Frequency Swept RF Excitation for Fast MRI Acquisition
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Solution Overview
Problem
Current magnetic resonance techniques, such as CW NMR and pulsed FT MRI, are limited by slow acquisition rates, making them impractical for in vivo and clinical applications, especially when imaging objects with broad distribution of relaxation times or fast relaxing spins, due to limitations in RF power and gradient slew rates.
Innovation Solution
The frequency swept excitation method, which uses a series of pulses with alternating excitation and quiescent segments, allowing for simultaneous or time-shared excitation and acquisition, enabling the acquisition of a time domain signal that is processed to correct for spin system spectrum, thus overcoming the limitations of conventional techniques by reducing echo time and improving sensitivity and resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If continuous wave (CW) technique is used with slow sweep rate to maintain steady state and avoid saturation, then signal saturation is avoided, but acquisition time becomes excessively long and impractical for clinical applications
Solution Approach 1:
The RF excitation pulse is segmented into multiple frequency components that are applied simultaneously rather than sequentially. This segmentation in frequency domain allows parallel excitation of multiple resonances, achieving fast acquisition without sacrificing signal stability.
Solution Approach 2:
The patent uses periodic pulse sequences with specific timing patterns to excite and acquire signals. The periodic structure allows for controlled saturation and recovery periods, maintaining signal stability while enabling faster overall acquisition compared to continuous slow sweeping.
2Productivity
If pulsed Fourier Transform (FT) spectroscopy uses short intense pulses to excite multiple resonances simultaneously, then acquisition speed and sensitivity are improved, but the range of excitation frequencies is limited and RF power requirements increase
Solution Approach 1:
The patent changes the frequency parameter of the RF pulse continuously across a broad range during the pulse duration. This frequency modulation allows a single pulse to excite multiple resonances across a wide frequency range, combining the speed of pulsed methods with the versatility of CW frequency sweeping.
Solution Approach 2:
The frequency-swept pulse design makes the RF excitation system universal by enabling it to excite multiple different resonances with a single pulse type. The pulse can be tuned to cover broad frequency ranges, making it adaptable to various spin systems and relaxation times without requiring multiple specialized pulses.
3Measurement precision
If fast gradient switching is used to reduce echo time and improve sensitivity for fast relaxing spins, then signal loss due to relaxation is reduced, but gradient slew rate limitations and hardware constraints are encountered
Solution Approach 1:
The frequency-swept excitation is designed to complete the excitation process before significant relaxation occurs. By pre-planning the frequency sweep to cover the required range within a short duration, the method achieves adequate excitation without requiring extremely fast gradient switching, thus reducing hardware complexity.
Solution Approach 2:
The patent replaces reliance on fast mechanical gradient switching with a frequency-modulated RF approach. Instead of using rapid gradient changes to achieve selective excitation and short echo times, the method uses frequency-swept RF pulses that naturally achieve broad excitation without demanding extreme gradient performance.
4Reliability
If stochastic NMR uses small flip angles to maintain linear region and avoid saturation, then signal artifacts are reduced, but signal amplitude and sensitivity become low
Solution Approach 1:
The frequency-swept pulse maintains continuous excitation across the frequency range without interruption or saturation. The pulse continuously sweeps through frequencies, constantly exciting new resonances while maintaining linear response, thereby preserving both signal linearity and adequate amplitude throughout the excitation 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 method allows for fast and sensitive magnetic resonance imaging with reduced signal loss due to motion and field inhomogeneity, enabling imaging of objects with short spin-spin relaxation times and improved signal-to-noise ratios, while avoiding artifacts and peak RF power constraints.
Implementation Method 1
The frequency-swept RF excitation pulse is used to sequentially excite a plurality of isochromats having different resonant frequencies
Implementation Method 2
the response of nuclear spins in a magnetic field is observed following excitation by a radio frequency (RF) field
Implementation Method 3
The time domain signal is based on evolution of the isochromats
Implementation Method 4
the response of nuclear spins in a magnetic field is observed following excitation by a radio frequency (RF) field
Data Source
AI summary
A method of magnetic resonance is provided that uses a frequency swept excitation wherein the acquired signal is a time domain signal is provided. In one embodiment, the sweeping frequency excitation has a duration and is configured to sequentially excite isochromats having different resonant frequencies. Acquisition of the time domain signal is done during the duration of the sweeping frequency excitation. The time domain signal is based on evolution of the isochromats.