Dose optimization is of great significance to eliminate the possible adverse effects of ionizing radiation exposure from CT. In this study, we employed the Deep IR algorithm to further reduce the radiation dose by using the combined low tube-voltage and tube-current technique. Our results indicated that our protocol with Deep IR algorithm could significantly reduce the radiation dose (5.09 ± 0.91 mSv vs. 1.53 ± 0.37 mSv) in portal-phase abdominal CT while improving the image quality. Specifically, Deep IR at levels 4 and 5 enabled a 69.69% reduction in the radiation dose while still providing improved SNR, CNR, and subjectively scored image quality.
The radiation dose is known to be proportional to the square of the tube voltage[23]. The study shows that[24] when the tube current is constant and the tube voltage is reduced from 120 kV to 100 kV, the radiation dose can be reduced by 33%; The radiation dose can be reduced by 65% when the tube voltage is reduced from 120 kV to 80 kV, and the radiation dose can be effectively reduced by reducing the tube voltage. Seung et al.[25] showed that the use of 80 kV could reduce the radiation dose by 45.2% with comparable or improved image quality in patients with abdominal tumors. Ren et al.[22] showed that in contrast-enhanced CT imaging of the abdomen, the 80 kV scanning scheme reduced the radiation dose by 63.3% and improved the quality of portal vein images in lean patients compared to the conventional 120 kV scanning scheme. Li et al.[26] used 80 kV scans combined with deep learning image reconstruction (DLIR) for late arterial-phase abdominal CT of patients with low BMI, and the results showed that DLIR could significantly reduce image noise and improve the overall image quality of low-dose abdominal CT while reducing the radiation dose by 57%. In our research, in group B, not only the tube voltage decreased from 100kV to 80KV, but also the tube current decreased from 205mAs to 154mAs. The results show that the combination of reducing the tube voltage and tube current and the image reconstruction algorithm can better reduce the radiation dose (by 69.94%) while ensuring the same or better abdominal image quality.
X-ray energy decreases and it approaches the K edge of iodine (33.2 keV). Through photoelectric effect, X-ray energy interacts more with iodine-based materials, which can cause greater X-ray attenuation and higher contrast enhancement. Yu et al.[27] phantom experiment showed that the iodine attenuation value of 80 kV CT increased by 70% and 100% on average compared with 120 kV CT and 140 kV CT. Zamboni et al.[28] found that pancreatic cancer was more obvious when low kV was used. The results of this study show that the CT values of all organs in group B are higher than those in group A, and the CT values of portal vein are significantly higher than those in group A, which is 33 HU more than that in group A. Sufficient CT values can increase the contrast of images and optimize the display of blood vessels. In addition, patients in both groups were treated with high-concentration iodine contrast agent (370 mgI/ml). Iezzi et al.[29] found that using high-concentration contrast agent and low kV can keep the image quality of abdominal aorta and reduce 74% radiation dose. Low tube voltage combined with high concentration iodine contrast agent can not only reduce the radiation dose, but also improve the image quality of solid organs and ensure the SNR. In this study, the SNR and CNR of liver, spleen and portal vein in group B are higher than those in group A, which can effectively enhance the display of parenchymal organs, blood vessels and their branches and improve the contrast between portal vein and its surrounding tissues.
The decrease of tube voltage and tube current will produce remarkable image noise and was sensitive to hardening artifacts, especially in abdominal CT with low soft tissue contrast, the increase of image noise will lead to the decline of image quality, thus affecting the diagnostic accuracy[6]. The image reconstruction algorithm can effectively improve the image quality. The Deep IR algorithm adopted in this study introduces artificial intelligence technology into the MBIR. By learning the noise and texture features in the image through this neural network, the useful signals and noises in the image can be automatically identified according to the input image, which can not only remove the fringe artifacts in the low dose image, but also achieve image denoising, so that the reconstructed image has better contrast and detail display[30]. In this study, compared with HIR algorithm, Deep IR algorithm can significantly improve the SNR and CNR of abdominal organs and portal vein at all reconstruction levels. The lower the Deep IR reconstruction level, the higher the SNR and CNR. From the analysis of subjective scoring results, Deep IR at levels 4 and 5 had the highest subjective scores, and the obtained abdominal images had low noise and no image distortion, and they also gave the best display of the portal vein and small structures, exceeding the image quality of the portal-phase images in group A. When the Deep IR algorithm was applied at levels 1 and 2, although the image noise was significantly reduced, the reconstructed images were subject to distortion and other alterations due to blurring effects and texture changes, resulting in loss of detail. These findings are similar to study by Li et al.[19], in which the proportion of low-frequency noise was higher when the Deep IR algorithm was applied at lower levels, which changed the textural structure of the image. The high-frequency noise in an image should be gradually reduced by increasing the reconstruction level, which is conducive to the display of details. Based on subjective and objective evaluation, appropriate Deep IR reconstruction levels can effectively improve the image quality of abdominal parenchymal organs and the clarity of portal vein, and Deep IR grades 4 and 5 have the best performance.
Some limitations still existed in this study. First, this was a single-center study with a relatively small sample size. Second, the maximum BMI of the patients in this study was 32, and the feasibility of improving abdominal image quality in patients with large BMI needs to be further studied. Finally, only the image quality of the portal venous phase of the abdomen was studied, the sensitivity and specificity of lesion detection and differentiation were not evaluated in this study, and the next step will be extended to the arterial phase and delayed phase of the abdomen to achieve the goal of multi-phase enhanced CT with low radiation dose.
In conclusion, Deep IR algorithm can meet the for clinical requirements and reduce the radiation dose by 69.94% in portal venous phase abdominal CT with low tube-voltage of 80 kV and tube-current. Deep IR at levels 4–5 can significantly improve the image quality of abdominal parenchymal organs and the clarity of the portal vein. We believe that this study provides a useful clinical application of Deep IR in low-dose abdominal CT and angiography to potentially benefit patients, especially patients who require periodic CT examinations during their treatments.