Abstract
Approaches for the quantitative mapping of water content, electrical conductivity and susceptibility have been developed independently. The purpose of this study is to develop a method for simultaneously acquiring quantitative water content, electrical conductivity and susceptibility maps based on a 2D multi-echo gradient echo sequence. Another purpose is to investigate the changes in these properties caused by brain tumours. This was done using a 3T hybrid magnetic resonance imaging and positron emission tomography (MR-PET) scanner. Water content maps were derived after performing T2* and transmit-receive field bias corrections to magnitude images essentially reflecting only the H2O content contrast. Phase evolution during the multi-echo train was used to generate field maps and derive quantitative susceptibility, while the conductivity maps were retrieved from the phase value at zero echo time. Performance of the method is demonstrated on phantoms and two healthy volunteers. In addition, the method was applied to three patients with brain tumours and a comparison to maps obtained from PET using O-(2-[18 F]fluoroethyl)-L-tyrosine and clinical MR images is presented. The combined information of the water content, conductivity and susceptibility may provide additional information about the tissue viability. Future studies can benefit from the evaluation of these contrasts with shortened acquisition times.
Highlights
Interest in quantitative magnetic resonance imaging for studying various brain pathologies has recently grown due the specificity of the information accessible with this technique
Quantitative susceptibility values are estimated based on the phase evolution over time, while the electrical conductivity is calculated from the transceiver phase, which is again obtained by interpolating the phase evolution to TE = 0
It is directly related to the degree of magnetic field inhomogeneity and by interpolation to TE = 0 it can be further utilised to retrieve a map of the water content by involving a series of correction and calibration steps
Summary
Interest in quantitative magnetic resonance imaging for studying various brain pathologies has recently grown due the specificity of the information accessible with this technique. The current protocols used in the aforementioned studies were usually performed separately, even though water content, electric and magnetic properties are internally linked through Maxwell’s equation and Maxwell mixture theory[11] This use of single contrast acquisitions has several drawbacks: long acquisition times are implicit and, independently measured images are not naturally co-registered and may cause spatial mismatch when performing further analysis. Quantitative susceptibility values are estimated based on the phase evolution over time, while the electrical conductivity is calculated from the transceiver phase, which is again obtained by interpolating the phase evolution to TE = 0 This method only makes use of phase information from the multi-echo GRE sequence.
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