## 1 Introduction  
Oral health plays a main role in people's overall health and quality throughout their lifetime, regardless of nationality, region, or religion. It is healthy conditions without mouth and facial pain, oral and throat cancer, oral infection and sores, periodontal (gum) diseases, tooth decay, tooth loss, and disorders that limit an individual's capacity in biting, chewing, speaking, and psychosocial wellbeing. The World Health Organization (WHO) estimated that around 3.5 billion people were affected by oral diseases in 2016 and continuedly increasing [1]. Caries, also known as tooth decay or oral cavities, is the most common disease that affects the quality of life worldwide. Around 60%-90% of school children and almost 100% of adults have dental cavities. Caries is the breakdown of teeth due to acid made by bacteria. The symptoms of untreated caries come in different forms or colors, such as yellow or black, resulting in oral pain, facial pain, tooth loss, and is a major cause of noncommunicable disease. Treatment of oral diseases is usually expensive and not part of universal health coverage. Dental treatment costs 5% of total health spending and is generally a 20% out-of-pocket expenditure in many developed countries. The condition seems to be worse in most developing countries where people cannot afford oral health treatment services. Most caries conditions are treatable and preventable in the early stage,therefore reducing the dentist's effort and expenditure. Figure 1 describes an example of a healthy tooth and a tooth with cavities. Detection of caries may consist of three phases: (1) segment (or isolate) the diagnosis tooth from other teeth; (2) preliminary diagnosis to determine whether a tooth has decay; (3) comprehensive diagnosis to make a treatment for the decaying tooth as well as classify the stage of decay into four groups (C1-C4) based on the condition and damage of the tooth. Although a nurse could perform phase one, phases two and three may need practical experience from a dentist. In this research, we aim to develop a method to make a preliminary diagnosis at phase two to reduce the dentist's effort on non-caries patients. Recently, with the development of medical imaging technology, computer-aided diagnosis systems (CADs) play the main role in the early detection of several diseases such as cancer, diabetes, or even caries [3; 4]. Caries can be detected using several different methods and techniques. Some researchers proposed detection using photoacoustic images, wavelengths, or ultrasound images [5; 6; 7]. Other research has detailed an approach using an RGB oral endoscope image [8; 9]; however, most approaches cannot provide a detailed structure of the tooth, especially the tooth root, and therefore struggle to provide caries diagnosis. Compared to oral endoscope imaging, dental radiographs provide greater image quality and detect detailed structure deformity in the tooth [10]; therefore, the dental radiographic image is the most well-known approach, and is preferable for the detection of caries in the early stage. Clinically, dental radiographs, which are used to identify teeth problems and evaluate oral health, were taken by X-ray with a low level of radiation to capture images of the interior of teeth and gums. Radiographs are usually shown in grayscale images or sometimes in color images; however, color radiographs require significant investment, which provides a barrier to entry for most hospitals, especially hospitals in low-income countries; to account for this, we focused on grayscale radiographs. Unfortunately, there is no reliable dataset that provides high-quality images, descriptions, and reliable ground truth. In this field, most data are only shared within some strict conditions, such as all the researchers must publish to a specific journal or must be a member of some group or event. Some researchers publish their private data used in their research.