A Phantom Study of Impact from Different Degree of the Main Magnetic Field Perturbations of MRI Scanner on T2* Measurements

Abstract

Objectives: To determine the levels of main magnetic field inhomogeneity that potentially affect to the accuracy of the T₂* measurements in phantom.

Methods: The magnetic field uniformity of a 1.5 Tesla MRI scanner was measured under normal condition and 7 different levels of perturbations. The change of T₂* of a gel phantom incorporated 8 different concentrations of Ferric iron, was monitored along with the levels of main magnetic field inhomogeneity. The correlation between the levels of main magnetic field inhomogeneity and measured T₂* was evaluated by 2 way analysis of variance (2 way ANOVA). The range of magnetic field inhomogeneity affected measured T₂* was evaluated by pair-Test based on Fisher’s least significant different (LSD) analysis at 95% confident interval. The fitting model used to evaluate for T₂* values was the simple mono-exponential model running on MATLAB 7.0.1 (Mathworks, Natick, MA, USA). The percent coefficient of variance (%CV) of mean T₂* in each concentration of Fe3+ was measured.

Results: The two-way ANOVA demonstrated that magnetic field inhomogeneity (ppm) significantly affected to T₂* values with 95% confident interval (P=0.000). Fisher’s least significant different analysis showed that magnetic field inhomogeneity under 0.90 ppm resulted in significant differences (95% confident interval with p=0.100) of mean T₂* measurements. In addition, main magnetic field inhomogeneity showed greater impact to T₂* values at upper 20 milliseconds than lower 20 milliseconds with the ranges of percent coefficient of variation (%CV) = 3.15-4.85 and 0.63-3.17 respectively.

Conclusion: Magnetic field homogeneity affected the accuracy of T₂* measurement. It was shown that when perturbation of main magnetic field was below 0.90 ppm, the accuracy of T₂* measurements tended to have no effect. The results also showed that ferric iron concentrations of 1.0-2.0 mg/g wet weight or T₂* values was lower than 20 milliseconds, the impact to T₂* values dued to external magnetic field perturbation tended to be reduced for T₂* measurements at the ranged of 10.2-50.7 milliseconds.