bSSFP MRI Phase Alignment for White-Marker Interface Artifacts
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Solution Overview
Problem
In magnetic resonance imaging (MRI) using balanced steady-state free precession (bSSFP) sequences, signal intensities at interfaces between materials with different resonance frequencies, such as fat and water, are undesirably increased due to white-marker gradients, leading to artifacts and reduced image quality.
Innovation Solution
Adjust the phase development between the magnetizations of materials at interfaces by setting the scanner frequency to the arithmetic average of their Larmor frequencies and introducing an additional phase progress, thereby aligning phases co-phasally to counteract the signal-enhancing effect of white-marker gradients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a white-marker gradient is provided to balance local dephasing caused by magnetic field-changing objects, then signal attenuation around these objects is reduced, but signal intensities at interfaces between materials with different resonance frequencies (such as fat and water) are undesirably increased
Solution Approach 1:
The patent applies different phase adjustment strategies to different spatial regions. By setting the scanner frequency to the arithmetic average of Larmor frequencies and introducing additional phase progress, the method creates co-phasal alignment specifically at material interfaces while maintaining the white-marker gradient effect around magnetic-field-changing objects. This local differentiation allows simultaneous optimization of both requirements.
Solution Approach 2:
The patent modifies key sequence parameters including scanner frequency (set to arithmetic average of Larmor frequencies) and phase progress (additional phase introduced). These parameter changes fundamentally alter the phase development between magnetizations at interfaces, transforming the signal enhancement effect into a suppression effect while preserving the beneficial white-marker gradient effect.
2Object-generated harmful factors
If the scanner frequency is set to the arithmetic average of Larmor frequencies and additional phase progress is introduced to align phases co-phasally at interfaces, then signal intensities at material interfaces are reduced, but the complexity of sequence parameter adjustment increases
Solution Approach 1:
The patent modifies key sequence parameters including scanner frequency (set to arithmetic average of Larmor frequencies) and phase progress (additional phase introduced). These parameter changes fundamentally alter the phase development between magnetizations at interfaces, transforming the signal enhancement effect into a suppression effect while preserving the beneficial white-marker gradient effect.
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 reduces signal intensities at material interfaces, improving image clarity and reducing artifacts without increasing signal intensity around magnetic-field-changing objects.
Implementation Method 1
Magnetic resonance apparatuses are imaging apparatuses that use a strong external magnetic field to align the nuclear spins of an object to be examined and to excite them by applying a radio-frequency excitation pulse for precession about the corresponding alignment. The precession or transition of the spins from this excited state to a state with less energy generates in response an electromagnetic alternating field that can be detected as an MR signal via receiver antennas.
Implementation Method 2
With the help of magnetic gradient fields, a position encoding can be impressed on the signals, which then allows the signal received to be assigned to a volume element of the object under examination.
Implementation Method 3
Objects, for example, made of a paramagnetic material situated in the object under examination can change a magnetic field in an area surrounding the object because of a magnetic susceptibility deviating from a surrounding material. As a result, the magnetic field in an area surrounding the object can have local magnetic field gradients caused by the object. Because of the field gradients, various local phase changes may take place in the area surrounding the object during a repetition time.
Implementation Method 4
A white-marker contrast can be generated by providing an extra gradient or omitting, reducing, and/or varying rephasing gradients typical of the sequence. The rephasing gradient can, for example, be a rephasing of a slice selection gradient, as described in Seppenwoolde et al. (2003). Increasing signal strengths using the white-marker gradient is impossible in spin echo sequences.
Implementation Method 5
A particular sequence form is a balanced steady-state free precession sequence, bSSFP for short. In this case, it is provided that, as with gradient echo sequences (GRE), magnetic field gradients are provided to generate echo signals. However, these are balanced out during a repetition time.
Implementation Method 6
Because of local field inhomogeneities, signal loss may occur close to the magnetic-field-changing object, as with the gradient echo sequences. The particular signal generation in the bSSFP can here result in an alternating phase at the echo time TE in a voxel, wherein this causes destructive interference to the spins.
Implementation Method 7
Adjust the phase development between the magnetizations of materials at interfaces by setting the scanner frequency to the arithmetic average of their Larmor frequencies and introducing an additional phase progress, thereby aligning phases co-phasally to counteract the signal-enhancing effect of white-marker gradients.
Data Source
AI summary
Capturing MR image data of an examination object using an MR apparatus, including: performing a balanced steady-state free precession sequence with phase progress of 180 degrees per repetition time using the MR apparatus; in the balanced steady-state free precession sequence, providing a white-marker gradient in order at least partially to balance a dephasing caused by a magnetic-field-changing object in the examination object; capturing image data of the examination object using the MR apparatus at an echo time; and adjusting a phase development between phase magnetization of a first and second materials, which form an interface in the examination object, in the balanced steady-state free precession sequence using the MR apparatus, wherein due to the adjusting of the phase development before an effect of the white-marker gradient, a co-phasal alignment of a magnetization of the first material and of the second material at the interface is effected at the echo time.