## 1 Introduction  
Computed tomography (CT) is one of the best modalities for low-contrast imaging in humans. In the 1970s, the first clinical CT scanner was introduced for performing head scans on patients [1, 2]. Pediatric head CT scans are the most common procedures performed for the diagnosis and follow-up of diseases, including trauma (23%), infections (22%), headaches (17%), and other medical concerns (38%) [3]. Advancements in CT image quality and reductions in acquisition time have led to a significant increase in CT utilization, establishing it as the primary source of diagnostic radiation exposure for pediatric patients [1, 2]. According to the National Council on Radiation Protection and Measurements (NCRP) Report No. 184, in 2016, approximately 5.2 million pediatric patients in the United States underwent CT examinations, accounting for 6.3% of the total annual CT examinations. Head procedures constituted 55% of the CT procedures performed among these pediatric patients, resulting in a collective effective dose of 5.7 person-Sv [4, 5]. The radiation dose delivered to patients, particularly children and infants, should be carefully considered due to their increased susceptibility to radiation compared with adults [3, 4, 6, 7]. Additionally, children have a heightened risk of radiation-induced cancer, and the lifetime attributable risk (LAR) of cancer resulting from a single head CT scan can be over ten times greater in infants than in middle-aged adults [8]. Radiation exposure from CT is typically quantified using specific dose metrics, including the volumetric CT dose index (CTDI\({}_{\text{vol}}\)) (mGy) and dose-length product (DLP) (mGy-cm). The CTDI\({}_{\text{vol}}\), which quantifies the output from the CT scanner and is displayed on the CT scanner control prior to the scan, depends on various parameters, including the tube current-time product (mAs), tube potential (kVp), pitch, and gantry rotation time [2,9,10]. The CTDI\({}_{\text{vol}}\) is a standardized method for comparing the radiation dose output among different CT scanners using a reference phantom. However, the CTDI\({}_{\text{vol}}\) does not account for patient size, shape, or attenuation, making it inaccurate as a patient dose metric since it is determined from a polymethyl methacrylate (PMMA) cylindrical phantom of a particular size. One phantom has a diameter of 16 cm, corresponding to the size of the head, whereas the other phantom has a diameter of 32 cm, corresponding to the size of the body. Both phantoms are 15 cm in length [9,11,12]. Indeed, the patient radiation dose should consider both the output radiation dose from the CT scanner and the patient's attenuation. The American Association of Physicists in Medicine (AAPM) introduced the size-specific dose estimate (SSDE) by developing conversion factors applied to the CTDI\({}_{\text{vol}}\) to estimate the patient radiation dose while considering patient size. The inception of the size-dependent conversion factor began with AAPM Report No. 204, which focused on patient geometry, such as the anteroposterior and lateral dimensions or effective diameter, specifically for CT imaging of the abdomen and pelvis [9]. AAPM Report No. 220 introduced the use of the water-equivalent diameter (Dw), which considers tissue attenuation and patient geometry size, to more accurately estimate the SSDE when applied to the abdominal and thoracic regions [9,13,14,15]. AAPM Report No. 293 introduced the SSDE based on the water-equivalent diameter (SSDE\({}_{\text{Dw}}\)) for head CT examinations, which estimates the patient radiation dose based on the CTDI\({}_{\text{vol}}\) and a conversion factor specific to the head region, determined based on the Dw. The head SSDE conversion factor was derived from a study combining physical measurements using phantoms and ionization chambers with Monte Carlo simulations across various models of CT scanners [14]. Many studies have investigated the SSDE in pediatric patients by applying conversion factors from AAPM Report No. 293 [7,16,17,18].