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.

Background: Diffusion weighted imaging which is clinically useful in the differential diagnosis of the benign and malignant thyroid nodules needs a distortion improvement as thyroid is located just below the skin which causes the susceptibility artifact on the image. Purpose: To reduce susceptibility induced distortion for diffusion weighted MRI(DWI) in the thyroid by compensating the anatomic geometry with the material similar to human tissue density and provide images with a better diagnostic value. Materials and methods: An exam was performed to select the most effective material in reducing the susceptibility difference between tissue and air by using the self produced phantom. On the basis of the result, the selected material was processed and the thyroid diffusion weighted images were acquired to estimate the optimum thickness comparing the distortion and error rate variation of the magnetic susceptibility. Results: As a result of the phantom experiment sulfur, borax, normal saline, silicon pad, flour, air, silicon and graphite were effective in reducing the tissue and air susceptibility differences in order. In the post analysis borax, sulfur, flour and silicone pad had no significant difference in reducing the tissue and air susceptibility differences. Considering the magnetic field becomes uneven at the both edges inside the MRI bore, the experiment was performed by dividing the phantoms into three sections (upper, mid and lower). The susceptibility difference was statistically same among the three sections. In the post analysis using the air as reference borax, sulfur and silicon pad showed little susceptibility difference than the air. The selection of the materials were performed using the Likert 5 point scaling, where the silicon pad was proved as the most effective material. Based on this, the silicone pad was engaged on the patients’ neck. The images obtained after application of the silicone pad showed less error and distortion on both lobes of thyroid and it was statistically meaningful. As a result of the variance analysis the optimum thickness of the silicon pad was 2cm as it was not statistically significant over 2cm on the both lobes. Conclusion: In conclusion, the use of a silicon pad can significantly improve the diagnostic value of the MRI image by compensating the complex magnetic environment commonly encountered in clinical thyroid imaging, not affecting the image contrast.