# New-Generation ASiR-V for Dose Reduction While Maintaining Image Quality in CT: A Phantom Study
## 4 Discussion
The goal of our phantom study was to demonstrate that obtaining images of good quality with low-dose protocols is possible when the ASiR-V IR algorithm is used. Radiologists' perception of image quality concerns subjective concepts, such as noise, resolution, and the detectability of different types of lesions. These characteristics can be expressed and quantified using specific parameters that strongly depend on the radiation dose. Thus, a Catphan 600 was used to perform CT acquisitions with an NI between 10 and 30 in order to obtain the optimal balance between image quality and dose. The quantitative analysis of such parameters was conducted in conjunction with a subjective/qualitative analysis, using a combination of different levels of NI and the novel iterative algorithm ASiR-V. Although studies have been conducted by many authors, the problem of parameter optimization in low-dose CT using iterative reconstruction algorithms is still insufficiently explored. Previous studies have almost exclusively focused on comparisons between ASiR and ASiR-V at a fixed NI. In order to properly address this question, we compared ASiR images at a fixed clinical NI (i.e., the NI suggested by CT vendors) with images reconstructed using ASiR-V at higher NIs. The analysis presented in this study showed that image noise, CNR, and SNR were improved with a varying percentage using the new ASiR-V iterative algorithm. Furthermore, the image noise decreased with an increase in blending, demonstrating the efficacy of the iterative algorithm. These results are also consistent with the recent findings of [PERSON] et al. [18], who studied the impact of ASiR-V on image noise performance. ASiR-V showed greater noise reduction than ASiR and FBP while maintaining the same shape of the noise power spectrum and hence image texture. However, our qualitative analysis showed that the images obtained with ASiR-V NI30 had high noise, which limited their use for diagnostic purposes. It could be argued that the use of NI30 might be used with an ultra-low-dose protocol to investigate well-defined clinical scenarios, such as CT colonography or renal stone disease. A previous study [24] demonstrated that, at low doses, ASiR must be used carefully because it cannot distinguish between a low-contrast object and noise. With reference to the SNR and CNR, both quantitative and qualitative analyses revealed that NI24 provided the best results. The contrast improved with blending, and from the radiologist's point of view, it also improved the detectability of the image. Finally, image resolution was evaluated, and both qualitative and quantitative (MTF) analyses showed that images with NI22 had good resolution. High resolution is of great importance in clinical practice, especially when dealing with lesions of small dimensions, such as hepatic metastasis. In the present study, we found (Figure 4) that in ASiR-V images, a higher number of pairs of lines per millimeter were easily distinguishable, confirming that even if ASiR-V is not always able to reproduce images with the same quality as FBP and ASiR, it improves the clinical performance of the images while highly reducing the radiation dose delivered to the patient. Several previous studies [31; 37] reported similar results, confirming that ASiR-V is a promising algorithm for use in clinical practice. However, to the best of our knowledge, there is still no unanimous agreement on the optimal combination of blending percentage and NI for ASiR-V. [PERSON] et al. [38] adopted an NI of 19 and a mean value of 40% of ASiR-V in their oncologic scan protocol, obtaining an image quality equal to (or greater for some selected parameters) than that of ASiR. A score-based qualitative analysis also stated that there was no significant difference in terms of subjective image quality for images reconstructed with ASiR or ASiR-V [37]. In our study, we identified a high blending percentage (70-100%) of ASiR-V as the best setup for low-dose CT scans.