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Original Research |
1 Department of Technical Radiology, Nagoya University School of Health
Sciences, Daikou-minami 1-1-20, Higashi-ku, Nagoya 461-8673, Japan.
2 Department of Radiology, Nagoya University Graduate School of Medicine, Nagoya
466-8550, Japan.
Received March 27, 2005;
accepted after revision May 30, 2005.
Address correspondence to S. Itoh
(shigeito{at}met.nagoya-u.ac.jp).
Abstract
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SUBJECTS AND METHODS. One hundred twelve patients were divided into three groups: contrast injection at 0.08 mL/kg body weight/s (an upper limit of 5 mL/s) over 30 seconds in patients 60 years old or younger (group 1, n = 49), the same contrast injection as group 1 in patients more than 60 years old (group 2, n = 32), and contrast injection at 0.07 mL/kg body weight/s (an upper limit of 4.5 mL/s) over 30 seconds in patients more than 60 years old (group 3, n = 31). Contrast enhancement in the aorta, portal venous system, pancreas, and liver was assessed quantitatively. Two radiologists blinded to the patients' clinical information and the injection protocol used to acquire the CT images graded the degree of contrast enhancement using a 5-point scoring system. The results for the different groups were statistically compared.
RESULTS. Contrast enhancement in the main phases for all organs was significantly more intense in group 2 than in groups 1 and 3. Cases in which pancreatic enhancement in the pancreatic phase was graded as excessive were more frequently observed in group 2. No statistically significant differences were observed between groups 1 and 3 in either quantitative or visual assessment for enhancement of any organ in any phase.
CONCLUSION. We recommend reducing the dose and rate of contrast material injection by at least 10% for elderly patients undergoing MDCT examination of the pancreatobiliary region.
Keywords: contrast media • dynamic CT • MDCT • pancreaticobiliary imaging
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Previous studies have reported that to evaluate anatomic structures and various pathologic conditions of the pancreatobiliary region on contrast-enhanced CT, it is essential to acquire pancreatic phase images showing intense enhancement of the pancreatic parenchyma [3-6]. However, when interpreting CT images acquired using that protocol in our clinical practice, we noted that in some examinations of elderly patients, because contrast enhancement of the pancreatic parenchyma in pancreatic phase images was too intense, the standard settings for window width and level used at our institution had to be adjusted to evaluate the parenchyma in detail. Therefore, we conducted the present study to assess whether it is possible to reduce the dose and rate of contrast material injection without adversely affecting the degree of contrast enhancement in elderly patients undergoing triple-phase (arterial, pancreatic, and portal venous) contrast-enhanced CT of the pancreatobiliary region with an MDCT scanner.
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In this prospective study, nonionic contrast material with an iodine concentration of 300 mg I/mL was injected at a fixed duration of 30 seconds, and a 5% dextrose flush was injected at a fixed rate of 5 mL/s over 6 seconds immediately after the end of contrast material injection. The patients were divided into the three groups on the basis of their age and the rate of contrast material injection. In groups 1 and 2, the same contrast material injection protocol was used. On the other hand, the injection rate was reduced by at least 10% in group 3. No statistically significant differences among the three groups were observed in sex distribution, as assessed by chi-square analysis, or in body weight or attenuation of the pancreatic and hepatic parenchyma on unenhanced CT, as assessed by one-way analysis of variance. Furthermore, there were no statistically significant differences between groups 2 and 3 in age as assessed by the Student's t test (Table 1). The injection of both contrast material and 5% dextrose was performed using two automatic power injectors (Auto Enhance A50 and Auto Injector 1205, Nemotokyorindo) through a 20-gauge IV catheter.
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CT Acquisition
CT was performed on a 16-MDCT scanner (Aquilion, Toshiba Medical Systems).
Five minutes before scanning, patients were instructed to drink 300 mL of
water for negative opacification of the gastrointestinal tract. All scanning
was performed with a peak tube voltage of 120 kVp and a gantry rotation speed
of 0.5 seconds. First, unenhanced images were acquired to determine the range
in the z-axis of the liver and pancreas in the cephalocaudal
direction using the following scanning parameters: a tube current of 300 mA, a
detector row configuration of 16 x 2 mm, a table increment of 30
mm/rotation, and a weighted CT dose index (CTDIw) of 14.9 mGy.
In this study, the range in the z-axis for arterial phase and pancreatic phase scanning was fixed at 150 mm from the porta hepatis to the end of the pancreas. These images were separately acquired with a tube current of 450 mA, a detector row configuration of 16 x 0.5 mm, a table increment of 11.5 mm/rotation, and a CTDIw of 19.3 mGy in the cephalocaudal direction for arterial phase scanning and in the caudocephalad direction for pancreatic phase scanning. The acquisition time for each phase was 8.5 seconds. Finally, portal images were acquired with a tube current of 400 mA, a detector row configuration of 16 x 1 mm, a table increment of 15 mm/rotation, and a CTDIw of 22.3 mGy in the cephalocaudal direction to include the pancreas and liver.
To determine the scan delay from the administration of contrast material to the start of arterial phase scanning, an automatic bolus-tracking method (Sure Start, Toshiba) was used in patients who were more than 60 years old, had cardiovascular disease, or both (specifically, two patients in group 1 and all patients in groups 2 and 3). In this study, a circular region of interest (ROI) with an area of 58 pixels was placed in the aorta at the same level as the start position of the arterial phase scan. The diagnostic scan was started as soon as possible (i.e., limited only by the intrinsic mechanical delay time of about 8 seconds in the system) after the enhancement threshold of aortic attenuation of 80 H was exceeded. In the other patients, the scan delay from the administration of contrast material to the start of arterial phase scanning was set at 24 seconds. Based on the results of previous studies [7, 8], the time intervals between arterial phase and pancreatic phase scanning and between pancreatic phase and portal venous phase scanning were fixed at 12 and 15 seconds, respectively.
Quantitative Assessment
The time interval from the administration of contrast material to the start
of arterial phase scanning was recorded. Attenuation values in the aorta,
portal vein, splenic vein, superior mesenteric vein, pancreas, and liver in
diagnostic scans were measured at a workstation (Alatoview, Toshiba) using a
circular ROI cursor (Table 2).
In all examinations, aortic attenuation was determined at the most superior,
middle, and most inferior levels of the aorta in the arterial and pancreatic
phase scans and only at the middle level in the unenhanced and portal venous
phase scans. Eight patients for the splenic vein, 1 for the superior
mesenteric vein, 7 for the pancreatic head, 13 for the pancreatic body and
tail, and 1 for the liver were excluded because of the presence of various
pathologic conditions that made it difficult to evaluate the degree of
contrast enhancement accurately. Measurements were obtained at the level of
the porta hepatis, pancreatic body and tail, and pancreatic head for the
portal vein, splenic vein, and superior mesenteric vein. For the liver,
measurements were obtained in three different hepatic segments at the level of
the porta hepatis. Attenuation values in the portal vein, splenic vein,
superior mesenteric vein, pancreas, and liver were determined in each
examination on the unenhanced scan and each contrast-enhanced scan. The degree
of contrast enhancement was calculated for each contrast-enhanced region as
the absolute difference in attenuation values between the unenhanced and
contrast-enhanced scans.
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Visual Assessment
All images, which were reconstructed with a 5-mm slice thickness at 5-mm
intervals and were displayed using the standard settings used at our
institution (window width, 345 H; window level, 45 H), were independently
interpreted by two experienced radiologists who were blinded to the patients'
clinical information and the injection protocol used to acquire the CT images.
The degree of contrast enhancement in each examination was determined using
the following 5-point scoring system: excessive, contrast enhancement was so
intense as to interfere with making a radiologic diagnosis; excellent,
contrast enhancement provided optimal information for making a radiologic
diagnosis; good, contrast enhancement provided adequate information for making
a radiologic diagnosis; fair, contrast enhancement provided acceptable
information for making a radiologic diagnosis, but image quality was
unsatisfactory; poor, contrast enhancement did not provide acceptable
information for making a radiologic diagnosis (Figs.
1A,
1B, and
1C).
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Statistical Analysis
The quantitative results for the different groups were compared by one-way
analysis of variance for the following items: first, the times to the start of
arterial phase scanning after the administration of contrast material; and,
second, the contrast enhancement values in the aorta, portal vein and its
branches, pancreas, and liver in each phase. In addition, pairwise comparisons
among them for the three study groups were performed using Scheffe's multiple
comparison tests or Tamhane's pairwise comparisons test according to the
homogeneity of variance. The results of visual assessment in each phase for
the different groups were compared by chi-square analysis. We used SPSS
software (version 12, SPSS) for all statistical analyses. A p value
of < 0.05 was accepted as statistically significant.
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Significant differences were observed among the three groups in the visual grades for enhancement of the pancreatic parenchyma in the pancreatic phase. Specifically, cases in which pancreatic enhancement was graded as excessive were more frequently observed in group 2. There were no statistically significant differences among the three groups in the visual grades for enhancement of the peripancreatic arteries in the arterial phase or the portal venous branches or hepatic parenchyma in the portal venous phase (Table 4).
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In previous studies, researchers have reported that the volume of contrast material needed to achieve adequate contrast enhancement is determined by patient weight [12, 13]. In addition, because it has also been reported that the time to peak contrast enhancement is nearly uniform with respect to the end of injection regardless of the rate [9-11, 14], it is desirable to adopt a fixed injection duration. Therefore, we use a protocol in which not only the volume but also the rate of contrast material injection are determined on the basis of patient weight for multiphase contrast-enhanced CT examination of the abdomen.
However, even using this protocol, there is some room for improvement with regard to individual variation in the degree of contrast enhancement. We speculated that patient age might be one factor that could easily be applied in clinical practice and might be useful in overcoming this problem. To our knowledge, there have been no large-scale studies to evaluate the effects of patient age on contrast enhancement.
The results of the present study have shown that, compared with patients 60 years old or younger, it is possible to achieve a 12.2% reduction in the volume and rate of contrast material injection in patients more than 60 years old while maintaining equivalent contrast enhancement in terms of both quantitative and visual assessment on contrast-enhanced CT studies of the pancreatobiliary region. The following factors account for these results. First, a reduction in cardiac output and in blood volume, which is commonly observed in elderly patients, helps to prevent the dispersion of contrast material [15]. This is beneficial in increasing the degree of contrast enhancement by delivering a better bolus of contrast material to the abdomen. Previous studies have also shown that contrast enhancement during the early phase after the injection of contrast material is inversely related to cardiac output and blood volume [9, 16, 17]. In addition, the fact that contrast material is more slowly excreted in patients with reduced cardiac output is beneficial in increasing the degree of contrast enhancement during the late phase [15].
Furthermore, in the present study, when the same contrast material injection protocol as that used for patients 60 years old or younger was used in patients more than 60 years old, images of the older patients frequently showed that pancreatic enhancement in the pancreatic phase was so intense that it interfered with interpretation unless the window width and level settings were adjusted in CT studies. These results are related to the circulatory dynamics of the pancreas, which is a well-perfused organ with an exclusively arterial blood supply from large arteries in the abdomen. We believe that it is desirable to reduce the dose and rate of contrast material injection in such patients.
In the present study, the automatic bolus-tracking method was not used in patients who were 60 years old or younger or in patients without cardiovascular disease to minimize the radiation dose (e.g., CTDIw of about 3.5 mGy in continuous monitoring scanning performed at a tube current of 50 mA, collimation thickness of 0.5 mm, and monitoring time of 10 seconds). The time to the start of arterial phase scanning determined by automatic bolus tracking ranged from 19.7 to 32.2 seconds. Thus, on average, maximum enhancement was observed in the aorta at the end of arterial phase scanning and at the start of pancreatic phase scanning in group 1 and in groups 2 and 3, respectively. These results suggest that the timing for the start of scanning after the administration of contrast material may differ somewhat between these two groups of patients.
However, we do not believe that the results of the present study can be attributed mainly to this difference in scanning protocol for the following reasons. First, given an injection duration of 30 seconds, the aortic time-enhancement curve rises sharply after the arrival of contrast material, followed by a relatively plateaulike and prolonged slope including the point of peak enhancement [17, 18]. The timing for this slope, not the peak, has a marked effect on the degree of contrast enhancement in the arterial and pancreatic phases obtained by an MDCT scanner [7]. In the present study, this timing corresponded to the period from the middle of arterial phase scanning to the start of pancreatic phase scanning in all groups. Second, because previous studies have shown that the use of automatic bolus tracking does not significantly improve the degree of contrast enhancement in patients without circulatory disturbances [7, 19-21], it is unlikely that the protocol used in the present study led to a deterioration in contrast enhancement in group 1. Actually, the pancreatic and hepatic peak enhancement values in group 1 observed in this study were somewhat higher than the maximum values reported in previous studies [10, 11]. Third, with regard to contrast enhancement of the aorta and pancreas in the portal venous phase, which were less strongly affected by differences in scan timing, group 2 was founded to be significantly superior to group 1.
There are some limitations in the present study. Because we arbitrarily determined the reduction from 0.08 mL/kg body weight/s with an upper limit of 5 mL/s to 0.07 mL/kg body weight/s with an upper limit of 4.5 mL/s in the patients more than 60 years old, it may be possible to achieve a further reduction, especially in more elderly patients. Furthermore, the number of patients was relatively small, and the capabilities of diagnosing pathologic conditions under the different protocols should be evaluated. Therefore, additional studies must be performed to determine an optimal contrast material injection protocol for elderly patients.
In conclusion, we recommend reducing the dose and rate of contrast material injection by at least 10% in patients more than 60 years old for multiphase contrast-enhanced CT studies of the pancreatobiliary region with an MDCT scanner. It should be noted that this helps to reduce both the cost of the examination and the risk of complications.
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