Offset RF Pulse for MRI Susceptibility Artifacts
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Solution Overview
Problem
High field MRI systems face challenges in reducing image artifacts caused by magnetic susceptibility differences near tissue/air interfaces, leading to asymmetric Larmor frequency distributions and inadequate RF preparatory pulse coverage.
Innovation Solution
A two-stage pulse sequence is employed with a non-selective RF preparatory pulse having a center frequency shifted relative to the nominal Larmor frequency, optimizing inversion coverage without increasing RF power or Specific Absorption Rate (SAR) penalties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a standard RF preparatory pulse at nominal Larmor frequency is used, then the pulse sequence is simple and RF power is minimized, but image artifacts increase due to inadequate coverage of asymmetric frequency distributions
Solution Approach 1:
The patent applies parameter changes by shifting the center frequency of the RF preparatory pulse from the nominal Larmor frequency to an offset frequency. This frequency shift parameter change enables the pulse to cover asymmetric frequency distributions more effectively, reducing image artifacts while maintaining pulse sequence simplicity and avoiding increased RF power requirements
2Reliability
If the RF preparatory pulse bandwidth is increased to cover asymmetric frequency distributions, then artifact coverage improves, but RF power and SAR increase
Solution Approach 1:
Instead of increasing bandwidth, the patent changes the center frequency parameter of the RF preparatory pulse. This frequency offset parameter allows the existing bandwidth to cover asymmetric frequency distributions more effectively, achieving improved artifact reduction without increasing RF power or SAR
Solution Approach 2:
The patent addresses the coverage problem by moving from a bandwidth dimension solution to a frequency offset dimension solution. By shifting the center frequency rather than expanding bandwidth, the method achieves better frequency distribution coverage while avoiding the energy costs associated with broader bandwidth pulses
3Measurement precision
If high field strength (3T or higher) is used to increase signal to noise ratio, then spatial and spectral resolution improve, but SAR limits are more easily exceeded
Solution Approach 1:
The patent applies parameter changes to the RF preparatory pulse frequency, which improves frequency distribution coverage and artifact reduction at high field strengths. This parameter optimization allows high field MRI to achieve better image quality without proportionally increasing SAR, as the frequency offset approach is more efficient than bandwidth expansion
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 approach results in MR images with fewer artifacts by effectively addressing magnetic field inhomogeneities, improving image quality without incurring SAR or RF power penalties, especially at higher magnetic fields.
Implementation Method 1
subjecting the object to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency such that the net aligned moment, Mz, may be rotated, or 'tipped', into the x-y plane to produce a net transverse magnetic moment Mt
Implementation Method 2
the nucleus precesses around this direction at a characteristic frequency that is termed the Larmor frequency, f0
Implementation Method 3
The amplitude, A, of the emitted NMR signal decays in an exponential fashion with time, t. The decay constant 1/T*2 depends on the homogeneity of the magnetic field and on T2, which is referred to as the 'spin-spin relaxation' constant
Implementation Method 4
After the excitation signal B1 (RF excitation pulse) is terminated, a nuclear magnetic resonance (NMR) signal is emitted by the excited spins and this signal is detected
Implementation Method 5
it is necessary to elicit NMR signals from specific locations in the subject, which is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes
Data Source
AI summary
A method for magnetic resonance imaging includes performing a preparatory stage of a MR pulse sequence with an MRI system in which a non-selective RF preparatory pulse is used having a bandwidth such that any spin species having corresponding Larmor frequencies within that bandwidth are affected and the bandwidth is centered at a selected frequency which is offset from a nominal Larmor frequency of the desired spin species being imaged. A time period (TI) elapses during which longitudinal spin magnetization recovers; and then an imaging stage is performed in which an RF excitation pulse is generated to produce transverse spin magnetization of the desired spin species, and in which a set of NMR signals are acquired. An image is reconstructed using the acquired set of NMR signals, and the reconstructed image has reduced artifacts due to B0 field inhomogeneities caused by magnetic susceptibility effects.