Multiband RF Pulse Phase Modulation for MRI Slice Registration
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Solution Overview
Problem
Current magnetic resonance imaging (MRI) techniques face challenges in reducing scan time while maintaining contrast and registering image data sets from multiple slices, especially for moving objects, where existing methods either limit contrast or introduce errors due to navigator echoes.
Innovation Solution
A method using a multiband radiofrequency pulse with varied phase, amplitude, duration, and pulse shape across slices to create image data sets with different contrasts, allowing for slice registration without additional complexity, employing techniques like MS-CAIPIRINHA and GRAPPA or SENSE reconstruction methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If multiple coils are used for reading out scan signals with undersampling to reduce scan time, then acquisition time is reduced, but aliasing artefacts occur in the reconstructed image
Solution Approach 1:
The patent applies CAIPIRINHA methods that modify the phase encoding parameters by introducing controlled aliasing patterns. The phase shifts applied to different slices change the spatial frequency sampling pattern in k-space, transforming the aliasing artefacts from overlapping structures into structured patterns that can be separated during reconstruction using parallel imaging algorithms like GRAPPA or SENSE.
2Loss of time
If different reconstruction algorithms are used to manage with fewer k-space lines, then scan time is reduced, but the complexity of the reconstruction process increases
Solution Approach 1:
The patent incorporates navigator echoes into the pulse sequence before the actual imaging data acquisition. These navigator echoes are used to characterize motion in advance, and the motion characterization results are stored and applied during the reconstruction phase. This preliminary motion characterization simplifies the reconstruction process by providing pre-computed motion correction factors that can be directly applied to the undersampled k-space data.
3Measurement precision
If navigator echoes are used to characterize motion in multiple slices, then motion correction is improved, but the scan time increases and the contrast is limited
Solution Approach 1:
The patent combines the navigator echo function with the multiband RF pulse excitation. The same multiband RF pulse that excites multiple imaging slices also generates navigator echoes from reference slices. This merging eliminates the need for separate navigator echo acquisitions, as the navigator data is obtained simultaneously with the imaging data during the same RF excitation and readout period, thus not increasing scan time.
4Loss of time
If multiband RF pulses are used to excite multiple slices simultaneously, then scan time is reduced, but the contrast differentiation between slices is limited
Solution Approach 1:
The patent applies slice-specific phase shifts to different slices excited by the multiband RF pulse. Each slice receives a unique phase modulation that is maintained throughout the echo train. This local phase differentiation allows each slice to have distinct signal characteristics that can be exploited during reconstruction to generate different contrast weightings for different slices from the same simultaneous acquisition, thereby achieving contrast differentiation without sacrificing scan time efficiency.
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
Enables the acquisition of image data sets with varied contrasts and facilitates registration of slices without additional complexity, improving temporal resolution and SNR, particularly suitable for moving examination objects by using a dual-band radiofrequency pulse and varying the deflection angles and amplitudes to achieve desired contrasts.
Implementation Method 1
A multiband radiofrequency (RF) pulse is radiated in a step (a) to excite at least two slices of a subject
Implementation Method 2
The phase of the scan signal in at least one slice is modulated in at least one step (a) to (c) in order to cause the phase of the scan signal of this at least one slice to be different from the phase of the scan signal of the other slices
Implementation Method 3
The scan signals of the excited slices are read out (detected) in a step (c) using each coil of a multi-coil array
Implementation Method 4
Image data sets are reconstructed dependent on the modulation of the phase of the scan signal in at least one slice
Data Source
AI summary
In a method and a magnetic resonance apparatus for generating at least two parallel two-dimensional image data sets of an examination object a multi-coil array radiates a multiband radiofrequency pulse that excites at least two slices, which are phase encoded by a phase encoding gradient. Scan signals produced by the excited slices are detected using each coil of the multi-coil array. In one of the excitation or the phase encoding, the phase of the scan signal in at least one slice is modulated such that the phase of the scan signal therefrom differs from the phase of the other slices. Image data sets are reconstructed based on the modulation of the phase of the scan signal in at least one slice.