Quantifying magnetic nanoparticles in non-steady flow by MRI
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OBJECTIVE: This work compares the measured R*2 of magnetic nanoparticles to their corresponding theoretical values in both gel phantoms and dynamic water flows on the basis of the static dephasing theory. MATERIALS AND METHODS: The magnetic moment of a nanoparticle solution was measured by a magnetometer. The R*2 of the nanoparticle solution doped in a gel phantom was measured at both 1.5 and 4.7 T. A total of 12 non-steady state flow experiments with different nanoparticle concentrations were conducted. The R*2 at each time point was measured. RESULTS: The theoretical R*2 on the basis of the magnetization of nanoparticles measured by the magnetometer agree within 11% of MRI measurements in the gel phantom study, a significant improvement from previous work. In dynamic flow experiments, the total R*2 calculated from each experiment agrees within 15% of the theoretical R*2 for 10 of the 12 cases. The MRI phase values are also reasonably predicted by the theory. The diffusion effect does not seem to contribute significantly. CONCLUSIONS: Under certain situations with known R*2, the static dephasing theory can be used to quantify the susceptibility or concentration of nanoparticles in either a static or dynamic flow environment at a given time point. This approach may be applied to in vivo studies.
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