Attenuation Map Determination Using Single MR Gradient Echo
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Solution Overview
Problem
Combined MR-PET devices require time-consuming pre-measurements for attenuation maps and shim settings, necessitating multiple MR image recordings and stressing patients, as existing methods like the Dixon method need two echoes to differentiate water and fat phases for accurate attenuation correction and homogeneity assessment.
Innovation Solution
A method utilizing a single MR image recording with a three-dimensional gradient echo sequence at two echo times to record 'in-phase' and 'opposed-phase' data, applying a mask to exclude voxels with low signal intensity, and using 2-point Dixon technology to determine attenuation maps and homogeneity information, thereby reducing the need for multiple scans and enabling automatic shim setting adjustments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple MR image recordings are performed to determine attenuation map and shim settings, then measurement precision is improved, but examination time increases and patient stress increases
Solution Approach 1:
The patent combines the determination of attenuation maps and shim settings into a single MR image recording process. By using a three-dimensional gradient echo sequence that captures both in-phase and opposed-phase information simultaneously, the method merges two previously separate measurements (attenuation correction and homogeneity assessment) into one unified acquisition, thereby reducing examination time while maintaining measurement precision.
Solution Approach 2:
The single MR image recording performs multiple functions: it determines fat/water ratios for attenuation correction, assesses magnetic field homogeneity for shim settings, and provides phase difference information. This multi-functional approach eliminates the need for separate dedicated scans for each purpose, resolving the contradiction between comprehensive measurement and examination duration.
2Measurement precision
If multiple MR image recordings are performed to determine attenuation map and shim settings, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent merges the protocols for attenuation map determination and shim settings determination into a single integrated measurement sequence. The three-dimensional gradient echo sequence simultaneously provides in-phase and opposed-phase data, eliminating the need for multiple separate scans and simplifying the overall measurement protocol while maintaining the precision required for both attenuation correction and homogeneity assessment.
3Measurement precision
If Dixon method is used to separate water and fat signals, then attenuation map accuracy is improved, but examination time increases due to requiring two echoes
Solution Approach 1:
The patent combines the acquisition of in-phase and opposed-phase information into a single three-dimensional gradient echo sequence. By carefully selecting echo times within this unified sequence, the method obtains both Dixon method requirements (in-phase and opposed-phase signals) simultaneously rather than through separate scans, thereby maintaining fat/water separation accuracy while reducing the time penalty associated with multiple dedicated scans.
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 allows for rapid and simplified determination of attenuation maps and shim settings within a single MR image recording, reducing examination time and enabling reliable homogeneity information for PET data correction and automatic shim control during MR-PET examinations.
Implementation Method 1
This method relates to the small difference in the Lamor frequency of protons bound in fat and water. To this effect, this frequency difference is noticeable such that the phases of protons in the fat and in the water diverge.
Implementation Method 2
A body introduced into the B0 field of a magnetic resonance facility interferes with the homogeneity of this field. It is therefore necessary to determine shim correction terms in order to reproduce homogeneity.
Data Source
AI summary
A method is disclosed for determining an attenuation map for use in positron emission tomography and for the use of homogeneity information relating to the magnetic resonance magnetic field, in particular for the purpose of determining shim settings, within the scope of a single magnetic resonance image recording. In at least one embodiment of the method, a first and a second image data record are firstly recorded with a three-dimensional gradient echo sequence during a first and a second echo time, respectively, with the phase difference between the water and the fat signal amounting to zero during the first echo time and amounting to 180 degrees during the second echo time. The attenuation map is determined from fat/water ratios obtained from the image data records by way of a Dixon technology, in particular a 2-point Dixon technology. In at least one embodiment, all voxels with a signal intensity below a first threshold value are excluded at least for the second image data record by using a mask and only the non excluded voxels of the first and second image data record are taken into consideration in order to determine the homogeneity information from the phase differences of adjacent voxels.