Portal vein anatomy is very important for the treatment of liver diseases, such as intervention of portal hypertension, liver tumor resection and preoperative assessment for tumor resection.[1–6] Understanding the portal vein tributaries and collateral circulation situation provides reliable assistance to the clinical treatment of liver diseases.[1–6] However, the contrast of portal vein with the surrounding tissue is poor. In order to improve the image quality, some researchers[3,7] increased the amount of contrast agent by using a high concentration or injection rate in the conventional computed tomography (CT) imaging for portal vein. However, this would increase the probability of side effects of the contrast agent.[3,7] In addition, multi-phase scan is often applied for abdominal scans, thus causing more radiation damage to patients. The development of low-kV technology in the chest, heart and abdominal arteries imaging has made great progress, in which the radiation dose is significantly lower than that in the conventional CT scanning.[8–11] The purpose of the present study was to prospectively evaluate whether low amount of contrast agent (1.2 mL/kg) with low tube voltage (90 kVp) and high mAs (395 mAs) could reduce the radiation dose without decreasing the image quality in CT portography.
2 Materials and methods
From March 2016 to June 2016, a total of 118 cirrhotic patients were enrolled in this study. All patients were at the decompensation period of liver cirrhosis. They were randomly divided into 2 groups according to the scanning tube voltage, with 59 cases in each group. The 90 kV group included 48 males and 11 females, and their ages ranged from 25 to 73 years old with a mean age of 54.1 ± 12.4 years old. The 120 kV group included 46 males and 13 females, and their ages ranged from 20 to 78 years old with a mean age of 53.2 ± 11.0 years old. Body mass indexes (BMI) of all patients were less than 25 kg/m2.[12,13] Prior written and informed consent was obtained from every patient and the study was approved by the ethics review board of Qianfoshan Hospital Affiliated to Shandong University.
2.2 CT technique
Multiple-phase CT scanning was performed using a 16-slice spiral CT scanner (Brilliance, Philips Healthcare, The Netherlands). All patients were positioned supine, with their feet first on the scanning table. The scanning range was from the top of the diaphragm to the lower edge of the liver. Two protocols were used: Protocol A, 90 kVp/395mAs, and Protocol B, 120 kVp/200 mAs. The pitch values of Protocols A and B were 0.813 and 0.938, respectively, which were the machine default. The other parameters used in the scan and reconstruction were the same in the 2 protocols. These parameters included: collimation 16 × 1.5 mm, reconstruction section thickness 0.625 mm, reconstruction interval 0.625 mm, field of view 350 mm, matrix 512 × 512, 2.0 mm thin slice thickness of portal venous phase image and 1.0 mm slice interval, window width 350 HU and window level 50 HU. The scan data was transferred to the post-processing Extended Brilliance Workspace (EBW) work station (Philips Healthcare, The Netherlands) and then the values were measured. The contrast agent (300 mgI/mL; Iohexol, Taizhou, China) was injected through the right antecubital vein via 18-gauge needle by power injector. The dosages for Protocols A and B were 1.2 and 1.5 mL/kg, respectively, and the rate was 2.8 to 3.0 mL/s. The delay time of portal phase was set at 50 seconds after injection.
2.3 Image processing and analysis
A radiologist with 7 years of experience in CT examinations measured the CT value and image noise within a 50 ± 1 mm2 circular region-of-interest (ROI) in the liver, portal vein and abdominal aorta. The CT value of liver in ROI (ROIL) was measured at the level of portal vein in three different parts, including the right anterior lobe, right posterior lobe and left liver lobe. Blood vessels and prominent artifacts in parenchymal density would be avoided carefully. The CT value of portal vein in ROI (ROIP) of 3 consecutive sections from the portal confluence down to the portal vein was measured. In the meantime, the abdominal aorta standard deviation (SDN) at the same level was measured as the noise value. The ROIL was recorded as the mean of 3 ROI readings placed in the parenchymal, and the ROIP and SDN were calculated in the same way. The signal-to-noise ratio of the liver parenchyma (SNR) was calculated as follows: SNR = ROIL/SDN. The portal vein-to-liver contrast-to-noise ratio (CNR) was calculated using the following formula: CNR = (ROIP-ROIL)/SDN.
For qualitative analysis, 2 radiologists with 7 years of experience in abdominal CT independently performed the blinded qualitative analysis of CT images obtained with each protocol during the portal venous phase. The images of axial, maximum intensity projection (MIP), and volume-rendering technique (VRT) were used for evaluation. Image quality was rated on a 5-point scale[14,15]: the main portal vein could be clearly shown; the left and right branches of portal vein were indicated well; the portal vein branches of liver lobe could be observed clearly; the portal vein branches of segment were clearly visualized; the portal vein branches of sub-segment were clearly visualized.
2.4 Measurement of radiation dose
The radiation doses of Protocols A and B were calculated, respectively. The CT dose index volume (CTDIvol, unit: mGy) and dose length product (DLP, unit: mGy cm), which were provided by the CT scanner, were recorded at the portal phase for each patient. The effective dose (ED, unit: mSv) was calculated as follows: ED = DLP × κ, where κ is the conversion factor, using the European quality standard Guide CT average of 0.015 (mSv/mGy cm).
2.5 Statistical analysis
Statistical analysis was performed with the statistical software SPSS 19.0 (SPSS Inc, Chicago, IL). All numeric values were reported as mean ± standard deviation (SD). To compare the ROIL, ROIP, SDN, CTDIvol, DLP, ED, SNR, CNR, scan length (SL), the image scores, and the amounts of contrast agent between Protocol A and Protocol B, independent sample t test was used. A P-value <.05 was considered as statistically significant. The inter-observer variability was estimated by the Cohen kappa test. The k values indicated poor (<0.40), moderate (0.41–0.60), good (0.61–0.80), and excellent (0.81–1.00) agreements, respectively.
3.1 Patient demographics
There was no significant difference in the age, height, weight, and BMI of the included patients (Table 1).
3.2 Image quality
The ROIL ROIP, SDN, SNR, and CNR were analyzed and compared between 2 protocols. The 90 kVp group showed significantly higher CT value than120 kVp group, in the liver and portal vein (both P = .001; Table 2, Fig. 1). The mean CT values of ROIL and ROIP in the images of 90 kVp group increased by 17.8, and 24.1%, respectively, compared to those of the 120 kVp group. The SDN of the 90 kVp group was 16.3 ± 9.2, while that of the 120 kVp group was 14.3 ± 2.3. There was no statistical significance in SDN between 90 kVp, and 120 kVp group (P = .053, Table 2). SNR and CNR in 90 kVp group were significantly higher than those in 120 kVp group (P = .015, .001; Table 2, Fig. 1). SL of the 90 and 120 kVp groups were 23.2 ± 2.4 and 23.2 ± 2.5, respectively, and there was no statistical significance (P = .511, Table 2).
The subjective image quality score had no significant difference between 2 groups (P = .139; Table 2, Fig. 2). Compared with 120 kVp group, the image quality in 90 kVp group was not obviously decreased. Moreover, the k-value of inter-observer agreement for the subjective image quality was 0.74, indicating good inter-observer agreement between the 2 observers.
3.3 Radiation dose and volume of contrast agent
As shown in Table 3, CTDIvol (12.4 ± 5.0 vs15.2 ± 6.8, P = .006), DLP (322.1 ± 28.0 vs 381.0 ± 35.1, P = .001), ED (4.8 ± 0.4 vs 5.7 ± 0.5, P = .001), between 90 and 120 kVp groups all had statistical significance. Compared with those in 120 kVp group, and the values of CTDIvol, DLP, and ED in 90 kVp group decreased by 18.2%, 16.0%, and 16.0%, respectively.
The volume of contrast agent in the 90 kVp protocol (75.0 ± 8.7 mL) was 17.8% lower than that in the 120 kVp protocol (91.2 ± 12.2 mL), and the difference had statistical significance (P = .001, Table 4).
The imaging quality of portal vein by CT depends on the contrast between portal veins and surrounding tissues.[2,16] A better contrast between the portal vein and liver parenchyma will lead to a better image quality.[2,16] In the past, scholars used to increase the dosage of contrast agent or the injection rate of contrast agent to enhance the density of the portal vein, thus improving the image quality of portal veins.[3,7] However, excessive use of contrast agents and increasing injection rate would cause more serious side effects.[17–21] Researches showed that low kVp scanner could significantly improve the CT values of blood vessels without increasing the amount of contrast agent, so that the radiation dose received by patients could be reduced.[22–24] In addition, the mAs should be increased appropriately in order to ensure the image quality. Therefore, low kVp and high mAs scan mode also achieved good clinical application in the CT examination of some organs. Nakayama et al  showed that when tube voltages decreased from 120 to 90 kVp and tube currents increased, there was no significant difference in the low contrast resolution capability and the radiation dose reduced by 35%. Nakayama et al  subsequently applied 120 kVp, 300 mAs, and 90 kVp, 300 mAs in clinical examination of patients, and the results showed that the SNR of images decreased, the radiation dose reduced by 56.8% and the image noise increased. Marin et al  applied 80 kVp, 540 mAs and used a new image reconstruction algorithm for image analysis in late arterial phase. The image noise was effectively reduced and the obtained images met the needs of clinical diagnosis, as well as the radiation dose reduced by 71%. In this study, the tube voltage dropped from 120 to 90 kVp, while the tube current was increased from 200 to 395 mAs. The results showed that the image noise had no significant difference between 120 and 90 kVp when applied in the patients with BMI less than 25 kg/m2. CTDIvol, DLP, and ED at 90 kVp were lower than those at 120 kVp, and decreased by 18.2, 16.0, and 16%, respectively. SNR and CNR of the images at 90 kVp were higher than those at 120 kVp, suggesting 90 kVp is more conducive to display the portal vein.
In this study, the amount of radiation dose reduction was less than that in the studies of Nakayama et al  or Marin et al . The tube current time in this study was 395 mAs, while that in Nakayama's research was 300 mAs. In Marin's research, the applied tube voltage was 80 kVp, while that was 90 kVp in this study. As the tube current time or tube voltage in this study was lower than those in Nakayama's research or Marin's research, the amount of reduced radiation dose was less.
The SDN in this study did not increase significantly compared to conventional scanning, but SNR increased. However, in the previous study,[11,28] SDN raised and SNR decreased. This might because the selected patients in this study had BMI less than 25 kg/m2. Therefore, the increased tube current led to good control of image noise, and the increased CT values of liver with low tube voltage in the portal phase increased the SNR. Nakaura et al used a 64-slice Philip CT scanner with automatically tube current regulation technology to examine a group of subjects with body weight less than 70 kg. The 80 and 120 kVp tube voltages were used, respectively. The image noise of 80 kVp was higher than that of 120 kVp, and in some subjects the window width had to be adjusted in the 80 kVp images. This study did not adjust the window width in the scanning application.
The CT plays a very important role in the assessment of portal vein diseases. However, the administration of high dosage contrast agent always yields high vascular attenuation in the large and tiny vessels. Particularly in the clinical practice, application of high dosage contrast agent might increase the risk of contrast-induced nephropathy. Application of low dosage contrast agent is essential to reduce the iodine burden in kidneys. Also, given the fact that CT values of organs with contrast agent would be increased significantly at low tube voltage scanning, the dosage of contrast agent was decreased from 1.5 to 1.2 mL/kg in this study. The results showed that the CT values of portal vein and liver parenchyma did not decrease, but significantly increased. The SNR of liver and portal vein also increased, but the average amount of contrast agent reduced by 17.8%. These findings indicate that our protocol is a promising approach, which can significantly reduce the radiation dose while provide superior diagnostic quality images. This protocol should be recommended in the portal vein examination.
However, there were some limitations in this study. Firstly, the selected subjects had normal weight (BMI < 25 kg/m2), but those subjects with a BMI greater than 25 kg/m2 was not included. This is because that the X-ray penetration ability is weakened with the increase of BMI. When using low tube voltage (90 kVp in this study), the image quality of patients with BMI > 25 kg/m2 will be impacted. Secondly, the amount of contrast agent could be further reduced. Thirdly, the amount of radiation dose reduction was less than the reported ones.[11,27]
In conclusion, limited to the lower BMI patients, CT portography at 90 kVp with low dosage of contrast agent results in a significant reduction in radiation dose and significantly improves SNR and CNR, without impairing the image quality.
Data curation: Sulan Liu, Hao Shi, Wei Li.
Formal analysis: Jingli Fan, Jingzhen He.
Funding acquisition: Hongjun Sun.
Resources: Huaqiang Sheng, Hao Shi, Wei Li.
Validation: Hongjun Sun.
Writing – original draft: Sulan Liu, Huaqiang Sheng, Hao Shi, Wei Li.
Writing – review & editing: Jingli Fan, Jingzhen He, Hongjun Sun.
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