Multi-Slice MR Imaging Fat Signal Suppression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current magnetic resonance imaging techniques face challenges in suppressing unwanted fat signal components outside and inside the field of view, particularly due to chemical shift differences, which lead to image artifacts and limitations in the number of slices that can be measured.
Innovation Solution
A method using an RF excitation pulse and magnetic field gradients to excite and refocus magnetization in multiple periodic slices, allowing for the acquisition of MR signals without the limitations of periodic slice selection, employing RF refocusing pulses with multiple frequency bands and coding magnetic field gradients to control phase patterns and separate signals from different layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If two-dimensional excitation pulse is used to reduce field of view and improve image quality, then the number of spatial coding steps is reduced and acquisition time is shortened, but fat signal components outside the field of view cannot be suppressed effectively
Solution Approach 1:
The excitation pulse is segmented into multiple frequency bands, each targeting specific slices. By dividing the frequency spectrum into distinct bands (e.g., water resonance band and fat resonance band), the method can selectively excite or suppress signals from different tissue types in different spatial locations, thereby addressing the fat signal suppression problem while maintaining efficient multi-slice acquisition.
Solution Approach 2:
Different regions of the spectrum are treated with different quality characteristics. The excitation pulse has enhanced frequency selectivity at specific resonance frequencies (water and fat), allowing localized suppression of fat signals in specific slices while maintaining water signal excitation. This local quality adjustment enables selective fat suppression without compromising overall acquisition efficiency.
2Productivity
If two-dimensional excitation pulse is used with periodic slice selection, then the number of layers is limited by the periodicity of excited slices, but this limits the number of slices that can be measured
Solution Approach 1:
The slice selection process is made dynamic by varying the frequency offset of the excitation pulse across different excitation cycles. Instead of using fixed periodic slice selection, the method dynamically adjusts which slices are excited in each cycle, allowing coverage of a larger number of slices beyond the periodicity limit. This dynamic approach enables measurement of multiple slices without being constrained by the repeating pattern of traditional periodic excitation.
Solution Approach 2:
The method uses periodic excitation pulses with varying frequency offsets to systematically cover multiple slices. By applying periodic action with different frequency settings in different cycles, the system can acquire data from multiple slices in a structured manner, overcoming the limitation of fixed periodicity while maintaining the efficiency of periodic excitation schemes.
3Object-affected harmful factors
If frequency-selective fat excitation is used to suppress fat signals, then homogeneity of polarization field B0 and RF field is required, but this is often not possible to generate
Solution Approach 1:
The method changes the frequency parameter of the excitation pulse to selectively target fat resonance frequencies. By adjusting the frequency offset to match the chemical shift of fat protons (approximately 3.5 ppm or 224 Hz at 1.5 Tesla), the system can suppress fat signals without requiring strict field homogeneity. This parameter-based approach is more robust to field inhomogeneities compared to spatially selective methods.
4Object-affected harmful factors
If one-dimensional focusing pulse is used to refocus water proton magnetization, then fat signal components outside and inside field of view are suppressed, but the number of slices is limited by periodicity
Solution Approach 1:
The excitation pulse is designed to perform multiple functions simultaneously: it excites water protons across multiple slices, suppresses fat signals through frequency selectivity, and enables multi-slice acquisition without periodicity limitations. By incorporating multiple frequency bands and adjustable frequency offsets, a single pulse sequence can achieve fat suppression, multi-slice coverage, and efficient acquisition, eliminating the need for separate optimization of each function.
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 acquisition of MR signals from multiple slices without the periodicity limitations, improving image quality by effectively suppressing unwanted fat signals and increasing the number of slices that can be measured, while maintaining signal-to-noise ratio.
Implementation Method 1
For imaging using magnetic resonance installations (MR installations), it is normal practice to direct a two-dimensional excitation pulse into an examination object in order to excite magnetization of nuclear spins in the object
Implementation Method 2
the chemical shift of the protons in fat signals lies approximately 3.5 ppm (parts per million) away from the water protons. This corresponds to the difference in the resonance frequency of 224 Hz at 1.5 Tesla or 447 Hz at 3 Tesla
Implementation Method 3
the RF excitation pulse limits the region in which the magnetization is excited in two spatial directions, which are substantially perpendicular to each other, using two associated magnetic field gradients during the application of the 2D excitation pulse
Implementation Method 4
an RF refocusing pulse is directed into the examination object, wherein the RF refocusing pulse has a number of frequency bands and refocuses the magnetization in the multiple periodic layers
Implementation Method 5
The MR signals in the multiple periodic slices are read out using multiple receive coils of the MR scanner
Data Source
AI summary
In a method and magnetic resonance (MR) apparatus for acquiring MR signals from an examination object an RF excitation pulse is directed into the examination object while activating magnetic field gradients in two different spatial directions, such that a magnetization in the examination object in the two different spatial directions is limited by the RF excitation pulse and the switching of the magnetic field gradients. The magnetization is excited in one of the two spatial directions, of a slice selection direction, in a number of periodic layers, so MR signals are generated in the multiple periodic slices. The MR signals in the multiple periodic layers are read out using multiple reception coils of the MR scanner.