Variable Flip Angle bSSFP MRI Reducing SAR
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Solution Overview
Problem
Conventional cardiac cine imaging using balanced steady-state free precession (bSSFP) techniques face challenges in reducing specific absorption rate (SAR) while maintaining high contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR), especially in patients with impaired cardiac function and those with implanted devices, due to the dependence on flip angle and field strength.
Innovation Solution
The method involves delivering radiofrequency pulses with modulated amplitudes to maintain a dynamic steady state during MRI pulse sequences, acquiring low spatial frequencies with high flip angles and high spatial frequencies with low flip angles, and using an asynchronous k-space acquisition scheme to reduce SAR while preserving image quality and contrast.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher flip angles are used in bSSFP imaging to increase blood SNR and CNR, then signal-to-noise ratio and contrast-to-noise ratio improve, but specific absorption rate increases substantially
Solution Approach 1:
The patent applies variable flip angle (VFA) schemes where the flip angle is dynamically adjusted during the imaging sequence rather than using a constant flip angle. This allows the system to optimize signal intensity while controlling SAR accumulation by varying the RF pulse amplitude across different time points and k-space segments.
Solution Approach 2:
The patent changes the flip angle parameter throughout the imaging sequence to balance CNR and SAR. By modulating the flip angle according to specific patterns (e.g., higher angles for early segments, lower angles for later segments), the system maintains diagnostic image quality while reducing overall RF energy deposition.
2Manufacturing precision
If segmented k-space acquisitions are used for cardiac cine imaging, then spatial resolution is improved, but SAR is not reduced efficiently and image artifacts increase
Solution Approach 1:
The patent divides the k-space acquisition into multiple segments acquired across different cardiac cycles, with each segment using optimized flip angles. This segmentation allows distribution of the total RF energy over time while maintaining high spatial resolution through complete k-space sampling.
Solution Approach 2:
The patent utilizes the periodic nature of the cardiac cycle to acquire different k-space segments during successive heartbeats. By synchronizing the segmented acquisition with the cardiac rhythm and applying VFA schemes across these periodic cycles, the system achieves resolution without excessive SAR accumulation.
3Measurement precision
If higher field strengths (≥3 T) are used for imaging, then SNR and CNR are improved, but SAR increases and restricts imaging capability
Solution Approach 1:
The patent implements field-strength-optimized VFA schemes that adapt the flip angle modulation pattern to the specific SAR characteristics of different field strengths. At higher fields where SAR is more restrictive, the VFA protocol adjusts the amplitude and duration of RF pulses to maintain SNR while staying within safety limits.
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 reduces SAR by at least 10-30% compared to conventional segmented constant FA bSSFP imaging while maintaining similar CNR and SNR, enabling safer imaging of patients with implanted devices and at higher field strengths, and improving blood-myocardium CNR.
Implementation Method 1
delivering a plurality of radiofrequency (RF) pulses in an MRI pulse sequence during consecutive cycles, wherein the RF pulse amplitude is modulated to maintain a dynamic steady state
Implementation Method 2
the acquisition is structured to acquire low spatial frequencies with high flip angles and high spatial frequencies with low flip angles
Data Source
AI summary
The present invention provides methods for reducing SAR during real-time MR imaging. The method improves the safety of cine imaging, while, in certain embodiments, do not decrease image quality. The method of the invention thereby allows for the use of higher field strengths that are sometimes necessary to provide the most diagnostic information. The present invention also provides methods for improving contrast-to-noise ratio, while not increasing SAR.