B1 Mapping via Quasi-Simultaneous FID and STEAM Signal Acquisition
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Solution Overview
Problem
Current B1 mapping techniques in MR imaging require multiple repetitions due to T1 relaxation, leading to excessive scan time and erroneous B1 maps, especially in abdominal imaging, where peristaltic motion causes phase shifts, and exhibit nonlinear flip-angle behavior limiting dynamic range.
Innovation Solution
A method where FID and stimulated echo signals are acquired quasi-simultaneously during the acquisition period of a stimulated echo sequence, allowing for voxel-wise intensity ratio-based B1 mapping without the need for refocusing pulses, enabling a single repetition and reducing scan time to a few seconds, thus improving robustness and dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple repetitions are performed for B1 mapping due to T1 relaxation, then measurement precision is improved, but loss of time increases excessively
Solution Approach 1:
The patent combines FID signal acquisition and stimulated echo signal acquisition into a single repetition of the pulse sequence. By acquiring both signals during the same acquisition period without requiring separate repetitions for T1 relaxation, the method merges multiple measurement functions into one execution, thereby reducing scan time while maintaining B1 mapping accuracy through the ratio-based calculation method.
2Measurement precision
If multiple repetitions are performed for B1 mapping, then measurement precision is improved, but device complexity increases due to motion-induced phase shifts
Solution Approach 1:
The patent merges FID and stimulated echo acquisitions into a single repetition, eliminating the need for multiple sequence executions that would be vulnerable to motion-induced phase shifts. This single-shot approach reduces the complexity of coordinating multiple repetitions and minimizes the impact of physiological motion during the measurement process.
3Reliability
If refocusing pulses are used in stimulated echo sequence, then reliability of signal acquisition is improved, but use of energy increases and scan time extends
Solution Approach 1:
The patent extracts and utilizes the FID signal that occurs naturally during the acquisition period of the stimulated echo sequence, without requiring additional refocusing pulses. By taking out the FID signal acquisition as a separate usable component from the same repetition, the method eliminates the need for extra RF pulses that would increase SAR burden and extend scan time, while still achieving reliable signal acquisition for B1 mapping.
4Ease of operation
If conventional B1 mapping methods are used, then ease of operation is maintained, but productivity decreases due to slow acquisition speed
Solution Approach 1:
The patent merges FID and stimulated echo acquisitions into a single repetition, doubling the information obtained per unit time compared to conventional methods that require separate repetitions. This merging approach maintains ease of operation through standardized pulse sequence implementation while significantly improving productivity by reducing B1 mapping acquisition time by at least a factor of 2.
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 significantly reduces scan time, increases B1 mapping speed by at least a factor of 2, and allows for simultaneous acquisition of B1 and B0 maps without additional measurement steps, enhancing spatial homogeneity and reducing SAR burden.
Implementation Method 1
a Bloch-Siegert B1 mapping approach is combined with a stimulated echo (STEAM) sequence for MR imaging. The off-resonant Bloch-Siegert RF pulse is applied during the preparation period of the stimulated echo sequence
Implementation Method 2
the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency
Implementation Method 3
a Bloch-Siegert B1 mapping approach is combined with a stimulated echo (STEAM) sequence for MR imaging. The off-resonant Bloch-Siegert RF pulse is applied during the preparation period of the stimulated echo sequence, i.e. between the two on-resonant preparation RF pulses. In this way, the Bloch-Siegert phase shift, which is due to B1 inhomogeneity, is stored along the longitudinal axis.
Implementation Method 4
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle.
Data Source
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AI summary
The invention relates to a method of MR imaging, wherein a portion of a body placed in the examination volume of an MR device is subjected to an imaging sequence of RF pulses and switched magnetic field gradients. The imaging sequence is a stimulated echo sequence including i) two preparation RF pulses (α) radiated toward the portion of the body during a preparation period (21), and ii) reading RF pulses (β) radiated toward the portion of the body during an acquisition period (22) temporally subsequent to the preparation period (21). FID signals (I1) and stimulated echo signals I2) are acquired during the acquisition period (22) with equal T2*-weighting. A B1map indicating the spatial distribution of the RF field of the preparation RF pulses within the portion of the body is derived from the acquired FID (I1) and stimulated echo (I2) signals.