Spectrally Designed MRI Pulses for B0 Inhomogeneity
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Solution Overview
Problem
Conventional MRI systems using balanced steady-state free precession (bSSFP) imaging suffer from significant signal loss and off-resonance artifacts due to B0 inhomogeneity, leading to undesirable dark bands and limited application, as well as difficulties in inserting magnetization preparation pulses to alter image contrast.
Innovation Solution
A method involving spectrally or spectrally-spatially designed RF pulses is applied to rotate spins from a longitudinal axis to a transverse plane and back, rapidly reestablishing longitudinal magnetization, thereby capturing more NMR signals and reducing signal loss, while being less sensitive to motion and compatible with magnetization preparation pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If balanced steady-state free precession (bSSFP) imaging is used, then fast imaging with good signal levels is achieved, but significant signal loss and off-resonance artifacts occur due to B0 inhomogeneity
Solution Approach 1:
The patent changes the fundamental parameters of the pulse sequence by using non-bipolar gradients and designing RF pulses with specific spectral properties tailored to the free precession interval. This transforms the sequence from bSSFP to a new type that is inherently less sensitive to B0 inhomogeneity while maintaining fast imaging capabilities
Solution Approach 2:
The patent applies magnetization preparation pulses (such as fat saturation or inversion recovery pulses) before the free precession interval to pre-establish desired magnetization states. This preliminary action allows for contrast modification without disrupting the steady state, solving the problem of inserting preparation pulses in conventional bSSFP
2Productivity
If bSSFP imaging is used, then fast imaging is achieved, but dark bands appear in the image due to signal loss at odd integer multiples of one-half inverse TR
Solution Approach 1:
The patent modifies the pulse sequence parameters by using non-bipolar gradient schemes and spectrally designed RF pulses that are insensitive to the frequency offsets causing banding artifacts. This parameter change eliminates dark bands while preserving fast imaging capabilities
Solution Approach 2:
The patent converts the harmful effect of B0 inhomogeneity that causes banding artifacts into a benefit by designing a sequence that deliberately exploits free precession with non-bipolar gradients, making the sequence inherently robust to frequency variations and eliminating the need for aggressive bandwidth increases
3Adaptability or versatility
If conventional RF pulses are used in bSSFP, then imaging is performed, but difficulty arises in inserting magnetization preparation pulses to alter image contrast
Solution Approach 1:
The patent structures the sequence to accept magnetization preparation pulses as preliminary actions before the free precession interval. This design allows fat saturation, inversion recovery, or other preparation techniques to be inserted without disrupting the steady state, enabling versatile contrast control
Solution Approach 2:
The patent creates a universal pulse sequence framework that can accommodate multiple types of magnetization preparation pulses (fat saturation, inversion recovery, magnetization transfer) while maintaining the same imaging mechanism. This multi-functionality allows a single sequence design to serve multiple contrast requirements
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 improves signal-to-noise ratio (SNR) efficiency, reduces banding artifacts, and allows for more robust imaging with comparable signal levels and tissue contrast to bSSFP, while being less sensitive to motion and compatible with magnetization preparation pulses, thus enhancing scan quality and reducing the need for additional scans.
Implementation Method 1
A magnetic field is applied to the object to align the spins along a longitudinal axis
Implementation Method 2
A first pulse is applied to the spins for rotating the spins from the longitudinal axis toward a transverse plane
Implementation Method 3
Image data is acquired from the spins during a free precession interval in which the spins precess in the transverse plane
Implementation Method 4
the spins induce a detectable nuclear magnetic resonance (NMR) signal
Implementation Method 5
A second pulse is applied to the spins for rotating the spins from the transverse plane to at least substantially along the longitudinal axis
Data Source
AI summary
A system and method for acquiring image data from an object that includes a plurality of spins is described. A magnetic field is applied to the object to align the spins along a longitudinal axis. A first pulse is applied to the spins for rotating the spins from the longitudinal axis toward a transverse plane. Image data is acquired from the spins during a free precession interval in which the spins precess in the transverse plane. A second pulse is applied to the spins for rotating the spins from the transverse plane to at least substantially along the longitudinal axis. At least one of the first and second pulses is spectrally or spectrally-spatially designed.