slSSFP MRI Pulse Sequence for Low SAR Imaging
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Solution Overview
Problem
Current T1ρ-weighted MRI techniques require time-consuming magnetization preparation and long delay times, leading to inefficiencies and significant signal loss, while establishing a steady-state T1ρ contrast with significant signal remains challenging due to low thermal polarization on-resonance and power constraints.
Innovation Solution
A spin locked steady-state free precession (slSSFP) pulse sequence is developed, delivering off-resonance RF pulses parallel to magnetization with adiabatic pulses, allowing continuous image acquisition and reducing power requirements, thereby improving contrast and signal-to-noise ratio while minimizing specific absorption rate (SAR).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional T1ρ pulse sequences are used to achieve T1ρ-weighted imaging, then contrast is improved, but acquisition time increases significantly due to required magnetization preparation and long delay times
Solution Approach 1:
The patent implements continuous steady-state signal acquisition during the entire scan, eliminating the need for separate magnetization preparation and delay periods. The spin-locking pulse train continuously maintains T1ρ weighting while signals are acquired throughout the imaging process, transforming discontinuous preparation-acquisition-delay cycles into continuous acquisition with persistent T1ρ contrast.
Solution Approach 2:
The patent applies preliminary spin-locking pulse trains before and during the imaging acquisition to establish and maintain T1ρ-weighted magnetization. By pre-preparing the magnetization in the rotating frame through off-resonance spin-locking pulses, the system achieves ready-to-acquire T1ρ contrast signal without requiring lengthy post-preparation delay times.
2Loss of time
If steady-state T1ρ contrast is pursued with continuous acquisition, then acquisition time is reduced, but signal loss occurs due to incomplete equilibrium restoration
Solution Approach 1:
The patent changes the RF pulse parameters by using off-resonance spin-locking pulses with specific frequency offsets and durations. This parameter modification enables the system to maintain T1ρ-weighted magnetization in a steady state without requiring complete equilibrium restoration between pulses, thereby achieving continuous acquisition with adequate signal intensity.
Solution Approach 2:
The patent employs periodic spin-locking pulse trains with carefully controlled durations and intervals. These periodic pulses continuously replenish T1ρ-weighted magnetization during the acquisition process, ensuring that signal intensity is maintained at adequate levels throughout the scan without requiring long recovery times between excitations.
3Measurement precision
If off-resonance spin locking RF pulses are delivered to achieve steady-state T1ρ contrast, then contrast is improved, but SAR increases due to continuous RF irradiation
Solution Approach 1:
The patent applies partial spin-locking pulse duration and amplitude rather than continuous maximum-power RF irradiation. By using off-resonance pulses with optimized parameters that provide sufficient T1ρ weighting without excessive energy deposition, the system achieves the desired contrast while minimizing SAR. The pulse duration and amplitude are carefully controlled to provide just enough effect for contrast generation without excessive heating.
4Reliability
If conventional bSSFP sequences are used for high signal intensity, then signal-to-noise ratio is improved, but power requirements increase significantly
Solution Approach 1:
The patent replaces the conventional bSSFP mechanism (which relies on high-power on-resonance RF pulses to maintain transverse magnetization) with an off-resonance spin-locking mechanism. This substitution allows the system to achieve comparable or superior signal intensity through a fundamentally different physical approach that requires significantly lower RF power, as the spin-locking pulses operate at reduced amplitude and frequency offset from resonance.
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 slSSFP sequence achieves high signal-to-noise efficiency with significantly lower power than conventional balanced steady-state free precession (bSSFP) sequences, reducing image acquisition time and SAR, particularly beneficial for high-field imaging systems.
Implementation Method 1
In the presence of radiofrequency (RF) irradiation, spin magnetization has different relaxation properties from T1 and T2. In the rotating reference frame, the components of the magnetization parallel and perpendicular to the effective field have characteristic relaxation times T1ρ and T2ρ, respectively.
Implementation Method 2
a spin locked steady-state free precession pulse sequence (slSSFP) has been developed that has a contrast similar to bSSFP, but with significantly lower power requirements
Implementation Method 3
a single, nonselective, off-resonance spin locking pulse or adiabatic pulse of amplitude ω1 and non-instantaneous duration TSL is delivered parallel to the magnetization
Implementation Method 4
The spin locking or adiabatic pulse is followed by a short period for frequency and phase encoding
Data Source
AI summary
A spin locked balanced steady-state free precession (slSSFP) pulse sequence combines a balanced gradient echo acquisition with an off-resonance spin lock pulse for fast MRI. The transient and steady-state magnetization trajectory is solved numerically using the Bloch equations and is shown to be similar to balanced steady-state free precession (bSSFP) for a range of T2/T1 and flip angles, although the slSSFP steady-state could be maintained with considerably lower RF power. In both simulations and brain scans performed at 7T, slSSFP is shown to exhibit similar contrast and SNR efficiency to bSSFP, but with significantly lower power.