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Gastrointestinal Imaging |
1 Department of Radiology, A113, Albany Medical Center, 43 New Scotland Ave, Albany, NY 12208.
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: Sixty-four contrast medium–enhanced abdominal helical CT scans (64 adult patients) were obtained with 150 mL of contrast medium. The injection rate was 2.5 mL/sec for the first 32 scans and 5.0 mL/sec for the remaining 32. Scans were obtained at 5-sec intervals, with an intermediate 8-sec breathing interval. Hepatic and pancreatic enhancement levels were measured and averaged, and time-attenuation curves were plotted for both groups. Differences in weight, age, time to peak pancreatic and hepatic enhancement, and peak enhancement were assessed with the Student t test.
RESULTS: Both peak enhancement and time to peak enhancement were significantly different between the two injection rates (P 
.002), with faster, more intense hepatic and pancreatic enhancement at the higher rate. At 2.5 mL/sec, the pancreas reached a peak attenuation level of 65 HU at 69 sec, and the liver reached a peak of 58 HU at 87 sec. At 5.0 mL/sec, the pancreas reached a peak attenuation of 84 HU at 43 sec, and the liver reached a peak of 75 HU at 63 sec.
CONCLUSION: Absolute values for and times to peak pancreatic and hepatic enhancement are directly related to the contrast medium injection rate.
Index terms: Computed tomography (CT), helical, 761.12115, 770.12115 • Liver, CT, 761.12112, 761.12114, 761.12115 • Pancreas, CT, 770.12112, 770.12114, 770.12115
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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It is universally accepted that intravenous administration of a contrast medium is required to maximize the conspicuity of pancreatic tumors at abdominal CT. Unfortunately, little agreement has been found with regard to the appropriate injection rate for contrast media and the appropriate scan acquisition timing for pancreatic CT. Some authors have shown improved tumor conspicuity during the venous phase of enhancement (10); however, others have found no difference in detection rate between arterial-phase and venous-phase scanning (11). Lu et al (12) demonstrated maximum tumor-pancreas contrast during an intermediate "pancreatic" phase of contrast enhancement.
On the basis of our experience with abdominal helical CT, we wondered if variations in the injection rate of contrast media might account for some of these discrepancies. Although the effect of the injection rate on hepatic enhancement has been studied, to our knowledge very few data have been published on the enhancement of normal pancreatic tissue at different injection rates. We therefore designed a prospective study to evaluate the effect of the injection rate of contrast medium on pancreatic and hepatic enhancement at abdominal helical CT, with the ultimate goal of devising appropriate protocols for examining the pancreas.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Scanning was performed with a commercially available scanner (Hi-Speed Advantage; GE Medical Systems, Milwaukee, Wis). An initial unenhanced localizer examination that consisted of eight to 10 scans was performed with helical acquisition, 10-mm collimation, a pitch of 1.0, and exposure factors of 120 kV and 250 mAs (Smart Scan; GE Medical Systems). Next, single-level dynamic scanning of the pancreas was performed during intravenous injection of 150 mL of iodinated contrast medium, with the section location selected from the set of unenhanced scans. Scanning was performed with 5-mm collimation and exposure factors of 140 kV and 60 mAs to minimize the dose of radiation. Scans were obtained with patients in suspended respiration every 5 sec from 25 to 45 sec and then again from 53 to 73 sec after injection, with an 8-sec breathing period between the two acquisitions (Fig 1).
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Hepatic and pancreatic attenuation values were measured on the enhanced scans with the use of three 8 x 8-mm regions of interest in the pancreas and one in the liver. All measurements were obtained in regions of uniform parenchymal attenuation, with care being taken to avoid vessels, artifacts, or other areas that might have spuriously increased or decreased the measurements. The absolute change in attenuation for each organ was then calculated by subtracting the baseline preinjection value. The three pancreatic measurements were averaged.
Time-attenuation curves for the pancreas and liver were plotted for the two groups of patients by using mean enhancement values for all of the subjects in each group. Neither of the curves for the lower injection rate (2.5 mL/sec) reached a definite plateau; therefore, a third group of 32 patients (16 men, 16 women; age range, 21–75 years; mean age, 52 years) was recruited (group 3). Patients in this group all received the nonionic contrast medium, iohexol, at an injection rate of 2.5 mL/sec. To ensure capture of peak attenuation, scans were acquired every 5 sec from 50 to 70 sec and again from 78 to 98 sec after injection, with an 8-sec breathing interval. Time-attenuation curves were plotted as they were for the first two groups of patients. Preliminary analysis showed that the curves for this group had definable peaks or plateaus. Therefore, patients in group 1 were excluded from further analysis.
To assess the significance of differences in enhancement between the remaining two groups, the mean times to peak pancreatic and hepatic enhancement and the mean peak enhancement levels were also calculated for each group. These differences were assessed with the Student t test, as were differences in weight and age.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Recent work has focused on defining the most appropriate phase of enhancement for pancreatic scanning. Several investigators have suggested that a brief scanning delay maximizes tumor conspicuity because of the exclusively arterial blood supply of the pancreas (9,12,15). In a study by Hollett et al (15), pancreatic enhancement was improved during the "arterial" phase of a biphasic scanning protocol with an injection rate of 5 mL/sec and a mean scanning delay of 32 sec; however, Lu et al (12) maximized pancreatic enhancement and tumor conspicuity by scanning the pancreas during what they termed the "pancreatic" phase of enhancement by using a scanning delay of 40–70 sec at a 3-mL/sec injection rate. Both groups emphasized the need for delayed scans of the liver for the evaluation of hepatic metastases; however, no attempt was made to assess the role of the injection rate of contrast medium in either study.
In a more recent study, Graf et al (10) found that tumor conspicuity and opacification of peripancreatic venous structures were greatest during the portal venous phase (60-sec scanning delay) with an injection rate of 4 mL/sec; however, this finding contradicts the results of Keogan et al (11), who compared tumor conspicuity and vascular opacification during biphasic arterial-phase (20-sec delay) and venous-phase (70-sec delay) scanning, also with a 4-mL/sec injection rate. They found no statistically significant difference in tumor conspicuity between the two phases.
In our clinical experience, we had consistently noted earlier and more intense pancreatic enhancement at higher injection rates, and we hypothesized that some of the reported discrepancies might be related to the different injection rates used in various studies. More important, we suspected that scanning protocols based on high injection rates might not be applicable to lower injection rates and vice versa. Although more intense enhancement might be exploited to improve pancreatic tumor conspicuity, high injection rates may not be feasible in patients with limited venous access. Therefore, an algorithm suitable for any arbitrary rate of injection would be desirable.
To the best of our knowledge, only one published report has examined the relationship between injection rate and pancreatic enhancement. In a study that compared injection rates of 2 and 6 mL/sec, Bonaldi et al (13) found that the time to peak pancreatic enhancement was inversely proportional to the injection rate, but no statistically significant difference in the degree of pancreatic enhancement was found between the slow and rapid injection rates. These same authors (13) concluded that a lower injection rate is appropriate for pancreatic scanning, largely because the lower rate affords a longer scanning window; however, their study was limited by the fact that helical scanning was performed only at the higher injection rate. Moreover, their population was heterogeneous (some patients had pancreatic pathologic abnormalities, and some did not), and the number of points plotted on the time-attenuation curves was limited.
A recent study by Bae et al (16) provides a theoretic framework that helps to explain our results and the results of other investigators. Using simulated and porcine models of contrast pharmacokinetics, Bae et al (16) showed that the time to aortic peak enhancement was the sum of the duration of injection plus the time for the contrast medium bolus to reach the aorta, which they estimated to be 5–10 sec. Time to peak hepatic enhancement was the sum of the duration of injection plus the time for the liver to reach equilibrium. Because the liver took longer to reach equilibrium at higher injection rates, the gap between peak aortic and peak hepatic enhancement also was wider at higher injection rates, despite the shorter duration of injection.
Because of the arterial blood supply of the pancreas, pancreatic enhancement probably parallels the aorta more closely than the liver. On the basis of this rough assumption, the work of Bae et al (16) would predict pancreatic enhancement peaks at approximately 70 sec and 40 sec with an injection volume of 150 mL at rates of 2.5 mL/sec and 5.0 mL/sec, respectively; these values are similar to our results of 69 and 43 sec. For the liver, predicted times to peak enhancement for the two injection rates would be approximately 92 sec and 72 sec, compared with our results of 87 and 63 sec. In our study, hepatic enhancement also followed the predicted trend, although not as closely as pancreatic enhancement did. This discrepancy may be partly related to biologic variability, which would be more likely to affect hepatic enhancement because of its complex pharmacokinetics. Furthermore, the hepatic attenuation curves in our patients tended to be flat, which makes it more likely that slight variations in peak enhancement level would affect the time to peak. Nevertheless, our work and the results of Bae et al (16) make it clear that any proposed scanning protocol must take the injection rate of the contrast medium into account.
This also suggests why Lu et al (12) achieved satisfactory tumor conspicuity by scanning the pancreas from 40 sec to 70 sec. With their contrast medium dose of 150 mL and their injection rate of 3 mL/sec, peak pancreatic enhancement would have occurred at approximately 60 sec. Therefore, they may have actually scanned the pancreas during the arterial phase, rather than the so-called pancreatic phase, as they suggested.
Unfortunately, using these results to design clinical pancreatic protocols is problematic. Historically, the assumption has been made that conspicuity of hepatic lesions is greatest when parenchymal enhancement is at its peak, which would also seem to be an appropriate initial assumption for pancreatic tumors; however, peak parenchymal enhancement may not correspond to peak lesion conspicuity, which we did not address in our study. Given the discrepant findings for tumor conspicuity reported in the literature and the biologic variability of tumor enhancement, biphasic scanning still seems prudent whenever possible.
Because the pancreatic time-attenuation curve is roughly symmetric in shape, the arterial-phase acquisition of a biphasic scan should be centered at peak pancreatic enhancement. The initial scanning delay can be calculated by adding 10 sec (the approximate aortic transfer time) to the injection duration (contrast medium volume divided by injection rate), and then subtracting one-half of the pancreatic scanning time. In most adult patients, it should be possible to encompass the pancreas in less than 30 sec, depending on its craniocaudal extent, scanning collimation, and pitch. For example, with a volume of contrast medium of 150 mL, an injection rate of 5 mL/sec, and a pancreatic scanning time of 20 sec, the arterial-phase scanning delay would be 10 + (150/5) – 10, or 30 sec. The timing for venous-phase scanning depends on whether the liver is scanned in the same helical acquisition as the pancreas is. In general, however, starting the venous acquisition approximately 7 sec after the end of the arterial acquisition should afford good venous-phase scanning at a 5-mL/sec injection rate.
The same principles apply at a slower injection rate; however, the shorter gap between the pancreatic and hepatic peaks may make it difficult to perform biphasic scanning. In these cases, a uniphasic scanning protocol may be an appropriate compromise, with acquisition roughly centered between the pancreatic and hepatic enhancement peaks: For a contrast medium volume of 150 mL and an injection rate of 2.5 mL/sec, a scanning delay of approximately 60 sec could be used (10 + [150/2.5] – 10 = 60). Whenever possible, however, patients who are known to have or who are suspected of having a hypervascular neoplasm, such as an islet cell tumor, should be scanned with a fast injection rate (5 mL/sec) and a short initial scanning delay (30 sec) to maximize detection of hyperattenuating primary lesions or hepatic metastases.
One of the inherent limitations in devising a universal scanning protocol is the inability to take biologic variability into account. Although one can try to adjust injection rates and scanning delays to account for differences in cardiac output, this is difficult, at best. Bolus-tracking techniques such as SMARTPREP integrated software (GE Medical Systems) might be used to further optimize uniphasic pancreatic scanning for individual patients (17). This was recently evaluated by Diehl et al (18), who used SMARTPREP to track aortic enhancement at a level immediately superior to the pancreas and used triggered scanning when the enhancement peak reached its plateau. Further work will be required to validate the usefulness of this approach for routine scanning.
Although our study was limited by the fact that we compared two different sets of patients, the groups were statistically similar in age and weight, and none had a history of cardiac or pancreatic disease that could have affected pancreatic enhancement. Moreover, our methods paralleled the protocols already used by other investigators to evaluate hepatic enhancement (19,20), and our findings for hepatic enhancement are similar to those reported in the literature (20–23).
We also recognize that peak hepatic or pancreatic enhancement could have occurred during the 8-sec breathing interval between our two helical acquisitions; however, had we not incorporated this breathing interval, it is highly likely that the peak would have occurred during one of the 5-sec gaps that separated our data points. Increasing the temporal resolution of our study by sampling more frequently was technically impossible for us, and determining the "true peak" of a relatively flat curve is always problematic; however, our primary intent was to demonstrate trends in enhancement. Finally, we studied only patients without evidence of pancreatic disease. Nevertheless, the general principles that we have outlined should still apply.
In conclusion, we have demonstrated that pancreatic and hepatic enhancement at helical CT depend on the injection rate of the contrast medium, with faster and more intense enhancement at higher rates. Scanning delays should be adjusted accordingly, and a high injection rate should be used for biphasic scanning of the pancreas whenever possible. Since performing this study, we have incorporated these recommendations into our clinical protocols, and we encourage others to do the same.
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Acknowledgments |
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Footnotes |
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From the 1996 RSNA scientific assembly.
Author contributions: Guarantor of integrity of entire study, M.E.T.; study concepts and design, M.E.T.; definition of intellectual content, M.E.T., F.N.T.; literature research, M.E.T., F.N.T.; clinical studies, M.E.T., P.C.M.; data acquisition, M.E.T.; data analysis, T.L.P., F.N.T., S.L.C.; statistical analysis, T.L.P., F.N.T., M.E.T.; manuscript preparation, editing, and review, F.N.T., M.E.T.
Received February 11, 1998;
revision requested March 31, 1998; revision received July 7, 1998;
accepted August 11, 1998.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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