## 1 Introduction  
Radiographic examinations are of great benefit to patients by visualizing conditions inside the body, which helps in the diagnosis of diseases and treatment strategies. However, there is a risk of radiation exposure due to the use of radiation [1; 2; 3; 4]. In Publication 118, the International Commission on Radiological Protection (ICRP) published a review of recent epidemiological evidence and concluded that the threshold doses for very late effects, such as cataracts, are lower than previously thought [5]. In 2011, ICRP revised its eye dose threshold for cataract induction downwards from 8 Gy to 0.5 Gy [5; 6]. Therefore, evaluation of lens exposure in particular is becoming more important [7; 8; 9; 10; 11; 12; 13; 14]. Since teeth and periodontal tissues that are the focus of dental radiography are close to the lens, it is critical to measure the exposure dose to the lens in dental radiography as well. Popular dental X-ray imaging procedures include intraoral radiography, panoramic radiography, and cone beam computed tomography (CBCT). The intraoral radiography provides high spatial resolution imaging of teeth and periodontal tissue and has often been used to detect periodontal disease and cavities [15; 16; 17; 18; 19]. Panoramic radiography, a single curved tomographic image, provides a wide range of information about the dentition, alveolar bone, and surrounding structures of both jaws. CBCT is a diverse 3D X-ray imaging technique that has enabled the clear visualization of the teeth and surrounding structures. It gives a lower radiation exposure to patients than multi-slice CT, but the exposure is more than that of intraoral and panoramic radiography [20; 21; 22; 23; 24; 25; 26; 27]. Childhood is the period when baby teeth become permanent; the development of the oral cavity is remarkable, and the dental structure is complex. Therefore, dental CBCT, which can capture the anatomical structure and positional relationship of teeth and periodontal tissues in three dimensions, is sometimes used for pediatric dental diagnosis and has been applied to very young patients. In the study conducted by [PERSON] et al. [28] in the UK, the youngest patients with CBCT requests were 5 years old. [PERSON] et al. [29] reported that the youngest patient was aged 4 years. Children are more sensitive to radiation than adults; therefore, it is necessary to reduce the exposure dose as much as possible [30; 31; 32; 33; 34; 35; 36]. [PERSON] et al. [37] reported a lifetime mortality risk imputed fraction of 0.00174% for 10 year olds using CBCT examinations, which was considerably higher than that for adults receiving the same dose (0.00075%). According to [PERSON] et al. [38], the lifetime attributable cancer risk, expressed as the probability of developing radiation-induced cancer, varied between 2.7 per million (age > 60 years) and 9.8 per million (age 8-11 years) with an average of 6.0 per million (age 8-83 years). It is important to assess and quantify radiation risks in children. The purpose of CBCT in children is to diagnose ectopic eruptions, impacted teeth, traumatized teeth, and intraosseous cysts or tumors. Among these, the rates of ectopic eruption and impacted teeth are particularly high [25; 28; 39; 40]. The number of ectopic eruptions and impacted teeth per pediatric patient is often a single tooth, and in most cases, a small field of view (FOV) is sufficient for the examination [41; 42; 43]. General guidelines recommend keeping the FOV as small as possible to include only the region of interest [44]. Several studies have measured radiation doses to highly radiosensitive organs using small FOVs (<40 cm\({}^{2}\)) in adults [45; 46; 47]. However, to the best of our knowledge, no studies have examined them in children. It is important to understand the FOVs and relative positions of radiation-sensitive organs, even in children, because of the differences in head and neck sizes between adults and children. One method to reduce radiation dose is 180\({}^{\circ}\) rotation.