RF Transmit Coil Frequency Response Correction for MRI
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Solution Overview
Problem
MRI systems with lower main magnetic field strengths, asymmetric gradients, and high gradient strengths face significant transmit power reduction due to non-uniform transmit power response of RF transmit coils, especially when exciting off-center slices, leading to variable flip angles and reduced efficiency.
Innovation Solution
Computing and applying RF amplitude scaling factors based on transmit power response data to adjust RF pulse amplitudes and durations, ensuring more uniform transmit power across varying frequencies and magnetic field strengths, thereby compensating for non-uniform power responses and maintaining consistent flip angles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If RF transmit coil is designed for high efficiency at nominal resonance frequency, then power conversion efficiency is improved, but transmit power becomes non-uniform when operating at offset frequencies
Solution Approach 1:
The patent applies parameter changes by computing RF amplitude scaling factors that adjust the RF pulse amplitude based on the transmit frequency offset from the nominal resonance frequency. The scaling factors are derived from measured transmit power response data, effectively changing the operating parameters (amplitude, frequency) to maintain uniform transmit power across different operating conditions while preserving coil efficiency at the resonant frequency.
2Adaptability or versatility
If off-center slices are excited with offset frequency, then slice selection capability is improved, but transmit power reduction occurs due to non-uniform response
Solution Approach 1:
The patent compensates for transmit power reduction by dynamically adjusting the RF pulse amplitude using pre-computed scaling factors. When an off-center slice requires offset frequency excitation, the system retrieves the appropriate scaling factor based on the frequency offset and applies it to the RF pulse amplitude, thereby maintaining adequate transmit power levels while preserving the ability to selectively excite slices at different positions.
3Reliability
If RF pulse amplitude is increased to compensate for power reduction, then transmit power uniformity is improved, but flip angle becomes variable
Solution Approach 1:
The patent simultaneously adjusts both the amplitude and duration parameters of the RF pulse using the computed scaling factors. By modifying both parameters in a coordinated manner, the system maintains a consistent flip angle across different transmit frequencies while achieving uniform transmit power. The scaling factors are specifically designed to compensate for frequency-dependent variations in both amplitude and duration requirements.
4Adaptability or versatility
If asymmetric gradient is used to enable slice offset, then gradient flexibility is improved, but additional non-zero gradient requires large offset frequency
Solution Approach 1:
The patent compensates for the increased offset frequency requirements imposed by asymmetric gradients by applying appropriate RF amplitude scaling factors. The system measures the transmit power response at the specific offset frequencies required by the asymmetric gradient configuration and computes scaling factors tailored to those conditions, thereby maintaining uniform transmit power despite the larger frequency offsets necessitated by the gradient design.
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 ensures more uniform RF transmit power and improved image quality by adjusting RF pulse amplitudes and durations, reducing the need for extended pulse durations and minimizing power limitations, thus enhancing MRI system performance, especially at lower magnetic field strengths and with asymmetric gradients.
Implementation Method 1
generating at least one RF pulse with an RF transmit coil
Implementation Method 2
magnetic resonance pulse sequence that includes generating at least one RF pulse with an RF transmit coil
Implementation Method 3
An image of the subject is reconstructed from the acquired data
Data Source
AI summary
Methods for correcting a non-uniform power response of a radiofrequency (“RF”) transmit coil used in magnetic resonance imaging (“MRI”) are described. Transmit power response data for an RF transmit coil are processed to compute RF amplitude scaling factors for the RF transmit coil as a function of transmit frequency offset. The RF amplitude scaling factors can be used to correct transmitted RF power, and thus flip angle, to be more uniform over a range of transmit frequency offsets, as may be encountered when imaging with lower field MRI systems or MRI systems with high strength or asymmetric gradients.