second magnetic field gradients (101).