B1 Field Map Correction Using Pulse Shape Factors
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Solution Overview
Problem
Magnetic resonance tomography scanners face challenges in achieving homogeneous B1 field excitation due to inhomogeneities, leading to variations in flip angles across slice thickness, which affects measurement quality and requires methods that either compromise on slice selection speed or increase RF pulse amplitudes, and phase-based methods are prone to interference and high SAR loads.
Innovation Solution
A method that determines a correction factor based on the pulse shape of the excitation pulse to calculate a corrected flip angle value, accounting for the pulse shape of all relevant excitation pulses, allowing for improved amplitude-based measurement of B1 field maps and reducing inhomogeneities by storing the corrected flip angle values in the B1 field map.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If sinc excitation pulses are used for homogeneous slice excitation, then the flip angle homogeneity is improved, but the pulse length becomes infinite making the method impractical
Solution Approach 1:
The patent applies parameter changes by modifying the excitation pulse shape from an ideal sinc function to a time-limited pulse using window functions. This transforms the infinite-length sinc pulse into a practical finite-length pulse, accepting controlled deviations in flip angle homogeneity in exchange for practical implementability. The measurement method then compensates for these deviations through correction factors.
2Duration of action of moving object
If window functions are applied to time-limit sinc pulses, then the pulse length is reduced to practical values, but amplitude ripple and frequency domain rounding occur causing flip angle variations
Solution Approach 1:
The patent implements feedback by measuring the actual flip angle achieved in the measurement region and comparing it to the assigned flip angle. Based on this comparison, a correction factor is calculated and applied to determine the actual B1 field strength. This closed-loop approach compensates for the flip angle variations introduced by windowing effects.
Solution Approach 2:
The patent replaces the mechanical approach of perfectly shaping excitation pulses to achieve homogeneous flip angles with a measurement and correction approach. Instead of relying on ideal pulse shapes, the system measures the actual flip angle and uses computational correction to achieve accurate B1 mapping.
3Manufacturing precision
If non-slice-selective preparation pulses are used to achieve homogeneous excitation, then the flip angle homogeneity is improved, but the ability to measure multiple adjacent slices in rapid succession is lost
Solution Approach 1:
The patent applies local quality by using slice-selective excitation pulses that are optimized for local measurement regions rather than using global non-slice-selective pulses. This allows different slices to be excited and measured independently and rapidly, maintaining both homogeneity within each slice and high measurement throughput across multiple slices.
4Measurement precision
If phase-based methods are used to determine flip angles, then the measurement is less affected by pulse shape variations, but the methods are prone to interference from main magnetic field inhomogeneities and have high SAR loads
Solution Approach 1:
The patent uses amplitude-based measurement of the B1 field map, which is a simpler, more robust approach compared to complex phase-based methods. The amplitude measurement is less susceptible to interference from main magnetic field inhomogeneities and does not impose high SAR loads, making it a more practical and reliable solution despite the need for correction factors.
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 spatially resolved and accurate determination of B1 field maps, minimizing errors from pulse shape variations and inhomogeneities, thereby improving measurement quality without increasing measurement time or SAR loads.
Implementation Method 1
the transmit antenna radiates multiple excitation pulses that change the magnetization of an excitation region
Implementation Method 2
the transmit antenna radiates multiple excitation pulses that change the magnetization of an excitation region encompassing the measurement region according to an assigned flip angle
Implementation Method 3
a first measured value is acquired in a first measuring interval and a second measured value is acquired in a second measuring interval by a reception antenna, the measured values relating to the magnetization in the measurement region
Data Source
AI summary
In a method and magnetic resonance apparatus for determining a B1 field map in a scanner of the apparatus, the B1 field map describing a local field distribution of a B1 field resulting from excitation pulses radiated in a measurement sequence, first and second measured values are acquired from a region in which nuclear spins are excited by an excitation pulse having an assigned flip angle, and a provisional flip angle is determined from the first and second measured values. A correction factor, dependent on the pulse shape of a selected excitation pulse, is then determined, and the provisional flip angle is multiplied thereby to obtain a corrected value for entry into said B1 field map.