T1-Compensated B1 Mapping for Accurate MR Imaging
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Solution Overview
Problem
Existing B1 mapping methods in MR imaging are limited by T1 relaxation, leading to systematic underestimation of flip angles and artifacts in MR images, especially at higher field strengths, constraining resolution and dynamic range.
Innovation Solution
A method involving two stimulated echo imaging sequences with different T1-weightings to compensate for T1 relaxation effects, using adiabatic RF inversion pulses and varying delays to ensure accurate B1 mapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a stimulated echo imaging sequence is used for B1 mapping, then the acquisition speed is improved, but T1 relaxation causes systematic underestimation of flip angles and artifacts
Solution Approach 1:
The patent applies preliminary action by introducing inversion pulses before the stimulated echo sequence to pre-manipulate the magnetization state. The inversion pulses are applied at specific times before the measurement to create known T1-weighted differences that can be mathematically compensated for in the B1 mapping calculation, thereby eliminating the systematic underestimation caused by T1 relaxation during the acquisition.
Solution Approach 2:
The patent changes physical parameters by varying the inversion time (TI) and using different inversion pulse flip angles (180° or 360°) to create multiple measurements with different T1-weightings. By acquiring data at different parameter settings and combining them through a compensation algorithm, the method eliminates T1 relaxation effects while maintaining fast acquisition speed.
2Measurement precision
If the main magnetic field strength is increased to improve image quality, then the signal-to-noise ratio is improved, but B1 inhomogeneity and T1 relaxation effects increase
Solution Approach 1:
The patent implements feedback by using the measured signal intensities from multiple stimulated echo sequences with different inversion weightings to calculate and compensate for T1 relaxation effects. The method measures the actual magnetization recovery at different time points and uses this feedback information to correct the B1 mapping calculation, thereby maintaining accuracy despite increased T1 effects at higher field strengths.
3Manufacturing precision
If the echo train duration is extended to improve spatial resolution, then the resolution is improved, but T1 relaxation causes greater signal decay and underestimation
Solution Approach 1:
The patent applies preliminary action by preparing the magnetization with inversion pulses before the extended echo train acquisition. This pre-preparation creates a known initial state that allows the system to tolerate longer echo train durations without excessive T1-related signal loss, because the inversion timing is specifically designed to account for the extended acquisition period.
Solution Approach 2:
The patent uses dynamics by adaptively adjusting the inversion timing and flip angles based on the echo train duration and desired spatial resolution. The method dynamically optimizes the inversion parameters for each specific acquisition configuration, allowing flexible trade-offs between resolution and T1 compensation effectiveness.
Data Source
AI summary
The invention relates to a method of MR imaging. It is an object of the invention to provide an improved B1 mapping method that is less affected by T1 relaxation. The invention proposes that a first stimulated echo imaging sequence (25) is generated comprising at least two preparation RF pulses (α) radiated during a first preparation period (21) and a sequence of reading RF pulses (β) radiated during a first acquisition period (22) temporally subsequent to the first preparation period (21). A first set of FID signals (IFID) and a first set of stimulated echo signals (ISTE) are acquired during the first acquisition period (22). A second stimulated echo imaging sequence (27) is generated comprising again at least two preparation RF pulses (α) radiated during a second preparation period (21) and a sequence of reading RF pulses (β) radiated during a second acquisition period (22) temporally subsequent to the second preparation period (21). A second set of FID signals (IFID) and a second set of stimulated echo signals (ISTE) are acquired during the second acquisition period (22). The first and second sets of FID signals (IFID) have different T1-weightings and/or the first and second sets of stimulated echo signals (ISTE) have different T1-weightings. A B1 map indicating the spatial distribution of the RF field of the RF pulses is derived from the acquired first and second sets of FID (IFID) and stimulated echo (ISTE) signals, wherein the different T1-weightings are made use of to compensate for influences on the B1 map caused by T1 relaxation. Preferably, either the first or the second preparation period (21) is preceded by an RF inversion pulse to obtain the different T1-weightings. Moreover, the invention relates to an MR device (1) and to a computer program for an MR device (1).