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 / 中华放射学杂志

ABSTRACT

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 includedL1BR,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 (SNRfrom 21.4±1.4 to 24.6±4.6; CNRfrom 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 noisefrom (5.8±0.2)HU to (3.9±0.6)HU; SNRfrom 26.3±4.5 to 33.1±6.9; CNRfrom 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.