Semi-Adiabatic Spectral-Spatial MRSI Sequence Reducing SAR at 7 T
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Solution Overview
Problem
Magnetic resonance spectroscopic imaging (MRSI) at higher field strengths faces challenges such as increased radiofrequency power deposition, B1 inhomogeneity, and severe chemical shift localization errors, leading to signal attenuation and SAR safety limit issues, particularly with conventional sequences like PRESS, which are sensitive to B1 variations and chemical shift errors.
Innovation Solution
A semi-adiabatic spectral-spatial spectroscopic imaging sequence (SASSI) is developed using a pair of adiabatic pulses and a linear phase pulse, transformed into spectral-spatial refocusing and excitation pulses, reducing SAR and improving spectral data uniformity at 7 T, with comparable signal-to-noise ratios to adiabatic alternatives like semi-LASER but with significantly lower SAR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional PRESS sequence with 180 degree RF pulses is used, then spatial localization and spectroscopic imaging can be achieved, but signal attenuation occurs due to B1 inhomogeneity and chemical shift errors at higher field strengths
Solution Approach 1:
The patent transforms conventional 180 degree RF pulses into adiabatic pulses with time-varying amplitude and frequency characteristics. This parameter change makes the refocusing pulses insensitive to B1 inhomogeneity and reduces chemical shift localization errors, thereby maintaining signal uniformity across the excited volume at high field strengths while preserving spatial localization accuracy
2Reliability
If adiabatic pulses are used to reduce B1 sensitivity and chemical shift errors, then signal uniformity improves, but SAR increases quadratically with field strength
Solution Approach 1:
The patent segments the 3D volume selection into two separate 2D selections using spectral-spatial adiabatic pulses. By selecting two slices intersecting at the desired slab orientation rather than using multiple adiabatic pulses for full 3D selection, the method reduces the total number of SAR-intensive adiabatic pulses from six (in LASER) to two, thereby reducing overall power deposition while maintaining B1 insensitivity and signal uniformity
3Reliability
If multiple adiabatic refocusing pulses are used for 3D volume selection, then B1-insensitive refocusing is achieved, but echo time is extended and scan time increases
Solution Approach 1:
The patent divides the volume selection process into two sequential 2D slice selections using spectral-spatial adiabatic pulses instead of using multiple adiabatic pulses for complete 3D selection. This segmentation reduces the number of adiabatic pulses from six to two, thereby shortening the minimum echo time and reducing total scan time while preserving B1-insensitive refocusing and spatial localization accuracy
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
The SASSI sequence provides more uniform spectral data and higher brain metabolite ratios with reduced SAR, achieving improved B1-insensitivity and chemical shift localization, while maintaining comparable signal-to-noise ratios to current adiabatic alternatives, thus overcoming the limitations of conventional MRSI sequences at higher field strengths.
Implementation Method 1
Adiabatic pulses create B1-insensitive refocusing and reduced CSL error
Implementation Method 2
chemical shift localization (CSL) errors
Data Source
AI summary
A method, magnetic resonance imaging computing device, and a non-transitory computer readable medium for producing a semi-adiabatic spectral-spatial spectroscopic imaging sequence for magnetic resonance imaging. A pulse control signal comprising a pair of adiabatic pulses and a linear phase pulse is generated. The pulse control signal is transformed into a pair of spectral-spatial refocusing pulses and an excitation pulse. The pair of spectral-spatial refocusing pulses and the excitation pulse are output to a waveform generator to produce the semi-adiabatic spectral-spatial spectroscopic imaging sequence.