SAR Hotspot Reduction via Temporal Averaging in Parallel Transmission MRI
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Solution Overview
Problem
Magnetic resonance imaging (MRI) is limited by specific absorption rate (SAR) hot spots, which increase with field strength and can cause patient safety issues, especially in multi-shot sequences where repetitive RF pulses lead to cumulative heating in critical tissues.
Innovation Solution
A method that varies the composition of repeated B1 pulses to shift SAR hot spots to different locations within the patient, utilizing multiple transmit coils and a control processor to apply varying pulse combinations, allowing for increased duty cycle and flip angle while minimizing hot spots in critical areas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the same RF pulses are repeatedly applied during multi-shot imaging sequence, then the image data acquisition is completed, but cumulative SAR hot spots are created in critical tissues
Solution Approach 1:
The patent applies dynamics by making the RF pulse composition variable rather than static. Different versions of the RF pulse are used in different repetitions, where each version has a different spatial distribution of SAR hot spots. This dynamic variation prevents cumulative heating in any single location while maintaining the necessary flip angle for image contrast.
Solution Approach 2:
The patent implements periodic action by cycling through multiple versions of the RF pulse in a repeating sequence. Each version is applied for a certain number of repetitions, then switched to the next version. This periodic variation ensures that SAR hot spots move to different locations over time, preventing cumulative heating while maintaining productivity.
2Object-affected harmful factors
If the amplitude of RF pulses is limited to reduce SAR hot spots, then patient safety is improved, but image contrast and duty cycle are reduced
Solution Approach 1:
The patent uses dynamics by varying the RF pulse composition across different repetitions rather than using a single static pulse with limited amplitude. This allows the system to maintain higher average duty cycle while distributing SAR hot spots spatially and temporally, preventing cumulative heating without sacrificing productivity.
Solution Approach 2:
The patent applies parameter changes by modifying the composition and timing of RF pulses across different repetitions. By changing parameters such as pulse amplitude, duration, and phase relationships between multiple transmit elements, the system achieves both SAR reduction and maintained duty cycle through temporal and spatial distribution of energy deposition.
3Object-affected harmful factors
If the duration of RF pulses is increased to reduce SAR hot spots, then SAR is reduced, but motion artifacts increase and image contrast is affected
Solution Approach 1:
The patent applies dynamics by using variable RF pulse compositions across repetitions rather than a single extended-duration pulse. This allows maintenance of short pulse durations (avoiding motion artifacts) while distributing SAR hot spots through temporal and spatial variation, preserving both image quality and reducing SAR.
Solution Approach 2:
The patent uses segmentation by dividing the imaging sequence into multiple repetitions, each using a different version of the RF pulse. This segmentation allows the total SAR to be distributed across multiple short pulses rather than concentrated in fewer long pulses, avoiding motion artifacts while achieving SAR reduction.
4Measurement precision
If higher field strength is used to improve image quality, then signal-to-noise ratio is improved, but SAR hot spots increase quadruply
Solution Approach 1:
The patent applies dynamics by implementing variable RF pulse compositions that change across repetitions when operating at higher field strengths. This dynamic variation distributes SAR hot spots spatially and temporally, enabling the use of higher field strengths (improved signal-to-noise ratio) without the quadratic increase in cumulative SAR that would otherwise occur from repeated pulse applications.
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 safer higher field strength MRI with improved image contrast and reduced acquisition times by distributing SAR hot spots across non-critical regions, enhancing patient safety and efficiency.
Implementation Method 1
Radiofrequency (RF) pulses applied during MR imaging causes potential heating of tissue due to the RF power or energy absorbed
Implementation Method 2
The rate at which radio frequency energy is deposited into the tissue is defined as the specific absorption rate (SAR)
Implementation Method 3
each pulse is the combination of the contributions of all of the elements
Implementation Method 4
SAR hotspot reduction by temporal averaging in parallel transmission MRI
Data Source
Figure 1
Figure 2
Figure 3~4B
AI summary
A magnetic resonance sequence includes a repetitively applied radiofrequency pulse capable of causing a specific absorption rate (SAR) hot spot. The composition of the repetitive pulse is varied to generate versions of the repetitive pulse such that the SAR hot spot changes locations with subsequent applications of the repetitive pulse. To generate versions of the pulse, a pilot scan is performed to generate a patient model. A simulation of the SAR response to each of the versions of the repetitive pulse is performed to determine the location of SAR hot spot(s). A plurality of versions of the repetitive pulse is selected to be used in the magnetic resonance sequence.