FLASH双源CT双能量扫描在肾脏肿瘤低kV扫描技术中的应用研究
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摘要
研究背景
现如今,多层螺旋CT在临床应用中已经得到了极大的普及及广泛的使用,已经成为临床检查中必不可少的一种检查方法。但是在它给我们带来方便的同时,随之而来的辐射也带给我们很大的危害,其潜在的致癌及致畸风险不容忽视。因此,如何降低CT检查患者所接受到的辐射剂量已经成为当前国内外研究的热点。目前,各种各样的措施和方法被应用的临床中,例如：厂家在硬件方面的改革和软件方面的更新改进,如管电流自动调节技术、智能管电压扫描技术、大螺距扫描、适当减少强化扫描次数、避免敏感器官的照射及敏感器官的屏蔽等等很多很多。但是,很多的研究表明,降低管电压是最直接、最有效的方法。目前,低剂量研究主要集中肺部、鼻窦等,国外在骨骼系统的低剂量研究也较多[1,2],这是因为这些部位的自然对比比较高,适当的降低扫描条件后,即使是图像的噪声增加,信噪比较低到一定的程度也不会影响其在临床中的应用。另外,CT血管成像(Computed Tomography angiography, CTA)也是国内外研究的热点[3],这是由于血管内高浓度的对比剂与周围组织器官的密度对比非常大,略有增加的噪声对图像尤其是疾病诊断的影响并不是很大。而腹部CT检查的低剂量研究较少[4,5,6],这是由于腹部的自然对比较差,各组织器官之间的密度差别不多,尤其是当实质脏器内的病变较小时、与周围正常实质的密度差别不是很大、或者是发现病变而定性困难时,就需要进行多期的动态增强扫描,这在临床中已经成为常规,但这就使得患者所接受的辐射剂量成倍地增加。因此,如何最大程度的降低腹部CT检查的辐射剂量这一问题非常迫切地需要我们解决。对于肾脏来说,肾皮质增强扫描图像强化程度非常强,皮髓质对比鲜明,尤其是肾脏肿瘤,多数是肾细胞癌,富血供,其增强扫描的特点是动脉期明显强化,而实质期或延迟期对比剂快速退出,这些特点为肾脏低kV扫描的研究提供了病理学基础。
另外,双源CT成像技术已经日趋成熟。双源CT有两套可以同时发射的X线球管及其探测器,分别安装于扫描机架上,互成90。。其管电压多数分别采用80kV和140kV。Siemens医疗公司推出的第二代SOMATOM Definition FLASHCT管电压还可以采用100kV和140kV[7]。双能量扫描每次可以获得以下三组图像：纯80kVp图像(或纯100kVp图像)、纯140kVp图像和融合图像(相当于120kVp的加权图像：是由80kVp图像和140kVp图像采用一定的权重因子产生的,其图像质量类似于120kV管电压条件下得到的图像),所以进行单次的双能量扫描就可从同一患者身上得到不同管电压的三组图像：80(或100) kV、120kV及140kV,并且与单源CT的单次扫描相比辐射剂量还不回增加。另外,利用双能量后处理软件系统,还可以得到虚拟平扫图像、碘图等[8]。因此,双源CT的双能量扫描为我们肾脏低管电压的扫描技术研究提供了便捷的途径。
本研究就是利用双源CT肾脏的双能量扫描采集到的不同管电压的多组图像数据进行比较、分析,旨在证明利用较低的管电压(80kV、100kV)就可以得到肾脏肿瘤足以满足诊断要求的图像,为以后单源CT低管电压扫描提供依据,从而大大地降低辐射剂量。
研究目的
利用双源CT对肾肿瘤患者增强扫描动脉期进行双能量扫描,对分别获得的三组图像(80kV或100kV图像、140kV图像、相当于120kV的融合图像)的图像质量进行比较、分析,研究不同管电压对肾脏动脉期扫描图像质量的影响,从而探讨肾脏肿瘤的低管电压CT扫描的可行性。
材料与方法
采用Siemens SOMATOM Definition FLASH双源CT扫描仪。随机选取查体发现肾脏肿瘤或临床怀疑肾脏肿瘤的患者40例作为研究对象。所有的病例均行增强扫描,动脉期均采用双能量扫描。根据高、低管电压采用的数值将研究对象平均分为两组：A组(20例)：管电压采用80kV和140kV；B组(20例)：管电压采用100kV和140kV。每一位患者的动脉期均能获得三组不同管电压的图像：低管电压(80或100kV)的图像、高管电压的图像(140kV)和融合图像(相当于120kV)。对每三组图像的图像质量分别进行评价,评价的方法分为两种：客观评价和主观评价。
客观评价：客观评价的指标采用背景噪声(Background noise, BN)、信号噪声比(Signal-noise ratio,SNR)及对比噪声比(Contrast-noise ratio, CNR).信号强度(Signal intensity, SI)采用正常肾皮质的CT值来表示。对每一个病人的不同管电压的3组图像选取肿瘤的最大截面或能体现肿瘤动脉期高强化特点的层面1-3层(根据肿瘤的截面大小来决定选取的层面数),分别来测量图像的背景噪声、肿瘤的明显高强化区域的CT值、同层面脊柱旁肌肉的CT值、对侧正常肾脏高强化的肾皮质的CT值及其同层面图像的背景噪声,并计算CNR、SNR。CNR=(肾肿瘤高强化区域的CT值-同层面脊柱旁肌肉的CT值)/景噪声(BN), SNR=正常肾皮质的CT值(SI)/景噪声(BN)。采用SPSS13.0for Windows统计学软件,对A、B两组研究对象的三组图像所获得的SNR及CNR分别进行统计学分析,统计学方法采用配对T检验进行两两比较,以P<0.05认为差别具有统计学意义。
主观评价：将A、B两组病例的厚层、薄层图像均传输到工作站,由2位有经验的副主任医师以上的CT诊断医师采用双盲法、在不知扫描条件的情况下单独对两组图像进行软阅片,并使用工作站提供的常规三维后处理方法(主要是MIP, VR)对图像进行重建、测量等分析,要求观察者对肾脏肿瘤的有无进行记录、计算出肿瘤的检出率,分别对三组横轴位图像及相应的三维后处理图像的图像质量进行综合评价并打分。评分标准采用“5级评分法”：1分：很好；2分：较好；3分：一般,但不影响肾肿瘤诊断和对肿瘤的周围侵犯、转移的评估；4分：较差,对诊断有一定的影响；5分：图像质量差,无法诊断。将A、B两组病人分别获得的三组图像的评分结果进行两两比较,采用威尔科克森(Wilcoxon)符号秩和检验进行统计学分析,P<0.05时认为差异具有统计学意义。通过Kappa检验对2位CT诊断医师进行一致性检验,当Kappa值≥0.75时认为诊断一致性较好；当0.75>Kappa值≥0.4时,认为诊断一致性一般；当Kappa值<0.4时,认为诊断一致性较差。结果
1.客观评价
1.1A组
80kV,140kV,融合图像(相当于120kV)共三组图像分别两两比较。
1.1.1背景噪声的比较
80kV,140kV,120kV图像的背景噪声分别为：80kV (11.69±3.35)、140kV (10.84±2.99)、120kV (9.08±2.42)。三者分别两两配对比较的结果：80kV (t=3.644, P=0.002<0.05)与140kV(t=4.264, P=0.001<0.05)的图像噪声均大于120kV的背景噪声。80kV与140kV图像的噪声相比较,其差异不具有统计学意义(t=1.030,P=0.318>0.05)。
1.1.2SNR的比较
80kV,140kV,120kV三组图像的SNR分别为：80kV(23.17±2.07):120kV (19.87±6.53):140kV(13.11±3.67).三者两两配对比较的结果：其中,80kV(t=4.507, P=0<0.05)与120kV图像(t=4.810,P=0<0.05)的SNR与140kV的相比较,均高于140kV的图像；80kV图像的SNR与120kV图像的相比,80kV的SNR高于120kV的(t=2.226,P=0.040<0.05)。
1.1.3CNR的比较
80kV,140kV,120kV三组图像的CNR分别为：80kV(13.53±6.92);140kV(3.58±2.07);120kV(7.46±4.00).三者两两配对比较的结果：其中,80kV(t=10.202, P=0<0.05)与120kV图像(t=6.615,P=0<0.05)的CNR均高于140kV的图像；80kV图像的CNR亦高于120kV的图像(t=5.565,P=0<0.05)。
1.2B组
100kV,140kV,融合图像(相当于120kV)共三组图像分别两两比较。
1.2.1背景噪声的比较
100kV,140kV,120kV三组图像的背景噪声分别为：100kV(9.56±3.99)；140kV(9.05±2.04)；120kV(6.66±1.70).100kV,140kV,120kV三组图像的背景噪声两两比较的结果：其中,100kV(t=3.234, P=0.04<0.05)与140kV(t=5.178, P=0<0.05)图像的噪声均高于120kV.100kV与140kV的图像背景噪声比较,其差异不具有统计学意义(t=0.716,P=0.481>0.05)。
1.2.2SNR的比较
100kV,140kV,120kV三组图像肾皮质的CT值分别为：100kV(196.25±49.52)；140kV(106.56±21.51)；120kV(153.73±33.15).SNR分别为：100kV(23.58±5.50),140kV(13.51±3.21)；120kV(27.29±8.83).SNR两两比较的结果：100kV和120kV比较,差异无统计学意义(t=1.766,P=0.121>0.05);120kV和140kV的SNR相比,120kV高于140kV的SNR(t=5.914, P=0.001<0.05);100kV和140kV的SNR相比,100kV的SNR高于140kV的(t=6.624,P=0.000<0.05)。
1.2.3CNR的比较
100kV,140kV,120kV三组图像的CNR分别为100kV (11.21±6.74);140kV (4.68±3.36);120kV (11.11±7.89)。100kV,140kV,120kV三组图像所获得的CNR分别两两比较的结果：100kV (t=11.701, P=0<0.05)与120kv(t=5.565, P=0<0.05)的CNR都高于140kV;100kV与120kV比较,二者的CNR差异不具有统计学意义(t=0.262,P=0.795>0.05)。
2主观评价
2.1A组
两位CT诊断医师对80kV、140kV、融合图像(相当于120kV)三组图像的质量评价一致性较好(Kappa值分别为0.819、0.954,P<0.05)。对80kV,140kV,融合图像(相当于120kV)三组图像的评分分别两两比较,统计分析比较的结果是：80kV与120kV的图像相比,二者的图像质量无明显差异(P=0.083>0.05);80kV (P=0<0.05)与120kV(P=0<0.05)的图像分别与140kV的图像相比,均比140kV的图像质量好。
2.2B组
两位CT诊断医师对100kV、140kV.融合图像(相当于120kV)三组图像的质量评价一致性较好(Kappa值分别为0.769、0.845,P<0.05)。对100kV,140kV,融合图像(相当于120kV)三组图像的评分分别两两比较。统计比较的结果是：100kV与120kV的图像相比,二者的图像质量无明显差异(P=0.157>0.05);100kV (P=0<0.05)与120kV (P=0<0.05)的图像与140kV的相比,均比140kV的图像质量好。
结论
1.肾脏CT增强扫描采用80或100kV的低管电压是可行的,可获得能够清晰地显示双肾实质、肾周及腹膜后解剖结构的、满足临床诊断需要的、优良的图像质量；
2.肾脏肿瘤的CT增强扫描采用80或100kV的低管电压是可行的,肿瘤的检出率均为100%,完全能够获得满足临床诊断需要的优秀图像,能清晰的显示肿瘤的内部结构特征、血供特点、对周围的影响及是否有静脉栓子的形成。
Background
Nowadays, 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.
Objective
The 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 methods
40cases 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.
Results
1. objective evaluation
1.1Group A
80kV,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 noise
The 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 SNR
The 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 CNR
The 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 B
Among100kv,140kv and fused images (120kV), there were a total of three sets of two two compared images.
1.2.1The comparison of background noise
The 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 SNR
The 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 CNR
The 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 evaluation
2.1Group 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 B
The 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. conclusions
1. 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%.
现如今,多层螺旋CT在临床应用中已经得到了极大的普及及广泛的使用,已经成为临床检查中必不可少的一种检查方法。但是在它给我们带来方便的同时,随之而来的辐射也带给我们很大的危害,其潜在的致癌及致畸风险不容忽视。因此,如何降低CT检查患者所接受到的辐射剂量已经成为当前国内外研究的热点。目前,各种各样的措施和方法被应用的临床中,例如：厂家在硬件方面的改革和软件方面的更新改进,如管电流自动调节技术、智能管电压扫描技术、大螺距扫描、适当减少强化扫描次数、避免敏感器官的照射及敏感器官的屏蔽等等很多很多。但是,很多的研究表明,降低管电压是最直接、最有效的方法。目前,低剂量研究主要集中肺部、鼻窦等,国外在骨骼系统的低剂量研究也较多[1,2],这是因为这些部位的自然对比比较高,适当的降低扫描条件后,即使是图像的噪声增加,信噪比较低到一定的程度也不会影响其在临床中的应用。另外,CT血管成像(Computed Tomography angiography, CTA)也是国内外研究的热点[3],这是由于血管内高浓度的对比剂与周围组织器官的密度对比非常大,略有增加的噪声对图像尤其是疾病诊断的影响并不是很大。而腹部CT检查的低剂量研究较少[4,5,6],这是由于腹部的自然对比较差,各组织器官之间的密度差别不多,尤其是当实质脏器内的病变较小时、与周围正常实质的密度差别不是很大、或者是发现病变而定性困难时,就需要进行多期的动态增强扫描,这在临床中已经成为常规,但这就使得患者所接受的辐射剂量成倍地增加。因此,如何最大程度的降低腹部CT检查的辐射剂量这一问题非常迫切地需要我们解决。对于肾脏来说,肾皮质增强扫描图像强化程度非常强,皮髓质对比鲜明,尤其是肾脏肿瘤,多数是肾细胞癌,富血供,其增强扫描的特点是动脉期明显强化,而实质期或延迟期对比剂快速退出,这些特点为肾脏低kV扫描的研究提供了病理学基础。
另外,双源CT成像技术已经日趋成熟。双源CT有两套可以同时发射的X线球管及其探测器,分别安装于扫描机架上,互成90。。其管电压多数分别采用80kV和140kV。Siemens医疗公司推出的第二代SOMATOM Definition FLASHCT管电压还可以采用100kV和140kV[7]。双能量扫描每次可以获得以下三组图像：纯80kVp图像(或纯100kVp图像)、纯140kVp图像和融合图像(相当于120kVp的加权图像：是由80kVp图像和140kVp图像采用一定的权重因子产生的,其图像质量类似于120kV管电压条件下得到的图像),所以进行单次的双能量扫描就可从同一患者身上得到不同管电压的三组图像：80(或100) kV、120kV及140kV,并且与单源CT的单次扫描相比辐射剂量还不回增加。另外,利用双能量后处理软件系统,还可以得到虚拟平扫图像、碘图等[8]。因此,双源CT的双能量扫描为我们肾脏低管电压的扫描技术研究提供了便捷的途径。
本研究就是利用双源CT肾脏的双能量扫描采集到的不同管电压的多组图像数据进行比较、分析,旨在证明利用较低的管电压(80kV、100kV)就可以得到肾脏肿瘤足以满足诊断要求的图像,为以后单源CT低管电压扫描提供依据,从而大大地降低辐射剂量。
研究目的
利用双源CT对肾肿瘤患者增强扫描动脉期进行双能量扫描,对分别获得的三组图像(80kV或100kV图像、140kV图像、相当于120kV的融合图像)的图像质量进行比较、分析,研究不同管电压对肾脏动脉期扫描图像质量的影响,从而探讨肾脏肿瘤的低管电压CT扫描的可行性。
材料与方法
采用Siemens SOMATOM Definition FLASH双源CT扫描仪。随机选取查体发现肾脏肿瘤或临床怀疑肾脏肿瘤的患者40例作为研究对象。所有的病例均行增强扫描,动脉期均采用双能量扫描。根据高、低管电压采用的数值将研究对象平均分为两组：A组(20例)：管电压采用80kV和140kV；B组(20例)：管电压采用100kV和140kV。每一位患者的动脉期均能获得三组不同管电压的图像：低管电压(80或100kV)的图像、高管电压的图像(140kV)和融合图像(相当于120kV)。对每三组图像的图像质量分别进行评价,评价的方法分为两种：客观评价和主观评价。
客观评价：客观评价的指标采用背景噪声(Background noise, BN)、信号噪声比(Signal-noise ratio,SNR)及对比噪声比(Contrast-noise ratio, CNR).信号强度(Signal intensity, SI)采用正常肾皮质的CT值来表示。对每一个病人的不同管电压的3组图像选取肿瘤的最大截面或能体现肿瘤动脉期高强化特点的层面1-3层(根据肿瘤的截面大小来决定选取的层面数),分别来测量图像的背景噪声、肿瘤的明显高强化区域的CT值、同层面脊柱旁肌肉的CT值、对侧正常肾脏高强化的肾皮质的CT值及其同层面图像的背景噪声,并计算CNR、SNR。CNR=(肾肿瘤高强化区域的CT值-同层面脊柱旁肌肉的CT值)/景噪声(BN), SNR=正常肾皮质的CT值(SI)/景噪声(BN)。采用SPSS13.0for Windows统计学软件,对A、B两组研究对象的三组图像所获得的SNR及CNR分别进行统计学分析,统计学方法采用配对T检验进行两两比较,以P<0.05认为差别具有统计学意义。
主观评价：将A、B两组病例的厚层、薄层图像均传输到工作站,由2位有经验的副主任医师以上的CT诊断医师采用双盲法、在不知扫描条件的情况下单独对两组图像进行软阅片,并使用工作站提供的常规三维后处理方法(主要是MIP, VR)对图像进行重建、测量等分析,要求观察者对肾脏肿瘤的有无进行记录、计算出肿瘤的检出率,分别对三组横轴位图像及相应的三维后处理图像的图像质量进行综合评价并打分。评分标准采用“5级评分法”：1分：很好；2分：较好；3分：一般,但不影响肾肿瘤诊断和对肿瘤的周围侵犯、转移的评估；4分：较差,对诊断有一定的影响；5分：图像质量差,无法诊断。将A、B两组病人分别获得的三组图像的评分结果进行两两比较,采用威尔科克森(Wilcoxon)符号秩和检验进行统计学分析,P<0.05时认为差异具有统计学意义。通过Kappa检验对2位CT诊断医师进行一致性检验,当Kappa值≥0.75时认为诊断一致性较好；当0.75>Kappa值≥0.4时,认为诊断一致性一般；当Kappa值<0.4时,认为诊断一致性较差。结果
1.客观评价
1.1A组
80kV,140kV,融合图像(相当于120kV)共三组图像分别两两比较。
1.1.1背景噪声的比较
80kV,140kV,120kV图像的背景噪声分别为：80kV (11.69±3.35)、140kV (10.84±2.99)、120kV (9.08±2.42)。三者分别两两配对比较的结果：80kV (t=3.644, P=0.002<0.05)与140kV(t=4.264, P=0.001<0.05)的图像噪声均大于120kV的背景噪声。80kV与140kV图像的噪声相比较,其差异不具有统计学意义(t=1.030,P=0.318>0.05)。
1.1.2SNR的比较
80kV,140kV,120kV三组图像的SNR分别为：80kV(23.17±2.07):120kV (19.87±6.53):140kV(13.11±3.67).三者两两配对比较的结果：其中,80kV(t=4.507, P=0<0.05)与120kV图像(t=4.810,P=0<0.05)的SNR与140kV的相比较,均高于140kV的图像；80kV图像的SNR与120kV图像的相比,80kV的SNR高于120kV的(t=2.226,P=0.040<0.05)。
1.1.3CNR的比较
80kV,140kV,120kV三组图像的CNR分别为：80kV(13.53±6.92);140kV(3.58±2.07);120kV(7.46±4.00).三者两两配对比较的结果：其中,80kV(t=10.202, P=0<0.05)与120kV图像(t=6.615,P=0<0.05)的CNR均高于140kV的图像；80kV图像的CNR亦高于120kV的图像(t=5.565,P=0<0.05)。
1.2B组
100kV,140kV,融合图像(相当于120kV)共三组图像分别两两比较。
1.2.1背景噪声的比较
100kV,140kV,120kV三组图像的背景噪声分别为：100kV(9.56±3.99)；140kV(9.05±2.04)；120kV(6.66±1.70).100kV,140kV,120kV三组图像的背景噪声两两比较的结果：其中,100kV(t=3.234, P=0.04<0.05)与140kV(t=5.178, P=0<0.05)图像的噪声均高于120kV.100kV与140kV的图像背景噪声比较,其差异不具有统计学意义(t=0.716,P=0.481>0.05)。
1.2.2SNR的比较
100kV,140kV,120kV三组图像肾皮质的CT值分别为：100kV(196.25±49.52)；140kV(106.56±21.51)；120kV(153.73±33.15).SNR分别为：100kV(23.58±5.50),140kV(13.51±3.21)；120kV(27.29±8.83).SNR两两比较的结果：100kV和120kV比较,差异无统计学意义(t=1.766,P=0.121>0.05);120kV和140kV的SNR相比,120kV高于140kV的SNR(t=5.914, P=0.001<0.05);100kV和140kV的SNR相比,100kV的SNR高于140kV的(t=6.624,P=0.000<0.05)。
1.2.3CNR的比较
100kV,140kV,120kV三组图像的CNR分别为100kV (11.21±6.74);140kV (4.68±3.36);120kV (11.11±7.89)。100kV,140kV,120kV三组图像所获得的CNR分别两两比较的结果：100kV (t=11.701, P=0<0.05)与120kv(t=5.565, P=0<0.05)的CNR都高于140kV;100kV与120kV比较,二者的CNR差异不具有统计学意义(t=0.262,P=0.795>0.05)。
2主观评价
2.1A组
两位CT诊断医师对80kV、140kV、融合图像(相当于120kV)三组图像的质量评价一致性较好(Kappa值分别为0.819、0.954,P<0.05)。对80kV,140kV,融合图像(相当于120kV)三组图像的评分分别两两比较,统计分析比较的结果是：80kV与120kV的图像相比,二者的图像质量无明显差异(P=0.083>0.05);80kV (P=0<0.05)与120kV(P=0<0.05)的图像分别与140kV的图像相比,均比140kV的图像质量好。
2.2B组
两位CT诊断医师对100kV、140kV.融合图像(相当于120kV)三组图像的质量评价一致性较好(Kappa值分别为0.769、0.845,P<0.05)。对100kV,140kV,融合图像(相当于120kV)三组图像的评分分别两两比较。统计比较的结果是：100kV与120kV的图像相比,二者的图像质量无明显差异(P=0.157>0.05);100kV (P=0<0.05)与120kV (P=0<0.05)的图像与140kV的相比,均比140kV的图像质量好。
结论
1.肾脏CT增强扫描采用80或100kV的低管电压是可行的,可获得能够清晰地显示双肾实质、肾周及腹膜后解剖结构的、满足临床诊断需要的、优良的图像质量；
2.肾脏肿瘤的CT增强扫描采用80或100kV的低管电压是可行的,肿瘤的检出率均为100%,完全能够获得满足临床诊断需要的优秀图像,能清晰的显示肿瘤的内部结构特征、血供特点、对周围的影响及是否有静脉栓子的形成。
Background
Nowadays, 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.
Objective
The 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 methods
40cases 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.
Results
1. objective evaluation
1.1Group A
80kV,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 noise
The 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 SNR
The 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 CNR
The 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 B
Among100kv,140kv and fused images (120kV), there were a total of three sets of two two compared images.
1.2.1The comparison of background noise
The 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 SNR
The 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 CNR
The 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 evaluation
2.1Group 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 B
The 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. conclusions
1. 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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