SSFP Imaging Spectral Selectivity via Weighted Combination
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Solution Overview
Problem
Existing MRI techniques using steady-state free precession (SSFP) struggle with robust fat suppression due to broad stop-bands, leading to residual fat signals comparable to water signals, especially in applications with moderate to large resonant frequency variations.
Innovation Solution
A weighted combination method for SSFP images is applied, using RF excitation pulses with different phase progressions and magnetic gradients to establish spectrally dependent steady-state magnetization, adjusting the trade-off between selective spectral suppression and signal-to-noise ratio (SNR) through a control parameter, effectively reducing stop-band artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If broad stop-bands are used for fat suppression in SSFP, then a wide range of frequencies can be suppressed, but the suppression effectiveness deteriorates with resonant frequency variations leading to residual fat signals
Solution Approach 1:
The patent divides the frequency suppression task into multiple narrow stop-bands instead of using a single broad stop-band. By segmenting the frequency response into multiple targeted suppression bands, each narrow stop-band can be precisely positioned at specific resonant frequencies (e.g., fat frequency), maintaining suppression effectiveness even when frequencies vary. This segmentation approach allows the system to achieve both wide overall coverage through multiple segments while keeping each segment narrow and precise.
Solution Approach 2:
The patent applies local quality by creating highly selective, localized suppression at specific frequency points rather than uniform broad suppression. Each narrow stop-band is tailored to target specific frequency ranges where fat or water resonances occur, providing localized spectral suppression with high precision. This allows the frequency response to maintain water signal integrity while selectively suppressing fat signals at their specific resonant frequencies.
2Measurement precision
If narrow stop-bands are used for selective frequency suppression, then suppression precision is improved, but the range of suppressible frequencies is reduced
Solution Approach 1:
The patent merges multiple narrow stop-bands into a composite frequency response that collectively covers a wide frequency range. By combining several narrowly targeted suppression bands at different frequency positions (e.g., at fat frequency, water frequency, and intermediate frequencies), the system achieves both the precision of narrow bands and the broad coverage of wide bands. The merged frequency response creates multiple discrete suppression points that together address the full spectrum of resonant frequencies.
Solution Approach 2:
The patent extends the frequency suppression approach by utilizing the temporal dimension through periodic flip angle variations. Instead of relying solely on spatial frequency filtering, the system modulates the RF excitation in the time domain with periodic variations that create additional spectral nulls. This temporal modulation adds another dimension to frequency control, enabling precise suppression at multiple frequency points while maintaining overall wideband coverage through the periodic nature of the modulation.
3Measurement precision
If periodic flip angle variations are applied for fat suppression, then spectral selectivity is improved, but signal-to-noise ratio deteriorates
Solution Approach 1:
The patent applies partial periodic flip angle variations rather than continuous full-period variations. By using partial modulation depths and selective application of flip angle variations only during specific portions of the TR cycle, the system achieves sufficient spectral selectivity to create the necessary stop-bands while minimizing the overall impact on signal intensity. This partial action approach provides just enough modulation to create frequency selectivity without excessively reducing the available signal for image formation.
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
The method achieves improved fat suppression robustness without affecting the pass-band, expanding the range of flip angles and T1/T2 ratios for effective fat or water suppression, even in the presence of large off-resonant frequency variations and higher resolutions.
Implementation Method 1
nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field
Implementation Method 2
A sequence of RF excitation pulses with a certain phase progression is applied at a repetition rate to give an SSFP image, in which a spectrally dependent steady-state magnetization is established
Implementation Method 3
Magnetic gradients are applied between said RF pulses
Implementation Method 4
The plurality of SSFP images is combined using a weighted combination in which the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR)
Data Source
AI summary
A method of collecting image data with selective spectral suppression for at least two species is provided. A sequence of RF excitation pulses is repeatedly applied, whereby a repeated sequence of at least two substantially different spectrally selective steady-state magnetizations is established. Magnetic gradients are applied between said RF pulses. A plurality of magnetic resonance image (MRI) signals is acquired. The plurality of MRI signals is combined using a weighted combination where the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR).