## 4 Discussion  
A modular phantom was developed to visualize and evaluate the unique ultrasound image characteristics of differing fat content. The characteristics of adipose tissue in ultrasound, which is typically seen in the organs of the human body, were seen through our phantom. We noticed that:  
1. The brightness and attenuation of fat content in the phantom were similar to those seen in mild, moderate, and severe fatty livers;
2. Posterior acoustic enhancement, which is mainly seen in anechoic cysts with clear boundaries, was also observed, along with the posterior acoustic shadowing caused by high attenuation such as oil cyst of the breast [23]. 3. Pure water and fat appeared to be echo-free, while an emulsion of water and fat showed a remarkably hyperechoic pattern [22]. A typically observed clinical phenomenon in the fatty liver, cyst, and oil cyst patients was verified with the gel-type modular phantom. Agarose gel was chosen as the tissue-mimicking material used in the phantom. The agarose gel is time- and cost-effective, easy to manufacture, and non-toxic [24]. Various materials other than agarose gel were considered but were ultimately excluded. Carbopol took a long time to completely mix as it became lumpy when mixed with water. Polyurethane takes a long time to degas, and there is a greater difference in the propagation speed of sound in soft tissue compared with other tissue mimics, while gelatin also took a relatively long time to degas and was softer than agarose gel. However, a well-known disadvantage of using agarose gel as a phantom is that it is very susceptible to mold growth and therefore has a relatively short lifespan. However, wrapping it in plastic wrap and storing it in the refrigerator can prolong its usefulness. In another study, phantoms were manufactured with an agarose gel content of 4% [25; 26; 27]. However, a hardening phenomenon appeared after stirring with a stirrer, and degassing was not performed properly. If degassing is not done properly, a lot of air is generated in the gel phantom. The 3% content also had a similar phenomenon to 4%, so the agarose content was set to 2% of DI water. Ultrasound imaging uses the reflection of waves for imaging. If the target object has inherent inhomogeneities, it can distort the incident and the reflected propagating waves. Consequently, the presence of air bubbles in the material can induce wave-speed fluctuations, and multiple scattering leads to artifacts or distortion in the resulting images. Therefore, when preparing the fat-mimicking material, air bubbles were removed by degassing and checked by MRI to confirm their absence. Fatty liver is evaluated to have high intensity and attenuation as fat content increases in ultrasound B-mode images. We first presented qualitative ultrasound imaging characteristics in the phantom, not in the human body. The attenuation coefficient according to fat content has been reported recently [28]. The ultrasound attenuation coefficient of the agar-based phantoms varied in the range of 0.30-1.49 dB/cm-MHz. The attenuation was  
Figure 7: Comparison of echo characteristics by injected water, oil, and emulsion. The water and oil appear echo-free, but the emulsions are hyperechoic. Unlike pure water or oil, emulsions have varying acoustic impedance. Water and oil contained in the emulsion solutions are mixtures, and they appear increasingly hyperechoic as the internal interface increases. increased in proportion to the concentration of agar and fatty evaporated milk. We are planning a systematic quantification study on the imaging characteristics of emulsified materials according to fat content. In addition, we plan to conduct a detailed analysis of the agarose gel content and stability when conducting quantitative research in the future. For the reproducibility of the acquisition of ultrasound images according to the fat content, it is essential to minimize the movement of the ultrasound probe when making the phantom.