Multi-Channel RF Array Mitigates Off-Resonance Angle in MRI
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Solution Overview
Problem
Steady-state coherent MRI techniques, such as TrueFISP, are sensitive to B0 field variations leading to phase accumulation and off-resonance angles, resulting in signal voids and dark bands in MR images, which conventional methods like shimming and multi-dimensional pulses have not adequately mitigated, especially due to their limitations in adaptability and duration.
Innovation Solution
The use of a multi-channel transmission array to adjust the phase profile and amplitude of the RF field based on an off-resonance angle map, employing single-dimensional pulses to excite spins that would otherwise remain unexcited, and specific pulse sequences like three-pulse and two-pulse methods to compensate for phase accumulation and maintain desired tip angles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multi-dimensional pulses are used to mitigate phase accumulation, then off-resonance angle effects are reduced, but sequence duration becomes very long
Solution Approach 1:
The pulse sequence is divided into multiple individual pulses (e.g., three separate pulses) rather than using a single long multi-dimensional pulse. Each pulse performs a specific function (excitation, rephasing, refocusing) to collectively address phase accumulation while maintaining short individual pulse durations that preserve the speed advantage of steady-state coherent MRI sequences
Solution Approach 2:
The patent employs periodic RF pulses applied at regular intervals (repetition time TR) to continuously compensate for phase accumulation. This periodic application of simplified pulses achieves mitigation of banding effects without requiring the extended duration of conventional multi-dimensional pulses
2Stability of the object's composition
If conventional shimming is used to address B0 field variations, then field homogeneity is improved, but adaptability to certain B0 situations is limited
Solution Approach 1:
The patent implements dynamic compensation by applying RF pulses with spatially varying phases that are specifically tailored to counteract the measured B0 inhomogeneity pattern. This dynamic approach adapts to different B0 situations by adjusting pulse parameters based on the actual field conditions, unlike static shimming which uses fixed field adjustments
Solution Approach 2:
The patent changes the phase parameter of the RF pulses across different spatial locations to compensate for B0 variations. By modifying the phase parameter of excitation pulses based on the off-resonance angle map, the system adapts to different B0 field conditions and achieves effective mitigation across various imaging scenarios
3Reliability
If RF field amplitude and phase are adjusted based on off-resonance angle map, then spin excitation is improved in previously unexcited locations, but system complexity increases
Solution Approach 1:
The patent performs preliminary mapping of the off-resonance angle distribution across the imaging field before the actual imaging sequence. This pre-acquired map is then used to pre-calculate the required phase and amplitude adjustments for each location, allowing the system to compensate for B0 variations without adding complexity during the imaging acquisition itself
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 eliminates banding in MR images by ensuring all spins are excited, improving image quality by mitigating phase accumulation and off-resonance effects without prolonging sequence duration.
Implementation Method 1
Spins that are not excited will not produce the correct level of nuclear magnetic resonance (NMR) signal
Implementation Method 2
those spins will not experience desired excitation due to the pulse sequence. Spins that are not excited will not produce the correct level of nuclear magnetic resonance (NMR) signal from which a magnetic resonance (MR) image can be formed
Implementation Method 3
Variations in the B0 field may lead to phase accumulation in steady-state coherent MRI
Data Source
AI summary
Systems, methods, and other embodiments associated with mitigating off-resonance angle in steady-state coherent magnetic resonance imaging (MRI) are described. One example method includes accessing a B0 map and a coil sensitivity profile associated with an MRI apparatus configured to produce a steady-state coherent MRI sequence to image an object. The MRI apparatus is configured with a multi-channel transmission array having individually controllable transmission channels. The method includes computing transmission control parameters for individual transmission channels as a function of the B0 map and the coil sensitivity profile. The transmission control parameters are configured to facilitate controlling the transmission array to create a spatially varying phase profile using a single dimensional radio frequency (RF) pulse.