Zero Echo Time MR Imaging Water Fat Separation
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Solution Overview
Problem
Current magnetic resonance (MR) imaging techniques, particularly zero echo time (ZTE) imaging, face challenges in achieving silent imaging while performing water/fat separation due to incomplete k-space data and sensitivity to main field inhomogeneities, making it difficult to apply known Dixon-type techniques.
Innovation Solution
The method involves varying the strength of the readout magnetic field gradient between ZTE sequence repetitions, allowing each k-space region to be sampled at multiple times, which induces specific phasing of FID signals based on chemical species precessional frequency differences, enabling signal contribution separation through phase difference analysis and using compressed sensing for reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If ZTE imaging is performed with radial k-space sampling, then imaging speed and silence are improved, but k-space data becomes incomplete leading to noise amplification and spatial resolution deterioration
Solution Approach 1:
The k-space sampling is segmented into multiple angular ranges, where fully sampled angular ranges provide high-quality data for accurate reconstruction while partially sampled ranges reduce overall acquisition time. This segmentation allows the system to achieve both fast imaging and maintained spatial resolution by strategically selecting which k-space regions to fully or partially sample.
2Measurement precision
If readout gradient strength is increased, then signal encoding accuracy is improved, but sensitivity to main field inhomogeneities increases
Solution Approach 1:
The readout gradient strength is dynamically adjusted as a variable parameter during the imaging sequence. By changing the gradient strength across different excitations and angular ranges, the system optimizes the balance between signal encoding accuracy and sensitivity to field inhomogeneities, allowing high gradient strengths where signal is strong and lower strengths where inhomogeneity effects are more problematic.
3Loss of information
If multiple k-space regions are sampled with different readout strengths, then water/fat separation is enabled, but sequence complexity increases
Solution Approach 1:
The readout gradient strength is made dynamic rather than static, allowing it to vary across different excitations and k-space regions. This dynamic adjustment enables the acquisition of multiple signal encodings necessary for water/fat separation while maintaining a relatively simple basic pulse sequence structure. The variability in gradient strength creates the phase differences needed for chemical species separation without requiring completely separate imaging sequences.
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 silent ZTE imaging with effective water/fat separation by accurately encoding signal contributions from different chemical species, reducing noise amplification and improving spatial resolution, even in areas with minimal sampling density below the Nyquist criterion.
Implementation Method 1
there is a known precessional frequency difference of hydrogen in fat and water
Implementation Method 2
the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images
Implementation Method 3
the strength of the readout magnetic field gradient is varied between at least some of the repetitions of the ZTE sequence such that each k-space region is 'visited' during the scan at least two times, each time with a different value of the readout strength
Data Source
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AI summary
The invention relates to a method of MR imaging of an object positioned in an examination volume of a MR device (1), the method comprises the steps of: - subjecting the object (10) to an imaging sequence of RF pulses (20) and switched magnetic field gradients(G), which imaging sequence is a zero echo time sequence comprising: i) setting a readout magnetic field gradient (G) having a readout direction and a readout strength; ii) radiating a RF pulse (20) in the presence of the readout magnetic field gradient (G); iii) acquiring a FID signal in the presence of the readout magnetic field gradient (G), wherein the FID signal represents a radial k-space sample; iv) gradually varying the readout direction; v) sampling a spherical volume in k-space by repeating steps i) through iv) a number of times, with the readout strength being varied between repetitions; - reconstructing a MR image from the acquired FID signals, wherein signal contributions of two or more chemical species to the acquired FID signals are separated. It is an object of the invention to enable silent ZTE imaging in combination with water/fat separation. This is achieved by varying the readout strength such that each position in k-space is sampled at least two times, each time with a different value of the readout strength. Moreover, the invention relates to a MR device and to a computer program for a MR device.