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Gastrointestinal Imaging |
1 From the Department of Radiology, Osaka University Medical School, 2-2 Yamadaoka, Suita City, Osaka 565-0871, Japan. From the 1997 RSNA scientific assembly. Received June 17, 1998; revision requested July 30; revision received September 22; accepted December 15. Address reprint requests to T.K. (e-mail: kim@radiol.med.osakau.ac.jp).
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: A total of 126 patients were divided at random into four groups with different injection rates and doses. Groups 1 and 2 underwent injection of 2 mL per kilogram of body weight of 300 mg of iodine per milliliter of contrast material, and groups 3 and 4 underwent injection of 1.5 mL/kg. The injection rate was 5 mL/sec for groups 1 and 3 and 3 mL/sec for groups 2 and 4. Single-level serial CT scanning was performed at the level of the pancreatic head, and the pancreatic enhancement value was calculated.
RESULTS: The maximum pancreatic enhancement value was 99 HU ± 18 (mean ± SD) for group 1, 90 HU ± 18 for group 2, 86 HU ± 15 for group 3, and 74 HU ± 13 for group 4. There were significant differences in the maximum pancreatic enhancement value between groups 1 and 2 (P = .045), between groups 3 and 4 (P = .001), between groups 1 and 3 (P = .016), and between groups 2 and 4 (P = .001).
CONCLUSION: Both a higher dose and a faster injection rate increased the maximum pancreatic enhancement value.
Index terms: Computed tomography (CT), contrast enhancement, 770.12112, 770.12119 • Pancreas, CT, 770.12112, 770.12119 • Pancreas, neoplasms, 770.32
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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One of the most common goals of pancreatic CT examination with use of contrast material is diagnosis of pancreatic ductal adenocarcinoma. Pancreatic adenocarcinomas are hypovascular (19), and theoretically, therefore, the greatest conspicuity of these tumors should be achieved when the pancreatic parenchyma is highly enhanced. Visualization of both the major peripancreatic arteries and portal venous system are also required for adequate staging of the tumor.
In this study, we evaluated the effects of different injection rates and doses of contrast material on pancreatic CT imaging through analysis of the enhancement of the pancreatic parenchyma, aorta, and superior mesenteric vein to identify the optimal injection technique.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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We explained to the patients the purpose of this study, and we explained that radiation levels would be slightly increased, that the added radiation level would be about equal to that of several sections of our routine abdominal CT, and that the study would not interfere with the patients' clinical examinations. The patients who did not agree to this study were excluded, and all the patients included in this study gave informed consent. This study adhered to the Declaration of Helsinki principles (20). Patients in whom a 19-gauge plastic cannula could not be placed in an antecubital vein at the time of the CT examination were also excluded from this study.
Ultimately, 126 patients (63 women, 63 men; age range, 21–88 years; mean age, 56 years; weight range, 30–80 kg; mean weight, 55 kg) who met all the criteria for inclusion were entered into this study. The patients were divided at random into four groups with different monophasic intravenous injection protocols for the contrast material. In this study, two different injection rates (faster rate: 5 mL/sec and slower rate: 3 mL/sec) and two different doses (higher dose: 2 mL/kg and lower dose: 1.5 mL/kg) were adopted. For the contrast material, 300 mg of iodine per milliliter of nonionic contrast material (iomeprol; Iomeron; Eisai, Tokyo, Japan) was used. Group 1 (29 patients) underwent injection of a dose of 2 mL per kilogram of body weight at a rate of 5 mL/sec, group 2 (30 patients) underwent injection of a dose of 2 mL/kg at 3 mL/sec, group 3 (32 patients) underwent injection of a dose of 1.5 mL/kg at 5 mL/sec, and group 4 (35 patients) underwent injection of a dose of 1.5 mL/kg at 3 mL/sec.
The patients were examined after fasting for at least 4 hours. All CT scanning was performed with a HiSpeed Advantage scanner (GE Medical Systems, Milwaukee, Wis). Several nonenhanced CT images covering the pancreas were obtained in the form of contiguous 10-mm sections. These nonenhanced images were used to establish a single reference level, namely the section that included the pancreatic head and the superior mesenteric vein and that was caudal to the confluence of the splenic and the superior mesenteric veins. Single-level serial CT images were obtained at this level during a single breath hold every 2 seconds from 22 seconds until 60 seconds after the initiation of contrast material injection with an automatic injector (Autoenhance A50; Nemotokyorindo, Tokyo, Japan) through a 19-gauge plastic cannula placed in an antecubital vein.
Single-level serial CT scanning was performed with a 1-second scanning time, a 1-second interscan delay, 10-mm sections, an 80-mA electric current, and 140 kV. The electric current used for this study was reduced to 80 mA, although a 240-mA current is used for routine CT imaging in our institution. This method of repeated single-level CT sections at reduced milliamperage is used routinely in the commercial packages that monitor CT enhancement to optimize timing of scans relative to a bolus administration of contrast material (21). After this single-level scanning, CT scanning of the liver during the portal venous phase, as routinely performed in our hospital, was initiated 70 seconds after the start of the injection of contrast material. This study involved a minimal increase in radiation levels and did not interfere with clinical examinations.
Quantitative data acquisition was followed by measurement in Hounsfield units of the resultant attenuation values of the pancreatic head and aorta by using a circular region-of-interest cursor placed on the images by one author (T.K.) before and every 2 seconds from 22 seconds until 60 seconds after the initiation of injection of contrast material for a total of 21 images. Every attempt was made to align the regions of interest on all the images and to maintain a constant region-of-interest area of at least 1 cm2 in the pancreatic head and aorta. Care also was taken to exclude vessels and ductal structures from the region of interest in the pancreatic head.
The enhancement value at each time point was calculated for each of the patients as the difference in attenuation value (in Hounsfield units) between the nonenhanced and the contrast material–enhanced images for the pancreatic parenchyma and aorta.
The maximum enhancement values of the pancreatic parenchyma within 60 seconds after the injection of contrast material were determined for each patient. If the enhancement value of the pancreas did not show a peak within 60 seconds after the initiation of injection and was still increasing at 60 seconds, the maximum enhancement value of the pancreas was regarded as the enhancement value at 60 seconds. Maximum enhancement values of the pancreas for the four groups were then compared statistically. An overall test was performed with the Kruskal-Wallis test. When the differences in ratings among the four groups were statistically significant, the Mann-Whitney U test was used to compare the data for the groups. A two-tailed P value less than .05 was considered statistically significant.
The mean enhancement values of the pancreas and the aorta at each time point for each group were calculated and plotted on graphs to show the enhancement patterns of the pancreas and the aorta according to the different injection protocols. From these graphs, the time point when the mean enhancement value of the pancreas reached 50 HU was obtained for each group, and this time point was defined as the average onset of the pancreatic phase during which the pancreatic parenchyma was optimally enhanced (10,11). The time points when the mean enhancement values of the pancreas and the aorta reached their maximum were also obtained for each group.
Two authors (T.K., T.M.) evaluated by consensus the enhancement of the superior mesenteric vein on the images obtained with single-level serial scanning. The time when the superior mesenteric vein was completely opacified without defects caused by the mixing of contrast material and blood was determined for each patient.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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The enhancement value of the pancreas reached its maximum within 60 seconds after the start of injection in all patients, except for four patients in group 2 and two patients in group 4. The maximum enhancement value of the pancreas was 99 HU ± 18 for group 1, 90 HU ± 18 for group 2, 86 HU ± 15 for group 3, and 74 HU ± 13 for group 4 (Fig 1). The resultant P values of statistical comparison of the maximum enhancement values were .045 between groups 1 and 2, .016 between groups 1 and 3, less than .001 between groups 1 and 4, .67 between groups 2 and 3, .001 between groups 2 and 4, and .001 between groups 3 and 4. There were significant differences between all groups except for between groups 2 and 3. The maximum enhancement value of the pancreas for group 1, in which the patients underwent injection with the higher dose at the faster rate, was greater than for the other groups.
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DISCUSSION |
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The pancreatic phase was defined by Lu et al (10) as the period during which the pancreatic parenchyma is most markedly enhanced, and the pancreatic phase in their study was 40–70 seconds after the infusion of 150 mL of contrast material at a rate of 3 mL/sec. For our study, we defined the time point when the mean enhancement value of the pancreas reaches 50 HU as the average onset of the pancreatic phase. A clinically valid definition of the start and end of a phase, such as the hepatic arterial phase, the hepatic portal venous phase, and the pancreatic phase, is difficult to obtain.
The results of our study revealed that the time until the average onset of the pancreatic phase was nearly identical for the groups with the same injection rate, which indicates that this period depends on the injection rate. The results of our study also support the definition of Lu et al (10) of the start of the pancreatic phase when the injection rate is 3 mL/sec, because the average time of onset of the pancreatic phase was 34–36 seconds after the start of injection in our groups with an injection rate of 3 mL/sec. However, the corresponding onset occurred 26–28 seconds after the start of injection in our groups with an injection rate of 5 mL/sec. Therefore, the pancreatic phase starts earlier when the injection rate is faster. It has been reported that the faster rate of injection decreases the time during which pancreatic enhancement is high (18). In addition, our study results showed that the duration of the pancreatic phase depended on the dose of contrast material because the mean pancreatic enhancement was maintained at a high level for a longer period in the groups with the higher dose.
Keogan et al (14) reported that "arterial phase" scans obtained 20–40 seconds after the injection of contrast material at a rate of 4 mL/sec, and obtained in addition to "venous phase" scans acquired 70–100 seconds after injection, did not improve the detection of pancreatic malignancies. Graf et al (16) also reported that the conspicuity of pancreatic adenocarcinomas was substantially greater on the "portal phase" CT scan started 60 seconds after the start of injection of 160 mL of contrast material at a rate of 4 mL/sec than on the "arterial-phase" CT scan started 18 seconds after the start of injection. However, Lu et al (10) reported that the mean tumor-pancreatic contrast was significantly greater during the "pancreatic phase" (40–70 seconds after the infusion of contrast material at a rate of 3 mL/sec) than during the "hepatic phase" (70–100 seconds after infusion). Our study results suggest that the arterial phase scan in the studies by Keogan et al (14) and Graf et al (16) started before the pancreatic parenchyma began to be greatly enhanced and that the pancreatic phase scan in the study by Lu et al (10) started after such notable enhancement.
Pancreatic ductal adenocarcinomas are hypovascular (19), and theoretically, therefore, the greatest conspicuity of these tumors should be achieved when the pancreatic parenchyma is greatly enhanced. Although, to our knowledge, there have been no reports comparing the efficacy of arterial phase imaging with that of pancreatic phase imaging for the detection of pancreatic adenocarcinoma, we hypothesize that pancreatic phase imaging is more effective for attaining the greatest conspicuity of hypovascular pancreatic adenocarcinoma.
Bonaldi et al (18) reported that a faster injection rate did not increase the maximum pancreatic enhancement. Our study results do not support the conclusion of Bonaldi et al because our results show that the faster injection rate of 5 mL/sec substantially increased the maximum enhancement value of the pancreas above that of the slower injection rate of 3 mL/sec for both the higher dose (2 mL/kg) and the lower dose (1.5 mL/kg) groups. The lower dose in our study was the same as the dose used by Bonaldi et al, and a comparison of the results of the groups in their study and of the lower dose groups in our study shows that the maximum enhancement value of the pancreas was 71 HU ± 15 for a rate of 6 mL/sec and 72 HU ± 17 for a rate of 2 mL/sec in the study of Bonaldi et al, and 86 HU ± 15 for a rate of 5 mL/sec and 74 HU ± 13 for a rate of 3 mL/sec in our study. The maximum enhancement values of the pancreas for the slower rates (2 mL/sec and 3 mL/sec) in the two studies were thus almost identical, but those for the faster rates (6 mL/sec and 5 mL/sec) were quite different.
Bonaldi et al (18) started CT scanning 30 seconds after the start of contrast material injection, while we started 20 seconds after the start of contrast material injection. In our study, moreover, the time point when the mean enhancement value of the pancreas reached its maximum was 32 seconds for the group of patients who underwent an injection of 1.5 mL/kg at a rate of 5 mL/sec. As a result of comparing the results of Bonaldi et al with ours, we presume that CT scanning could have been started after the actual peak pancreatic enhancement in some patients in the study by Bonaldi et al and that therefore they may have underestimated the maximum enhancement value of the pancreas for a rate of 6 mL/sec.
To our knowledge, there have been no reports to evaluate the effects of different doses of contrast material injection on pancreatic CT imaging. Our study results clearly showed that a higher dose substantially increases the maximum pancreatic enhancement. Both a higher dose and a faster rate for the injection of contrast material thus appear to increase the maximum pancreatic enhancement. A higher dose results in both greater pancreatic enhancement and more constant maintenance of high pancreatic and arterial enhancement during the pancreatic phase. A faster injection rate also results in greater pancreatic enhancement. However, if the dose is reduced, a faster injection rate may decrease the duration of high pancreatic and arterial enhancement in the pancreatic phase.
Of the four injection protocols evaluated in this study, an injection of a dose of 2 mL/kg at a rate of 5 mL/sec achieved the greatest maximum enhancement value of the pancreas, but this injection did not produce constant maintenance of high arterial enhancement throughout the pancreatic phase. Such high arterial enhancement was maintained most constantly with the injection of a dose of 2 mL/kg at a rate of 3 mL/sec.
For staging of pancreatic adenocarcinomas, evaluation of arterial involvement is required. For this purpose, high arterial enhancement should be maintained constantly throughout the pancreatic phase. The dose of 2 mL/kg used as the higher dose in this study is the same as the usual clinical dose. When injected at a rate of 5 mL/sec, however, this dose is not enough to maintain constantly high arterial enhancement throughout the pancreatic phase. We therefore think that the arterial structures can be evaluated better if a dose of more than 2 mL/kg is injected at a rate of 5 mL/sec for the diagnosis of pancreatic adenocarcinoma, unless a patient has renal dysfunction. If intravenous access is poor and a higher injection rate cannot be achieved in some patients, increasing the dose of contrast material may result in an increase in pancreatic enhancement.
The involvement of veins, including the superior mesenteric vein, should also be evaluated for the staging of pancreatic adenocarcinomas. Lu et al (10) reported that substantially greater enhancement in all vessels, including the superior mesenteric vein, was attained during the pancreatic phase rather than the hepatic phase. However, our study results showed that complete opacification of the superior mesenteric vein without defects caused by the mixing of contrast material and blood could not be attained consistently throughout the pancreatic phase. And if the superior mesenteric vein is not completely opacified with contrast material, venous involvement by pancreatic adenocarcinomas cannot be precisely evaluated. Therefore, our study results indicate that second-phase imaging performed after the pancreatic phase is needed for the evaluation of venous involvement, regardless of injection technique. Portal venous phase imaging is usually performed as the second-phase imaging (10–17). Portal venous phase imaging is necessary not only for the detection of hypovascular liver metastases from pancreatic adenocarcinomas but also for the precise evaluation of the venous involvement of pancreatic adenocarcinomas.
In summary, our study results indicate that the average time until the onset of the pancreatic phase depends on the injection rate. A higher dose results in both greater pancreatic enhancement and more constant maintenance of high pancreatic and arterial enhancement during the pancreatic phase. A faster injection rate also results in greater pancreatic enhancement. However, if the dose of contrast material is reduced, a faster injection rate may decrease the duration of high pancreatic and arterial enhancement in the pancreatic phase. Our study results lead us to hypothesize that pancreatic phase and portal venous phase helical CT scans with the injection of contrast material at a higher dose and at a faster rate can provide better results for the detection and staging of pancreatic adenocarcinoma. Further clinical study in patients with pancreatic adenocarcinoma is needed to prove this hypothesis.
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Footnotes |
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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