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1 Comparison of pure iterative reconstruction techniques with hybrid iterative reconstruction technique and conventional filtered back projection: viability assessment of hepatocellular carcinoma in liver dynamic CT after TACE Award: Cum Laude Poster No.: C-1078 Congress: ECR 2015 Type: Scientific Exhibit Authors: M. Bang, J. G. Nam, S. H. Choi, Y. K. Jeong, B. S. Kang, S. W. Shim; Ulsan/KR Keywords: Abdomen, Computer applications, CT, Image manipulation / Reconstruction, Computer Applications-Detection, diagnosis DOI: /ecr2015/C-1078 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 27

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3 Aims and objectives Since the invention of CT in the early 1970s, CT images have been reconstructed from raw data using filtered back projection algorithm (FBP)[1]. Although image quality of FBP at lower radiation was rough and less optimal, it was widely used owing to fast image reconstruction time and simplicity of method[2,3]. Recently, remarkable improvements in the computational speed of processors have enabled the implementation of more complex reconstruction algorithms such as iterative reconstruction [2,]. The first generation iterative method "idose " is a hybrid iterative reconstruction that is applied by combining filtered back projection and iterative reconstruction algorithm [5,6]. The latest advanced algorithm, knowledge based iterative model reconstruction algorithm "Iterative Model Reconstruction" (IMR), is a pure iterative reconstruction algorithm [3,7]. IMR models the process of physical data acquisition as accurately as possible through the iterative minimization of the difference between measured raw data and the estimated image (figure1 ) [7]. Page 3 of 27

4 Fig. 1: Three reconstruction techniques:filetered back projection(fbp), hybrid iterative reconstruction technique(idose) and pure iterative reconstruction technique(imr) References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Until now, the usefulness of IMR on liver dynamic CT to evaluate the therapeutic response of transcatheter arterial chemoembolization (TACE) has been unknown. In the case of a nodule with incompact retention of lipiodol, it is difficult to detect the viable portion owing to its ambiguity and beam hardening artifact8). Several studies that have evaluated the quality of images acquired with IMR have shown a reduction of image noise and artifact with better image quality than those acquired with idose and FBP [3,,7,9]. Therefore, we hypothesized that IMR, compared with idose and FBP, would result in a more accurate viability assessment of hepatocellular carcinoma (HCC) on liver dynamic CT after TACE with lower noise and greater CNR. The purpose of this study was to investigate the impact of pure iterative reconstruction, "Iterative Model Reconstruction" (IMR) for viability assessment of HCC in liver dynamic CT after TACE, compared with hybrid reconstruction,"idose ", and filtered back projection. Page of 27

5 Images for this section: Fig. 1: Three reconstruction techniques:filetered back projection(fbp), hybrid iterative reconstruction technique(idose) and pure iterative reconstruction technique(imr) Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 5 of 27

6 Methods and materials Patients From April 2013 to December 2013, 17 patients who underwent liver dynamic CT for assessment of the therapeutic response after TACE were enrolled in this study. A total of 70 nodules in these 17 patients were evaluated. The interval period between TACE and enrolled CT acquisition ranged from 21 days to 3 years. The etiology of HCC in these patients were Hepatitis B virus infection in 12 patients, alcoholic hepatitis in three patients and Hepatitis C virus infection in two patients. Mean patient age was 62 years old, ranging from 9 to 75 years old. Thirteen patients were men and four patients were women. Data acquisition and scanning protocols All patients were examined using a 256-slice CT system. The parameters for the contrast enhanced helical CT scan were as follows; collimation: 128 x 0.625, gantry rotation time: 0. sec, and pitch: The tube potential was fixed to 100kV and the automatic tube current modulation technique (Z-axis Dose modulation) was used with a maximum current of 300mAs. Iodinated contrast medium at a rate of 3ml/sec was injected using a power injector. The average CTDIvol was 8.36 mgy in late arterial phase, 8.36 mgy in portal venous phase, 8.53 mgy in delayed phase. Image reconstruction We reconstructed the raw data using the same parameters, i.e. a 35-cm FOV and 3-mm section thickness at 2.7-mm intervals. Images including late arterial, portal venous and delayed phase were reconstructed with IMR, idose and FBP. For the current study, we used idose level as recommended by the vendor and as used in the previous study [9]. Lesion categorization and confirmation A truth reader (Y.K.J) with 20 years of experience of abdominal imaging, reviewed all three image sets obtained with the different reconstruction algorithms for each nodule and categorized the nodules into one of three groups: 1) non-lipiodolized enhancing nodule, 2) nodule with incompact retention of lipiodol, 3) nodule with compact retention of lipiodol. The truth reader noted the liver segment and slice number where each lesion was best seen. Referring to follow-up CT, MRI and TACE, the truth reader determined whether there was a viable portion in the nodule with incompact retention of lipiodol. The diagnostic criteria for viable tumor on imaging were as follows: a dense deposit of lipiodol on CT after TACE; definite growth; hyper-enhancement on late arterial phase and washout on portal venous or delayed phase during follow-up CT and MRI. Page 6 of 27

7 Quantitative image analysis Objective measurements were performed for the three image datasets of the 17 patients (a total of 51 image sets) by a radiologist (M.S.B.) with 6 years of abdominal imaging experience. The standard deviation of Housefield units within regions of interest (ROI) was recorded as noise. Circular ROI (10-50 mm2) was drawn in the homogeneous part of the subcutaneous fat layer of the anterior abdominal wall and the paravertebral back muscle in late arterial phase. The size, shape, and position of the ROI were kept constant by applying the copy-and-paste function at the work station for all measurements in each patient. This study also evaluated the contrast noise ratio (CNR) of viable portion to adjacent liver parenchyma. The CNR was calculated using the following formula; CNR = (ROI viable - ROI liver) / SD noise Where ROI viable is the mean attenuation of the viable portion, ROI liver is the mean attenuation of the adjacent liver parenchyma and SD noise is the image noise in the paravertebral back muscle and the subcutaneous fat layer of the anterior abdominal wall. Qualitative image analysis The randomized image sets were reviewed individually by two reviewers (M.S.B., J.C.H.) with 6 years and 20 years of abdominal imaging experience, respectively. Both were blinded to the reconstruction types. Two blinded reviewers individually detected and evaluated all three categories of nodules at the selected slice and decided presence of viable portion in those nodules with incompact retention of lipiodol in each of the three image sets. Reviewers were encouraged to vary the window width and level at will. Similar to previous studies of liver lesion detection in patients with HCC, lesion assessment was performed in late arterial phase but used evidence from all phases2). To improve interobserver agreement, the criteria for image grading were established by consensus between the two reviewers. Subjective visual conspicuity of the viable portion of nodules with incompact retention of lipiodol and non-lipiodolized enhancing nodule was graded on 1- scale, where 1 was barely perceptible with presence debatable, 2 was subtle finding but likely a lesion, 3 was a detected definite lesion, and was strikingly evident and easily detected. Artifact was also graded on 1- scale, where 1 was unacceptable, 2 was suboptimal, 3 was average, and was minimal. Statistical analysis All statistical analyses were performed using Statistical Product and Service Solution and R package. For quantitative and qualitative analysis, general linear model (GLM) was used to compare the three methods (IMR, idose and FBP) with patients as random factor. Intra-class correlation coefficients were calculated to confirm agreement between Page 7 of 27

8 two judgments for qualitative analysis including visual conspicuity and artifact. The level of significance was set at p< Page 8 of 27

9 Results The truth reader categorized all 70 nodules into 11 non-lipiodolized enhancing nodules, 31 nodules with incompact retention of lipiodol, and 28 nodules with compact retention of lipiodol. Forty three nodules were confirmed by follow- up CT, 17 nodules were confirmed by follow-up MRI, 9 nodules were confirmed by TACE and one nodule was confirmed surgically. Of the 31 nodules with incompact retention of lipiodol, 26 lesions were judged to have a remaining viable portion by the truth reader. Quantitative image analysis Image noise in the paravertebral back muscle and the subcutaneous fat layer of the anterior abdomen wall were significantly lower with IMR compared with idose and FBP. Image noise in the paravertebral back muscle was 55% lower with IMR compared with idose and 67% lower with IMR compared with FBP. Image noise in the subcutaneous fat layer of the anterior abdominal wall was 37% lower with IMR compared with idose and 58% lower with IMR compared with FBP (Table 1). CNR of viable portion to adjacent liver parenchyma with IMR was greater compared with idose and FBP. For IMR, CNR with calculated with noise in the paravertebral back muscle was 118% greater compared with idose and 212% greater compared with FBP. For IMR, CNR with calculated with noise in the subcutaneous fat of the anterior abdominal wall was 81% greater compared with idose and 138% greater compared with FBP (Table 1). Page 9 of 27

10 Table 1: Image noise with IMR was significantly lower than with idose and FBP in the paravertebral back muscle and the subcutaneous fat layer of the anterior abdominal wall (p<0.001, general linear model). Contrast to noise ratio with IMR was significantly greater than with idose and FBP ( p<0.001, general linear model). References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Qualitative image analysis Two blinded reviewers detected all 70 nodules and decided correctly on viable portion with IMR, whereas the viable portion in one nodule with incompact retention of lipiodol was missed with idose and the viable portion of two nodules with incompact retention of lipiodol and one non-lipiodolized enhancing nodule were missed with FBP. All 28 nodules with compact retention of lipiodol were detected by two blinded reviewers using the three different reconstruction algorithms. All viable portions of 26 nodules with incompact retention of lipiodol were also judged correctly on IMR. However, two viable portions of the 26 viable portions were not detected with FBP (Figure 2) and one viable portion detected by one reviewer was missed by the other reviewer using idose (Figure 3). One Page 10 of 27

11 of all 11 non-lipiodolized enhancing nodules which appeared with both IMR and idose was not detected with FBP by two blinded reviewers (Figure ). Fig. 2: A 56-year-old man with a nodule with incompact retention of lipiodol in the caudate lobe. The viable portion did not appear with FBP (A) but was detected with idose (B) and IMR (C). The viable portion was confirmed by definite growth on followup CT (D). The viable portion was more apparently seen with IMR (C) than with idose (B) (same section position using different reconstruction algorithm). References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 11 of 27

12 Fig. 3: A 59-year-old woman with a nodule with incompact retention of lipiodol at segment VI. The viable portion in IMR (C) was prominently seen due to better contrast of viable portion to adjacent liver parenchyma compared to FBP (A) and idose (B). The viable portion was confirmed surgically (D, precontrast image). References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 12 of 27

13 Fig. : A 70-year-old man with a non-lipiodolized enhancing nodule at segment VIII in late arterial phase. Although the enhancing nodule was not detected with FBP (A), it was apparently detected with idose (B) and IMR (C) (same section position using different reconstruction algorithm). Follow up CT (D) showed that lipiodol had accumulated in the enhancing nodule at segment VIII. References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Subjective visual conspicuity was significantly better with IMR compared with idose and FBP for the viable portion of nodules with incompact retention of lipiodol and nonlipiodolized enhancing nodules (p<0.001) in all reviewers (Table 2). Page 13 of 27

14 Table 2: Subjective visual conspicuity was significantly better with IMR than idose and FBP for viable portion of nodules with incompact retention of lipiodol and enhancing nodules (scale 1: barely perceptible with presence debatable, 2: subtle finding but likely a lesion, 3: detected definite lesion, : strikingly evident and easily detected) (p<0.001, general linear model). References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Artifact was significantly lower with IMR than with idose and FBP (p<0.001) (Table 3). The interclass correlation coefficient for interobserver agreement of subjective visual grading was above moderate in all three reconstruction algorithms. Page 1 of 27

15 Table 3: Artifact was significantly lower with IMR compared with idose and FBP (scale 1: unacceptable, 2: suboptimal, 3 :average, : minimal) (p<0.001, general linear model). References: Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 15 of 27

16 Images for this section: Table 1: Image noise with IMR was significantly lower than with idose and FBP in the paravertebral back muscle and the subcutaneous fat layer of the anterior abdominal wall (p<0.001, general linear model). Contrast to noise ratio with IMR was significantly greater than with idose and FBP ( p<0.001, general linear model). Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 16 of 27

17 Fig. 2: A 56-year-old man with a nodule with incompact retention of lipiodol in the caudate lobe. The viable portion did not appear with FBP (A) but was detected with idose (B) and IMR (C). The viable portion was confirmed by definite growth on follow-up CT (D). The viable portion was more apparently seen with IMR (C) than with idose (B) (same section position using different reconstruction algorithm). Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 17 of 27

18 Fig. 3: A 59-year-old woman with a nodule with incompact retention of lipiodol at segment VI. The viable portion in IMR (C) was prominently seen due to better contrast of viable portion to adjacent liver parenchyma compared to FBP (A) and idose (B). The viable portion was confirmed surgically (D, precontrast image). Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 18 of 27

19 Fig. : A 70-year-old man with a non-lipiodolized enhancing nodule at segment VIII in late arterial phase. Although the enhancing nodule was not detected with FBP (A), it was apparently detected with idose (B) and IMR (C) (same section position using different reconstruction algorithm). Follow up CT (D) showed that lipiodol had accumulated in the enhancing nodule at segment VIII. Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 19 of 27

20 Table 2: Subjective visual conspicuity was significantly better with IMR than idose and FBP for viable portion of nodules with incompact retention of lipiodol and enhancing nodules (scale 1: barely perceptible with presence debatable, 2: subtle finding but likely a lesion, 3: detected definite lesion, : strikingly evident and easily detected) (p<0.001, general linear model). Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 20 of 27

21 Table 3: Artifact was significantly lower with IMR compared with idose and FBP (scale 1: unacceptable, 2: suboptimal, 3 :average, : minimal) (p<0.001, general linear model). Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital - Ulsan/KR Page 21 of 27

22 Conclusion To the best of our knowledge, this study is the first study to compare image quality using three different reconstruction algorithms, IMR, idose and FBP in the viability assessment of hepatocellular carcinoma in liver dynamic CT after TACE. In this study, we detected all 70 nodules using IMR including three nodules which were not detected with idose or FBP. We also founded improved subjective visual conspicuity and reduced artifact with lower image noise and greater CNR using IMR, compared with idose and FBP. We demonstrated that IMR resulted in significant improvements in the viability assessment of hepatocellular carcinoma in liver dynamic CT after TACE. Liver dynamic CT is commonly used for the assessment of the therapeutic response to TACE for HCC. During follow-up, the focal defect or washout of lipiodol in the mass with the contrast enhanced area suggests the presence of a viable portion [10]. However, CT has several disadvantages for the evaluation of therapeutic response after TACE. First, the widely used embolizing agent lipiodol can generate beam hardening artifacts on CT[8,10,11]. Second, some viable tumor is difficult to detect due to subtle enhancement and poor contrast to the adjacent liver parenchyma. Compared to previous report using CT exam as a follow-up imaging tool for evaluating the therapeutic response after TACE, the diagnostic accuracy of the viable portion was relatively high in this study using IMR[10]. There are several factors for improved diagnostic accuracy with IMR, including reduction of artifact. Our study demonstrated that artifact was significantly reduced with IMR compared with idose and FBP. Previous studies have reported that iterative reconstruction reduced artifact in cervicothoracic lesion and prosthetic heart valves[11, 12]. Katsura et al.[11] investigated pure iterative reconstruction and found reduction of subjective streak artifact from shoulder in cervicothoracic area, compared with FBP. Sucha et al.[12] used IMR to study about artifact related to prosthetic heart valve and found reduction in artifact at a reduced dose acquisition. A significant improvement of the CNR was also a factor leading to increased diagnostic accuracy. In this study, CNR with IMR was 212% greater than seen with FBP. Several studies have reported an increased CNR with the use of variable methods such as varying the contrast concentrations and injection techniques to improve the diagnostic confidence in HCC[13]. IMR decreased artifact and increased CNR to overcome disadvantages of CT exam and improved the evaluation of therapeutic response in liver dynamic CT after TACE. Both idose and IMR are members of the iterative reconstruction algorithms aimed at providing alternative approaches to improve image quality[3]. "idose ", a hybrid iterative reconstruction, involves an initial FBP reconstruction followed by iterative modeling of noise statistics. It involves two de-noising components, that is, iterative maximum Page 22 of 27

23 likelihood- type sonogram restoration and local structure model fitting on image data that iteratively decrease uncorrected noise[3]. "IMR", pure iterative reconstruction model, is a updated iterative reconstruction algorithm[7]. It not only incorporates modeling of noise statics like idose, it also involves modeling system optics[11]. IMR involves sequential estimation of image from raw data. The forward projection of data from CT images are compared with the actual measured data according to statistical metrics, and the computed difference is itself back projected to create an image update [6]. In this study, CT image obtained with IMR were described by our reviewers as being "artificial" or "pixilated". Iterative reconstruction algorithms have previously been criticized for appearing blotchy, pixilated and plastic like [2,3]. This appearance was not prominently with idose images because of dynamic frequency noise removal methods used in recent hybrid iterative reconstruction algorithm. Considerable computational time for pure iterative reconstruction is another weakness[3,6]. Innovative hardware design and powerful super computers may eventually lead to a significant reduction in the reconstruction time for IMR. Currently, IMR has reduced the reconstruction time to 10 slices/sec, approximately less than one minute for three phases liver dynamic CT [3,6]. LIMITATION First, our study included a small number of patients. Large scale prospective study would be necessary for validation of our results. Second, while reviewers were blinded to reconstruction type, this blindness was not perfect because each reconstruction produced different image appearances. For accurate evaluation, our reviewers were asked to ignore different image appearance before evaluation. Third, lesion conspicuity was assessed in late arterial phase representatively although reviewers referred to other dynamic phases. According to the purpose of our study, we focused on HCC which appeared as enhancing lesion in late arterial phase[1]. In clinical practice, we evaluated liver lesion more commonly in routine abdominal enhanced CT rather than liver dynamic CT. Therefore, study including or focusing on portal venous phase in liver dynamic and abdominal CT should be evaluated in future studies. Fourth, we used only one raw image data with the same radiation parameter for comparing different reconstructions in this study. In prior report comparing IMR to other reconstruction algorithm concerning other body part, IMR image with radiation dose reduction resulted in reliable diagnostic confidence without compromising image quality[7,15]. For example, Deedar et al.[15] reported that lesion detection and diagnostic confidence on IMR image were not compromised in submsv chest CT with radiation dose reduction of 69% and Oda et al.[7] demonstrated that image noise using lower radiation dose and IMR was significantly lower with higher visual score compared with FBP at super-low-dose cardiac CT using 20% of the standard tube current. In the future, study of liver lesion should be performed with radiation dose reduction. Page 23 of 27

24 CONCLUSION This study showed that IMR reconstruction technique provided significantly enhanced quantitative and qualitative image quality compared to idose and FBP in liver dynamic CT. In conclusion, IMR could be a useful reconstruction technique for the evaluation of viability assessment of HCC in liver dynamic CT after TACE. Page 2 of 27

25 Personal information Minseo Bang, M.D. Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Jeong gu Nam, M.D. Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Seong Hoon Choi, M.D Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Yoong Ki Jeong, M.D. Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Byeong Seong Kang, M.D. Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Sang woo Shim, M.D. Department of Radiology, Ulsan university hospital, Ulsan University, Ulsan, Korea. Page 25 of 27

26 References 1. Halpern EJ, Gingold EL, White H, Read K. Evaluation of coronary artery image quality with knowledge-based iterative model reconstruction. Acad Radiol 201; 21(6): Shuman WP, Green DE, Busey JM, Kolokythas O, Mitsumori LM, Koprowicz KM, et al. Model-based iterative reconstruction versus adaptive statistical iterative reconstruction and filtered back projection in liver 6-MDCT: focal lesion detection, lesion conspicuity, and image noise. AJR Am J Roentgenol 2013; 200(5): Yuki H, Utsunomiya D, Funama Y, Tokuyasu S, Namimoto T, Hirai T, et al. Value of knowledge-based iterative model reconstruction in low-kv 256-slice coronary CT angiography. J Comput Assist Tomogr 201; 8(2): Funama Y, Taguchi K, Utsunomiya D, Oda S, Katahira K, Tokuyasu S, et al. Image quality assessment of an iterative reconstruction algorithm applied to abdominal CT imaging. Physica medica 201; 30(): Dobeli KL, Lewis SJ, Meikle SR, Thiele DL, Brennan PC. Noise-reducing algorithms do not necessarily provide superior dose optimisation for hepatic lesion detection with multidetector CT. Br J Radiol 2013; 86(1023): Willemink MJ, de Jong PA, Leiner T, de Heer LM, Nievelstein RA, Budde RP, et al. Iterative reconstruction techniques for computed tomography Part 1: technical principles. Eur Radiol 2013; 23(6): Oda S, Utsunomiya D, Funama Y, Katahira K, Honda K, Tokuyasu S, et al. A knowledge-based iterative model reconstruction algorithm: can super-low-dose cardiac CT be applicable in clinical settings? Acad Radiol 201; 21(1): Yu JS. Hepatocellular carcinoma after transcatheter arterial chemoembolization: difficulties on imaging follow-up. Korean journal of radiology 2005; 6(3): Song JS, Lee JM, Sohn JY, Yoon JH, Han JK, Choi BI. Hybrid iterative reconstruction technique for liver CT scans for image noise reduction and image quality improvement: evaluation of the optimal iterative reconstruction strengths. La Radiologia medica 201. Aug 29. [Epub ahead of print] Page 26 of 27

27 10. Minami Y, Kudo M. Therapeutic response assessment of transcatheter arterial chemoembolization for hepatocellular carcinoma: ultrasonography, CT and MR imaging. Oncology 2013; 8 Suppl 1: Katsura M, Sato J, Akahane M, Matsuda I, Ishida M, Yasaka K, et al. Comparison of pure and hybrid iterative reconstruction techniques with conventional filtered back projection: image quality assessment in the cervicothoracic region. Eur J Radiol 2013; 82(2): Sucha D, Willemink MJ, de Jong PA, Schilham AM, Leiner T, Symersky P, et al. The impact of a new model-based iterative reconstruction algorithm on prosthetic heart valve related artifacts at reduced radiation dose MDCT. Int J Card Imaging 201; 30(): Bendik E, Noel PB, Munzel D, Fingerle AA, Henninger M, Markus C, et al. Evaluation of a method for improving the detection of hepatocellular carcinoma. Eur Radiol 201; 2(1): Taylor AJ, Carmody TJ, Quiroz FA, Erickson SJ, Varma RR, Komorowski RA, et al. Focal masses in cirrhotic liver: CT and MR imaging features. AJR Am J Roentgenol 199; 163(): Khawaja RD, Singh S, Gilman M, Sharma A, Do S, Pourjabbar S, et al. Computed tomography (CT) of the chest at less than 1 msv: an ongoing prospective clinical trial of chest CT at submillisievert radiation doses with iterative model image reconstruction and idose technique. J Comput Assist Tomogr 201; 38(): Page 27 of 27