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Head and Neck Imaging |
1 From the Department of Radiology (C.d.M., F.C., T.T.d.W., A.v.d.L.) and Neurology (D.A.M.S., D.W.J.D.), Erasmus Medical Center, Dr Molewaterplein 40, 3015 GD Rotterdam, the Netherlands. From the 2003 RSNA Annual Meeting. Received April 9, 2004; revision requested June 22; final revision received November 10; accepted December 14. Supported by Revolving Fund 2002, Erasmus Medical Center, Rotterdam. Address correspondence to A.v.d.L. (e-mail: a.vanderlugt{at}erasmusmc.nl).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: Institutional Review Board approval and informed consent were obtained. Seventy-five consecutive patients (44 men, 31 women; mean age, 63 years; range, 22–85 years) were allocated to one of three protocols: group 1, 80 mL of contrast material; group 2, 80 mL of contrast material followed by 40 mL of saline; and group 3, 60 mL of contrast material followed by 40 mL of saline. Bolus tracking was used to synchronize contrast material injection with CT scanning. The attenuation in Hounsfield units was measured from the ascending aorta to the intracranial arteries at 1-second intervals. Differences were tested with the Student t test.
RESULTS: The maximum attenuation was reached in the proximal internal carotid artery in all groups. The addition of a bolus chaser to 80-mL contrast material resulted in a higher mean attenuation (323 HU ± 39 vs 351 HU ± 60, P = .06), higher maximum attenuation (393 HU ± 53 vs 425 HU ± 76, P = .09), and higher minimum attenuation (240 HU ± 34 vs 264 HU ± 48, P < .05). Group 3 had lower mean, maximum, and minimum attenuation than did groups 1 and 2 (P < .001).
CONCLUSION: The addition of a bolus chaser to 80 mL of contrast material results in a slightly higher attenuation. Decreasing the volume of contrast material from 80 to 60 mL results in a significantly lower attenuation.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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With the introduction of spiral computed tomography (CT), CT angiography entered clinical practice (4,5). Evaluation of CT angiography for the assessment of significant stenosis (>70%) in the carotid artery has already revealed a high sensitivity and specificity (6–11). However, single-section CT angiography has not gained much popularity in the diagnostic work-up of patients suspected of having symptomatic carotid artery stenosis. This may have been related to limitations in the required volume of contrast material (>100 mL), scan range (<120 mm), section thickness (
2 mm), and available postprocessing techniques.
Multi–detector row CT, and in particular, 16–detector row CT, has eliminated several of these limitations (12–15). It allows CT angiography of the carotid arteries to be performed with an increased coverage from the aortic arch to the circle of Willis, an improved spatial resolution of less than 1-mm section thickness, shorter acquisition times of less than 15 seconds, and lower doses of contrast material.
The application of a saline bolus chaser after the injection of contrast material may further reduce the volume of contrast material (16–20), but an optimal contrast material injection protocol has not yet been established for 16–detector row CT angiography of the carotid arteries. Thus, the purpose of our study was to prospectively compare different volumes of intravenously administered contrast material with and without a bolus chaser at 16–detector row CT angiography of the carotid arteries.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Each patient was allocated to one of three contrast material administration protocols. The first 25 patients (group 1: 14 men and 11 women; mean age, 60 years; range, 36–81 years) received 80 mL of contrast material without bolus chaser, the next 25 patients (group 2: 17 men and eight women; mean age, 68 years; range, 27–84 years) received 80 mL of contrast material followed by 40 mL of saline bolus chaser, and the last group of 25 patients (group 3: 13 men and 12 women; mean age, 60 years; range, 22–85 years) received 60 mL of contrast material followed by 40 mL of saline bolus chaser. In case of bolus tracking, which allows synchronization of CT scanning with the passage of contrast material, the amount of contrast material should be equal to or more than the scan time times the injection rate as follows: ±15 seconds x 4 mL/sec = 60 mL. As a precaution, we started the study with 80 mL of contrast material and tried to reduce the dose to 60 mL. With an injection rate of 4 mL/sec, 40 mL of saline is a reasonable amount of fluid to flush the vein for injection and to push the tail of the contrast material bolus to the superior vena cava. For each patient, age, sex, and weight were recorded.
Scanning Protocol
Patients underwent CT angiography of the carotid arteries with a 16–detector row CT scanner (Sensation 16; Siemens Medical Solutions, Forchheim, Germany). Patients were positioned supine on the CT table with the arms along the chest. A lateral scout view that included the thorax, neck, and skull was acquired. The scan range reached from the ascending aorta to the intracranial circulation (2 cm above the sella turcica). Scanning parameters were 0.75-mm collimation, 12-mm (pitch of 1) table feed per rotation, 0.5-second rotation time, 120 kV, 180 mAs, caudocranial scanning direction, and 10–14-seconds scan time (depending on individual patient's size and anatomy). The entire examination took 15 minutes.
The contrast material iodixanol (320 mg of iodine per milliliter, Visipaque; Amersham Health, Little Chalfont, UK) was injected intravenously through an 18–20-gauge cannula, depending on the size of the vein, into the antecubital vein by using a power injector (EnVision; MedRAD, Pittsburgh, Pa). The right antecubital vein was preferentially used because it provides the shortest path for the contrast material through the venous system and therefore the least dilution. When venous access on the right side could not be achieved, the left antecubital vein was used. The saline bolus chaser was injected immediately after the contrast material injection was completed by using a second power injector (EnVision; MedRAD). Both power injectors were connected to the injection cannula with a T-shaped tube (MedRAD), with an integrated one-way valve attached to the power injector that contained the saline bolus chaser to prevent reflux of the contrast medium. Contrast material and saline bolus chaser injection rates were 4 mL/sec.
Synchronization between the passage of contrast material and data acquisition was achieved with real-time bolus tracking. The arrival of the injected contrast material was monitored in real time by using a series of dynamic transverse low-dose monitoring scans (120 kV, 20–40 mAs) at the level of the ascending aorta at intervals of 1 second. The monitoring sequence started 5 seconds after the initiation of contrast material administration. CT angiography was triggered automatically on the basis of a threshold measured in a region of interest (ROI) in the ascending aorta. The size of the ROI in the ascending aorta for the bolus triggering was adjusted to the size and composition of the ascending aorta but was always greater than 5 mm in diameter. The trigger threshold was set at an increase of 75 HU over the baseline (approximately 150 HU in absolute value). When the threshold was reached, the table was moved to the caudal start position while the patient was instructed not to swallow. Four seconds after the trigger threshold was reached, CT angiographic data acquisition was started automatically. All bolus timing procedures and CT angiographic scans were successfully completed. No adverse reactions to contrast material were observed.
Data Collection and Analysis
Images were reconstructed with an effective section width of 1 mm, reconstruction interval of 0.6 mm, field of view of 100 mm, and a medium-smooth convolution kernel (B30f; Siemens Medical Solutions). The images were transferred to a stand-alone workstation and evaluated by using dedicated analysis software (Leonardo; Siemens Medical Solutions). For clinical analysis, two curved planar reformations of each carotid artery from the aortic arch to the carotid siphon were created in perpendicular planes (Fig 1).
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Transverse images were used for attenuation measurements. The Digital Imaging and Communications in Medicine layout of the images showed the time at which the scanning was performed. At intervals of 1 second (each 40th section), an ROI was drawn throughout the data sets in two regions: (a) the ascending aorta to the right internal carotid artery (ICA) and (b) the ascending aorta to the left ICA. The location of the measurements was recorded as follows: the ascending aorta, aortic arch, proximal common carotid artery (CCA) (first two measurements in the CCA), distal CCA, proximal ICA (first two measurements in the ICA), distal ICA, carotid siphon, and intracranial part of the ICA; measurements in the brachiocephalic trunk were considered to be measurements in the CCA.
One observer (C.d.M.) with 3 years of experience with CT angiography measured and recorded all data. The attenuation was measured by drawing a circular ROI in the center of the vessel lumen. The ROIs were drawn as large as the anatomic configuration of the lumen allowed in the transverse section.
The mean value of the measurements on the left and right side at each time point was calculated. Time-attenuation curves were generated for each patient. Subsequently, the attenuation at time 0; the mean, minimum, and maximum attenuation; and the time to reach the maximum attenuation were assessed. Because attenuation above 200 HU was considered optimal, the number of measurements below 200 HU was counted.
The aforementioned analysis resulted in one to three measurements per location, depending on the size of the patient. To analyze the attenuation in these locations, the measurements obtained in these locations were averaged. The relationship between the mean attenuation and weight was analyzed in all three groups.
Statistical Analysis
Differences between measurements on the left and right side were analyzed with paired Student t test. Baseline characteristics and attenuation parameters (value at time 0, minimum and maximum attenuation, and time to maximum attenuation) in the three groups were compared with a one-way analysis of variance (ANOVA) test or 
2 test. In case of a significant difference, a pair-wise comparison was performed with the Student t test. The mean attenuation and the attenuation at different locations in the three groups were compared with repeated-measures ANOVA. In case of a significant difference, a pair-wise comparison with repeated-measures ANOVA was performed. In addition, a pair-wise comparison of attenuation parameters with adjustment for differences in weight, age, and sex was performed with a linear regression model. The relationship between weight and mean attenuation in the three groups was analyzed with linear regression analysis.
Statistical analysis was performed by using software (SPSS, version 9.0, SPSS, Chicago, Ill; and SAS Proc Mixed, SAS Institute, Cary, NC). P < .05 was considered to indicate a significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Group 2: 80-mL Contrast Material with 40-mL Saline
The mean arterial attenuation per patient was 351 HU ± 60. The minimum and maximum arterial attenuations per patient were 264 HU ± 48 and 425 HU ± 76, respectively. Five measurements in two of the 25 patients (330 measurements) had an attenuation of less than 200 HU. Two of these measurements were in the ascending aorta or aortic arch, and three were in the carotid siphon or intracranial arteries.
Group 3: 60-mL Contrast Material with 40-mL Saline
The mean arterial attenuation per patient was 273 HU ± 53. The minimum and maximum arterial attenuations per patient were 185 HU ± 43 and 331 HU ± 64, respectively. Fifty-three of the attenuation measurements in 16 of the 25 patients (326 measurements) were less than 200 HU. These measurements were obtained in the ascending aorta, CCA, ICA, and intracranial arteries in two, 15, nine, and 27 cases, respectively.
Comparison of Time-Attenuation Curves
The attenuation value at time 0 was significantly higher with the application of 40-mL bolus chaser (group 1 vs group 2); in addition, the maximum attenuation was higher (425 HU in group 2 vs 393 HU in group 1), although not significantly (P = .09). The minimum attenuation was significantly higher with the bolus chaser (P < .05). The time to maximum attenuation was shorter (6.9 seconds in group 2 vs 7.8 seconds in group 1, P < .05) (Table 3, Fig 3).
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Comparison of Locations
In all groups, the attenuation first increased to a maximum and then decreased during the course of the CT angiographic examination (Table 4, Fig 4). The maximum attenuation was reached in the proximal ICA in all three groups. The addition of a bolus chaser resulted in a higher attenuation at all locations, although no significant difference was reached except for the aortic arch (P < .05).
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Relationship of Weight and Mean Attenuation
In the group that received 80 mL of contrast material, there was a weak, though almost significant, relationship between weight and the mean attenuation (slope, –1.2; P = .06; R2 = 0.16). With the addition of a bolus chaser in groups 2 and 3, this relationship became stronger and more significant, with a slope of –4.4 and –2.6 (P < .01) and R2 of 0.56 and 0.42, respectively (Fig 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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When the left carotid artery was compared with the right carotid artery, a slightly higher mean attenuation and a shorter time to maximum attenuation were found on the left side. This may be caused by the way the time-attenuation curves were assessed. The time scale does not correspond to the length of the path the contrast material has passed through the vessel: The contrast material reaches the left carotid artery, which originates directly from the aortic arch, earlier than the right carotid artery, where the contrast material has to follow a longer path through the brachiocephalic trunk to reach the right carotid artery.
Injection of 80 mL of contrast material resulted in an attenuation above 200 HU in all but two measurements. Comparison of the mean time-attenuation curve of 80 mL of contrast material with and without a saline bolus chaser demonstrated higher mean and maximum attenuations along the carotid arteries, although the differences were not significant.
Hopper et al (19) found higher attenuation in the ascending aorta with the addition of a 50-mL saline bolus chaser to 75 or 100 mL of contrast material, although the differences were not significant. The results from their study can be explained by the lack of the analysis of a time-attenuation curve of the aorta, which allows the assessment of maximum and mean attenuations. However, Irie et al (16), who did perform such an analysis, failed to demonstrate a significant increase in the maximum attenuation with the application of a saline bolus chaser. Failure to demonstrate a significant effect of a bolus chaser on attenuation in these two studies and in our study may be explained by the small number of patients (n = 15–25) in the groups with different contrast material protocols.
On the basis of the attenuation curves of 80 mL of contrast material with and without a saline bolus chaser, we analyzed whether a decrease in the volume of contrast material was possible without compromising the attenuation. However, 60 mL of contrast material followed by 40 mL of saline bolus chaser resulted in a significantly lower mean and maximum attenuations in comparison to those with 80 mL of contrast material with and without saline bolus chaser. Previous studies have not revealed such a decrease in attenuation with the replacement of contrast material by a saline bolus chaser. Haage et al (18) found the same attenuation in the ascending aorta by comparing 60 mL of contrast material and 30-mL saline bolus chaser with 75 mL of contrast material alone (240 HU for both). In another study, almost the same attenuation in the ascending aorta was observed by using 75 mL of contrast material with 50-mL saline bolus chaser and 125 mL of contrast material alone (254 vs 225 HU, respectively) (19). In both studies, the attenuation was measured in one region instead of several levels. Irie et al (16), who assessed the time-attenuation curve in the aorta, found the same maximum attenuation with 75 mL of contrast material alone and with 63 mL of contrast material and a 25-mL saline bolus chaser. Cademartiri et al (20) found the same mean and maximum attenuations in a time-attenuation curve in the descending aorta with 140 mL of contrast material alone and with 100 mL of contrast material followed by a 40-mL saline bolus chaser. The discrepancy of our results could partly be explained by the difference, although not significant, in weight between the groups in our study, especially between groups 2 and 3. However, adjustment for weight in a regression model still showed a difference in mean and maximum attenuations between group 3 and groups 1 and 2.
Patient weight is inversely correlated with arterial enhancement (17,21–23). Our study revealed only a relationship between weight and mean attenuation in the groups with a bolus chaser. The best correlation was found in the group that received 80 mL of contrast material and a 40-mL saline bolus chaser. This could be explained by a better circulation of the contrast material with the addition of a bolus chaser to the contrast material protocol. On the basis of our results, a consideration could be made to adjust for weight; for example, use 60 mL of contrast material followed by a 40-mL saline bolus chaser in patients weighing 75 kg or less and use 80 mL of contrast material followed by a 40-mL saline bolus chaser in patients weighing more than 75 kg to establish the mean and minimum attenuations of more than 250 and 200 HU, respectively.
The search for the lowest volume of contrast material necessary for optimal analysis has several motives. First, the high cost of contrast material forces the radiologist to search for ways to further decrease contrast material volume. However, one should realize that replacement of contrast material by saline necessitates the purchase of a dual-head power injector or an additional single-head power injector and the extra use of saline, a T connector, and a syringe. Schoellnast et al (24) showed that by taking these extra costs into account there is still a cost reduction. Second, the risk of nephrotoxicity is related to the volume of contrast material, and decreasing the volume of contrast material may influence the risk of subsequent nephrotoxicity (25–27). Third, in patients with acute stroke, CT angiography of the carotid artery has been combined with CT perfusion of the brain, which required an additional injection of 50 mL of contrast material (28,29). In such studies, the total volume of contrast material should be restricted to what is necessary for optimal analysis. Finally, it may be possible that, in comparison to what is commonly thought, the best contrast material protocol is not the one with the highest intraluminal attenuation. Which attenuation in CT angiography allows the best evaluation of the presence and severity of vessel disease is not well studied. Attenuation levels obtained with 60 mL of contrast material followed by 40 mL of saline may be high enough for an excellent interpretation of the vessel, owing to a better contrast with calcifications in the vessel wall or atherosclerotic plaque. This may have an effect on both visual analysis and semiquantitative analysis of the vessel dimensions.
Our study had several limitations. First, the patients were not randomly assigned to the groups. However, baseline characteristics were not substantially different except for the injection side. The higher frequency of the left-sided injection in group 3 may have led to a lower mean attenuation, because the longer path of the contrast material through the venous system may have diluted the contrast material. Second, the groups may be too small to lead to a significant result. Third, we analyzed attenuation from the aortic arch to the circle of Willis. Ideally, a single-level dynamic CT study will result in a more precise analysis of the contrast media dynamics (30). Such a study will cause extra radiation exposure to the patients, and we were reluctant to do this. Nevertheless, in clinical practice we are dealing with attenuation along the supraaortic arteries, which reflects the way contrast material passes through the vessels.
In conclusion, of the protocols we tested the integration of a saline bolus chaser in the contrast material protocol for evaluation of the carotid artery with 16–detector row CT leads to optimization of the attenuation but does not allow decrease in contrast material volume from 80 to 60 mL in all patients. Future studies may focus on a less strong reduction in contrast material volume, weight-adjusted dose of contrast material, or adjustment of the scanning protocol; increase in pitch will shift the maximum attenuation distally.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Abbreviations: ANOVA = analysis of variance • CCA = common carotid artery • ICA = internal carotid artery • ROI = region of interest
Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, C.d.M., A.v.d.L., F.C.; study concepts and design, C.d.M., F.C., A.v.d.L.; literature research, C.d.M., F.C., A.v.d.L.; clinical studies, C.d.M., D.A.M.S., D.W.J.D.; data acquisition, C.d.M., T.T.d.W.; data analysis/interpretation, all authors; statistical analysis, C.d.M.; manuscript preparation and editing, C.d.M., A.v.d.L.; manuscript definition of intellectual content, revision/review, and final version approval, all authors
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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