# New-Generation ASiR-V for Dose Reduction While Maintaining Image Quality in CT: A Phantom Study
## 1 Introduction  
The European Directive 2013/59/Euratom [1], establishing the basic safety standards for protection against the dangers arising from exposure to ionizing radiation, is bringing relevant and constructive changes in European radiodiagnostic and radiotherapeutic procedures. Among other relevant radiation protection issues, the Directive places great emphasis on radiation dose optimization in diagnostic and interventional radiology, i.e., the actions that need to be undertaken in order to keep \"the exposure of patients to the minimum necessary to achieve the required diagnostic or interventional objective\" [2]. To date, the effective dose (E) remains the best single descriptor for stochastic risks despite being widely debated [3]. The effective dose associated with most radiological procedures is in the range of fractions of mSv (chest X-ray) to approximately 10-15 mSv (computed tomography (CT)) [4]. Despite being the most frequently used imaging modality, traditional radiography involves a relatively small effective dose per exam, typically below 3 mSv. On the other hand, CT use worldwide has surged in recent decades and is widely used for a number of medical diagnostic purposes. It is well known that compared withtraditional diagnostic X-rays, CT delivers much higher radiation doses. In comparison with planar radiography, CT images provide higher contrast resolution, thus offering a much higher level of detail and enabling whole-body image scanning with a submillimeter resolution. More importantly, in recent years, CT-based radiomics is playing a key role in differentiating histological subtypes and predicting histopathological outcomes in a number of cancers. CT radiation dose may vary significantly among different types of CT studies. Past studies show that the overall median effective doses may range from 2 mSv for a routine head CT scan to about 30-35 mSv for a multiphase abdomen and pelvis CT scan [5; 6]. Notably, recent findings show that a large number of patients may fall under a higher-dose group, exceeding 100 mSv of effective dose [7; 8; 9]. As a result, the general consensus is that dose reduction in body CT is today considered a critical issue because of the potential risk associated with ionizing radiation [10; 11; 12]. The 2009 National Council on Radiation Protection and Measurements (NCRP) report n. 160 showed a substantial increase in medical exposure and reported that CT and nuclear medicine contributed to 75% of the medical radiation exposure of the United States population [13]. The NCRP report, in particular, suggests an increase in average per capita radiation effective dose from medical sources of almost six times that amount (from 0.53 mSv to roughly 3.0 mSv, excluding radiation therapy). Most of this increase is attributed to the increased use of diagnostic and therapeutic applications of ionizing radiation in medicine, among which are CT scans and nuclear medicine procedures. Due to the increasing concern surrounding the potential attributable risks of medical exposure, the radiological community and CT manufacturers have introduced a variety of strategies to reduce the CT radiation dose, mostly related to the technological evolution of CT equipment [14; 15; 16]. One of the most effective approaches is to optimize CT protocols using iterative reconstruction (IR) algorithms that evolved beyond the traditional reconstruction method of filtered back projection (FBP) [17; 18; 19]. In fact, CT images have been reconstructed from raw data using FBP algorithms since the inception of this diagnostic modality. However, the back-projection algorithm has two main limitations: noise and streak artifacts. The combination of these major drawbacks and advances in computing power have led to the use of iterative reconstruction (IR) algorithms in diagnostic CT, which allow for the preservation of good overall image quality even in low-dose conditions [20; 21; 22; 23].