| BackgroundNowadays, MDCT has been widespreadly applied in clinical practice, which has became one kind of the indispensable examination method.It brings us convenience greatly, while in the same time the accompanying radiation also brings us a lot of harm. The potential carcinogenic harm and birth defects harm of radiation cannot be ignored absolutely. Therefore, how to reduce the radiation dose received by patients with CT examination has became a hot area of research both at home and abroad. At present, all kinds of measures and methods have been used in clinical practice.For example:the hardware innovation and the software upgrade of the manufacturer, such as tube current automatic regulation technology, intelligent optimal kV scanning technology, big pitch scanning, appropriately decreaseing the scan times of enhancement, avoiding sensitive organs’exposure and screening the sensitive organs, and so on. However, many studies have shown that lower tube voltage is the most direct and effective way. At present, the low dose research mainly concentrates on the lung and sinuse, the skeletal system is more oversea[1,2], because the natural contrast of these places is high. When we appropriately reduce the scanning conditions, the image backgroud noise increases and the signal-noise ratio (signal-noise ratio, SNR)decreases to a certain degree,but that will not affect its application in clinical. In addition, CT angiography (Computed Tomography angiography, CTA) is also research hot spot domestic and overseas[3], that is due to density contrast of the high concentration of contrast medium in blood vessels and surrounding tissues is very hign, a slight increase of noise may not influence the disease diagnosis. There are less low-dose studies about abdominal CT scan[4,5,6]because of the low nature contrast of abdominal organs. When the density difference between organs is not much markedly, especially when the lesion within the solid organ is very small, and the density difference between the lession and the normal parenchyma around the lession is not very apparent, or we find lesion but have difficulties in qualitative diagnosis, multiphase dynamic enhancement CT scan is needed. It has became a routine method in the clinical applications, but this makes the radiation dose increased exponentially. Therefore, how to reduce the abdominal radiation dose of CT examination to the greatest extent needs us to solve very urgently. For kidney, enhancement degree of the renal cortex is very strong, and the contrast between cortex and medulla is very distinct. Especially the majority of kidney tumors are renal cell carcinomas, which blood supply is rich. Compared with kidney parenchyma, the renal carcinoms showed obvious high enhancement in arterial phrase and low enhancement in the substantial period. These features provide pathology basis for the renal low kV scan research.In addition, dual-source CT imaging technology has been mature. Dual-source CT has two sets of the X-ray tube and detector system installed on the scanner frame with90°for each other,which can launch x-ray at the same time. The tube voltage is selected to be80kv and140kv most of the time. Siemens medical healthcare introduced the second generation of the DSCT of SOMATOM Definition, which tube voltage can be selected to be100kV and140kV [7]. We can get the following three groups of images from dual energy scanning every time: pure80kVp image (or pure100kVp image), pure140kVp images and fused images (images equivalent to120kVp weighted by80kVp and140kVp images using a such weighting factor, the image quality is similar to the image of120kV tube voltage). So for a single dual energy scan we can obtain three sets of images of the same level at the same time from the same patients:80or100kV,100kV and140kV. In addition, through the dual-energy post-processing software system, we also can get virtual non-contrasted images and iodine map, etc[8]. Therefore, dual energy scanning provides great convenience about the kidney low tube voltage scanning technology for us.This study is to use the multiple set of image data obtained from the kidney dual energy scans for comparison and analysis, and aim to prove that using the lower tube voltage (80kV,100kV) we can get images of the renal tumor enough to meet the requirements of diagnosis, thus greatly reduce the radiation dose.ObjectiveThe dual-energy scans in arterial phase of enhancement CT were performed in40renal tumors patients, through which we could got three groups of images(images of80kV or100kV, images of140kV, fused images of120kV). The purpose of this study was to research influence of the different tube voltage on the image quality of renal arterial scaning image, so as to explore the the feasibility of the low kilovoltage kidney tumors CT scan with comparing and analysising the images quality.Materials and methods40cases of patients found renal tumors with examination or suspected kidney renal tumors clinically were randomly selected to be research objects. All of the patients were examined by the dual-energy scans in arterial phase of enhancement CT. According to the tube voltages, all of the patients were divided into two groups: Group A (20cases)with tube voltage of80kV and140kV; Group B (20cases)with tube voltage of100kV and140kV. Each patient could obtain three sets of images:the lower tube voltage (80or100kV) images, the higher tube voltage images (140kV) and the fused images (be equal to the image of120kV). For each three groups of images, image quality was evaluated respectively. Evaluation methods could be divided into two kinds:the objective evaluation and subjective evaluation. Objective evaluation:Objective evaluation index were background noise(Background noise, BN), Signal to noise ratio (Signal-noise ratio, SNR) and contrast to noise ratio (Contrast-noise ratio, CNR). Signal intensity(Signal intensity, SI))was indicated by CT value of the normal renal cortex. We measured the BN, CT value of the tumor area with obvious enhancement, CT value of paraspinal muscle on the same section image that has the maximum cross section of the tumor or where the tumor showed the characteristics of obvious enhancement. The CT value of the contralateral normal renal cortex and background noise were measured on the same section image. At last we calculated the CNR and SNR. CNR=(CT value of renal tumor area with obvious enhancement-CT values of paraspinal muscle)/background noise(BN);SNR=CT value of the normal renal cortex (SI)/background noise (BN).Using SPSS13.0for Windows statistical software, the BN, SNR and CNR of three groups of images were analysised statistically. The paired T test was use to compare the difference of the BN, SNR, and CNR among the three groups of images. A statistically significant difference was defined as a two-sided P less than0.05.Subjective evaluation:All the thick and thin layer images of A, B two groups were transmitted to the workstation. The images were read separately by two experienced vice director physicians or above of CT diagnosis by double-blind method under the condition of having no ideas of the scan conditions.The readers can use the regular three-dimensional post-processing methods (mainly MPR, MIP, VR) provided by the workstaion to reconstruct, measure or analysis images. The observers were asked to write down the sum of the kidney tumor and calculate the tumor detection rate, respectively to evaluate and grade the image quality of transverse images and three-dimensional post-processing images between the three groups. The scoring criteria was by the method of five points:1score:very good;2score:better;3score:generally, but not affecting the diagnosis of the kidney tumor and evaluating of the local aggressive and distant metastasis;4score:poor, has a certain influence on diagnosis;5score:the image quality is to poor to diagnose. The scores of the three groups of images in the two groups of patients were compared respectively with the statistical method of Wilcoxon Signed Ranks Test. The difference was statistically significant when p is less than0.05. Through the Kappa consistency test to detect the consistency of the two CT diagnostic doctors, when the Kappa value was equal or greater than0.75the diagnostic consistency was better, when Kappa value was equal or greater than0.4and less than0.75the diagnostic consistency was general; when the Kappa values was less than0.4the diagnostic consistency was poor.Results1. objective evaluation1.1Group A80kV,140kV, the fused image (be equal to the image of120kV), three sets of images were compared between any two means.1.1.1The comparison of background noiseThe background noise of80kV,140kV,120kV image respectively is: 80kV (11.69±3.35),140kV (10.84±2.99),120kV (9.08±2.42).80kV,140kV,120kV three pair of comparative results:The image noise of80kV (t=3.644, P=3.644<0.05) and140kV(t=4.264, P=4.264<0.05) was bigger than120kV. There were no statistically significant difference between the image noise of80kV and140kV(t=1.030, P=1.030>0.05).1.1.2The comparison of SNRThe SNR of80kV,140kV,120kV of three groups of images respectively:80kV (23.17±2.07),120kV(19.87±6.53),140kV(13.11±13.11).Three two paired comparison results was:Among them,80kV(t=4.507, P=0<0.05)and120kV image (t=4.810, P=0<0.05) compared with140kV, the SNR of images were higher than140kV; The SNR of80kV image was higher than that of120kV image (t=2.226, P=0.040<0.05).1.1.3The comparison of CNRThe CNR of the three groups respectively were:80kV (13.53±6.92),140kV (3.58±2.07),120kv (7.46±4.00). Three pair of comparative results:Among them, the CNRs of80kV (t=10.202, P=0<0.05) and120kV (t=6.615, P=0<0.05) were higher than140kV; The CNR of80kV was higher than the CNR of120kV (t=5.565, P=0<0.05).1.2Group BAmong100kv,140kv and fused images (120kV), there were a total of three sets of two two compared images.1.2.1The comparison of background noiseThe background noises respectively were:100kV(9.56±3.99);140kV(9.05±2.04);120kV(6.66±6.66). The comparative results of BN of 100kV,140kV,120kV in the three groups:Among them, the noise of the100kV (t=3.234, P=3.234<0.05) and140kV (t=5.178, P=0<0.05) were higher than120kV. There were no statistically significant difference (t=0.716, P=0.716>0.05) between the BN of100kV and140kV.1.2.2The comparison of SNRThe CT value of renal cortex on100kV,140kV,120kV of three groups images respectively were:100kV (196.25±49.52);140kV(106.56±21.51);120kV(153.73±33.15). The SNR respectively was:100kV(23.58±5.50),140kV(13.51±3.21);120kV(27.29±8.83). The three two paired comparative results were:There was no statistically significant difference (t=1.766, P=1.766>0.05)between the SNR of100kv and120kv; Comparing the SNR of120kV to140kV, the SNR of120kV was higher than140kV (t=5.914, P=0.0O1<0.05); Comparing the SNR of100kV and140kV, the SNR of100kV was higher than that of140kV(t=6.624, P=0.000<0.05).1.2.3The comparisons of CNRThe CNR of the100kV,140kV,120kV three groups respectively were:100kV(11.21±6.74);140kV(4.68±3.36);120kV(11.11±7.89). The CNRs of the three groups images of100kV,140kV and120kV respectively were compared:The CNRs of100kV(t=11.701, P=0<0.05) and120kV(t=5.565, P=0<0.05) were higher than140kV; There was no statistically significant difference (t=0.262,P=0.795>0.05) between the CNR of100kV and120kV.2subjective evaluation2.1Group A The image quality assessment consistency of the two CT diagnosis physicians to the80kV,140kV, the fusion image (120kV) of the three groups of images was good (Kappa values respectively were0.819,0.954, P<0.05). The scores of80kV,140kV, fused images (120kV), A total of three sets of images were compared by pairwise comparison. Statistical results was:comparing the score of80kV to120kV there was no obvious difference in image quality with P=0.083>0.05;The image quality of80kv (P=0<0.05)and120kV (P=0<0.05)both were better than that of the140kV.2.2Group BThe image quality assessment consistency of the two CT diagnosis physicians to the100kV,140kV, the fusion image (120kV) was good (Kappa values respectively were0.769,0.845, P<0.05). The scores of100kV,140kV, fused images (120kV), A total of three sets of images were compared by pairwise comparison. Statistical results:Between the image quality of100kv and120kV,there was no obvious difference (P=0.157>0.05);The image quality of100kV (P=0<0.05) and120kV(P=0<0.05) both were better than the image quality of140kV. conclusions1. Kidney enhanced CT scanning using low tube voltage of80or100kV is feasible. The excellent image quality meeting the need of clinical diagnosis is available that can clearly show bilateral kidneys, the structures around the kidney and retroperitoneal dissection structures.2. The enhanced CT scanning of kidney tumors using80kV or100kV low tube voltage is feasible. We can gain completely excellent images meeting the needs of the clinical diagnosis, which can clearly show the characteristics of internal structure of tumor,characteristics of blood supply, the impact on the surrounding and whether there is the formation of venous emboli. The tumor detection rate is100%. |
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