MRI Phase Compensation via Off-Resonance RF Pulses
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Solution Overview
Problem
Magnetic resonance imaging (MRI) using off-resonance RF pulses faces challenges in improving contrast and image quality due to phase shifts and artifacts, particularly with non-uniform B1 intensity caused by strong static magnetic fields.
Innovation Solution
A magnetic resonance imaging apparatus and method that applies a first off-resonance RF pulse to generate a phase shift in magnetic resonance signals, followed by a second off-resonance RF pulse to compensate this phase shift, enhancing image quality and contrast by adjusting off-resonance frequencies and flip angles within the SSFP sequence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If an off-resonance RF pulse is applied to generate phase shift for contrast enhancement, then image contrast is improved, but phase artifacts and image quality deteriorate
Solution Approach 1:
A phase compensation RF pulse is applied after the off-resonance RF pulse to pre-compensate and cancel the phase shift artifacts before image reconstruction, thereby maintaining both contrast enhancement and image quality
2Illumination intensity
If a gradient magnetic field spoiler pulse is applied to spoil transverse magnetization signals, then contrast is enhanced, but signal loss and image quality deteriorate
Solution Approach 1:
The gradient magnetic field spoiler pulse is removed from the pulse sequence, extracting the harmful element that caused signal loss while preserving the contrast enhancement achieved through off-resonance RF pulsing
3Measurement precision
If high static magnetic field intensity is used to improve resolution, then image resolution is improved, but B1 intensity becomes non-uniform due to electric and dielectric losses
Solution Approach 1:
Off-resonance RF pulsing with phase compensation is used to measure and correct B1 intensity non-uniformity, allowing high field operation while maintaining signal uniformity through parameter adjustment and compensation
4Productivity
If fast imaging sequences are used to reduce scan time, then productivity is improved, but image quality and contrast may deteriorate
Solution Approach 1:
The phase compensation RF pulse is continuously applied in each TR period of fast imaging sequences, maintaining contrast enhancement and image quality throughout the accelerated imaging process
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 effectively cancels phase shifts within a TR, improving image quality and contrast, allowing for high-speed imaging without degrading image quality, and enabling efficient measurement of B1 intensity distribution.
Implementation Method 1
MRI is an imaging method which magnetically excites nuclear spin of an object set in a static magnetic field with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation
Implementation Method 2
an off-resonance RF pulse having a carrier frequency which is different from the resonant frequency by about several kHz is applied to an object
Implementation Method 3
The excitation for slice selection is carried out by applying a gradient magnetic field for SS (slice selection)
Implementation Method 4
the measurement of B1 intensity using the off-resonance RF pulse uses the Bloch-Siegert shift. The Bloch-Siegert shift is the shift in the resonant frequency which arises with a constant rate when MR signals are acquired with applying an RF signal having a frequency close to the observed frequency
Data Source
AI summary
According to one embodiment, a magnetic resonance imaging apparatus includes a data acquiring unit and a data processing unit. The data acquiring unit is configured to acquire magnetic resonance signals according to an imaging condition for applying a first off-resonance radio frequency pulse after an application of an excitation pulse and before a readout of the magnetic resonance signals, and applying a second off-resonance radio frequency pulse after the readout of the magnetic resonance signals and before an application of a following excitation pulse. The first off-resonance radio frequency pulse generates a phase shift in the magnetic resonance signals. The second off-resonance radio frequency pulse compensates the phase shift. The data processing unit is configured to obtain information to be obtained by data processing of the magnetic resonance signals.