迭代模型重组设置对不同辐射剂量下肝脏增强CT图像噪声及质量的影响

Knowledge-based iterative reconstruction technique： effect of different reconstruction settings on noise reduction and image quality in hepatic contrast-enhanced CT at different radiation dose levels

目的 探讨迭代模型重组技术（IMR）的不同重组设置对不同辐射剂量下肝脏增强CT噪声及图像质量的影响.方法 前瞻性收集因病情需要行肝脏增强CT检查的患者纳入研究,将患者按照数字表法随机分为常规组和低剂量组2组.常规组29例,低剂量组25例.患者均行上腹部平扫和肝脏3期增强扫描.常规组门静脉期管电压120 kV,管电流250 mAs;低剂量组门静脉期管电压80 kV,管电流500 mAs.将门静脉期原始数据进行传统滤波反投影法（FBP）重组和IMR重组,IMR重组得到不同重组水平（L1～ L3）的体部常规重组（BR）和体部软组织重组（BST）图像,分别为L1BR、L2BR、L3BR、L1BST、L2BST、L3BST.对各组图像进行主观评价[包括低对比分辨率（LCD）评分、图像失真（ID）评分和诊断信心（DC）评分]、客观评价[包括肝脏噪声、信噪比（SNR）和对比噪声比（CNR）],并记录有效辐射剂量.采用FriedmanH检验和随机区组方差分析比较各组图像的主观和客观评价指标,采用两独立样本t检验比较常规组和低剂量组的辐射剂量.结果 常规组图像中,LCD评分L2BST组、L3BST组较高,中位数均为4分;ID评分L3BST、L2BST组较低,中位数为3分,FBP及L1BR组较高,中位数均为4分;DC评分L1BR、L2BR及L1BST组较高,中位数均为4分,L3BST组最低,中位数为3分.低剂量组图像中,LCD、ID评分分布与常规组相似,DC评分L1BST组最高（4分）而FBP组最低（2分）.不同剂量组各重组图像的LCD、ID和DC评分差异均有统计学意义（P＜0.05）.常规组FBP图像的噪声为（9.8±2.0）HU,SNR为13.3±3.3,CNR为6.0±1.9;低剂量组FBP图像图像噪声、SNR和CNR分别为（16.2±4.1）HU、12.9±3.3和6.6±2.3.常规组随着IMR设置从L1BR到L3BST依次增高,图像噪声依次降低,从（4.5±0.9）HU降低到（3.2±0.2） HU,SNR及CNR则依次升高,SNR从21.4±1.4升高到24.6±4.6,CNR从9.4±2.2升高到10.9±2.9;低剂量组图像噪声、SNR及CNR的变化规律同常规组,从L1BR到L3BST图像噪声从（5.8±0.2）HU降低到（3.9±0.6） HU,SNR从26.3±4.5升高到33.1±6.9,CNR从13.6±4.0升高到17.4±5.1.常规组和低剂量组不同重组设置的7组图像间噪声、SNR及CNR的差异均有统计学意义（F值为15.50～ 131.39,P值＜0.01）.常规剂量组和低剂量组的有效辐射剂量分别为（7.32±0.58）、（4.19±0.22） mSv,差异有统计学意义（t=15.27,P＜0.01）.结论 IMR重组技术可降低肝脏增强CT图像噪声,并能提高图像质量.在常规辐射剂量CT扫描时,推荐选择重组设置L1BR、L2BR和L1BST;在低剂量扫描时,推荐选择L1BST.

Objective To evaluate the effects of different reconstruction settings of knowledge-based iterative reconstruction technique （IMR） on noise reduction and image quality in hepatic contrast-enhanced CT（CECT） at different radiation dose levels.Methods Patients who underwent hepatic CECT because of their diseases were enrolled in this prospective study.According to random number table,patients were randomly assigned to two groups （standard dose group,SD-group,n=29; low dose group,LD-group,n=25）.All patients underwent both non-enhanced CT and triphasic CECT scan including hepatic arterial phase （HAP）,portal-venous phase （PVP） and delayed phase.PVP images of SD-group were acquired with tube voltage of 120 kVp and tube current-time products of 250 mAs,and 80 kVp and 500 mAs for LD-group.PVP images were reconstructed with both filtered back projection （FBP） and IMR techniques.Settings applied in IMR reconstructions consisted of body routine （BR） and body soft tissue （BST） with three levels （L1 to L3）,and image series included：L1BR,L2BR,L3BR,L1BST,L2BST and L3BST.Subjective assessment of image quality including low contrast detectability （LCD）,image distortion （ID） and diagnostic confidence （DC） as well as objective image quality including image noise,signal to noise ratio （SNR） and contrast to noise ratio （CNR） were compared between groups.Effective radiation dose was recorded.Objective image quality and subjective image quality were compared by using Friedman H test and ANOVA,respectively.In addition,Student t test was used to compare effective radiation doses between groups.Results In SD-group,IMR settings of L2BST,L3BST scored highest in LCD with median score of 4;L3BST and L2BST scored lowest in ID with median score of 3,while FBP and L1BR scored highest with median score of 4; L1BR,L2BR and L1BST scored highest in DC with median score of 4,while L3BST scored lowest with median score of 3.In LD-group,the distribution of all reconstruction settings scores in LCD and ID were similar to those in SD-group; however,L1BST scored highest with median score of 4,and FBP scored lowest with median score of 2 in DC.There were statistical differences in LCD,ID and DC among all the reconstruction settings for both groups （P＜0.05）.In terms of FBP technique,the image noise,SNR and CNR were （9.8±2.0）HU,13.3±3.3 and 6.0±1.9 in SD-group,and （16.2±4.1）HU,12.9±3.3 and 6.6± 2.3 in LD-group,respectively.In SD-group,image noise exhibited a trend of decrease from L1BR to L3BST [from （4.5±0.9） HU to （3.2±0.2）HU],while SNR,CNR trended to increase （SNR：from 21.4±1.4 to 24.6±4.6; CNR：from 9.4±2.2 to 10.9±2.9,respectively）.The image noise,SNR and CNR in LD-group showed the same trends as SD-group [image noise：from （5.8±0.2）HU to （3.9±0.6）HU; SNR：from 26.3±4.5 to 33.1±6.9; CNR：from 13.6±4.0 to17.4±5.1,respectively].In both groups,there were statistically differences in image noise,SNR and CNR among all IMR settings and FBP （F values were 15.50 to 131.39,P＜0.01）.The effective radiation dose of the LD-group was significantly lower than that of the SD-group [（4.19±0.22） versus （7.32±0.58） mSy,t=15.27,P＜0.01）.Conclusions IMR can reduce image noise and improve image quality in hepatic CECT at both standard and low dose levels.L1BR,L2BR and L1BST are the most optimized reconstruction settings for the standard dose protocol,while L1BST performs best for the low dose protocol.