MRI Velocity-Selective Pulse Trains with Refocusing and Phase Cycling
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Solution Overview
Problem
Existing velocity-selective pulse trains in MRI are susceptible to B0 and B1 field inhomogeneity, leading to inaccurate excitation profiles and degraded performance, especially at high field strengths and off-resonance conditions.
Innovation Solution
Incorporating a pair of refocusing pulses, such as adiabatic pulses, within each velocity encoding step of the pulse train and applying phase cycling, to enhance robustness against B0/B1 inhomogeneity and eddy currents, with specific configurations like MLEV-8 and MLEV-16 phase cycling schemes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single composite refocusing pulse is incorporated in each velocity encoding step, then B0 field inhomogeneity susceptibility is alleviated, but the tolerable B0 offset is limited to ±80 Hz and B1 inhomogeneity sensitivity remains an issue
Solution Approach 1:
The single refocusing pulse is divided into a pair of refocusing pulses (first and second refocusing pulses) with different phase cycling patterns. This segmentation allows each pulse to handle specific aspects of B0 and B1 inhomogeneity, collectively extending the tolerable B0 offset range beyond the ±80 Hz limitation of a single pulse while maintaining robustness.
Solution Approach 2:
Phase cycling is applied periodically to the refocusing pulses, where the phase of the first refocusing pulse is cycled through multiple values (e.g., 0, 90, 180, 270 degrees) and the second refocusing pulse is cycled with a different pattern. This periodic phase variation compensates for B1 inhomogeneity and extends B0 offset tolerance by averaging out errors across multiple excitations.
2Adaptability or versatility
If non-selective RF pulse trains with embedded velocity-encoding gradients are used, then arbitrary velocity-selective profiles can be produced, but off-resonance effects cause excitation profile shifting along velocity
Solution Approach 1:
The phase cycling scheme acts as a feedback mechanism that compensates for off-resonance effects. By cycling the phases of refocusing pulses and selectively combining signals, the system identifies and corrects for excitation profile shifts, maintaining accurate velocity selection despite B0 inhomogeneity and off-resonance conditions.
Solution Approach 2:
The invention changes the phase parameters of refocusing pulses dynamically through phase cycling. This parameter variation allows the system to compensate for frequency offsets and maintain accurate velocity-selective profiles across a wider range of B0 conditions without sacrificing the flexibility of arbitrary profile design.
3Reliability
If phase cycling is applied to refocusing pulses, then robustness against B1 inhomogeneity is improved, but the sequence complexity increases
Solution Approach 1:
The phase cycling is segmented into systematic patterns (e.g., MLEV-8, MLEV-16 schemes) where phases are cycled through predetermined sequences. This segmentation makes the complexity manageable by organizing it into repeatable modules that can be implemented efficiently on standard MRI systems without requiring complex real-time control algorithms.
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 proposed pulse trains maintain accurate velocity-selective profiles across varying B0/B1 conditions, improving the fidelity of MRI images by reducing distortions and enhancing sensitivity, as demonstrated by improved Mz-velocity responses and MR angiography results.
Implementation Method 1
Magnetic Resonance Imaging (MRI) based hemodynamic evaluation
Implementation Method 2
The pair of refocusing pulses can take the form of a pair of adiabatic pulses, which can be rectangular, composite, or tanh/tan adiabatic pulses
Implementation Method 3
The combination of non-selective RF pulse trains with embedded velocity-encoding gradients, based on the Fourier-transform, can produce almost arbitrary velocity-selective profiles
Implementation Method 4
Fourier-transform based VS magnetization-prepared MRA has been introduced for visualization of vessels based on the designated flow velocity
Implementation Method 5
applying phase cycling for the refocusing pulses in the velocity-selective pulse train
Data Source
AI summary
The present invention is directed to a system and method for magnetic resonance imaging including an extended Fourier transform-based velocity-selective pulse train design with a pair of refocusing pulses within each velocity encoding step and accompanying phase cycling between different velocity encoding steps. The present invention is robust to B0/B1 field inhomogeneity and eddy current effects. The utility of this technique, through a velocity-selective inversion pulse, is demonstrated in a 2D velocity-selective arterials spin labeling study, which shows a reasonable agreement in CBF quantification with the standard PCASL method.