Purpose：Recently, studies have been conducted to improve the signal intensities by increasing the filling factor, which is the volume ratio of the subject in the coil using a compensating material that does not appear as a magnetic resonance signal. Previous studies note that the magnetic susceptibility difference between the air and the compensation material should be minimized. Therefore, in this study, we tried to investigate and compare the materials that can reduce the susceptibility differences most effectively among various materials similar to human tissue density.
Materials and Methods：The customized phantom was constructed using 7 different compound(borax powder, saline solution, sulfur powder, silicon power, graphite powder, wheat flour and silicon pad). A Bland-Altman plot was used to measure the magnetic susceptibility difference between the air and the phantom compound. Image acquisition was performed using a 3.0 T magnetic resonance imaging system (Skyra, Siemens, Germany) and a 15-channel head coil. Validation of a quantitative susceptibility mapping acquisitionw were carried out on T2 and T2* apping using Matlab (Ver.7.10, Mathworks, USA) and Image J (Ver.1.47v, NIH, USA) program. Results：The mean values of graphite (upper: -88.4, center: -100.5, lower: -101.6) and silicone (upper: -74.0, center: -37.1, lower: -96.6) were appeared negative which meant the susceptibility differences were higher than air. In contrast, wheat flour(Upper: 9.0, center: 0.3, lower: 11.3), silicon pad (upper: 13.9, center: -0.7, lower: 13.5), saline (upper: 11.9 , center: 1.0, lower: 16.3), borax (upper: 19.1, center: 1.2, lower: 13.6)and sulfur (upper: 19.0, center: 1.1, lower: 13.9)showed positive value which meant lower susceptibility difference compared to air. As a compensating material to overcome difference in susceptibility uniform performance and less deviation were observed in the compounds other than the graphite and silicon regardless of their position.
Conclusion：The magnetic susceptibility is the intrinsic physical quantity of the elements in the periodic table such as mass and spin. In other words, when an element is exposed to an external magnetic field, it is a measure of how much the element is magnetized. Therefore, in selection of the magnetic susceptibility compensating material, it is important to select one the with the small magnetic susceptibility difference compared to air. In this study, the differences of susceptibility among wheat flour, silicon pad, physiological saline, borax and sulfur were observed to be appropriate. Increasing the filling factor with magnetic susceptibility compensating materials proposed in this study can improve the signal intensities.

• Dae-Keon Seo(Department of Radiology, Asan Medical Center)