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Original Research |
1 Department of Radiology, Taichung Veterans General Hospital, No. 160, Sec. 3,
Taichung Harbor Rd., Taichung 407, Taiwan, R.O.C.
2 Faculty of Medicine, Medical College of Chung Shan Medical University, Taiwan,
R.O.C.
3 Department of Radiology, Fong-Yuan Hospital, Department of Health, Executive
Yuan, Taiwan, R.O.C.
Received July 11, 2006;
accepted after revision October 4, 2006.
Address correspondence to T. Lee
(sillyduck{at}vghtc.gov.tw).
Abstract
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SUBJECTS AND METHODS. A total of 113 pediatric patients referred for thoracic CT aortography were classified into four groups: group 1 (1-5 years old, CCT), group 2A (6-10 years old, CCT), group 2B (6-10 years old, empiric setting), and group 3 (11-15 years old, CCT). The CT attenuation values from the left common carotid artery to the descending aorta were recorded every 0.5 second. The quantitative bolus geometric analysis of each group included average enhancement, SD within the patient, and slope of enhancement. Groups 2A and 2B were compared to determine whether better bolus geometry could be obtained with the CCT concept than with the traditional empiric setting. Groups 1, 2A, and 3 were compared to determine whether homogeneous bolus geometry could be obtained in different age groups.
RESULTS. More homogeneous enhancement was obtained with the CCT concept than the empiric setting with a smaller SD of enhancement (25.5 ± 8.5 H vs 49.3 ± 16.2 H, p < 0.001). Furthermore, in the three different age groups (groups 1, 2A, and 3) examined using the CCT concept, there was no significant difference in the average enhancement (415.7 ± 83.6 H, 422.8 ± 97.1 H, 392.0 ± 78.5 H, respectively; all p > 0.05), SD of enhancement (28.5 ± 9.8 H, 25.5 ± 8.5 H, 28.5 ± 14.6 H, respectively; all p > 0.05), or enhancement slopes (-5.6 ± 18.0 H, -2.7 ± 10.7 H, -5.4 ± 12.3 H, respectively; all p > 0.05).
CONCLUSION. The CCT concept yields more homogeneous enhancement than the empiric setting. It also can routinely obtain homogeneous bolus geometry in patients in different age groups.
Keywords: contrast media • CT • CT technique • pediatric imaging • pediatric radiology
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After reviewing the literature regarding contrast injection [11-14], we developed a new concept of contrast injection called contrast-covering time (CCT). Our initial experience with CCT indicated that it outperformed the empiric setting in obtaining precise bolus geometry. In this study, we focused on nongated pediatric thoracic CT aortography, which is one of the most commonly performed procedures in pediatric MDCT angiography. We prospectively compared the CCT concept and the empiric setting to determine which method could obtain better bolus geometry. Then, we evaluated whether the CCT concept could also obtain good bolus geometry in patient groups with different ages and body weights.
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The exclusion criteria were as follows: First, patients with body weight of more than 40 kg were excluded because these patients had body habitus similar to that of adult patients and thus were beyond the scope of our study. These excluded patients were scanned with the adult protocol. Second, patients with coarctation of the aorta, atrial septal defect, ventricular septal defect, or patent ductus arteriosus due to interference with the bolus geometry were excluded. And third, pregnant patients and those with a previous allergic reaction to iodinated contrast media were excluded.
Oral and written informed consents were obtained from all patients or their parents. The study was approved by the institutional review board of our hospital.
CT Scanning
Before the examination, IV angiocatheters were inserted into the back of
the right hand by a senior nurse with 10 years of experience. We used this
location instead of the antecubital vein because it is easier to monitor by
video camera to observe any extravasation. The largest bore of angiocatheter
was chosen according to the condition of each patient's vein.
All examinations were performed on a 40-MDCT scanner (Brilliance 40, Philips Medical Systems). The scanning parameters were tube voltage, 120 kV; a weight-based tube current adjustment from 30 to 140 mAs per section; collimation, 40 x 0.625 mm; pitch, 0.876 without ECG gating; rotation time, 0.5 second; slice thickness, 0.67 mm; and reconstruction interval, 0.33 mm. The automatic online dose modulation (D-DOM Brilliance 40, Philips Medical Systems) was then turned on. Another set of images was reconstructed with a slice thickness and reconstruction interval of 5 mm for the later bolus geometric analysis.
All contrast injections were made using a power injector with a double syringe (Stellant, Medrad) for precise flow rate control and for saline chasing. A contrast medium ([iohexol] Omnipaque 350, Amersham) volume of 1.7 mL/kg of body weight was used. A routine saline chaser with a volume of 0.5 mL/kg of body weight plus 5 mL was also used to compensate for the connection tube.
Using the scout film, a scan range from the fourth vertebral body of the cervical spine to the diaphragm was determined. Then the region of interest (ROI) of the bolus-tracking technique was placed in the ascending aorta. After contrast injection, when the ROI reached 150 H, the scan was started craniocaudally after a 5-second postthresh-old delay. The inherent 5-second delay in the bolus-tracking technique is necessary to move the scan table to the start of the scan, give breath-hold instructions to the patient, and tune the gantry parameters.
CCT Concept
The key point of the CCT concept is to emphasize the total duration of
contrast injection, not just the contrast volume or flow rate alone. After the
ROI reaches 150 H, it appears as though an imaginary catheter in the ascending
aorta is starting to inject the contrast medium. According to the literature
[11-13],
the vessel attenuation will rise to a peak and then decrease. Good and
homogeneous enhancement will be obtained if the scan is in the middle of the
contrast injection interval. On the time-attenuation curve, the bolus geometry
will be optimal if the scanning time window "rides on" the peak.
Thus, we duplicated the postthreshold delay after the scanning time as a safe
margin to situate the scanning time in the middle of the contrast
injection.
In clinical practice, the equation for calculating the flow rate of CCT is as follows:
Contrast-covering time = postthreshold delay + scanning time + safe margin = 5 seconds + scanning time + 5 seconds = scanning time + 10 seconds.
Flow rate = contrast volume / contrast-covering time = (1.7 mL x body weight) / (scanning time + 10 seconds).
The calculated flow rate is applied to both the contrast medium and saline chaser.
Empiric Setting
The empiric flow rate of the control group (group 2B) was determined
according to the size of the IV line
[5], body weight of the
patient, and clinical experience by a senior pediatric radiologist with 10
years of experience in pediatric CT angiography in a tertiary referral medical
center
[1-7].
The empiric flow rate was applied to both the contrast medium and the saline
chaser. Other parameters, such as CT scanning, bolus-tracking technique,
contrast volume, and saline chaser volume, were the same as for group 2A.
Bolus Geometric Analysis
All the reconstructed images with a slice thickness of 5 mm were loaded in
a viewer program (Viewer, Extended Brilliance Workstation, Philips Medical
Systems). From the left common carotid artery to the descending aorta at the
diaphragm level, an ROI of one third the diameter of the target vessel was
placed in the center to avoid partial volume effect (Fig.
1A,
1B,
1C,
1D). Perivenous streak
artifacts and beam-hardening artifacts caused by shoulder bones were also
avoided. The attenuation value of the ROI was measured every 0.5 second of the
scanning procedure.
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The average enhancement means the overall enhancement of the examination. The further the scanning time window is from the enhancement peak, no matter whether it is earlier or later, the lower the average enhancement will be. Even if two groups have the same average enhancement, the homogeneity might still differ. Thus, the following parameters were analyzed.
The SD of enhancement means the homogeneity of the enhancement in each patient. As the SD of enhancement gets smaller, the variation of enhancement from the left common carotid artery to the descending aorta is also reduced. This occurs when the scanning time window rides on the peak of enhancement. If the value is large, then the scanning time window missed the peak timing, either earlier or later. We averaged the SD of each patient in a group to get the average SD of enhancement. When comparing two groups with the same average enhancement, the group with the smaller average SD of enhancement has less variance, meaning the contrast injection method is more stable and reliable in obtaining similar bolus geometry.
The slope of enhancement can indicate an earlier or later timing of the scan [14]. If the scan is earlier than the peak enhancement timing, then we will obtain an ascending pattern of time-attenuation curve and positive slope. If the timing is late, we will have a descending pattern of time-attenuation curve and a negative slope. If the slope is zero, we will obtain the best timing with the scanning time window riding on the peak enhancement. If this method is used when comparing two groups with similar average enhancement, the group with the slope closer to zero will have timing that is closer to the peak, which also means better bolus geometry. If two groups have similar enhancement and slope, the group with the smaller SD of slope will have better results because the homogeneity of slopes within the group is better, which results in a more stable bolus geometry.
Comparison Between Groups
After collecting the data, we conducted a comparison between CCT and
empiric setting (groups 2A and 2B). The demographic data were compared to
determine whether the two groups were comparable. Then the average
enhancement, average SD of enhancement, and slope of enhancement were also
compared to check for respective differences.
Next, the bolus geometric parameters of different age groups (groups 1, 2A, and 3) using the CCT concept were compared to determine whether the CCT could obtain the same bolus geometry regardless of age. Then we combined the patient groups using the CCT concept and reclassified them into three groups with the same number of patients according to body weight. We then compared the average enhancement, average SD, and slope of enhancement among the three groups. A good contrast injection setting concept should obtain the same bolus geometry regardless of a patient's body weight.
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Flow Rate-Related Injection Safety
To confirm the safety of the CCT concept in flow rate setting and IV
injection, we checked for the presence of contrast medium extravasation
immediately after the CT scan. The flow rates used in each patient were also
recorded for analysis with regard to IV angiocatheter size. In the outpatient
follow-up 1 week later, we also checked for evidence of phlebitis.
Statistical Analysis
Statistical analysis was performed by using commercially available software
(version 13.0, SPSS). Quantitative variables are expressed as mean values
± SDs. The comparison of demographic data, injection parameters, and
bolus geometry parameters between groups 2A and 2B was performed using a
two-tailed independent Student's t test. The sex distribution was
compared using a chi-square test. Analysis of variance with the Fisher's least
significant difference procedure was performed to compare the bolus geometric
parameters when more than two groups were compared. Differences of p
< 0.05 were considered statistically significant.
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Comparison Between CCT and Empiric Setting (Group 2A vs Group 2B)
Table 1 shows the comparison
between the CCT concept (Fig.
2B) and the empiric setting (Figs.
3A,
3B and
4A,
4B,
4C,
4D). The demographic data are
comparable, but the CCT concept used a lower flow rate than the empiric
setting. There was no difference in average enhancements between the two
groups. However, the average SD of enhancement was smaller in the CCT group,
meaning that CCT produced a more homogeneous enhancement than the empiric
setting. The homogeneity was also reflected in the SD of the enhancement
slope, which was smaller in the CCT group.
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Comparison Among Different Age Groups Using CCT (Groups 1, 2A, and 3)
Comparing the different age groups (groups 1, 2A, and 3) (Figs.
2A,
2B,
2C and
4A,
4B,
4C,
4D), we found that,
even though the age, height, body weight, scanning time, and contrast medium
volume were all significantly different, the CCT concept still produced
similar average enhancement, average SD, and enhancement slope (p
> 0.05 in all) by setting an individualized flow rate
(Table 2). This means the CCT
concept produced stable bolus geometry regardless of differences in age.
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Comparison Among Different Body Weight Groups Using CCT
We retrospectively reclassified the patients in the CCT concept groups
according to body weight into three groups: group BW1 (9-20 kg, n =
26), group BW2 (21-32 kg, n = 27), and group BW3 (33-40 kg,
n = 27). Comparing the three groups, we found that the height, body
weight, scanning time, and contrast volume were all different (p <
0.05 in all), but the CCT concept produced similar bolus geometric parameters
(p > 0.05 in all), including average enhancement, average SD, and
enhancement slope, with an individualized flow rate setting
(Table 3). This means that the
CCT concept produced stable bolus geometry regardless of the difference in
patients' body weights.
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Comparison Among Groups with Different Time-to-Threshold Values
We retrospectively reclassified the patients in the CCT concept groups
according to the time-to-threshold into three groups: group TSI1 (8-15
seconds, n = 27), group TSI2 (16-18 seconds, n = 27), and
group TSI3 (19-25 seconds, n = 26). Comparing the three groups, we
found that even the time-to-threshold was different (p < 0.05 in
all), but the CCT concept produced similar bolus geometric parameters
(p > 0.05 in all), including average enhancement, average SD, and
enhancement slope (Table
4).This means the CCT concept produced stable bolus geometry
regardless of the difference in time-to-threshold.
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Flow Rate Versus IV Size in CCT
Table 5 summarizes the flow
rates used in the CCT concept groups. No evidence of extravasation or
phlebitis was identified immediately after scanning or during the outpatient
follow-up.
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The result is encouraging. In clinical practice, we only need to know the body weight and scanning time to calculate the desired flow rate to obtain good bolus geometry. Unlike with the empiric setting, we do not need a long-term trial-and-error learning curve or an experienced radiologist to stand by. We can still obtain high and homogeneous enhancement in every pediatric thoracic CT aortography.
In the literature [1-10], the suggestions for flow rate setting were ambiguous. Some studies recommended setting the flow rate according to the IV size [5]. However, the IV size is determined by the skill of the nurse and the condition of the patient's vein, not by how much flow rate the patient needs. Our method is derived from the actual time-attenuation curve documented in the literature [11-13], which is closer to the actual need. Furthermore, according to the results of our study, the empiric setting flow rate based on the IV size uses a larger flow rate than the CCT concept but produces poorer bolus geometry. This assumes that the radiologist will spontaneously increase the flow rate to prevent inadequate enhancement. Even though a higher enhancement was found in group 2B than in group 2A, which was not statistically different, the bolus geometry could not cover all the scanning time, resulting in a larger variation of the slope and larger SD of the enhancement. Some studies have recommended using biphasic injection to provide homogeneous enhancement in CT aortography [15]. However, to the best of our knowledge, no studies have applied the biphasic technique to a pediatric population. This might be because adult patients are a more homogeneous group in body habitus compared with pediatric patients. Considering the heterogeneity of the pediatric population, the biphasic setting combined with body weight-based flow rate adjustment would be too complex to apply in clinical routine. Besides, the purpose of our study was to find a simple and reproducible contrast injection method that could replace the empiric setting. The complexity of the biphasic technique would have compromised our original purpose.
Due to its simplicity and underlying theory, we think the CCT concept can be applied on different scanners from 4- to 64-MDCT. Once we know the body weight and the scanning time of the patient, we can easily set most pediatric body CT angiography. We have applied this technique in more than 300 cases of pediatric CT angiography, including the abdomen, lower limb, and cervicocranial vasculature. In most cases, good bolus geometry was routinely obtained.
There were some limitations in this study. First, we did not include patients under 1 year old because these patients are scanned using 80 kV for radiation reduction in our institution. Because iodine attenuation is different at 80 and 120 kV, the comparison of bolus geometry parameters cannot be done. In our experience, however, the CCT concept can also be used in neonates (Lee T et al., presented at the 2005 annual meeting of the Radiological Society of North America). Second, we chose only hemodynamically healthy patients. The bolus geometry and applicability of CCT in patients with congenital heart disease, such as coarctation of the aorta, septal defects, and patent ductus arteriosus, were not addressed in this study. We intentionally excluded these patients to avoid shifting the focus of the study. Nevertheless, further study on the application of the CCT concept in patients with proven congenital heart disease is needed. Third, CT scanning has its inherent radiation limitation. Thus, we used a weight-based tube current adjustment and automatic online modulation to reduce the radiation dose.
In conclusion, the CCT concept yields more homogeneous enhancement than the empiric setting. It also can routinely provide homogeneous bolus geometry in patients in different age groups.
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