UTE MRI Refocusing Pulse Correction for Side Lobe Artifacts
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Solution Overview
Problem
In ultra-short echo time (UTE) sequences for MRI, the excitation profiles of positive and negative polarity data do not always have 180-degree inverted phase distributions in their side lobes, leading to incomplete cancellation of side lobe signals, resulting in artifacts and poor image quality due to mixed signals from unintended positions.
Innovation Solution
Adjusting the refocusing pulse of the slice gradient magnetic field to ensure that the excitation profiles of positive and negative polarity data have 180-degree inverted phase distributions in their side lobes, using measured slice gradient magnetic field waveforms to calculate correction values for refocusing pulses, irradiation frequencies, and phase differences, and applying these corrections in the pulse sequence to improve image reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a half RF pulse is used in UTE sequence with positive and negative polarity slice gradient magnetic fields, then the main lobe excitation is achieved, but the side lobe signals are not completely canceled due to phase distribution deviations
Solution Approach 1:
The patent adjusts the refocusing pulse parameters (application timing, duration, amplitude) to compensate for phase distribution deviations in the side lobe portions of the excitation profile. By changing these parameters, the phase difference between positive and negative polarity data is corrected to achieve complete side lobe cancellation while maintaining accurate main lobe excitation.
2Object-generated harmful factors
If refocusing pulses are applied to correct phase distribution, then side lobe cancellation is improved, but the pulse sequence complexity increases
Solution Approach 1:
The patent performs preliminary measurement of the actual slice gradient magnetic field waveforms before the main imaging sequence. Based on these measurements, correction values for the refocusing pulse parameters are pre-calculated and stored. During the actual imaging, these pre-determined correction values are applied, avoiding the need for complex real-time calculations and reducing pulse sequence complexity while maintaining effective side lobe cancellation.
3Measurement precision
If correction values are calculated based on measured gradient waveforms, then image quality is improved, but the measurement and calculation time increases
Solution Approach 1:
The measurement of slice gradient magnetic field waveforms and calculation of correction values are performed as a preliminary step before the actual imaging sequence. This allows the time-consuming measurement and calculation to be completed in advance, so that during the actual imaging only the application of pre-determined correction values is needed, minimizing the time impact on the main imaging process.
Solution Approach 2:
The patent uses a simplified measurement sequence that copies the essential gradient waveform characteristics without requiring full imaging data acquisition. By measuring only the gradient waveforms rather than complete images, the measurement time is significantly reduced while still obtaining sufficient information for accurate correction value calculation.
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 enables the suppression of side lobe signals and enhances image quality by ensuring accurate phase alignment and position correction, resulting in high-quality images without artifacts from unintended signal mixing.
Implementation Method 1
measures an NMR signal (echo signal) generated by an object, especially, the nuclear spins that form human tissue
Implementation Method 2
The nuclear spins that form an object precess at a frequency corresponding to the gyromagnetic ratio and the strength of the inclined magnetic field. The frequency of the precession is called a Larmor frequency
Implementation Method 3
it is possible to excite only the nuclear spins at an arbitrary position by irradiating the RF pulse that matches the Larmor frequency
Implementation Method 4
measures an NMR signal (echo signal) generated by an object, especially, the nuclear spins that form human tissue
Data Source
AI summary
In order to obtain a high-quality image even in multi-slice imaging in a UTE sequence that uses a half RF pulse, a refocusing pulse of the slice gradient magnetic field is adjusted and applied so that the excitation profiles of positive polarity data and negative polarity data have phase distributions that are 180 [deg] inverted with respect to each other in side lobe portions. In addition, the irradiation frequency of the half RF pulse is adjusted so as to eliminate a position shift between the intensity distributions of the positive polarity data and the negative polarity data.