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Original Research |
1 Service d'Imagerie Guilloz, University Hospital of Nancy, 29, ave. Marechal de
Lattre de Tassigny, Nancy 54 035, France.
2 Service d'Epidémiologie et évaluation cliniques, Centre
d'Epidémiologie Clinique, Hôpital Marin, University Hospital of
Nancy, Nancy 54 035, France.
Received June 15, 2004;
accepted after revision April 6, 2005.
Address correspondence to A. Blum
(a.blum{at}chu-nancy.fr).
Abstract
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SUBJECTS AND METHODS. We enrolled 407 patients undergoing abdominal exploration on MDCT, including an early arterial phase and a portal phase. Group 1 received contrast medium at a dose of 1.5 mL/kg. Group 2 received contrast medium at a dose of 1.5 mL/kg less 20 mL of iodine followed by 30 mL of saline solution. Group 3 received the full 1.5 mL/kg dose of medium followed by a 30-mL saline chaser. Attenuation values were obtained from the aorta in the arterial phase and from the liver in the portal phase.
RESULTS. The groups were comparable in mean body weight and heart rate. None of the differences between them in aortic enhancement in the early arterial phase were statistically significant (group 1, 206 ± 3 H; group 2, 204 ± 3 H; group 3, 209 ± 4 H). There was a negative correlation between weight and aortic enhancement (r = -0.42, p < 0.0001) and a positive correlation between weight and hepatic enhancement (r = 0.19, p < 0.0001). A significant reduction (p = 0.0002) in hepatic enhancement was observed in group 2 (group 1, 53 H; group 2, 46 H; group 3, 54 H). Hepatic enhancement greater than 45 H was observed in all groups except for the group 2 subgroup of patients weighing less than 55 kg.
CONCLUSION. In MDCT, reducing the amount of contrast medium does not affect aortic enhancement in the early arterial phase but decreases hepatic enhancement at the portal phase. The saline chaser technique had no influence on the phases studied. Greater than 1.5 mL/kg reduction in the dose of contrast medium followed by saline flushing may not be advisable for liver CT in low- and medium-weight patients.
Keywords: abdominal CT • aorta • contrast media • liver • saline chaser
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In a 2003 study, Dorio et al. [1] found that use of a saline chaser (also known as saline flushing) may allow a reduction in the amount of contrast medium needed in abdominal imaging examinations. Those investigators compared two fixed-dose injection protocols, one with 150 mL of contrast material and one with 100 mL of contrast material followed by a 50-mL saline chaser. Despite the reduced volume of contrast medium, there were no clinically significant differences in hepatic enhancement between the two protocols. Administration of a fixed amount of contrast medium can have limitations. A volume insufficient for enhancement of the liver in a large patient may cause contrast medium-induced nephropathy in someone thin and frail. It has therefore become usual practice to consider the patient's weight in decisions about the amount of medium to use [2-4]. The objective of this study was to explore the effects of using a saline chaser in abdominal CT when the dose of contrast medium is based on the patient's weight. Another objective was to evaluate the use of saline solution when doses of contrast medium are reduced to less than 1.5 mL/kg.
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Image Acquisition
Contrast-enhanced MDCT (Sensation 16, Siemens Medical Solutions) was
performed. Each examination included one acquisition of unenhanced images from
the hepatic dome to the pelvis with the following parameters: detector row
configuration, 16 x 1.5 mm; pitch, 1 (table feed per gantry rotation, 24
mm/s); slice thickness, 8 mm at 10-mm intervals; 120 kV; and product of tube
current and exposure time in milliampere-seconds (mAs), equal to the patient's
body weight.
Early arterial phase acquisition with an automated bolus-tracking device (Carebolus, Siemens) was initialized when enhancement in the aorta exceeded 100 H. Acquisition lasted approximately 6 seconds and was performed with a 16 x 1.5 mm detector row configuration and a pitch of 1 (table feed per gantry rotation, 24 mm/s; gantry rotation time, 0.5 seconds). Images were reconstructed with a 5-mm slice thickness at 7-mm intervals.
Forty seconds after the end of the first acquisition, data were acquired for the entire abdomen in the portal phase. A 16 x 0.75 mm or 16 x 1.5 mm detector row configuration was chosen with a pitch of 1. Slice thickness was 5 mm at 7-mm intervals. Other slices were obtained for examination and interpretation elsewhere. In all cases, 120 kV was used; the product of tube current and exposure time in milliampere-seconds was equal to two to three times the body weight.
Contrast medium was injected through an 18-gauge IV catheter into an antecubital vein. At the beginning of the study, the saline chaser technique required two interconnected power injectors, each holding a 200-mL syringe as described by Haage and colleagues [5]. The two long tubes on the injectors were connected in line with a nonreturn valve and then with a Y-adapter leading to the IV catheter. To save time, dual-head injectors (Stellant, Medrad) were used when they became available toward the end of the study. Reports in the literature indicate the volume of chaser saline solution used varies from 20 to 50 mL, and there is no consensus on the optimal amount. Bae et al. [6] estimated that the dead space caused by the brachial vein is approximately 40 mL in a 70-kg patient. Because we believe dead space is minimally influenced by the patient's weight, a fixed dose of 30 mL was adopted to simplify the procedure and make it reproducible.
Data Acquisition
For each of the 407 scans, one radiologist measured arterial and hepatic
enhancement after identification of consistent regions of interest with areas
ranging from 15 to 30 mm2. Measurements first were obtained from
unenhanced images of the aorta and liver. In the arterial phase, circular
regions of interest were identified at the level of the celiac trunk, the
superior mesenteric artery, and the iliac bifurcation. In the portal phase,
measurements free of artifacts and enhanced vessels were obtained in the
hepatic parenchyma and the aorta.
Statistical Analysis
Comparability between groups was assessed with tests based on the
chi-square test and Student's t test for categorical and continuous
variables, respectively (level of type 1 error, 5%). Quantitative data
obtained with the various protocols were compared through one-way analysis of
variance. Bonferroni adjustment was used for multiple comparisons. A
difference was considered statistically significant at p < 0.05.
Pearson's correlation coefficient was calculated to test the hypothesis that
aortic enhancement and hepatic enhancement are related to patient weight.
Analysis of variance was used to determine the effect of weight (< 55 kg,
55-79 kg, and 
80 kg) on aortic and hepatic enhancement. Statistical
analysis was performed with SAS system software (release 8.2, SAS Institute
[7]).
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Contrast enhancement data for the three groups are summarized in Tables 2, 3, 4, 5 and illustrated in Figures 1A, 1B, 2A, and 2B. The magnitudes of observed enhancement were comparable in the aorta during the early arterial phase: group 1, 206 H; group 2, 204 H; and group 3, 209 H (Table 2). Differences between groups did not reach statistical significance. A negative correlation was observed between patient weight and aortic enhancement (r = -0.42, p < 0.0001) (Table 3).
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Average hepatic enhancement in the three groups was 53 H, 46 H, and 54 H. A statistically significant reduction in hepatic enhancement was observed in group 2 compared with groups 1 and 3 (6.8 H, p = 0.002; 7.8 H, p < 0.001). When doses of contrast medium were similar (groups 1 and 3), hepatic enhancement was unaffected by the saline chaser (Table 4). Patient weight and hepatic enhancement were positively correlated (r = 0.19, p < 0.0001). Hepatic enhancement was greater than 45 H in all patient categories other than group 2 subjects weighing less than 55 kg. The only significant difference in hepatic enhancement was an increase among group 3 patients weighing 55-79 kg compared with the same weight category in group 2 (p = 0.01) (Table 5).
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Hopper et al. [11] looked at flushing techniques as a way of reducing the dose of contrast medium in thoracic CT. Their hypothesis was that medium remaining in the brachiocephalic vein and the right-heart cavities provided few data and that saline pushing would reduce the transit time of the column of medium and decrease the amount of iodine in these so-called dead spaces. A 75-mL dose of contrast medium pushed with saline solution was shown to enhance the thoracic vasculature to a degree equivalent to that obtained with 125 mL of contrast material alone and to cause significantly fewer beam-hardening artifacts.
To the best of our knowledge, there have been few studies of the value of saline flushing in abdominal CT. Dorio and colleagues [1] evaluated 86 hypoattenuating metastatic lesions of the liver in 26 patients who underwent CT in two sessions separated by a mean of 85 days. At the first session, 150 mL of contrast material (iohexol; Omnipaque 300, Nycomed) was administered at a rate varying from 1.0 to 3.0 mL/s depending on venous access. At the second session, 100 mL of contrast medium was followed by a 50-mL saline chaser. Liver and tumor attenuation was slightly better with the 150-mL dose, but the difference was of questionable clinical significance (95 H vs 89 H for the liver).
Schoellnast and colleagues [12] conducted a study in which 41 patients (average weight, 65 kg) with malignant tumors or inflammatory disease underwent two successive sessions of abdominal MDCT. The subjects received 100 mL of nonionic contrast medium (concentration, 300 mg/mL) either alone or pushed with 20 mL of saline solution through a double-syringe power injector at a rate of 2.5 mL/s. Mean enhancement of the liver improved 9 ± 9 H when saline solution was used.
Itoh et al. [13] studied the effects of a 5% dextrose flush on aortic, portal, and hepatic enhancement in 180 patients given 2.1 mL/kg of contrast medium (concentration, 300 mg/mL) in four different protocols. No statistically significant difference in hepatic enhancement in the portal phase was observed between protocols with or without 5% dextrose.
Our results contradict those of Dorio et al. [1] and Schoellnast et al. [12] but are in accord with those of Itoh et al. [13]. Our data do not show that bolus flushing improves hepatic enhancement in the portal phase or allows significant reduction in the amount of iodine administered. Numerous factors, such as dose of contrast medium, injection rate, venous site of injection, and the liver disease involved, may account for the discrepancies. Dorio et al. and Schoellnast et al. used a fixed dose of contrast medium, whereas we determined the dose according to the patient's body weight.
Our results suggest that hepatic enhancement depends on the total dose of contrast medium and not on whether a saline chaser is administered. In the portal phase, all the contrast material is available to contribute to hepatic enhancement because the dead spaces of the right-heart cavities and the brachiocephalic veins have already been washed out.
One of the aims of this study was to determine whether the use of saline solution allows reduction in doses of contrast medium. Some investigators have devised strategies to lower iodine doses, but their findings have been contradictory. Most authors consider maximal hepatic enhancement greater than 50 H to be adequate. Bree and colleagues [14] found that 125 mL of iohexol 300 (43.7 g iodine) resulted in quantitatively optimal hepatic enhancement but that qualitatively optimal enhancement required 150 mL (45 g iodine). Baker et al. [15] found no significant difference in hepatic enhancement in a 168-patient study in which one group received 150 mL of iopamidol 300 (45 g iodine) and another received 125 mL of ioversol 320 (40 g iodine). Blumeke et al. [16] extrapolated from data on rabbits and concluded that 429 mg/kg is the lowest dose with which to produce a 30-H increase in hepatic enhancement. This figure is close to the 457 mg/kg used by Brink et al. [3]. In a large, clinical multicenter trial, Megibow and colleagues [2] established that a dose of 450 mg/kg (1.5 mL/kg of iopromide at a concentration of 300 mg/mL) produces acceptable scans in most patients. The effect of patient weight has also been noted in previous studies. Brink's group [3] found that in heavy patients an iodine dose of 38 g was required for adequate enhancement, whereas in thin patients doses as low as 26 g might be sufficient. In a prospective randomized study by Yamashita et al. [17], hepatic parenchymal enhancement at visual analysis was graded as good or excellent in 64% of patients receiving 1.5 mL/kg, 85% of those receiving 2 mL/kg, 94% of those receiving 2.5 mL/kg, and 65% of those receiving a fixed dose.
We chose a basic dose of 1.5 mL/kg because it is close to the minimum recommended by Megibow and colleagues [2]. A fixed dose reduction of 20 mL for all patients in group 3 was adopted because it was simple and reproducible. Moreover, this amount was considered a reasonable approximation of the dead space in the peripheral vein that does not contribute to aortic or hepatic enhancement. Because it corresponds to the vascular compartment, we considered it unlikely this amount would be significantly affected by body weight. These hypotheses were not confirmed because hepatic enhancement apparently was unaffected by flushing. The dose used for thin patients therefore is clearly too low.
That reductions in contrast medium were not made on the basis of weight is a potential limitation, because thin patients were disproportionately sensitive to dose lowering. The observation of less enhancement in thin patients in all groups is in accord with the finding by Megibow et al. [2] that the larger percentage of unacceptable enhancement was found in patients in the low-weight group. We observed that the relation between a patient's weight and hepatic enhancement was not linear and that the highest levels of hepatic enhancement occurred in the highest weight category, even in groups 1 and 3. The reason for this effect remains hypothetical. It may be that in large patients, the ratio between the diffusion space of contrast medium (i.e., interstitial and blood pools) and the intracellular pool is lower.
It is interesting that hepatic enhancement greater than 45 H was seen in all subjects except for the thin and medium-sized patients in group 2 (< 55 kg, hepatic enhancement, 38 H; 55-79 kg, hepatic enhancement, 45 H). Even if it is agreed that maximal hepatic enhancement greater than 50 H produces 100% acceptable scans, important individual variability occurs in acceptability levels. Thus in group 2, scans were acceptable for large patients. Therefore, if hepatic enhancement alone is considered, one might use a lower dose of contrast medium for these patients. This practice, however, is not acceptable for vascular structures because aortic enhancement is lower in large patients.
With regard to aortic enhancement, attenuation values remained high even when the dose of contrast medium was reduced. This finding is consistent with recent evidence that MDCT allows dramatic reduction in contrast medium in vascular studies [18]. In our study, there was no difference among the three groups. This finding is in agreement with the finding by Itoh et al. [13] of no significant difference in aortic enhancement in the early arterial phase between four injection techniques. The lack of discrepancy between groups can probably be attributed to assessment of aortic enhancement before the arterial peak, which is known to occur (approximately) at the end of continuous injection after a continuous increase in arterial opacification due to recirculation effects [3]. With MDCT, acquisition time is very short. Because it lasted 6 seconds, the arterial phase in this study was completed before the end of injection in many patients.
Absence of late arterial data was a limitation of the investigation. The effect on aortic enhancement of the volume injected and the chaser was probably greater in the late arterial phase, as suggested by the results of Itoh et al. [13]. In their study, a statistically significant increase in aortic enhancement was observed in the late arterial phase and corresponded to the aortic peak of enhancement, when saline flush was used. In this study, multiphasic acquisitions with early and late arterial phases were not clinically justified.
In summary, reducing the volume of contrast medium in this context leads to a reduction in hepatic enhancement not compensated by saline flushing. In the portal phase, it is unlikely that contrast medium in the arm vein will be available for saline flushing. However, if most examinations are to be acceptable, greater than 1.5 mL/kg reduction in contrast bolus may not be advisable for scanning of the livers of thin and medium-sized patients. With regard to aortic enhancement in the early arterial phase, a low dose of contrast medium is sufficient to produce the same enhancement as 1.5 mL/kg with or without a bolus chaser.
Acknowledgments
We thank Jean Bérard for his assistance and William Francis for
reviewing the manuscript.
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