Multidetector CT of the Liver and Hepatic Neoplasms: Effect of Multiphasic Imaging on Tumor Conspicuity and Vascular Enhancement

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1 Isaac R. Francis 1 Richard H. Cohan 1 Nancy J. McNulty 1 Joel F. Platt 1 Melvyn Korobkin 1 Achamyeleh Gebremariam 2 Kartik Ragupathi 2 Received June 3, 2002; accepted after revision October 21, Presented at the annual meeting of the Radiological Society of North America, Chicago, Illinois, November 28 December 3, Department of Radiology, University of Michigan Hospitals, 1500 E. Medical Center Dr., Ann Arbor, MI Address correspondence to I. R. Francis. 2 Department of Internal Medicine, University of Michigan Hospitals, Ann Arbor, MI AJR 2003;180: X/03/ American Roentgen Ray Society Multidetector CT of the Liver and Hepatic Neoplasms: Effect of Multiphasic Imaging on Tumor Conspicuity and Vascular Enhancement OBJECTIVE. Our aim was to determine which of three contrast-enhanced phases (early arterial, late arterial, or portal venous) was optimal for achieving maximal enhancement of the celiac artery, portal vein, and hepatic parenchyma. We also wanted to learn which phase provided the maximal tumor-to-parenchyma difference when using multidetector CT (MDCT) with fixed timing delays. MATERIALS AND METHODS. Fifty-two patients with suspected or known hepatic tumors underwent multiphasic contrast-enhanced MDCT using double arterial (early and late arterial) and venous phase acquisitions with fixed timing delays. All patients were administered 150 ml of IV contrast material at an injection rate of 4 ml/sec. Images were acquired at 20 sec for the early arterial phase, 35 sec for the late arterial phase, and 60 sec for the portal venous phase. Attenuation measurements of the celiac artery, portal vein, normal hepatic parenchyma, and the hepatic tumor were compared. Three reviewers independently and subjectively rated tumor conspicuity for each of the three phases. Ratings were compared using kappa statistics. RESULTS. Late arterial phase images showed maximal celiac axis attenuation, whereas portal venous phase images revealed the highest portal vein and normal hepatic parenchymal attenuation. Maximal tumor-to-parenchyma differences for hypovascular tumors was superior in the portal venous phase, but we found no significant differences in maximal tumor-to-parenchyma differences for hypervascular tumors among the evaluated phases. On subjective analysis, interobserver agreement was moderate to very good for the three phases. All three reviewers graded both hypovascular and hypervascular tumor conspicuity as superior in either the late arterial phase or the portal venous phase in most patients. In only one patient was the early arterial phase graded as superior to the late arterial and portal venous phases (by two of the three reviewers). CONCLUSION. When MDCT of the liver is performed using fixed timing delays, maximal vascular and hepatic parenchymal enhancement is achieved on either late arterial phase or portal venous phase imaging. In most patients, early arterial phase imaging does not improve tumor conspicuity by either quantitative or subjective analysis. H elical CT is the most commonly used imaging modality for both detection and characterization of hepatic neoplasms. To ensure optimal tumor detection, tumor conspicuity has to be maximized. This can be achieved by maximizing the tumor-to-hepatic parenchymal contrast differences. Initial studies using single-detector helical CT scanners have found that arterial phase scanning improves detection of hypervascular tumors [1 14]. The arterial phase in these studies, defined as occurring sec after initiation of IV contrast media administration, has been found to be most useful for maximizing enhancement of vascular tumors. In comparison, portal venous phase imaging, defined as occurring sec after the initiation of IV contrast media administration, has been found to be most useful for obtaining maximal hepatic parenchymal enhancement and thus maximizing tumor-to-parenchyma differences for hypovascular tumors [15 18]. With the introduction of multidetector CT (MDCT), image acquisition of a larger volume of anatomy in the z axis is now possible in a shorter time (< 10 sec), with no sacrifice in image quality [19, 20]. This development has created even more possibilities for varying the AJR:180, May

2 Francis et al. timing of image acquisition after contrast material injection. MDCT has the potential to further improve tumor detection and characterization because even more dramatic hepatic tumor and hepatic parenchymal enhancement differences might now be achieved. Preliminary studies have shown that MDCT using two sets of arterial phase imaging maximizes hepatic tumor detection rates [20, 21]. Foley et al. [20] found that when using a double arterial phase acquisition on MDCT, maximal tumor-to-parenchyma differences for hypervascular tumors were observed on the delayed arterial acquisition images (late arterial phase). In a more recent study of patients with hepatocellular carcinoma, Murakami et al. [21], using a double arterial phase acquisition on an MDCT scanner, showed that although most of the tumors had maximal conspicuity in the late arterial phase when compared with the early arterial phase, a combination of the two phases maximized the number of tumors detected. Using two arterial phases instead of one may be beneficial in hepatic tumor detection, but doing so adds more complexity, exposes the patient to more radiation, requires additional film and data-storage capacity, and requires more radiologist time for reviewing the additional images. Although several methods are available to determine optimal delay times for arterial phase imaging, no conclusive data are available as to which of these is superior [22 24]. The minibolus and automated techniques can result in a more consistent and reliable enhancement of the upper abdominal vasculature, but they have some disadvantages [23]. These drawbacks include limited availability, added cost of the automated-technique software, and increase in table time needed for review of minibolus images to calculate the time delay, thereby decreasing patient throughput. In addition, the potential exists for computer malfunctions to occur, leading to mistiming of the IV contrast bolus and, thus, suboptimal studies [24]. Use of a test bolus can also potentially decrease the conspicuity of a hepatic lesion because of the liver enhancement that is present when the diagnostic scanning begins. The timing-bolus technique also has an inherent delay before scanning begins once the target threshold is reached. These reasons led us to use fixed timing delays for a variety of examinations, including hepatic imaging, at our institution. In our study, we sought to determine which of three contrast-enhanced phases (early arterial, late arterial, or portal venous) was optimal for achieving maximal enhancement of the celiac artery, portal vein, and hepatic parenchyma, as well as which phase provided maximal tumor-to-parenchyma differences when MDCT was used with fixed timing delays. Materials and Methods Subjects The study population for this retrospective study consisted of all patients with known or suspected hepatic tumors who underwent multiphase imaging of the liver by MDCT between December 9, 1998, and October 26, 1999, for suspected or known hepatic neoplasms. The role of the double arterial phase in the detection of hepatic neoplasms was assessed. On the basis of our early results, we discontinued performing the early arterial phase of hepatic imaging because we found that it was not of added value for lesion detection. After this period, 52 patients (27 men and 25 women) with hepatic neoplasms (primary and metastatic tumors) were included. Patient ages ranged between 25 and 81 years (mean age, 61 years). The tumors included a variety of primary hepatic neoplasms (15 hepatocellular carcinomas, two cavernous hemangiomas, two focal nodular hyperplasias, one hepatic adenoma, and one intrahepatic cholangiocarcinoma), and 31 metastases from various primary tumors (16 colorectal tumors, three gastrointestinal stromal tumors, two pancreatic cancers, two carcinoid tumors, two lung cancers, one breast cancer, one melanoma, one gastrinoma, one uterine cancer, one islet cell tumor, and one ovarian cancer). We obtained institutional review board approval for this retrospective study. Imaging Technique In all patients, CT was performed using an MDCT scanner (Lightspeed QX/i; General Electric Medical Systems, Milwaukee, WI). One hour before scanning, patients received 320 ml of dilute barium orally (Scan C: 2.1% weight by weight barium sulfate; Lafayette Pharmaceuticals, Yorba Linda, CA) for gastrointestinal tract opacification. A timing bolus was not used to determine scanning delays. Via a power injector, 150 ml of Omnipaque 300 mg I/mL (iohexol 300 mg I/mL; Nycomed, Princeton, NJ) was injected IV at a rate of 4 ml/sec. Double arterial phase scanning consisting of early and late arterial phase image acquisition was performed during a single breath-hold using, respectively, 20- and 35-sec scanning delays from initiation of contrast material injection. These timing delays were explicitly chosen so that both arterial phases could be performed within a single breath-hold that ranged from sec (which could be performed in all our patients). We elected to perform these two phases during a single breath-hold to minimize the effect of variation on lesion conspicuity and detection. The entire liver was scanned in a cephalad-to-caudad direction using a detector collimation of 5 mm with a table speed per rotation of 15 mm/0.8 sec, a pitch of 3 in the scanner s HQ mode, and an image thickness of 5 mm. After a brief period of quiet breathing, portal venous phase imaging of the entire abdomen was performed in a cephalad-to-caudad direction during a breath-hold, using a scanning delay of 60 sec, detector collimation of 5 mm, table speed of 15 mm/ 0.8 sec, pitch of 3 (HQ mode), and an image thickness of 5 mm. Using these parameters, the combined scanning duration for both arterial phase acquisitions was sec and for portal venous phase, sec. The scanning parameters used were 120 kvp and mas. Image Analysis All images were loaded and reviewed on a workstation (Windows Advantage 3.1; General Electric Medical Systems). Regions-of-interest (ROI) measurements ranging in area between 15 and 30 mm depending on vessel size were made by one of the authors who did not participate in the qualitative evaluation portion of the study. Measurements were made of the celiac axis just distal to its takeoff from the aorta and of the portal vein in the portal hilum during each of the three phases. Hepatic parenchymal ROI measurements were obtained from the most homogeneous regions of the liver. Three regions mm in area were chosen in the cranial aspect, mid portion, and caudal aspect of the liver, taking care to avoid regions with focal fatty change, tumor involvement, and normal vessels. Thus, nine measurements of hepatic attenuation values were obtained in each patient for each phase. Hepatic parenchyma attenuation for each phase was then calculated in each patient as the mean of the nine attenuation measurements on each scan. The same author who made the ROI measurements of the vessels and liver performed quantitative analysis of hepatic tumor using the ROI measurements obtained on images acquired during each of the three phases of contrast enhancement. These ROI measurements encompassed most of the tumors and varied in area according to tumor size, ranging between 15 and 750 mm. When multiple liver masses were present, one of the authors who was not a reviewer for the study selected the most conspicuous or, if all were equally conspicuous, the largest hepatic tumors for ROI measurements. For each tumor, the maximal tumor-to-parenchyma difference was compared for each of the three contrast-enhanced phases. Separate analyses of maximal tumor-to-parenchyma difference for hypervascular tumors (such as hepatocellular carcinoma, hypervascular metastases from neuroendocrine tumors, hepatic adenomas, and focal nodular hyperplasia) and hypovascular tumors (such as metastases from colon and pancreatic adenocarcinoma) were also carried out. Three reviewers performed qualitative analysis of tumor conspicuity for the three phases of IV contrast enhancement. The tumors selected for this analysis were the same as those chosen for obtaining quantitative ROI measurements. Tumor conspicuity was graded on a 4-point scale (0 = not seen, 1 = poorly seen, 2 = moderately well seen, 3 = very well seen) AJR:180, May 2003

3 Multidetector CT of the Liver and Hepatic Neoplasms Statistical Analysis Quantitative analysis. Because more than one attenuation measurement was obtained for each patient, repeated measures analysis of variance was used for evaluating the effect of phase on attenuation for celiac axis, portal vein, liver, and tumor-to parenchyma differences [25]. Tumors were analyzed as two separate groups: 22 hypervascular neoplasms (hepatocellular carcinoma; metastatic neuroendocrine tumors including carcinoid, islet cell tumor, gastrinoma; and focal nodular hyperplasia) and 30 hypovascular neoplasms (metastases from colon, pancreas, lung, gastrointestinal stromal, breast, melanoma, uterus, ovary, intrahepatic cholangiocarcinoma, large cavernous hemangiomas, and a hypovascular adenoma). The three metastases from gastrointestinal tumors were necrotic, and the solitary case of metastasis from melanoma was hypovascular on qualitative analysis. We therefore included them in the hypovascular group. When the phase effect was found to be significant, multiple comparisons were made using the Tukey-Kramer test. The rating for phase effect on tumor conspicuity was analyzed using the general estimating equations approach for repeated ordinal data [26]. Statistical analysis was performed using the SAS software (Statistical Analysis System, Cary, NC) [25]. Power calculations were performed for assessing the variance among the means of the three phases using nquery Advisor 4.0 (Statistical Solutions, Boston, MA) [27]. Tumor conspicuity reviewer analysis. For each of the phases of imaging, four kappa statistics were computed for interobserver variability with accompanying 95% confidence intervals A (CI). For the pair-wise comparisons, weighted kappa statistics were used. Results Celiac Axis Enhancement The greatest average celiac axis attenuation was observed in the late arterial phase and was significantly superior ( p < ) when compared with that measured during each of the other two phases. Portal Vein Enhancement The greatest average attenuation of the portal vein was observed on the portal venous phase. This attenuation was significantly supe- B Fig year-old man with metastases from colon carcinoma. A, Enhanced multiphasic axial CT scan obtained in arterial phase shows no portal vein enhancement. B, Enhanced multiphasic axial CT scan obtained in late arterial phase shows some enhancement in portal vein (arrow). Arrowhead shows metastasis. C, Enhanced multiphasic axial CT scan obtained in portal venous phase shows most marked enhancement of portal vein (arrow) in comparison with A and B. C AJR:180, May

4 Francis et al. TABLE 1 Normal Hepatic Parenchymal Enhancement CT Imaging Phase Estimate a SE Early arterial Late arterial Portal venous Note. SE = standard error. a Expressed in Hounsfield units. rior to that observed in the late arterial phase (p < ) (Fig. 1). Normal Hepatic Parenchymal Enhancement Hepatic parenchymal attenuation was greatest in the portal venous phase and was significantly higher (p < ) than that of each of the other phases. Although the hepatic attenuation in the late arterial phase was significantly greater than that in the early arterial phase, the difference was not as great (Table 1). TABLE 3 TABLE 2 a Expressed in Hounsfield units. Interobserver Agreement Among Three Reviewers Tumor-to-Parenchyma Differences Tumor Type CT Imaging Phase Estimate a Standard Error Hypervascular Early arterial Late arterial Portal venous Hypovascular Early arterial Late arterial Portal venous Quantitative Analysis: Tumor-to-Parenchyma Differences For the subgroup of hypervascular tumors, although maximal tumor-to-parenchyma differences were greater during the late arterial and portal venous phases than during the early arterial phase, the differences were not statistically significant (Table 2). In comparison, in the subgroup of hypovascular tumors, maximal tumor-to-parenchyma differences were significantly greater during the late arterial and portal venous phases than during the early arterial phase. However, no significant difference was found between the late arterial and portal venous phases (Table 2). The available data suggests that a univariate two-group repeated measures analysis of variance will have 99% power to detect a variance among the tumor vascularity group marginal means of , will have 99% power to detect a variance of among the means of the three phases (early arterial, late arterial, and portal venous), and will have 99% power to detect an interaction between tumor vascularity groups and phases with a variance of These outcomes assume that the between-groups error term is 35.22; the within-groups error term is 21.93; and the measure of sphericity of the covariance matrix, epsilon, is 0.79 (its estimate, the Greenhouse-Geisser correction, has an expected bias of approximately g1 / [2n 2] where g1 is 0.28) when the significance level is and the sample size in each of the two groups is 23. Power calculations were performed using nquery Advisor 4.0 software. CT Imaging Phase Reviewers Kappa a Standard Error 95% CI Early arterial 1 vs (0.5331, ) 1 vs (0.5325, ) 2 vs (0.3504, ) Overall (0.3980, ) Late arterial 1 vs (0.4106, ) 1 vs (0.4329, ) 2 vs (0.4935, ) Overall (0.3479, ) Portal venous 1 vs (0.6468, ) 1 vs (0.6646, ) 2 vs (0.6425, ) Overall (0.5883, ) Qualitative Analysis We used kappa statistics to determine interobserver variability for tumor conspicuity. The level of overall interobserver agreement for all three phases for the three reviewers ranged from 0.41 to 0.80 (moderate to good), with interobserver agreement being best in the portal venous phase, ranging from to (Table 3). On the basis of qualitative analysis, the overall tumor conspicuity score for all 52 hepatic tumors as determined by all three reviewers was generally graded as superior on the late arterial or portal venous phase but not on the early arterial phase images. Reviewers 1, 2, and 3 graded tumor conspicuity as being superior in the late arterial phase images for 15, 17, and 15 tumors, respectively. Portal venous phase images were graded as being superior for 12, 19, and 18 tumors. In comparison, the three reviewers graded early arterial phase images as being superior for only one, zero, and four tumors, respectively. TABLE 4 CT Imaging Phase Subjective Analysis of Tumor Conspicuity by Three Reviewers Preferred by at Least Two Reviewers for Hypervascular Tumors (n = 22) Hypovascular Tumors (n = 30) Late arterial 11 6 Portal venous 4 12 Early arterial 0 1 Late arterial or portal venous a 7 10 No preference 0 1 Note. CI = confidence interval. a Values representing qualitative degree of agreement: < 0.2 = poor, = fair, = moderate, = good, = very good. a Equally preferred over early arterial phase AJR:180, May 2003

5 Multidetector CT of the Liver and Hepatic Neoplasms For 11 (50%) of 22 hypervascular tumors, late arterial phase images were thought to be superior to early arterial and portal venous phase images for tumor conspicuity by at least two of three reviewers (Figs. 2 and 3). In seven tumors (32%), late arterial and portal venous phase images were graded as equivalent but superior to early arterial phase images by at least two reviewers, and in the remaining four tumors (18%), portal venous phase images were rated as superior. In no patient were early arterial phase images thought to be superior by any reviewer (Table 4). For 12 of 30 hypovascular tumors, portal venous phase images were thought to be superior to late arterial phase and early arterial phase images by at least two reviewers (Fig. 4). Tumor conspicuity was rated as equiva- lent on late arterial and portal venous phase images but superior to early arterial phase images by at least two reviewers for 10 of the 18 remaining tumors. In six others, late arterial phase images were rated as superior to both portal venous and early arterial phase images. Conspicuity was rated as equivalent on all phases in one patient. In only one instance in a patient with metastasis in a liver with fatty changes were early arterial phase images rated superior to late arterial and portal venous phase images (Table 4). Discussion Helical CT scanning protocols for evaluating hepatic tumors have been previously described in numerous studies [1 18]. Many A authors have reported the findings of biphasic or dual-phase CT for detection of hepatic tumors using single-detector CT scanners [1 18]. When using single-detector CT scanners and scanning delays of sec for arterial phase imaging, most authors have found that vascular or hypervascular tumors are better detected on arterial phase images [1 14]. In these studies, the range of the scanning duration for the arterial phase was sec. Minor variations in the results of these studies can be largely attributed to differences in rate and volume of IV contrast administration, scanning delays, time of image acquisition, scanner differences, as well as differences in such patient variables as weight and cardiovascular status. MDCT provides substantial improvement in volume coverage over single-detector CT. B Fig year-old man with hepatocellular carcinoma. A and B, Enhanced multiphasic axial CT scans obtained in early arterial phase (A) and late arterial phase (B) show hypervascular hepatocellular carcinoma (arrow, B) poorly in early arterial phase (A), but best in late arterial phase (B). C, Enhanced multiphasic axial CT scan obtained in portal venous phase shows that hypervascular hepatocellular carcinoma is now hypodense to liver. C AJR:180, May

6 Francis et al. More rapid image acquisition allows greater separation of arterial and venous phases, thus facilitating multiphasic acquisition [20]. This improvement does not sacrifice image quality: a recent study has shown that images of comparable diagnostic quality were achieved with three times the volume coverage with MDCT than with single-detector CT [19]. Several methods have been used to determine the optimal timing of arterial phase image acquisition during helical CT. These include the use of fixed timing delays, as used in this study; a timing bolus with an initial minibolus of iodinated contrast material; and semiautomated computer bolus-tracking techniques, such as SmartPrep (General Electric Medical Systems) and Care Bolus (Siemens Medical Systems, Forkheim, Germany). Although timing bolus or bolus tracking methods may seem to offer the unique advantage of tailoring the CT technique to each patient, a number of studies have shown that in clinical practice, these techniques are not consistently superior to that of fixed timing delays. Platt et al. [22] have shown that the test injection computed time delay does not correlate well to the actual time to peak aortic enhancement in patients undergoing CT angiography. Additionally, in a study specifically evaluating ways to achieve optimal arterial phase hepatic imaging, Sandstede et al. [23] reported that optimal hepatic arterial phase imaging was obtained in only 41% of patients with the use of an automated bolustracking technique. Conversely, Macari et al. [24] found that using empiric fixed time delays resulted in adequate qualitative and quantitative enhancement of the aorta and iliac arteries. Because no consensus exists that either the bolus-tracking or bolus-tracking method is superior to the use of fixed timing delays for determining the optimal scanning delay times for achieving arterial phase imaging, we elected to use fixed timing delays for our study. We found that this technique was easier for our technologists to use and, thus, to be most applicable to our busy clinical practice. In our investigation using MDCT, we chose to use a fixed scanning delay of 20 sec for early arterial, 35 sec for late arterial, and 60 sec for portal venous phase imaging. When we used this timing and a contrast agent injection rate of 4 ml/sec, maximal celiac axis attenuation occurred in the late arterial phase rather than in the early arterial phase, as has been seen in studies using single-detector scanners [1, 2]. This finding can be explained by the more rapid scanning of MDCT. Given the larger volume of anatomic coverage achieved in a shorter time when our protocol is used, only approximately 80% of the total IV contrast material dose (120 of the 150 ml in our study) is injected by the end of the early arterial phase. Therefore, the remaining 20% of the total dose of IV contrast material is still being injected at the beginning of the late arterial phase. Our results also differ from those observed in the more recent studies by Foley et al. [20] and Murakami et al. [21], who also used MDCT scanners and found that when using a double arterial acquisition, maximal arte- Fig year-old woman with hypervascular metastases from primary extrahepatic neuroendocrine tumor. A, Enhanced multiphasic axial CT scan obtained in early arterial phase shows small hypervascular metastases poorly, except for largest one in left lobe (arrow). B, Enhanced multiphasic axial CT scan obtained in late arterial phase clearly shows large hypervascular (arrow) and smaller hypervascular metastases (arrowheads). C, Enhanced multiphasic axial CT scan obtained in portal venous phase shows large hypervascular metastasis (arrow ); other metastases are isodense to liver. A B 1222 C AJR:180, May 2003

7 Multidetector CT of the Liver and Hepatic Neoplasms rial enhancement occurred in the early arterial phase. This difference is likely explained by the fact that in those studies, a timing bolus was used to individually optimize the early arterial phase, whereas in our study, a fixed scanning delay was used. We also found that statistically significant greater opacification of the portal vein occurred during the portal venous phase. These findings are similar to the results of Foley et al. [20]. Tublin et al. [28] have recently shown that enhancement of the normal liver is dependent on the total amount and rate of contrast material injected, as well as on the timing of the scan acquisition. These authors showed that both peak enhancement and time to peak enhancement are directly related to the rate of contrast material injection. In their study, at an injection rate of 2.5 ml/sec, peak enhancement of the liver was reached in 87 sec, in comparison to a peak enhancement at 63 sec with an injection rate of 5 ml/sec. Thus, in our study, in which a total of 150 ml of nonionic iodinated contrast material was injected at a rate of 4 ml/sec in all patients, it would be expected that using a scanning de- lay of 60 sec for portal venous phase image acquisition and a mean scanning duration of 14.5 sec would result in portal venous phase images showing the highest liver attenuation. This observation is similar to that reported by Foley et al. [20] who found that liver parenchymal enhancement tended to progressively increase from early arterial phase to portal venous phase images. Our results with MDCT concerning conspicuity of hypervascular tumors also differ from those observed on earlier studies using single detector scanners, in that in our study tumor conspicuity for hypervascular tumors was qualitatively superior on the late arterial phase images rather than on the early arterial phase images [1 14]. Two of three reviewers rated the late arterial phase to be superior to early arterial and portal venous phase images in 11 (50%) of 22 patients and, in another seven patients (32%), superior to early arterial phase and equivalent to portal venous phase images. In 4 (18%) of 22 patients, portal venous phase images were judged to be superior to late arterial phase images. In no instances were hypervascular tumors judged to be best seen on early arterial phase images. Our findings with respect to conspicuity of the subgroup of hypervascular liver tumors are similar to those of recent maximal tumor-to-parenchyma differences reported by Foley et al. [20]. In that study, maximal tumor-to-liver contrast difference occurred in the second pass (late arterial phase) for patients both with and without cirrhosis. A study by Murakami et al. [21] showed that although the sensitivity for detection of hepatocellular carcinoma was superior in the late arterial phase, a combination of both late and early arterial phase imaging resulted in improved detection; more tumors were detected when both early and late arterial phase images were analyzed in combination than when they were analyzed individually. We did not find this to be true in our study, which included a smaller number of hepatocellular carcinomas, but the differences may in part be explained by the use of a timing bolus to optimize early arterial phase images in the study by Murakami et al. When we used quantitative analysis, we found no significant difference in tumor-to-parenchyma differences for the three phases in the hypervascular tumor group in our study. However, inclusion of larger numbers of patients might have yielded different results. Fig year-old woman with metastases from colon carcinoma. A, Enhanced multiphasic axial CT scan obtained in early arterial phase shows hypovascular metastasis (arrow) poorly. B and C, Enhanced multiphasic axial CT scans obtained in late arterial phase (B) and portal venous phase (C) show hypovascular metastasis (arrows) less clearly in arterial phase (B) than in portal venous phase (C). A B AJR:180, May 2003 C 1223

8 Francis et al. Also in our series, for 22 of 30 hypovascular tumors, portal venous phase images were rated as superior or equivalent to late arterial phase images, but superior to early arterial phase images. In only one instance were early arterial phase images rated as superior to late arterial and portal venous phase images. One limitation of our study is its small sample size. We also did not tailor the IV contrast dose on the basis of patient weight, but instead chose a standard dose of 150 ml for all patients. Although prior studies have shown that aortic and hepatic enhancement can be adversely influenced if patient weight is not factored into account when determining the total dose and rate of IV contrast material administered, few institutions tailor contrast dose to patient weight [22, 29]. In summary, although we found that late arterial and portal venous phase images had better MDCT results than early arterial phase images for hypervascular liver tumors, this finding was not statistically significant. For hypovascular liver tumors, late arterial and portal venous phase images had statistically significant superior maximal tumor-to-parenchyma differences compared with early arterial phase images. On subjective analysis of tumor conspicuity, late arterial phase images were also thought to be superior to early arterial phase images for most (18/22) hypervascular tumors. In 22 of 30 hypovascular tumors, portal venous phase images were judged to be superior or equivalent to late arterial phase images, but superior to early arterial phase images. Because maximal celiac arterial and portal venous opacification is obtained in the late arterial and portal venous phases, respectively, and because use of early arterial phase images does not contribute to tumor detection in most patients, we no longer routinely perform early arterial phase imaging in patients with known or suspected hepatic neoplasms. At our institution, patients referred for CT with possible or known hepatic tumors undergo MDCT using only late arterial and portal venous phase imaging. The early arterial phase images are acquired only if CT angiography is required to depict hepatic arterial anatomy before surgery. Although the arterial enhancement is superior in the late arterial phase, some venous contamination occurs, which makes the early arterial phase more suitable for CT angiography. 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