MR Imaging Flip Angle Adjustment for Motion Artifact Reduction
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Solution Overview
Problem
Magnetic resonance imaging (MRI) techniques, particularly spin-echo-based imaging, face challenges in reducing artifacts caused by patient movements such as pulsatile blood flow, heartbeat, and respiration, which limit the flexibility in selecting repetition times and contrast settings.
Innovation Solution
A method that applies a static magnetic field, a 90° excitation pulse, and a refocusing pulse, while detecting object movement to set the repetition time and flip angle, allowing for adaptive contrast adjustment between tissue types based on the movement, thereby reducing artifacts and enhancing image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If triggering recording with heart rate is used to reduce flow artifacts, then flow artifacts are reduced, but the repetition time is determined by patient's heart rate and is no longer freely selectable
Solution Approach 1:
The patent applies dynamics by making the flip angle adjustable and adaptable during the imaging sequence. Instead of fixing both repetition time and flip angle, the system dynamically adjusts the flip angle based on the predetermined repetition time to achieve desired tissue contrast. This resolves the contradiction by allowing freedom in repetition time selection while maintaining artifact reduction through controlled flip angle modulation.
Solution Approach 2:
The patent changes the flip angle parameter to compensate for the fixed repetition time imposed by heart rate triggering. By adjusting the flip angle, the system can achieve the desired T1-weighted contrast between different tissues even when the repetition time is constrained by the patient's heart rate, thus resolving the contradiction between artifact reduction and parameter flexibility.
2Manufacturing precision
If standard spin-echo sequence is used to achieve T1 contrast, then T1 contrast is obtained, but motion artifacts from pulsatile blood flow and patient movements interfere with image quality
Solution Approach 1:
The patent applies preliminary action by setting a predetermined repetition time before the imaging sequence begins, based on expected heart rate ranges. This allows the system to pre-calculate appropriate flip angles that will achieve the desired T1 contrast while accounting for the timing constraints imposed by heart rate triggering, thereby reducing motion artifacts before they occur.
Solution Approach 2:
The patent implements feedback by using the known repetition time (determined by heart rate triggering) to calculate and adjust the flip angle. This feedback mechanism ensures that the flip angle is optimized for the specific repetition time being used, maintaining T1 contrast quality while working within the constraints of heart rate-triggered imaging to minimize motion artifacts.
3Object-affected harmful factors
If repetition time is fixed by heart rate triggering, then flow artifacts are reduced, but the ability to optimize contrast for different tissue types is limited
Solution Approach 1:
The patent applies dynamics by making the flip angle adjustable and adaptable during the imaging sequence. Instead of fixing both repetition time and flip angle, the system dynamically adjusts the flip angle based on the predetermined repetition time to achieve desired tissue contrast. This resolves the contradiction by allowing freedom in repetition time selection while maintaining artifact reduction through controlled flip angle modulation.
Solution Approach 2:
The patent changes the flip angle parameter to compensate for the fixed repetition time imposed by heart rate triggering. By adjusting the flip angle, the system can achieve the desired T1-weighted contrast between different tissues even when the repetition time is constrained by the patient's heart rate, thus resolving the contradiction between artifact reduction and parameter flexibility.
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 enables flexible selection of T1 contrast independently of the patient's heart rate, reduces motion artifacts, and maintains optimal contrast by dynamically adjusting the flip angle in response to movement, improving the quality of MR images.
Implementation Method 1
The static magnetic field causes alignment of the dipole moments of the examination object in the field direction (z-direction). This results in externally measurable magnetization in the direction of the external static field (longitudinal magnetization).
Implementation Method 2
An alternating magnetic field with a suitable frequency, strength and duration (in the present case, also called excitation pulse or 90° excitation pulse), can tip the magnetization out of the longitudinal direction (z-direction) such that it precesses in the x-y plane
Implementation Method 3
the refocusing pulse is radiated in the middle of the time interval between the (90°) excitation pulse and the data readout (half echo time). This pulse causes synchronization of the gyroscopic motions resulting in maximum signal amplification (echo) at the point in time of the data readout (echo time TE).
Implementation Method 4
applying an RF pulse at the point in time of an echo caused by the pulses as a result of which the magnetization is deflected in the negative z-direction by a flip angle
Data Source
AI summary
High-quality magnetic resonance (MR) recordings are triggered with movements of an object, for example the heartbeat. In a method and apparatus for obtaining raw data reconstruction for an MR image, a spin-echo-based sequence is executed that includes applying a static magnetic field and applying a magnetization pulse train. A movement of the object to be imaged is detected and a target contrast for two tissue types of the object is prespecified. The repetition time of the pulse train is set in dependence on the movement of the object to be imaged, and the flip angle is set such that prespecified target contrast for the two tissue types is obtained at the set repetition time.