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Vascular and Interventional Radiology |
1 From the Department of Radiology, Kinki University School of Medicine, Osaka, Japan (K.A.); and Department of Radiology, Rinku General Medical Center, Osaka, Japan (K.H., S.H.). Received August 22, 2002; revision requested October 29; final revision received May 16, 2003; accepted June 18. Address correspondence to K.A., Department of Radiology, Graduate School of Medical Sciences, Kumamoto University, 1-1-1 Honjyo, Kumamoto 860-8556, Japan.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: One hundred ninety-nine patients were randomly divided into three groups in which the fixed duration of contrast material injection was 25 seconds (group A) or 35 seconds (group B) or the fixed injection rate was 4.0 mL/sec (group C). Computed tomography (CT) at the L3 vertebral level was performed before and after contrast material injection. Aortic peak time, aortic peak enhancement, and period when aortic enhancement is 200 HU or greater (T200) were calculated. The Pearson product-moment correlation coefficient (r) was used to investigate relationships between patient weight and aortic peak time, aortic peak enhancement, and T200 in each group.
RESULTS: A significant correlation between aortic peak time and patient weight (r = 0.91, P < .001) was observed in group C. No significant correlations between patient weight and aortic peak time were observed in group A (r = 0.16, P = .21) or B (r = -0.05, P = .69). A significant inverse correlation between aortic peak enhancement and patient weight (r = -0.70, P < .001) was observed in group C. No significant correlations between patient weight and aortic peak enhancement were observed in group A (r = 0.09, P = .48) or B (r = 0.10, P = .41). A significant correlation between T200 and patient weight (r = 0.72, P < .001) was observed in group C. No significant correlations between patient weight and T200 were observed in group A (r = 0.12, P = .34) or B (r = 0.001, P = .99).
CONCLUSION: Aortic peak time, aortic peak enhancement, and T200 were closely related to injection duration in the protocol with contrast material dose determined according to patient weight.
© RSNA, 2004
Index terms: Aorta, CT, 981.12916 • Computed tomography (CT), contrast enhancement, 981.12916
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Although most radiologists administer a uniform dose of contrast material in all patientsundergoing abdominal CT (1,3,6–20), some have adopted a contrast material injection protocol in which the doses are tailored to the patients’ weights (2,4,21–25). Yamashita et al (23) reported that the dose of intravenous contrast material should be adjusted to the patient’s weight to achieve adequate contrast enhancement and recommended the use of 2.0–2.5 mL of contrast material per kilogram of body weight, with an iodine concentration of 300 mg/mL for abdominal helical CT. Furthermore, Onishi et al (25) reported that at least 1.0–1.3 mL/kg contrast material with an iodine concentration of 300 mg/mL is necessary for CT angiography of the aorta. From these investigations, it appears reasonable to determine the dose of contrast material for abdominal CT and CT angiography according to patient weight.
When a dose of contrast material is determined according to the patient’s weight, there are two methods of administering the agent in a uniphasic injection protocol. One method, which is currently the most commonly used, involves administering the contrast material at a fixed injection rate for all patients (2,4,21–24). The other method involves administering the contrast material for a fixed injection duration.
The purpose of our study was to investigate the effect of the duration and rate of contrast material injection on aortic peak time and peak enhancement in an injection protocol in which the dose of contrast material is adjusted according to patient weight.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Sixty-two patients who had not undergone echocardiography within 3 months before CT examination also were excluded. Furthermore, seven patients with arterial partial pressures of oxygen of 60 mm Hg or lower were excluded because of the possibility that they would not be able to breathe shallowly and regularly during single-level serial CT scanning and thus their attenuation values for the aorta would not be measured correctly. For similar reasons, we also excluded six patients whose general health statuses were judged to be bad and who were judged by the radiologist in charge of the CT examinations to have difficulty breathing shallowly and regularly.
At Rinku General Medical Center, we performed portal venous phase CT of the liver to evaluate liver metastasis. We explained to the patients the purpose of the study and that the study would not interfere with their clinical examination. We estimated the effective radiation dose for this study to be about 5 mSv and the effective radiation dose for routine abdominal CT to be about 20–25 mSv (26). We explained to the patients that in this study, radiation exposure levels would be increased to about 20%–25% more than that delivered at routine abdominal CT. Thirty-one patients who did not give informed consent to be scanned for this study were not included. All other patients provided informed consent. The study received institutional review board approval from Rinku General Medical Center.
A total of 199 patients (115 men, 84 women; mean age 62.6 years ± 12.0 [SD]; age range, 30–92 years; mean weight, 56.8 kg ± 10.3; weight range, 35–83 kg) were included in the study. The mean age of the male patients was 63.0 years ± 12.1 (age range, 30–92 years), and the mean age of the female patients 61.7 years ± 12.3 (age range, 31–86 years). There was no significant difference in mean age between the male and female patients at two-tailed Student t test analysis (P = .44).
The pathologic diagnoses for the 199 patients were lung cancer (n = 52), colorectal cancer (n = 39), gastric cancer (n = 23), lymphoma (n = 23), breast cancer (n = 21), esophageal cancer (n = 6), cholangiocarcinoma (n = 5), bladder cancer (n = 5), thymoma (n = 5), renal cell carcinoma (n = 4), gallbladder cancer (n = 3), ovarian cancer (n = 2), uterine cancer (n = 2), testicular cancer (n = 2), pancreatic cancer (n = 2), cancer of ampulla of Vater (n = l), prostatic cancer (n = l), maxillary cancer (n = l), nasopharyngeal cancer (n = 1), and adult T cell leukemia (n = 1).
Contrast Material Infusion and CT Protocols
The 199 patients were randomly assigned to one of three groups on the basis of the contrast material injection protocol used, as summarized in the Table. Heiken et al (27) and Brink et al (28) reported that a maximum hepatic enhancement of at least 50 HU is needed to perform high-diagnostic-quality hepatic CT and that 521 mg/kg of iodine is necessary to achieve this level of hepatic enhancement. This dose corresponds to about 1.7 mL/kg of contrast material with an iodine concentration of 300 mg/mL. Therefore, in all protocols, each patient received 1.7 mL/kg of iopamidol 300 (Iopamiron; Nihon Schering, Osaka, Japan).
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Initially, transverse CT scanning was performed to obtain a baseline attenuation value for the aorta at the level of the third lumbar vertebra. Then, 10–60 seconds after the initiation of the contrast material injection, single-level serial CT scans were obtained at the same level in 2-second intervals. All patients were scanned with a multi–detector row CT scanner (LightSpeed QX/i; GE Medical Systems, Milwaukee, Wis). Nonenhanced scanning and single-level, contrast-enhanced serial scanning were performed with the following parameters: 1-second rotation time, 5-mm detector row width, 10-mm section thickness, 50-cm field of view, and 120 kV. The electric current used for these examinations was reduced to 20 mA to minimize radiation exposure, although 220–280 mA is used for routine abdominal CT scanning in our hospital. During CT scanning, the patients were instructed to practice shallow and regular breathing.
After single-level serial CT scanning, 70 seconds after the initiation of the contrast material injection, routine multi–detector row helical CT scanning of the abdomen during the portal venous phase was started with the following parameters: 0.8-second rotation time, 5-mm detector row width, 7-mm section thickness and image interval, helical pitch of 3.0, 50-cm field of view, 120 kV, and 220–280 mAs. Image reconstruction was performed in a 25–35-cm display field of view, depending on the patient’s physique. We additionally performed nonenhanced helical CT of the entire liver in the patients with renal cell carcinoma or breast cancer to keep from overlooking hypervascular liver metastases (30). The scanning parameters used for nonenhanced helical CT were the same as those used for routine contrast-enhanced helical CT of the abdomen.
Quantitative Analysis
We measured the attenuation values for the abdominal aorta in all patients by placing a circular region-of-interest cursor on the nonenhanced CT image and on all single-level, contrast-enhanced serial CT images. An attempt was made to consistently use a region of interest of approximately 1 cm2. The contrast enhancement of the aorta was calculated as the absolute difference in attenuation (in Hounsfield units) between the nonenhanced CT image and each of the single-level serial CT images. The attenuation values for the aorta were measured by a radiologist (K.H.) who was unaware of the injection protocol used. This radiologist constructed a time-enhancement curve for each patient by connecting each time point by using the spline interpolation.
We determined the aortic peak time—that is, the time to peak aortic enhancement from the aortic arrival time—and the peak enhancement value for the aorta. We defined aortic arrival time as 2 seconds before the time when the attenuation value on the time-enhancement curve for the aorta increased to 10 HU more than the attenuation value at nonenhanced CT. Furthermore, we defined T200 as the period during which enhancement of the aorta was 200 HU or more and determined the T200 in each patient. Mitsuzaki et al (4), Kim et al (13), and Yamashita et al (23) reported that when the injection rate was 3 mL/sec or more, the maximum enhancement of the aorta was 238 HU or more. According to these reports, we arbitrarily selected an enhancement value of 200 HU to indicate adequate enhancement of the aorta.
Statistical Analysis
All data, including patient age and weight, aortic peak time, aortic peak enhancement value, and T200, were reported as means ± SDs. One-way analysis of variance was used to investigate intergroup differences in patient age and weight. When the overall differences were statistically significant, post hoc analysis was performed by using Bonferroni adjustment.
The Pearson product-moment correlation coefficient (r) was used to investigate the relationships between patient weight and aortic peak time, aortic peak enhancement, and T200 in each group.
The mean aortic peak time, mean aortic peak enhancement value, and mean T200 were compared between groups A and B by using the two-tailed Student t test. Comparisons of mean aortic peak time, mean aortic peak enhancement value, and mean T200 by patient weight, divided into 10-kg ranges, between groups A and C also were performed by using the two-tailed Student t test. P < .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Figure 1 shows scatter diagrams demonstrating the relationships between patient weight and aortic peak time and between injection rate and aortic peak time in groups A, B, and C. For group A, the Pearson correlation coefficient and corresponding P value for the relationship between either patient weight and aortic peak time or injection rate and aortic peak time were 0.16 and .21, respectively; for group B, corresponding values were -0.05 and .69, respectively. There were no significant correlations between either patient weight and aortic peak time or injection rate and aortic peak time in group A or group B.
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For group C, the Pearson correlation coefficient and corresponding P value for the relationship between patient weight and aortic peak time were 0.91 and less than .001, respectively. There was a significant positive correlation between patient weight and aortic peak time. For group C, the mean aortic peak time was 19.7 seconds ± 4.4.
Figure 2 shows scatter diagrams demonstrating the relationships between patient weight and peak enhancement value for the aorta and between injection rate and peak enhancement value for the aorta in groups A, B, and C. For group A, the Pearson correlation coefficient and corresponding P value for the relationship between either patient weight and peak enhancement value or injection rate and peak enhancement value were 0.09 and .48, respectively; for group B, corresponding values were 0.10 and .41, respectively. There were no significant correlations between either patient weight and peak enhancement value or injection rate and peak enhancement value.
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For group C, the Pearson correlation coefficient and corresponding P value for the relationship between patient weight and aortic peak enhancement were -0.70 and less than .001, respectively. There was a significant negative correlation between patient weight and peak enhancement value.
Figure 3 shows scatter diagrams demonstrating the relationships between patient weight and T200 and between injection rate and T200 in groups A, B, and C. For group A, the Pearson correlation coefficient and corresponding P value for the relationship between either patient weight and T200 or injection rate and T200 were 0.12 and .34, respectively; for group B, corresponding values were 0.001 and .99, respectively; there were no significant correlations between either patient weight and T200 or injection rate and T200.
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For group C, the Pearson correlation coefficient and corresponding P value for the relationship between patient weight and T200 were 0.72 and less than .001, respectively. There was a significant positive correlation between patient weight and T200.
Figure 4a is a graph illustrating the comparison of aortic peak times between groups A and C, with patients divided into groups by weight in 10-kg ranges. For the patients weighing less than 50 kg, mean aortic peak times for groups A and C were 21.4 seconds ± 2.0 and 14.8 seconds ± 2.0, respectively. In this weight group, the mean aortic peak time for group C was significantly shorter than that for group A. For the patients weighing 50–59 kg, mean aortic peak times for groups A and C were 20.8 seconds ± 2.3 and 17.9 seconds ± 1.6, respectively. In this weight group, the mean aortic peak time for group C was significantly shorter than that for group A. For the patients weighing 60–69 kg, mean aortic peak times for groups A and C were 21.9 seconds ± 2.1 and 22.1 seconds ± 2.4, respectively. In this weight group, there was no significant difference in aortic peak time between groups A and C. For the patients weighing 70 kg or more, mean aortic peak times for groups A and C were 22.0 seconds ± 3.2 and 25.5 seconds ± 1.8, respectively. In this weight group, the mean aortic peak time for group C was significantly longer than that for group A.
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Figure 4c is a graph illustrating the comparison of T200 values between groups A and C, with patients divided into groups according to weight in 10-kg ranges. For the patients weighing less than 50 kg, mean T200 values for groups A and C were 20.4 seconds ± 2.3 and 17.9 seconds ± 1.9, respectively. In this weight group, the mean T200 for group C was significantly shorter than that for group A. For the patients weighing 50–59 kg, mean T200 values for groups A and C were 19.8 seconds ± 3.3 and 20.1 seconds ± 2.3, respectively. In this weight group, there was no significant difference in T200 values between groups A and C. For the patients weighing 60–69 kg, mean T200 values for groups A and C were 21.6 seconds ± 2.5 and 22.7 seconds ± 3.4, respectively. In this weight group, there was no significant difference in T200 values between groups A and C. For the patients weighing 70 kg or more, mean T200 values for groups A and C were 21.1 seconds ± 3.0 and 25.5 seconds ± 4.1, respectively. In this group, the mean T200 for group C was significantly longer than that for group A.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In our study, when the injection duration was fixed, the aortic peak time was almost constant with both the 25-second and the 35-second injection durations, regardless of patient weight or injection rate. The aortic peak times in group A, for which the injection duration was 25 seconds, were significantly shorter than those in group B, for which the injection duration was 35 seconds. In group C, there was a strong correlation between aortic peak time and patient weight. Because the contrast material dose was determined according to patient weight and the injection rate for all patients was constant in group C, the injection duration was directly proportional to the patient weight. Thus, in group C, aortic peak time also correlated strongly with injection duration.
Bae et al (34) reported that the "time to peak aortic enhancement was the sum of injection duration and bolus transfer time of contrast material from the injection site to the aorta" on the basis of pharmacokinetic analysis results. In our study, aortic peak time was defined as the period from the aortic arrival time to the aortic peak enhancement; thus, the aortic peak time must be consistent with the injection duration. Our results seem to agree with those of Bae et al. According to our results, the mean aortic peak time in group A, in which the contrast material injection duration was 25 seconds, was 21.4 seconds ± 2.3, and that in group B, in which the injection duration was 35 seconds, was 29.2 seconds ± 2.0. In our study, the aortic peak time was slightly shorter than the injection duration in all groups. However, the model of Bae et al may yield data that are somewhat different from those actually observed in humans.
In groups A and B, the aortic peak enhancement value was almost constant, and that in group A was significantly higher than that in group B. In group C, the aortic peak enhancement value showed an inverse correlation with patient weight. As mentioned earlier, in group C, injection duration was directly proportional to patient weight. Thus, the aortic peak enhancement value was inversely correlated with the injection duration in these patients. The results for groups A, B, and C show that the maximum enhancement value for the aorta was significantly correlated with the injection duration when the contrast material dose was determined according to patient weight.
Bae et al (34) reported that "contrast medium is injected into the central blood compartment and distributed rapidly to the well-perfused extracellular compartment and slowly to the poorly perfused extracellular compartment." By using the compartmental model of Bae et al, this phenomenon can be explained conceptually as follows: When investigating the aortic contrast enhancement soon after contrast material injection, as we did in this study, the blood flow to the poorly perfused extracellular compartment can usually be ignored. Therefore, in this case, the compartmental model can be simplified by differentiating between two compartments: a central blood compartment and a well-perfused extracellular compartment.
Both the central blood compartment and the well-perfused extracellular compartment are thought to be of a certain percentage of weight (35). Blood flow from the central blood compartment to the well-perfused extracellular compartment and recirculated blood flow from the well-perfused extracellular compartment to the central blood compartment are always estimated to be nearly constant in a patient with normal circulatory dynamics.
Determining the contrast material dose on the basis of patient weight and using a fixed injection duration are equivalent to injecting a certain volume of contrast material into the central blood compartment in a given amount of time. Therefore, if the injection duration is fixed, then the concentrations of contrast material in the central blood compartment in different patients may be almost constant, and, thus, the degree of enhancement of the aorta also may be nearly constant. With longer injection durations, more contrast material flows from the central blood compartment to the well-perfused extracellular compartment. Thus, the longer the injection duration, the lower the concentration of contrast material in the central blood compartment may be.
Our study results show that aortic peak time, aortic peak enhancement, and T200 correlated significantly with injection duration in the contrast material injection protocol in which dose was determined according to patient weight. These values are very important in CT angiography and arterial phase dynamic CT, in which scanning must be performed during the time after contrast material administration when the highest concentration is in the aorta.
Multi–detector row CT, which has come into wide use recently, is much faster than single–detector row CT. For example, with multi–detector row CT, one can scan the entire liver in about 5 seconds. At arterial phase CT of the liver, scanning must be performed during the peak time of contrast material concentration in the abdominal aorta. However, if the contrast material is injected at a fixed rate, then the aortic enhancement peak may change considerably according to patient weight. Therefore, in a protocol in which contrast material is injected at a fixed rate, the timing of arterial phase CT scanning should be optimally tailored to each patient. On the other hand, in a protocol involving a fixed injection duration, the time of arterial phase CT scanning may remain unchanged, and, thus, the scanning protocol can easily be specified because the aortic peak time and the period during which contrast enhancement is 200 HU or greater are almost constant.
In our study, when the rate of contrast material injection was fixed, the aortic enhancement value decreased as patient weight increased. Therefore, if the injection rate is fixed, sufficient enhancement of the aorta may not be achieved in a heavy patient. In contrast, if an adequate injection duration is fixed, then images with stable diagnostic capability can always be obtained because almost constant enhancement of the aorta can be achieved by fixing the injection duration independently of patient weight.
In general, in heavier patients, adipose tissue, which is considered to represent the poorly perfused extracellular compartment, accounts for a higher proportion of the weight. In other words, in obese patients, the well-perfused extracellular compartment accounts for a lower proportion of the weight. Our study data (Fig 4b) showed no significant difference in aortic peak enhancement values between the patients in groups A and C who weighed 70 kg or more. These data suggest that a small amount of contrast material flows from the central blood compartment to the well-perfused extracellular compartment, and it is believed that the well-perfused extracellular compartment accounts for a relatively low proportion of the weight of heavier patients. Therefore, we think that when we determine contrast material doses according to patient weight in the future, we should use body weight corrected according to patient height and extent of obesity.
There were potential limitations in our study. First, the ranges and mean values of body weight of the patients examined in our study are smaller than those of people in North America and Europe. Whether our results apply to populations of heavier patients needs to be verified. Second, we excluded from our study patients with heart failure, respiratory failure, and poor general health. However, these patients represent an important subset of patients who are imaged with CT in daily clinical practice. Thus, the data for patients of various weights and health conditions might be different from those obtained in our study.
Our study results show that variations in aortic peak times and aortic peak enhancement values can be reduced by using an injection protocol in which contrast material is injected during a fixed injection duration. However, the optimum injection duration has not been determined. This subject should be addressed after investigating the extent to which maximizing the aortic peak enhancement would be diagnostically useful in arterial phase CT scanning of hepatic tumors and CT angiography. Furthermore, because the duration of adequate enhancement of the aorta is associated with the duration of contrast material injection, consideration of the scanning duration according to the anatomic territories scanned and the scanning techniques used also is needed to determine adequate injection durations.
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FOOTNOTES |
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Abbreviation: T200 = time when aortic enhancement is 200 HU or greater
Author contributions: Guarantors of integrity of entire study, K.A., S.H.; study concepts and design, K.A.; literature research, K.A.; clinical studies, K.A., K.H., S.H.; data acquisition, K.A., K.H.; data analysis/interpretation, K.A.; statistical analysis, K.A.; manuscript preparation, K.A.; manuscript definition of intellectual content and editing, K.A., S.H.; manuscript revision/review, K.A., K.H., S.H.; manuscript final version approval, S.H.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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