| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Original Research |
1 Department of Diagnostic Radiology, Severance Hospital and Research Institute
of Radiological Science, Yonsei University College of Medicine, Seodaemun-ku
Shinchon-dong 134, Seoul 120-752, Republic of Korea.
2 Brain Korea 21 Project for Medical Science and Institute of Gastroenterology,
Yonsei University College of Medicine, Seoul 120-752, Republic of Korea.
Received June 10, 2004;
accepted after revision January 10, 2005.
Address correspondence to M.-J. Kim
(kimnex{at}yumc.yonsei.ac.kr).
Abstract
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
MATERIALS AND METHODS. Three hundred eighteen patients underwent a
helical CT examination using a bolus-tracking technique. All the examinations
were performed after administering a nonionic contrast medium (300 or 370 mg
I/mL; 2 mL/kg of body weight for patients weighing 
75 kg, 150 mL for
those weighing > 75 kg). The patients were assigned to one of three groups
according to the injection protocol. The injection rate was alternated to 3 or
4 mL/sec in group 1. The injection duration was 38 or 47 sec in groups 2 and
3, respectively. The aortic arrival time and the 100-H threshold time in each
patient were measured. The mean values and the variations in the aortic
arrival time and 100-H threshold time according to the injection protocols and
the contrast media were compared.
RESULTS. The mean variations (± SD) of aortic arrival times and 100-H thresholds in group 2 (aortic arrival time = 16.1 ± 2.7 sec, 100-H threshold time = 19.6 ± 2.9 sec) were smaller than in groups 1 (16.3 ± 3.0 sec and 19.9 ± 3.7 sec, respectively) and 3 (16.8 ± 3.5 sec and 20.4 ± 4.1 sec, respectively). However, the range of aortic arrival times and 100-H threshold times was more than 10 sec for all groups. The mean aortic arrival time and 100-H threshold time for all patients were 16.5 and 20.0 sec, respectively, and did not vary significantly with the injection protocol and concentration of contrast medium.
CONCLUSION. The individual variations of the aortic arrival and 100-H threshold times can be reduced using a fixed-duration injection technique, but there are still substantial variations. Therefore, a bolus-tracking technique is recommended for optimal timing of arterial phase scanning.
Keywords: abdominal imaging • contrast media • CT technique • MDCT
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Most authors have used a fixed-rate injection protocol to administer contrast medium—that is, 3-5 mL/sec—regardless of the patient's body weight [10, 16-23]. However, some have used a fixed-duration injection technique. This technique involves adjusting the dose and injection rate to the patient's body weight while fixing the total duration of the injection [24-26]. Using this technique, Awai and colleagues [27] hypothesized that the timing of the arterial phase enhancement is almost constant and the scanning window can be specified easily.
The hypothesis in our study is that if the individual variations of the aortic arrival time could be reduced substantially using a fixed-duration injection protocol, the optimal scan delay time can be determined without the need for bolus-tracking or test-bolus injection techniques. Therefore, the aim of this study was to determine whether the aortic arrival of contrast medium depends on injection protocols and the concentration of contrast medium.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The study subjects were divided into three groups according to the purpose of the CT examinations. Groups 1 (n = 94) and 2 (n = 92) included patients undergoing hepatic CT for evaluation of known or suspected hepatic lesions. Group 3 (n = 132) included patients undergoing a routine abdominopelvic examination to evaluate either known or suspected primary malignancies in the abdomen and pelvis. For groups 1 and 2, we believed that contrast medium should be injected rapidly to increase the maximum enhancement of the arterial phase to improve the detection of hypervascular liver lesions. For group 3, a relatively slower injection rate was used because the detection of a hypervascular liver tumor was not the primary concern. On the basis of findings of a preliminary study (Kim et al., unpublished data), we assumed that the dissimilarity of the CT indication would not affect the time to aortic enhancement. The patients in groups 1 and 2 were randomized according to the date of their examination.
The patients were randomly assigned to receive one of the following two nonionic contrast agents: iopamidol (Iopamiro, Bracco) at a concentration of 370 mg I/mL or iohexol 60% (Omnipaque 300, Nycomed Amersham) at a concentration of 300 mg I/mL. A dose of 2 mL/kg of body weight of contrast medium was administered to patients weighing 75 kg or less, and the total dose was fixed to 150 mL for those weighing more than 75 kg. The contrast medium was injected into the antecubital vein by a power injector (EnVision CT, Medrad) using an 18- to 22-gauge needle.
For group 1, the contrast medium was administered at a fixed injection rate of either 3 or 4 mL/sec according the body habitus and the accessibility of the venous route. An injection rate of 3 mL/sec was used in 73 patients (43 men and 30 women; mean body weight ± SD = 62.8 ± 9.6 kg) and 4 mL/sec was used in 21 patients (20 men and one woman; 70.9 ± 12.2 kg). In group 2, the contrast medium was administered using a fixed short-duration injection protocol, and the total injection duration was 38 sec. This injection time was used to limit the maximum injection rate to less than 4 mL/sec for most patients while maintaining the gradual increase in the injection rate according to body weight. In this group, the maximum injection rate was 4 mL/sec for patients weighing 75 kg or more. In group 3, the contrast medium was administered using fixed long-duration injection of 47 sec. With this injection duration, the maximum injection rate was 3.2 mL/sec for patients weighing 80 kg or more (Table 1).
|
CT Technique
All CT examinations were performed using bolus-tracking software. For bolus
tracking, a series of nonhelical sequential images were obtained 10 sec after
administering the contrast agent. These images were acquired with a scanning
time of 0.5 sec (360°) using a low-dose radiation technique (120 kV, 20
mA) with a cycle time of 1.5 sec. In each image, an approximately
1-cm2 circular region of interest (ROI) was placed around the
abdominal aorta at the level of the celiac artery, and the attenuation value
was measured in Hounsfield units (H)
[28].
The aortic arrival time was defined as the time interval between the beginning of the contrast medium injection and when the aortic enhancement reached more than 20 H compared with the baseline level. The 100-H threshold time was defined as the time interval between the start of the injection and the point at which the aortic enhancement reached more than 100 H.
Analysis
A Web browser page [29]
developed by Eng [30] was used
to calculate the appropriate sample size for this study. For this calculation,
the minimum expected difference in the mean aortic arrival time and 100-H
threshold time was set to 2 sec and the estimated SD was set to 4 sec. Using
these values, we estimated that the sample size for two groups was 125
patients with a p value of less than 0.05 and a statistical power of
0.80. Because each group exceeded 50% of this value, the sample size was
considered to be sufficiently large to detect a difference of at least 2 sec
in the aortic arrival time and 100-H threshold time values.
A test of the homogeneity of the variance was performed using the Levene
test to determine whether the variations in the aortic arrival time and 100-H
threshold time were different according to the type and concentration of
contrast medium and the injection protocol. A one-way analysis of variance
test was used to compare the aortic arrival times and 100-H threshold times
according to the injection protocols, and an unpaired Student's t
test was used to compare the aortic arrival times and 100-H threshold times
according to the different contrast medium (iopamidol vs iohexol) and between
the groups divided according to body weight (small [ 75 kg] vs large [>
75 kg]). The injection rate was varied between 3 or 4 mL/sec in group 1, so an
unpaired Student's t test was again performed to compare the aortic
arrival times and 100-H threshold times between these subgroups in group 1.
Because the same volume of contrast medium was used for the patients weighing
more than 75 kg, the aortic arrival times and 100-H threshold times of
patients weighing 75 kg or less and those weighing more than 75 kg in each
group were also compared. Pearson's correlation coefficients were calculated
to determine the correlation between body weight and the aortic arrival time
and the 100-H threshold time for each group. The correlation coefficients
between the aortic arrival times and 100-H threshold times and body weight
were also calculated in each group. Pearson's correlation coefficients were
also calculated between aortic arrival times and 100-H threshold times in each
group.
|
|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The ranges between the minimum and maximum aortic arrival times and 100-H threshold times were smaller in group 2 (aortic arrival time, 10-24 sec; 100-H threshold time, 14-28 sec) than in groups 1 (10-28 sec and 12-32 sec, respectively) and 3 (10-28 sec and 12-34 sec, respectively) (p = 0.004 and p = 0.065, respectively) (Table 1). In group 1, aortic enhancement in two patients was greater than 20 H at 10 sec. The corresponding number of patients in groups 2 and 3 were two and eight patients, respectively. None of the patients showed aortic enhancement of more than 100 H at 10 sec.
A test of the homogeneity of the variances showed that the dispersion of the aortic arrival times and 100-H threshold times in groups 1 to 3 were not the same (p = 0.004 and p = 0.065, respectively). However, the mean aortic arrival time and 100-H threshold time values were similar in all groups: 16.5 ± 3.2 sec for the aortic arrival times and 20.2 ± 3.7 sec for the 100-H threshold times, respectively (p > 0.05 for all comparisons) (Figs. 1A and 1B).
The mean aortic arrival time and 100-H threshold time tended to be longer in the patients who weighed more than 75 kg than in those who weighed 75 kg or less in all groups, but the difference was not significant (p > 0.05 for all comparisons) (Table 2). The mean and range of aortic arrival times and 100-H threshold times were also comparable between the subgroups of patients with an injection rate of 3 mL/sec (16.4 ± 3.1 sec and 20.1 ± 3.8 sec, respectively) and 4 mL/sec (15.6 ± 2.1 sec and 18.9 ± 2.6 sec, respectively) (p = 0.255 for aortic arrival time, p = 0.205 for 100-H threshold time). When we excluded the patients with an injection rate of 4 mL/sec, the mean aortic arrival time and the mean 100-H threshold time of group 1 were similar to those of groups 2 (p = 0.427 and p = 0.321, respectively) and 3 (p = 0.510 and p = 0.580, respectively).
|
When the mean and the dispersion of aortic arrival time and 100-H threshold time values of iopamidol (aortic arrival time = 16.7 ± 2.8 sec, 100-H threshold time = 20.1 sec) and iohexol (16.1 ± 3.5 sec and 19.9 ± 3.9 sec, respectively) were compared, the mean values were comparable to each other (p = 0.103 and p = 0.508, respectively), and the dispersion could be assumed to be equal (p = 0.127 and p = 0.287, respectively) (Table 3).
|
A plot of the body weight as a function of the aortic arrival times and 100-H threshold times shows no significant correlation in any group (Figs. 2A, 2B, 2C, 3A, 3B, and 3C). The results were also the same when patients weighing 75 kg or less and those weighing more than 75 kg were analyzed separately. In addition, the results were not changed for group 1 when those given an injection rate of 4 mL/sec were excluded from the analysis (r = 0.045, p = 0.706 for aortic arrival time; r = 0.122, p = 0.307 for 100-H threshold time). The aortic arrival time and 100-H threshold time values showed a strong positive correlation with each other in all groups: r = 0.855 for group 1, r = 0.858 for group 2, r = 0.924 for group 3 (p < 0.001 for all groups) (Figs. 4A, 4B, and 4C).
|
|
|
|
|
|
|
|
|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The results showed less variation in aortic arrival times and 100-H threshold times in group 2 than the other groups and similar variations in groups 1 and 3. The ranges of the aortic arrival times and 100-H threshold times were 10-24 sec and 14-28 sec, respectively, for group 2 and were wider for groups 1 and 3. Therefore, the ranges of the aortic arrival time and 100-H threshold time values can be larger than the total scanning time for scanning the entire liver or abdomen and pelvis, which is approximately 6-10 sec on current MDCT systems. Without a bolus-tracking technique, the scan delay can be too early or too late if a fixed scan delay is used. This is particularly so when acquiring arterial phase images: The scanning window for an arterial phase scan cannot be fixed for all patients even if a fixed-duration technique were used; therefore, the scan delay should be optimized individually using a bolus-tracking technique. In addition, the results of this study that the mean aortic arrival times and 100-H threshold times of the three groups were comparable suggest that the variation in aortic arrival times or 100-H threshold times depends on the intrinsic parameters of each patient, such as the cardiac output and circulation time, rather than on the injection protocol.
The results of this study are comparable with those reported by Irie and Kusano [26], who compared the time to peak hepatic enhancement and aortic transit time of three groups using different fixed-duration injection protocols (30, 45, and 60 sec). They reported that the aortic transit time, which was defined as the time to peak aortic enhancement from the beginning of the contrast agent injection, was comparable for all three groups, ranging from 10 to 30 sec. Their results also suggested that the time to the aortic peak enhancement can vary significantly even when a fixed-duration injection technique is used.
In a recent report by Awai et al. [27], the transit time to an aortic enhancement of 200 H or greater was compared in three groups: a fixed-duration injection protocol of either 25 or 35 sec was used in two groups and a fixed-rate injection protocol of 4 mL/sec was used in another group. In their study, the range of the transit times was approximately 15-25 sec and 15-30 sec for the two groups in which a fixed-duration injection was used, respectively. These results are also comparable to the range of 14-28 sec seen in the 38-sec injection group (group 2) in this study. However, in the 25-sec injection group in the study conducted by Awai et al., the mean transit time to an aortic enhancement of 200 H or greater and the range were slightly shorter. In this study, the mean 100-H threshold time and the range in the 47-sec injection group were slightly longer. Therefore, it is reasonable to assume that the peak aortic enhancement can be more variable and delayed when an injection technique with a longer duration is used. However, in any case, the range of the peak or certain threshold values using a current MDCT system will be larger than the total scanning time of the liver.
The results of our study are different from those reported by Awai et al. [27] in some aspects. In our study, neither the aortic arrival times nor the 100-H threshold times showed significant correlation with patient body weight, but the time to peak aortic enhancement showed a strong correlation with body weight in the fixed-rate injection protocol reported by Awai et al. The reason for this discrepancy might be differences in the study populations and study designs. We included all consecutive patients who were eligible for contrast-enhanced CT examinations, but Awai and colleagues excluded patients with cardiopulmonary dysfunction from their study. Inclusion of patients with cardiopulmonary function might have reduced the homogeneity of the population in our study and reduced the correlation between transit time and body weight.
Another difference between our study and that of Awai et al. [27] is in study design. In our study, the total dose of the contrast medium was fixed to 150 mL for patients weighing more than 75 kg. This limit in the total dose of contrast medium might have caused a decrease in the peak enhancement of the solid organs and the vascular system. However, the aortic arrival time or 100-H threshold time values did not vary significantly in the subgroups of patients weighing more than 75 kg. Therefore, this dose limit could not have been responsible for the difference in the results of this study and that conducted by Awai and colleagues. Another difference is that group 1 in this study used an alternate injection rate based on body habitus and accessibility of the venous route. Accordingly, the injection rate was not fixed for all patients. However, the results were the same when patients who received contrast medium at a rate of 4 mL/sec were excluded from the analysis.
There are several limitations to this study. First, the patient population was not homogeneous. Groups 1 and 2 included patients with similar reasons for CT examination— that is, for evaluation of known or suspected focal hepatic lesions. However, group 3 included patients with different reasons for CT examination. Although this might have affected the difference in the variation of the aortic arrival times and 100-H threshold times in the three groups, the results suggest that the variation was substantially high even in group 2, who received the short-duration injection, and the group was composed of a relatively homogeneous patient population. Therefore, we believe that the results of this study were not affected by this difference in the patient population.
Second, this study compared only the aortic arrival and the 100-H threshold times with the different injection protocols. This was because the scan protocols were different between groups 1 and 2 and group 3. In groups 1 and 2, double arterial and delayed phase images of the liver were obtained, whereas portal venous or hepatic parenchymal phase images were acquired in group 3. Hence, the differences in the peak attenuation values or times for the peak aortic or hepatic enhancement with the different injection protocols could not be determined. However, the results of this study showed that the aortic arrival time and 100-H threshold time values strongly correlated with each other. Accordingly, the time to peak aortic enhancement can also be expected to correlate with them and vary according to the aortic arrival time or 100-H threshold time values.
Third, the injection rate used in group 1 patients was 3 or 4 mL/sec, which was determined by body habitus and venous accessibility. Although the injection rate was not truly fixed in this group, we believe that the use of an alternating injection rate according to body habitus or venous accessibility may be more practical in a routine CT examination. There were no significant differences in aortic arrival times between the groups who received contrast medium at an injection rate of 3 mL/sec and those who received contrast medium at an injection rate of 4 mL/sec. When the results were analyzed after excluding the smaller portion of patients with an injection rate of 4 mL/sec, the major results did not change. Finally, bolus-tracking was begun 10 sec after the start of contrast administration because we expected that the aortic arrival time would be longer than that in most patients and to ensure that the injection of the contrast medium had been started uneventfully during that time. A small portion of patients showed an aortic arrival time of less than 10 sec, but none showed 100-H threshold times of less than 10 sec. A slightly larger proportion (6%) of the aortic arrival times of less than 10 sec was noted in group 3 using a longer injection duration with a relatively slow injection rate than in the other groups (2%). This suggests that a short-duration injection protocol may not increase the incidence of an early aortic arrival.
In conclusion, the results of this study showed that there are still substantial variations in the aortic arrival times and 100-H threshold times individually, although these variations may be reduced using a fixed-duration injection technique. The scan delay for acquiring arterial phase images cannot be fixed for all patients using either a fixed-rate injection or a fixed-duration injection technique. Therefore, the bolus-tracking technique is recommended for determining the optimal timing of arterial phase scanning.
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
1.5 cm) malignant hepatic neoplasms.
AJR 1995; 164:879
-884
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
This article has been cited by other articles:
|
|
 |
|
  L. L. Chu, B. N. Joe, A. C. A. Westphalen, E. M. Webb, F. V. Coakley, and B. M. Yeh Patient-specific Time to Peak Abdominal Organ Enhancement Varies with Time to Peak Aortic Enhancement at MR Imaging Radiology, December 1, 2007; 245(3): 779 - 787. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
