MRI-based quantification of magnetic susceptibility: Assessment of measurement and calculation accuracy
Sammanfattning: An object in an external magnetic field will be magnetized and the degree of magnetization is dependent of the magnetic susceptibility of the object. The local magnetic field inside and around an object in the MR scanner will change due to the magnetization of the object. The phase shift is proportional to the local magnetic field, which means that phase images hold information about the susceptibility distribution. Since the magnetic susceptibility is a material property, it will have a specific value for a certain substance. Different tissues and compartment of the brain show different susceptibilities, which makes it possible to use susceptibility as a source of contrast in MRI. The magnetic field can be described as a convolution of the susceptibility distribution with a dipole field. The susceptibility distribution cannot easily be resolved from this relationship because of the angular dependence of the dipole field. When the dipole field is zero, at the magic angle, the magnetic field will be zero independently of the susceptibility. In Quantitative Susceptibility Mapping (QSM), the susceptibility is calculated through an iterative process comparing the field calculated from an estimated susceptibility distribution to the measured phase. In this master's thesis, a procedure for QSM has been tested to investigate which conditions and parameters are important for the accuracy of the method. For this purpose, three phantoms were constructed with cylinders of varying susceptibility and geometry. Experiments were made with varying measurement and calculation parameters, and the results were compared to simulated and analytically calculated data. The results show that the susceptibility images differ substantially in contrast from the original phase images. The method seems to be quite stable for changes in measurement and calculation parameters and it provides an expected linear relationship between estimated susceptibility and concentration of contrast agent. The method does not, however, always retrieve accurate values of the susceptibility for cylindrical objects at an angle about or larger than the magic angle, relative to the main magnetic field. The results seem to be more accurate for large objects or high spatial resolution, large volume coverage and with the slice direction applied along the long axis of the cylindrical object of interest.
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