In our study we investigated whether DLIR-H could improve image quality in CTA of pediatric patients with TAK under normal scan conditions, in comparison with the traditional FBP and the state-of-the-art 50%ASIR-V algorithm. From the objective evaluation point of view, our results showed that DLIR-H provided about 50% image noise reduction, while 50%ASIR-V provided about 25% reduction compared with FBP. From the subjective evaluation point of view, our results showed that DLIR-H did not affect the visualization of small arteries and image texture while reducing image noise. The overall appearance of DLIR-H images was similar to that of FBP and 50%ASIR-V images, without "blotchy" feeling, which was conducive to the display of detailed structures such as small arteries. In fact, since the overall image noise was significantly reduced, the ability of displaying small vessels was judged to be even better than that of FBP and 50%ASIR-V (3.53 vs. 2.94 and 3.03). All the subjective image quality scores of DLIR-H were higher than 3.0, and its comprehensive score was better than other reconstruction algorithms. On the other hand, even though 100%ASIR-V also significantly reduced image noise, it also altered image texture making images appeared too "smooth" with some spatial resolution and structure detail loses (Fig. 2), which affected the subjective diagnostic confidence and the diameter measurement for small arteries.
CTA is an important evaluation method for TAK (18–19). Since TAK can involve the whole artery system, the scanning range is general much larger (including neck, chest and abdomen simultaneously) than that of conventional CTA. In addition, it is also necessary to observe whether the arterial branches with small arteries are involved, high spatial resolution and thin-layer images are desirable. All these clinical requirements put more pressure on the radiation dose in CTA for TAK patients (22), and efficient solutions are urgently needed to reduce the radiation dose while maintaining or improving image quality.
IR algorithms have been widely used to tackle the problem of higher image noise with thin-layer images and/or under reduced radiation conditions. However, IR algorithms with high weights tend to over smooth the high frequency noise and reduce the average frequency of the noise power spectrum (NPS) of the images. Although the image noise is greatly reduced, images may lose the normal image texture (Fig. 2), making images look "overly smooth", "plastic", or simply "unnatural" (16, 17, 23). Our results indicated that although it did not affect the visualization of large arteries (Fig. 1), it was not conducive to display image details such as small arteries (Fig. 2) using 100%ASIR-V algorithm. Therefore, in general, 50% weight of IR is recommended to maintain the balance between noise and image resolution in clinical routine work. Only when the radiation dose is very low, 100% IR weighted is recommended as a mean to salvage the scans (24). This is the major limitation of low-dose CTA applicated in TAK. DLIR is a new generation of image reconstruction algorithm. The deep learning method is used to analyze the characteristics of image noise generation and distribution in different organizations. In theory, the targeted reduction of noise will have minimum impact on the image resolution. In the process of DLIR reconstruction, the integrity of the image information remains largely unchanged when the image noise is greatly reduced, and there is no artificial addition or loss of image resolution. Our results indicated that when DLIR algorithm was used in the assessment of TAK, the image details were maintained, while image noise was significantly reduced. Ever though our study was focused on image noise reduction and image quality improvement, since at the current dose level with 50%ASIR-V, image quality was acceptable, the lower image noise and better image quality performance of DLIR-H could be traded with radiation dose reduction in the future.
There were still some limitations in this study. Firstly, the case number in the study was rather small, because TAK is not a common disease, so it was hard to collect large samples, and impossible to compare children of different ages in groups. In the future, the sample size will be increased to enrich the data information. Secondly, subjective image evaluation was performed using a non-validated scale developed by the authors. Third, our study focused on the image quality improvement over the state-of-the-art ASIR-V algorithm using the same scan under the standard-dose condition. Although the image quality of some ASIR-V images was different from those of DLIR images, no children had interventional radiology results as the gold standard, so it was impossible to confirm the diagnostic accuracies of the different reconstruction algorithms. Further research is needed to demonstrate the improvement in diagnostic efficacy. In addition, further research is also needed to focus on dose reduction to investigate whether DLIR could achieve the same image quality and diagnostic efficacy as the standard-dose ASIR-V images under lower-radiation dose or lower-contrast agent dose.