## 1 Introduction  
Adipose tissue is a connective tissue composed mostly of fat cells that plays an important role in absorbing body heat as a buffer and as an insulating layer under the skin [1]. Adipose tissue also helps to regulate glucose, cholesterol, and hormone metabolism. A healthy person's total body weight has a fat content of approximately 20-25% [2].  
Although a certain amount of adipose tissue is beneficial to the body, an excessive amount leads to obesity, which increases the risk of developing type 2 diabetes [3]. Non-alcoholic fatty liver disease (NAFLD), which is associated with type 2 diabetes, is becoming increasingly common, affecting approximately 20% of the general population [4]. NAFLD is associated with metabolic comorbidities such as cardiovascular disease, hypertension, and hyperlipidemia. Some groups of people with NAFLD are more likely to develop a higher stage of liver fibrosis or non-alcoholic steatohepatitis [5]. These diseases increase liver-related morbidity and mortality, leading to a higher risk of cirrhosis; liver failure; and, occasionally, liver cancer [6].  
An early diagnosis of steatosis is critical if the patient is to recover [7]. There are various methods for quantitatively diagnosing steatosis, such as liver biopsy and ultrasound imaging [8; 9; 10; 11; 12; 13]. A liver biopsy is a representative evaluation method for diagnosing steatosis. However, it is an invasive procedure, which can have side effects such as pain, bleeding, infection, and even death in rare cases [8; 11]. Furthermore, it is affected by sampling variability due to small tissue samples [9]. Because of these limitations, non-invasive methods have been proposed as potential diagnostic alternatives with different sensitivities and accuracies [12].  
As a non-invasive evaluation method, ultrasound imaging is widely used for diagnosing steatosis [11; 13]. From both a convenience and economic perspective, ultrasound can be the primary diagnostic tool for diagnosing NAFLD since it allows for not only the evaluation of hepatic steatosis but also a comprehensive study of other abdominal organs [12; 14]. In a healthy liver, intrahepatic blood vessels are clearly distinguishable on an ultrasound image, and the posterior aspect of the liver is well delineated. The liver parenchyma is uniform and appears brighter than the kidney parenchyma. In a fatty liver, however, the echo of the liver parenchyma increases, which makes it difficult to observe the deeper portions [11; 15]. In addition, intrahepatic blood vessels in a fatty liver are not always clearly distinguishable.  
However, it is difficult to understand the characteristics of a fatty liver because of complicated factors from the structure of the human body. For instance, the intensity of the high echo changes depending on the fat content ratio, and the fat may appear as a low echo rather than a high echo in an ultrasound image as a result of the surrounding anatomical and physiological influences [16; 17]. Despite the differences in the characteristics of ultrasound images depending on fat content and anatomical characteristics, previous studies have attempted to analyze the ultrasound image characteristics of fat content throughout the human body [15; 18; 19].  
In this study, we developed a module phantom to visualize and qualitatively evaluate the unique ultrasound image characteristics for different fat contents. A fat-mimicking material was prepared to demonstrate fat properties in the ultrasound image according to the fat content. Generally, the ultrasound images depend on the level of training and experience of the sonographer because the transducer is held by hand during ultrasound scanning. In this study, fat-mimicking materials and ultrasound probes were fixed to a LEGO-compatible module frame for reproducibility. Based on our results, ultrasound images of varying fat content can be reproduced using our module phantom, which can then be used to understand its characteristics.