Multidetector CT: Diagnostic Impact of Slice Thickness on Detection of Hypervascular Hepatocellular Carcinoma

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1 Shuji Kawata 1 Takamichi Murakami 1 Tonsok Kim 1 Masatoshi Hori 1 Michael P. Federle 2 Seishi Kumano 1 Eiji Sugihara 1 Shigeru Makino 1 Hironobu Nakamura 1 Masayuki Kudo 3 Received October 15, 2001; accepted after revision December 27, Department of Radiology, Osaka University Graduate School of Medicine D1, 2-2 Yamadaoka, Suita City, Osaka, Japan. Address correspondence to T. Murakami. 2 Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop St, Pittsburgh, PA General Electric Yokogawa Medical Systems, Asahigaoka 4-chome, Hino City, Tokyo, Japan. AJR 2002;179: X/02/ American Roentgen Ray Society Multidetector CT: Diagnostic Impact of Slice Thickness on Detection of Hypervascular Hepatocellular Carcinoma OBJECTIVE. The objective of our study was to evaluate the diagnostic impact of varying slice thickness on multidetector CT to optimize detection of hypervascular hepatocellular carcinoma. MATERIALS AND METHODS. Forty-three patients with 87 hypervascular hepatocellular carcinomas (diameter: range, 3 80 mm; mean, 22 mm) and 19 patients with either chronic hepatitis or liver cirrhosis and without hepatocellular carcinoma who had undergone early arterial and late arterial phase imaging of the entire liver on multidetector CT were retrospectively enrolled in this study. The detector row configuration was mm, the pitch was 6, and the scanning time was 10.5 sec for each phase. All patients received contrast medium (2 ml/kg of body weight) at a rate of 5 ml/sec; the mean scanning delay for the early arterial phase was 19.0 sec, and the mean delay for the late arterial phase was 34.5 sec. Eighty 2.5-mm-thick reconstruction images, forty 5-mm-thick reconstruction images, and twenty-six 7.5-mm-thick reconstruction images were obtained for each phase. Each image set was interpreted separately by three observers to detect hypervascular hepatocellular carcinoma by viewing images on a workstation monitor. Sensitivity, positive predictive value, and area under the receiver operating characteristic curve (A z ) were calculated. We used retrospectively excellent follow-up and imaging or pathologic proof as the gold standard. RESULTS. The mean sensitivity and positive predictive value for hypervascular hepatocellular carcinoma were 76% and 69% on 2.5-mm images, 73% and 69% on 5-mm images, and 67% and 76% on 7.5-mm images, respectively. No significant difference in sensitivity among the images was detected, except by one observer who reported a significant difference in the sensitivity between 2.5- and 7.5-mm images (p < 0.05) and between 5- and 7.5-mm images (p < 0.05). The mean A z values were 0.79, 0.80, and 0.78 for 2.5-, 5-, and 7.5-mm images, respectively. No significant difference in A z values among the images obtained with different slice thicknesses was detected. CONCLUSION. For multidetector CT identification of hypervascular hepatocellular carcinoma, we found little or no advantage in reducing slice thickness to less than 5 mm. H epatocellular carcinoma is usually a hypervascular tumor [1 3], and helical CT has improved our ability to detect hepatocellular carcinoma by allowing acquisition of both hepatic arterial dominant and portal venous dominant sets of images during separate breath-holds [4 6]. Baron et al. [6] established that the addition of hepatic arterial dominant phase images statistically significantly increases hepatocellular carcinoma tumor detection over imaging performed only during the portal venous dominant phase using singledetector helical CT. Multidetector CT scanners acquire multiple CT data sets with each rotation of the X-ray tube [7] and the data for several images simultaneously; they can scan through large anatomic areas three to eight times faster than single-detector helical CT scanners. The ability to scan through the entire liver in 10 sec or less allows acquisition of two separate sets of CT images of the liver within the time generally regarded as the hepatic arterial dominant phase (double arterial phase). Murakami et al. [8] emphasized the usefulness of double arterial phase imaging for the detection of hypervascular hepatocellular carcinoma with multidetector CT. Multidetector technology also enables us to obtain images of thinner slices during the same breath-hold and to retrospectively reconstruct images with different slice thicknesses obtained from the original CT data. The images of thinner slices are useful to reconstruct three-dimensional CT angiography [9 12] and have been AJR:179, July

2 Kawata et al. shown to have a higher sensitivity for the detection of liver lesions [13]. The ability to detect smaller hepatocellular carcinoma may lead to earlier institution of therapy, including percutaneous ablation, and may help to better stage the disease, especially in patients being considered for liver transplantation. However, disadvantages include the need to obtain and interpret an increased number of images. Another disadvantage of multidetector CT, especially if thin sections are used routinely, is the increased radiation exposure. The purpose of this study was to evaluate the diagnostic impact of slice thickness and determine the optimal slice thickness in the detection of hypervascular hepatocellular carcinoma by comparing 2.5-, 5-, and 7.5-mm-thick reconstruction images. Materials and Methods Sixty-two consecutive patients, (43 men and 19 women; age range, years; mean age, 64 years) who had chronic hepatic disease and were clinically suspected of having hepatocellular carcinoma were retrospectively enrolled in this study. All the patients included in this study who had undergone CT at our institution during the 10-month period of March 1999 to December 1999 gave informed consent, following the Declaration of Helsinki principles [14], for CT examinations with IV or intraarterial administration of contrast medium for detection of hepatocellular carcinoma. Of these 62 patients, 43 had hypervascular hepatocellular carcinoma; a total of 87 tumors were detected. Of these tumors, 20 were 10 mm or smaller in diameter, 30 were mm in diameter, and 37 were larger than 20 mm in diameter (range, 3 80 mm; mean, 22 mm). Hypervascularity was defined as focal lesion hyperattenuation relative to the liver during at least one of the two arterial phases of CT. Proof of hypervascular hepatocellular carcinoma was confirmed using a combination of pathologic and radiologic criteria. For confirmatory imaging studies, the 43 patients with hepatocellular carcinoma and one patient without hepatocellular carcinoma underwent CT hepatic arteriography and CT during arterial portography using a technique previously described [15]. Of the 43 patients with hepatocellular carcinomas, 14 hepatocellular carcinoma nodules in 12 patients were pathologically proven after surgery or after percutaneous liver biopsy, which was performed within 1 month after double arterial phase dynamic CT. With regard to the stage of differentiation, one nodule was well differentiated, 12 nodules were moderately differentiated, and one was completely necrotic due to transcatheter hepatic arterial chemoembolization with iodized oil. The remaining 31 patients with hepatocellular carcinoma underwent angiography and transcatheter hepatic arterial chemoembolization with iodized oil. In these 31 patients, the lesions were diagnosed with certainty as hepatocellular carcinoma by assessing a combination of images on the basis of findings in the previous literature [1, 2, 16]. The diagnostic criteria for hepatocellular carcinoma on imaging were as follows: a lesion that showed hypoperfusion on CT during arterial portography; a round hyperenhancement on CT hepatic arteriography; a round, dense deposit of iodized oil on CT after transcatheter hepatic arterial chemoembolization with iodized oil; or both a definite growth and hypervascularity on arterial phase CT during follow-up. The follow-up CT examinations were performed more than 2 months later (range, 2 31 months; mean, 14 months) for 44 patients who had undergone CT hepatic arteriography and CT during arterial portography. The diagnosis for the other 18 patients without hepatocellular carcinoma was confirmed by follow-up CT more than 6 months later (range, 6 24 months; mean, 13 months) and by an unremarkable interval change in the serum level of the tumor marker α-fetoprotein. The 14 pathologically proven hepatocellular carcinoma nodules also satisfied the radiologic criteria. The CT scanner (LightSpeed QX/i; General Electric Yokogawa Medical Systems, Tokyo, Japan) detector configuration was mm in the interspaced high-speed mode, in which four interspaced helical data sets are collected from eight 1.25-mm detector rows. The high-speed mode is equivalent to a pitch of 6 with the table speed set at a 15-mm rotation. One rotation of the X-ray tube was 0.8 sec. Electric pressure of 120 kvp and current of 270 ma were used. All patients received a low-osmolarity contrast medium, iohexol (Omnipaque 300 [300 mg I/mL]; Daiichi Pharmaceutical, Tokyo, Japan) by means of a power injector (Autoenhance A-50; Nemotokyorindou, Tokyo, Japan) at a rate of 5 ml/sec through a 20-gauge plastic IV catheter placed in an antecubital vein. The volume of contrast medium delivered was 2 ml/kg of body weight. Patient weight ranged from 43 to 96 kg (mean, 60 kg); therefore, the volume of contrast medium administered ranged from 86 to 192 ml (mean, 120 ml). The scanning delay was determined using a test bolus of 15 ml of contrast medium administered at a rate of 5 ml/sec followed by the acquisition of a series of single-detector CT scans at a low dose (120 kvp, 10 ma). The scanning location was at the dome of the liver, and the monitoring scans were acquired every 2 sec for sec. A cursor was placed over the abdominal aorta at this level, and the time to peak aortic enhancement was used to determine the scanning delay for the early arterial phase images. Scanning began from the dome of the liver, with the location determined by means of a scout digital radiograph, and proceeded in a caudal direction for 10.5 sec, covering a z-axis distance of 20 cm. After an interscan delay of 5.0 sec for table movement, scanning resumed from the dome of the liver in a caudal direction for 10.5 sec. These images constituted the late arterial phase images. The total acquisition time was 26 sec and was accomplished during a single breathhold. The mean scanning delay for the early arterial phase was 19.0 sec (range, sec), whereas the mean delay for the late arterial phase was 34.5 sec (range, sec). Eighty reconstruction images of 2.5-mm-thick slices and 40 reconstruction images of 5-mm-thick slices were obtained of each phase from the original CT data. Twenty-six images of a 7.5-mm slice thickness were reconstructed from the data sets of 2.5- mm-thick and 1.25-mm-thick interval images (159 images for each phase) by a radiology technologist on a workstation (Advantage Windows 3.1; General Electric Medical Systems, Milwaukee, WI) using the axial multiplanar reformation technique. These reconstructed images of different slice thicknesses (2.5, 5, and 7.5 mm) were interpreted separately and independently by three experienced abdominal radiologists who viewed the images on a workstation monitor. Image review consisted of three sessions. The images of the 62 patients were divided into three groups: 1 (n = 21 patients), 2 (n = 21 patients), and 3 (n = 20 patients). The first session included 2.5-mm-thick images of group 1, 5-mmthick images of group 2, and 7.5-mm-thick images of group 3. The second session included 2.5-mmthick images of group 2, 5-mm-thick images of group 3, and 7.5-mm-thick images of group 1. The last session included 2.5-mm-thick images of group 3, 5-mm-thick images of group 1, and 7.5-mm-thick images of group 2. During each session, the images of patients appeared in random order. Information that identified a specific patient was removed from the images on the workstation. The three review sessions were performed at 4-week intervals. The observers knew that the patients were at risk for hepatocellular carcinoma but did not know how many, if any, hepatocellular carcinomas were present in any patient. The observers recorded the size of each focal hepatic hypervascular lesion and its location by segment number (Couinaud classification) and indicated one of the four following confidence levels for the diagnosis of a malignant tumor: 1, probably absent; 2, equivocal; 3, probably present; and 4, definitely present. After all blinded review sessions had been completed, each lesion was compared with the proven tumor burden mentioned earlier. To assess interobserver variability, we calculated the kappa statistic for multiple observers using the nonweighted binary kappa statistic. The lesions that were among the 87 proven hepatocellular carcinoma lesions that had been assigned a confidence level of 3 or greater were considered true-positive findings. The lesions not assigned or assigned a confidence level of 1 or 2 when a lesion was actually present were considered false-negative findings. The degree of disagreement was not factored into the calculation. A kappa value of was considered slight agreement; , fair; , moderate; , substantial; and , almost perfect. Sensitivity and positive predictive values for each slice thickness were also calculated. Sensitivity for each slice thickness was compared by means of the McNemar test. A two-tailed p value of less than 0.05 was considered significant. For imaging of each slice thickness, alternative free response receiver operating characteristic (ROC) curve analysis was performed on a tumor-by-tumor basis. Although the conventional ROC method allows 62 AJR:179, July 2002

3 Multidetector CT of Hypervascular Hepatocellular Carcinoma TABLE 1 Sensitivity and Positive Predictive Value for the Detection of Hypervascular Hepatocellular Carcinomas on Dynamic CT Using Three Slice Thicknesses Size of HCCs Total No. of HCCs Observer 1 Observer 2 Observer 3 Mean for Observers mm 5 mm 7.5 mm 2.5 mm 5 mm 7.5 mm 2.5 mm 5 mm 7.5 mm 2.5 mm 5 mm 7.5 mm 1 cm (no. of HCCs detected) Sensitivity (%) a cm (no. of HCCs detected) Sensitivity (%) >2 cm (no. of HCCs detected) Sensitivity (%) Total no. of HCCs detected Sensitivity (%) a Positive predictive value (%) Note. HCCs = hepatocellular carcinomas. a Significantly (p < 0.05) lower than sensitivities of images obtained with a 2.5-mm slice thickness and with a 5-mm slice thickness. only one response per image, the alternative free response ROC method allows an observer response for all the lesions present, and we analyzed all 87 lesions in this study [17]. An alternative free response ROC curve was fitted to each observer s confidence rating using a maximum-likelihood estimation (ROCKIT 0.9B; Metz CE, Chicago, IL). The diagnostic accuracy of imaging of each phase for each observer and their composite data were estimated by calculating the area under the ROC curve (A z ). Differences between the imaging techniques in terms of the mean A z values were analyzed statistically using the two-tailed Student s t test for paired data. A two-tailed p value of less than 0.05 was considered significant. Results The kappa values for the three observers, calculated on the basis of each observer s confidence level for the ROC analysis, were 0.52 for reconstruction images with a 2.5-mm slice thickness, 0.47 for those with a 5-mm slice thickness, and 0.60 for those with a 7.5-mm slice thickness. The kappa values for the three observers showed moderate agreement with regard to the presence of lesions on images obtained using a slice thickness of 2.5, 5, and 7.5 mm. The detection sensitivity for tumors of three size categories (diameter: 1 cm, >1 2 cm, or >2 cm) and the positive predictive values of each of the three observers are shown in Table 1. The total number of hypervascular hepatocellular carcinomas detected by three radiologists ranged from 60 to 73 (mean, 66) on the images of 2.5-mm slices, (mean, 64) on those of 5-mm slices, (mean, 58) on those of 7.5- mm slices (Fig. 1). The sensitivity was 69 84% (mean, 76%) for 2.5-mm-thick images, 66 82% (mean, 73%) for 5 mm, and 59 79% (mean, 67%) for 7.5 mm. The sensitivity for lesions more than 1 cm in diameter was higher than that of smaller lesions (Table 1). No significant differences in sensitivity among the slice thicknesses were recorded by the three observers except observer 1; this observer detected lesions 1 cm or smaller in diameter (Figs. 2 and 3) with a significantly higher sensitivity for images obtained with a 2.5- rather than a 7.5-mm slice thickness (p = ) and with a 5- rather than a 7.5-mm slice thickness (p = ). The mean positive predictive value was 69% for images obtained with a 2.5- mm slice thickness, 69% for 5 mm, and 76% for 7.5 mm. The positive predictive value for the 7.5-mm images was slightly higher than that for the 2.5- and 5-mm images; however, no significant difference between each slice thickness was found. Eight small hepatocellular carcinomas (diameter: range, 3 13 mm; mean, 8.9 mm) and two large hepatocellular carcinomas (diameter: 22 and 23 mm) could be detected only on 2.5-mm images by at least one observer. Eleven small hepatocellular carcinomas (diameter: range, 5 18 mm; mean, 11 mm) and two large hepatocellular carcinomas (diameter: 22 and 29 mm) could be detected only on images obtained with a 2.5- and 5-mm slice thickness by at least one observer. No lesion was detected only on 7.5-mm images. Results of calculating the A z values for each phase are shown in Table 2. The mean A z values were 0.79 for images obtained with a 2.5-mm slice thickness, 0.80 for 5 mm, and 0.78 for 7.5 mm. There was no significant Fig. 1. Bar chart shows number of lesions detected by each of three observers on images obtained with different slice thicknesses. Note interobserver agreement is substantial for 2.5- and 7.5-mm slice thicknesses and moderate for 5-mm slice thickness. Black bars = observer 1, white bars = observer 2, gray bars = observer 3. AJR:179, July

4 Kawata et al. A B C Fig. 2. Hypervascular hepatocellular carcinoma nodule, 14 mm in diameter, in 71-year-old man. A C, CT scans obtained in late arterial phase with slice thicknesses of 2.5 (A), 5 (B), and 7.5 mm (C) show hypervascular tumor (arrow). Note lesion is conspicuous at each slice thickness. difference in the Az values between each slice thickness for all observers. Discussion For dynamic CT with a single-detector helical CT scanner, a section thickness (collimation) of from 7 to 10 mm has been commonly used [2 6, 18 20]. The advent of multidetector CT scanners allows acquisition of images of thin sections through large areas of anatomy without concern for breathing artifacts or X-ray tube cooling problems. Reconstruction of thin sections ( mm) is essential for goodquality CT angiography [21, 22], but the opti- TABLE 2 Slice Thickness 2.5 mm 5 mm 7.5 mm Az Values for the Detection of Hypervascular Hepatocellular Carcinoma Observer Mean Note. No significant difference among the A z values for the three slice thicknesses was found. Az = area under the receiver operating characteristic curve. 64 mal collimation for detection of focal hepatic lesions is less well established. Using a single-detector helical scanner with a very thin collimation has the disadvantage of delivering an increased radiation dose to the patient. Using a multidetector scanner to image thin sections has the disadvantages of a significantly increased radiation burden and of the creation of a very large set of images that must be viewed and possibly filmed. Therefore, it is important to document the optimal collimation for detection of focal liver lesions. Weg et al. [13] used a dual-detector CT scanner to study the usefulness of thin collimation to evaluate small (diameter < 10 mm) liver lesions, primarily hypovascular metastases. These researchers detected more lesions on images obtained with a 2.5-mm collimation than on those obtained with a 5-, 7.5-, or 10-mm collimation. Using multidetector CT, other investigators have described findings regarding the detection rate for focal hepatic lesions that conflict in terms of optimal slice thickness [23, 24]. The value of a relatively thin (4 7 mm) collimation has been well documented for detection of most hypovascular or hypoattenuating focal liver lesions. We focused our investigation on the detection sensitivity of hypervascular or hy- perattenuating lesions using double arterial phase CT of hypervascular hepatocellular carcinoma as our model. Only one of the three observers in our study had a significantly improved detection rate when viewing thin sections (2.5- vs 7.5-mm collimation and 5- versus 7.5-mm collimation). There was no significant difference of detection sensitivity for 2.5- versus 5-mm sections by all observers. Although we did not document increased sensitivity or positive predictive value for 5- versus 7.5-mm sections, we are reluctant to recommend routine use of 7.5-mm sections because the resulting partial volume artifact may obscure the size of the lesion and render the attenuation measurement of small lesions inaccurate. Because observer 1 had a significantly improved rate of detection especially for small lesions ( 1 cm in diameter), we suggest the use of at least a 5-mm collimation for detecting hypervascular lesions. The use of very thin sections (e.g., 1.25 mm) was not evaluated in our study, but seems a poor choice for the evaluation of focal hepatic lesions because of the decreased signal-to-noise ratio, a more prolonged scanning time necessary to cover the entire liver, and significantly increased radiation burden. The 5-mm-collimation images seem to provide the AJR:179, July 2002

5 Multidetector CT of Hypervascular Hepatocellular Carcinoma A B Fig. 3. Hypervascular hepatocellular carcinoma nodule, 8 mm in diameter, in 74year-old man. A C, CT scans obtained in late arterial phase with slice thickness of 2.5 (A), 5 (B), and 7.5 mm (C) show hypervascular tumor (arrow). Hypervascular nodule in lateral segment (arrow) is conspicuous on images obtained with 2.5- (A) and 5-mm (B) slice thickness, whereas lesion is very subtle on image obtained with 7.5-mm (C) slice thickness because of partial volume effect. optimal combination of adequate signal-tonoise ratio and spatial and contrast resolution. Urban et al. [25] have shown that overlapping reconstructions detect more lesions than contiguous reconstructions. These researchers compared an 8- versus a 4-mm reconstruction interval with an 8-mm collimation and detected 7% more lesions with the thinner sections. The increased detection rate AJR:179, July 2002 was primarily for small lesions (diameter, <2 cm). In our study, we evaluated only contiguous nonoverlapping reconstructions and showed that reducing slice thickness to less than 5 mm offers little or no advantage in detection. We think that using a 5-mm slice thickness with overlapping reconstructions could yield better images than using a very thin slice thickness without overlapping recon- C structions, and we believe that further study may be necessary. We did not evaluate portal venous phase images in this study, because hypervascular tumors are usually not detected well in this phase. Nevertheless, we believe that portal venous phase images should be obtained routinely in any CT evaluation for known or suspected primary or metastatic tumor. Images 65

6 Kawata et al. obtained in the portal venous phase are usually optimal for additional characterization of liver tumors, which can improve differential diagnosis between hepatocellular carcinoma and other malignant and benign tumors, and are useful for the visualization of vascular anatomy and pathology. When very thin sections on multidetector CT are used to evaluate hypervascular lesions, more lesions could possibility be detected including false-positive lesions such as small hemangiomas and shunts. These findings are difficult to characterize or biopsy. Moreover, an improved sensitivity for the detection of hypervascular lesions leads to all sorts of problems an increased need for follow-up, performance of unnecessary invasive procedures, increased patient and clinician anxiety, and so on. However, as we mentioned earlier, dynamic studies including arterial and portal venous phase imaging will help in the differential diagnosis of these lesions. The other limitation of our study could be the lack of histologic proof for every lesion that we believed to represent hepatocellular carcinoma. However, all lesions had several confirmatory studies such as CT hepatic arteriography, CT during arterial portography, CT after arterial infusion of iodized oil, and follow-up CT. Each of these studies, especially in combination, has been found to detect hypervascular hepatocellular carcinoma with an accuracy approaching 100%. Moreover, we were able to follow the course of most lesions over time and in response to therapy, especially transcatheter arterial chemoembolization. In summary, we found no improvement in detection sensitivity of hypervascular hepatocellular carcinoma using a very thin collimation and believe that a 5-mm collimation is optimal for evaluation of focal hypervascular hepatic lesions. References 1. Ohishi H, Uchida H, Yoshimura H, et al. Hepatocellular carcinoma detected by iodized oil: use of anticancer agents. Radiology 1985;154: Merine D, Takayasu K, Wakao F. Detection of hepatocellular carcinoma: comparison of CT during arterial portography with CT after intraarterial injection of iodized oil. Radiology 1990; 175: Yoshimatsu S, Inoue Y, Ibukuro K, Suzuki S. Hypovascular hepatocellular carcinoma undetected at angiography and CT with iodized oil. Radiology 1989;171: Ohashi I, Hanafusa K, Yoshida T. Small hepatocellular carcinomas: two-phase dynamic incremental CT in detection and evaluation. Radiology 1993;189: Hollett MD, Jeffrey RB Jr, Nino-Murcia M, Jorgensen MJ, Harris DP. Dual-phase helical CT of the liver: value of arterial phase scans in the detection of small ( 1.5 cm) malignant hepatic neoplasms. AJR 1995;164: Baron RL, Oliver JH III, Dodd GD III, Nalesnik M, Holbert BL, Carr B. Hepatocellular carcinoma: evaluation with biphasic, contrast-enhanced, helical CT. Radiology 1996;199: Hu H, He HD, Foley WD, Fox SH. Four multidetector-row helical CT: image quality and volume coverage speed. Radiology 2000;215: Murakami T, Kim T, Takamura M, et al. Hypervascular hepatocellular carcinoma: detection with double arterial phase multi-detector row helical CT. Radiology 2001;218: Rubin GD, Dake MD, Napel SA, McDonnell CH, Jeffrey RB Jr. Three-dimensional spiral CT angiography of the abdomen: initial clinical experience. Radiology 1993;186: Winter TC III, Freeny PC, Nghiem HV, et al. Hepatic arterial anatomy in transplantation candidates: evaluation with three-dimensional CT arteriography. Radiology 1995;195: Raptopoulos V, Steer ML, Sheiman RG, Vrachliotis TG, Gougoutas CA, Movson JS. The use of helical CT and CT angiography to predict vascular involvement from pancreatic cancer: correlation with findings at surgery. AJR 1997;168: Kamel IR, Kruskal JB, Pomfret EA, Keogan MT, Warmbrand G, Raptopoulos V. Impact of multidetector CT on donor selection and surgical planning before living adult right lobe liver transplantation. AJR 2001;176: Weg N, Scheer MR, Gabor MP. Liver lesions: improved detection with dual-detector-array CT and routine 2.5-mm thin collimation. Radiology 1998; 209: Forty-First World Medical Assembly. Declaration of Helsinki: recommendations guiding physicians in biomedical research involving human subjects. Bull Pan Am Health Organ 1990;24: Murakami T, Oi H, Hori M, et al. Helical CT during arterial portography and hepatic arteriography for detecting hypervascular hepatocellular carcinoma. AJR 1997;169: Kanematsu M, Hoshi H, Imaeda T, et al. Detection and characterization of hepatic tumors: value of combined helical CT hepatic arteriography and CT during arterial portography. AJR 1997;168: Ward J, Chen F, Guthrie JA, et al. Hepatic lesion detection after superparamagnetic iron oxide enhancement: comparison of five T2-weighted sequences at 1.0 T by using alternative-free response receiver operating characteristic analysis. Radiology 2000;214: Kim T, Murakami T, Oi H, et al. Detection of hypervascular hepatocellular carcinoma by dynamic MRI and dynamic spiral CT. J Comput Assist Tomogr 1995;19: Oi H, Murakami T, Kim T, Matsushita M, Kishimoto H, Nakamura H. Dynamic MR imaging and earlyphase helical CT for detecting small intrahepatic metastases of hepatocellular carcinoma. AJR 1996;166: Yamashita Y, Mitsuzaki K, Yi T, et al. Small hepatocellular carcinoma in patients with chronic liver damage: prospective comparison of detection with dynamic MR imaging and helical CT of the whole liver. Radiology 1996;200: Foley WD, Mallisee TA, Hohenwalter MD, Wilson CR, Quiroz FA, Taylor AJ. Multiphase hepatic CT with a multirow detector CT scanner. AJR 2000;175: Takahashi S, Murakami T, Kim T, Takamura M, Narumi Y, Nakamura H. Multi-detector row helical CT angiography of the hepatic vessel: separate depiction of hepatic arterial, portal, and venous anatomy. (abstr) Radiology 1999;213 (P): Kopka L, Rodenwaldt J, Hamm BK. Biphasic multislice helical CT of the liver: intraindividual comparison of different slice thickness for the detection and characterization of focal liver lesion. (abstr) Radiology 2000;217(P): Basilico R, Filippone A, Ricciardi M, Iezzi A, Bonomo L. Impact of slice thickness on the detection of liver lesions with multislice CT. (abstr) Radiology 2000;217(P): Urban BA, Fishman EK, Kuhlman JE, Kawashima A, Hennessey JG, Siegelman SS. Detection of focal hepatic lesions with spiral CT: comparison of 4- and 8-mm interscan spacing. AJR 1993;160: AJR:179, July 2002