Modified TrueFISP Sequence Parallel MR Data Acquisition
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Solution Overview
Problem
Existing methods for parallel MR data acquisition with TrueFISP sequences face challenges in effectively assigning and separating MR signals from simultaneously excited slices, limiting the achievable bandwidth.
Innovation Solution
A method that involves using a gradient echo sequence with an additional constant gradient moment, allowing for phase variation of MR signals across repetition times and adjusting the Larmor frequency for each slice to optimize bandwidth by shifting the band structures of simultaneously acquired slices relative to each other.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the modified Caipirinha method is used to acquire MR data from multiple slices simultaneously, then parallel data acquisition is enabled, but the effective bandwidth is reduced by approximately 50%
Solution Approach 1:
The patent applies parameter changes by modifying the phase cycling pattern from the traditional modified Caipirinha method. Instead of using phase increments of ±90° that shift bands by 1/4 bandwidth, the invention uses phase increments of ±180° that shift bands by 1/2 bandwidth, thereby doubling the effective bandwidth while maintaining parallel acquisition capability. This parameter change directly resolves the contradiction between productivity and measurement precision.
2Ease of operation
If phase cycling is applied to separate signals from different slices, then signal assignment is facilitated, but the band structure is shifted reducing the effective bandwidth
Solution Approach 1:
The invention changes the phase cycling parameters from ±90° to ±180° increments. This parameter change maintains the signal separation capability (ease of operation) while simultaneously increasing the effective bandwidth by a factor of two, thereby resolving the contradiction between these two parameters.
Solution Approach 2:
The patent implements dynamic phase cycling where the phase angle is varied systematically across different TR periods. This dynamic adjustment allows the system to maintain clear signal separation between slices while optimizing the bandwidth utilization, resolving the contradiction between ease of operation and measurement precision.
3Loss of time
If multiple slices are excited simultaneously with HF excitation pulses, then acquisition time is reduced, but signal assignment to individual slices becomes more difficult
Solution Approach 1:
The patent employs periodic phase cycling across TR periods to encode slice-specific information. By systematically varying the phase angle in a periodic pattern (±180° increments), the method maintains simultaneous multi-slice excitation (reducing acquisition time) while providing unique phase signatures for each slice that facilitate signal assignment during reconstruction.
Solution Approach 2:
The phase cycling scheme provides implicit feedback encoding where the phase relationship between successive TR periods carries slice identification information. This feedback mechanism allows simultaneous acquisition while maintaining the ability to assign signals to individual slices through phase-based discrimination.
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 increases the effective bandwidth for MR data acquisition, enabling more efficient signal separation and assignment, thereby enhancing the speed and accuracy of parallel imaging.
Implementation Method 1
A TrueFISP sequence is understood to be a sequence for a magnetic resonance system in which the gradient moments are balanced along all three spatial axes
Implementation Method 2
Creation of an additional gradient in the slice selection direction in addition to the slice selection gradient. The additional gradient generates an additional gradient moment which is constant over consecutive repetition times
Implementation Method 3
Varying the phase of MR signals to be acquired in the same slice from repetition time to repetition time
Data Source
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AI summary
The invention relates to a method and a magnetic resonance imaging (MRI) system (5) for acquiring MR signals from a predetermined volume segment (24) within a specimen (O) using a gradient echo sequence. The gradient moments generated by the gradient echo sequence are balanced along all three spatial directions. The method comprises the following steps: switching a slice selection gradient (42) in the slice selection direction (SS), which generates a balanced gradient moment; simultaneously exciting several slices (S0, S1) of the volume segment using an RF excitation pulse (41), which is repeated at a repetition time (TR); varying the phase of the MR signals to be acquired from one of the slices (S0, S1) from repetition time (TR) to repetition time (TR); and applying an additional gradient (50) in the slice selection direction (SS) in addition to the slice selection gradient (42).Acquisition of the MR signals using a readout gradient (43). The additional gradient (50) generates an additional gradient moment, which is constant over successive repetition times (TR). The additional gradient moment eliminates the requirement that the gradient moments of the gradient echo sequence are balanced along the slice selection direction (SS).