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Vascular and Interventional Radiology |
1 From the Department of Radiology, Duke University Medical Center, Box 3808, Room 2529 Blue Zone, Durham, NC 27710. From the 2001 RSNA scientific assembly. Received June 17, 2003; revision requested August 27; revision received December 18; accepted January 28, 2004. Supported in part by Bracco Diagnostics, Princeton, NJ. Address correspondence to L.M.H. (e-mail: ho000004@mc.duke.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Forty-five patients with abdominal aortic aneurysm were randomized to undergo multi–detector row helical CT with either an interactive protocol (n = 23) or a standard protocol (n = 22). Scanning delays in all patients were determined with automated triggering. Patients in the standard protocol group received 150 mL of contrast medium intravenously at 4 mL/sec. The same injection rate was used for the interactive protocol group, but the dose was reduced with discontinuation of injection at start of scanning. Quantities of contrast medium used and contrast-enhanced aortic attenuation achieved were compared. Aortoiliac enhancement was evaluated qualitatively by using a five-point scale (1 = poor, 5 = excellent). Quantitative and qualitative data were analyzed with the two-tailed t test and Wilcoxon rank sum test, respectively, to determine significance of differences (P < .05).
RESULTS: Data from six patients were excluded because of technical errors. Data were analyzed from 20 patients in the interactive protocol group and 19 in the standard protocol group. Mean contrast medium volume was 107 mL ± 20 (standard deviation) in the interactive protocol group and 148 mL ± 3 in the standard protocol group (P < .001). Mean contrast-enhanced attenuation at initial, peak, and final measurements was 257 HU ± 38, 285 HU ± 46, and 269 HU ± 54, respectively, for the interactive protocol group, and 261 HU ± 65, 288 HU ± 66, and 269 HU ± 61 for the standard protocol group (P > .05). Mean qualitative enhancement scores for interactive and standard protocol groups were 4.47 and 4.44, respectively (P = .47).
CONCLUSION: The interactive method is a simple, efficient, and reproducible way to optimize aortoiliac enhancement while reducing contrast medium dose.
© RSNA, 2004
Index terms: Aneurysm, aortic, 981.73 • Aorta, CT, 981.12913, 981.12915 • Computed tomography (CT), angiography, 981.12913, 981.12915 • Computed tomography (CT), contrast enhancement, 981.12913
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Various protocols have been described for performing angiography with single–detector row helical CT scanners. The simplest, easiest, and perhaps most widely used technique includes a fixed contrast medium dose and fixed scanning delay based on empiric data (4). Because of the narrow temporal window for achieving optimal attenuation of the target vessels, however, various methods have been investigated for improving the consistency of vascular contrast enhancement (5,6). In the method we use, contrast material is injected rapidly into a peripheral vein via a power injector, and the upper abdominal aorta is monitored visually for contrast enhancement by using automated-triggering hardware and dedicated software (SmartPrep; GE Medical Systems, Milwaukee, Wis). This system for optimizing the scanning delay in individual patients has been described in the literature (7–9). When the appearance of the aorta is qualitatively enhanced, two events occur sequentially: The diagnostic portion of the examination is triggered, and the injection of contrast material is immediately interrupted manually. It is our hypothesis that even though the bolus is interrupted at the beginning of scanning, the multi–detector row CT scanner is so fast that there is enough contrast material upstream to achieve adequate enhancement of both the abdominal aorta and the iliac arteries.
Thus, the purpose of our study was to prospectively evaluate a technique for aortoiliac enhancement at multi–detector row helical CT with both the scanning delay and contrast medium dose determined by using an interactive method.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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All patients were referred for assessment of a known infrarenal abdominal aortic aneurysm (n = 15) or for imaging follow-up after aneurysm repair (n = 30). Three patients had undergone open surgical repair. Fifteen of the 23 patients in the interactive protocol group and 12 of the 22 patients in the standard protocol group had undergone placement of an aortic stent graft.
The institutional review board approved this study, and informed consent was obtained from all patients.
Imaging
Imaging was performed by using a multi–detector row CT scanner (QX/i LightSpeed Plus; GE Medical Systems) capable of acquiring four sections per gantry rotation. A detector configuration of 4 x 2.5-mm and pitch of 1.5 (table speed, 15 mm per rotation) were used in both groups. Gantry rotation time was 0.5 second (11 interactive protocol patients and 12 standard protocol patients) or 0.6 second (11 interactive protocol patients and eight standard protocol patients). Incorrect gantry rotation times were used in three patients (0.7 second was used in one interactive protocol patient, and 0.8 second was used in two standard protocol patients). Gantry rotation speed was somewhat arbitrarily chosen by a technologist, but the primary consideration was patient habitus. In general, patients with a larger habitus (ie, more than 175 cm in length or more than 90 kg in weight) were scanned by using the 0.6-second gantry rotation speed. Tube voltage of 140 kVp and current of 190–200 mA were used in all patients. The reconstructed section thickness was 2.5 mm with an interval of 1.0 mm (60% overlap). Imaging was performed in the cephalocaudal direction, from the celiac axis to the common femoral arteries.
The contrast medium used in all patients was iopamidol (Isovue 300; Bracco Diagnostics, Princeton, NJ) with a concentration of 300 mg of iodine per milliliter. The contrast medium was administered with a mechanical power injector (Percupump II; E-Z-Em, Westbury, NY) at 4 mL/sec through a 20-gauge 1
-inch cannula inserted in an antecubital vein in all patients. The power injector was loaded with 150 mL of contrast material for both the standard protocol group and the interactive protocol group.
Low-radiation monitoring images (140 kVP, 40 mA) were acquired in a single transverse section at the level of the upper abdominal aorta (automated triggering) immediately after the initiation of intravenous contrast material injection. This anatomic level was chosen to coincide with the most cephalic location of the diagnostic data set so that no time would be required to move the table between the monitoring and diagnostic phases of the examination. When the aorta began to enhance qualitatively, the diagnostic portion of the study of the aorta was immediately triggered and the power injector was stopped manually, thus determining the scanning delay and contrast medium dose for patients in the interactive protocol group. The technologist triggered diagnostic scanning immediately after increased attenuation was visualized in the upper abdominal aorta. The aortic enhancement increased so rapidly that the choice of an enhancement threshold was not necessary. Patients in the standard protocol group received a fixed dose of 150 mL of contrast material. The scanning delay for that group was also determined with automated triggering by using the software.
Quantitative Assessment and Statistical Analysis
To facilitate efficiency and to simulate a normal practice situation in which weight scales are not readily available, the technologist who performed scanning asked patients to state their current weight. The volume of contrast material injected for each patient and the scanning delay for each acquisition were recorded on a data sheet by the technologist.
Contrast-enhanced attenuation measurements were obtained by placing a manually defined region of interest with an area of 150–400 mm2 in the lumen of the abdominal aorta on every section, beginning with the first section at or near the level of the celiac axis and continuing caudad to the level of the aortic stent-graft bifurcation. One additional measurement was also obtained in the right or left common iliac artery (region of interest, 20–50 mm2) and the right or left common femoral artery (region of interest, 10–20 mm2), just above the origin of the deep femoral artery. The region of interest measured was as large as possible, while wall calcifications and metallic stents that might cause artifacts were avoided. The right or left artery was chosen on the basis of which side had the largest region of interest with the least potential for stent-related or wall calcification–related artifacts. All measurements were performed by a board-certified radiologist with 6 years of experience in reading abdominal CT scans (L.M.H.). Contrast-enhanced attenuation was measured during the dynamic phase of aortoiliac enhancement. For the interactive protocol group, attenuation measurements were obtained after the end of contrast medium administration, as the injection was interrupted manually when the diagnostic study was initiated. For the standard protocol group, attenuation measurements were obtained during or after the completion of contrast medium administration, depending on the duration of the acquisition.
From among these data, initial, peak, and final attenuation measurements were recorded separately for each patient. The initial attenuation measurement was derived from the first image obtained at or near the level of the celiac axis. The peak attenuation measurement was obtained by examining all attenuation measurements recorded for each patient and choosing the highest value. The final attenuation measurement was derived from the last image obtained at the level of the common femoral artery.
Contrast-enhanced attenuation–time curves (expressed as change in Hounsfield units over time) were also generated. Because of slight differences in data sampling that were associated with the gantry rotation speed used (0.5 or 0.6 second), the attenuation values measured at 1-second time intervals (ie, in six or seven sections) were averaged. In addition, patient groups were compared by using analysis of covariance to adjust for patient weight. These data were also depicted graphically as attenuation-time curves.
In addition, weight-adjusted estimates of the attenuation differences at several time points between the interactive and standard protocol groups were calculated. Confidence intervals for these differences were then determined, to indicate the possible extent of attenuation differences.
Differences in contrast medium volume used, patient weight, scanning delay, and acquisition times between the interactive and standard protocol groups were compared by using a two-tailed t test. For all statistical analyses, a P value less than .05 was considered to indicate a significant difference.
Mean attenuation for each patient was calculated by adding all of the aortoiliac attenuation values recorded for that patient and dividing the total by the number of measurements performed in that patient.
The relationship between patient weight and mean attenuation was also examined and depicted graphically for the interactive and standard protocol groups.
Qualitative Assessment and Statistical Analysis
Each image data set was reconstructed and hard copies were printed by using a soft-tissue window (width, 350 HU; level, 40 HU). Three board-certified abdominal radiologists (R.C.N., J.T., E.I.G.) then independently evaluated the quality of contrast enhancement at the following five locations: celiac axis, abdominal aorta at the level of the renal arteries, aortic bifurcation, common iliac arteries, and common femoral arteries. (R.C.N. and E.I.G. each had more than 15 years, and J.T. had 4 years, of experience in reading abdominal CT scans.) The following five-point scale was used: 1, poor; 2, fair; 3, average; 4, good; and 5, excellent. The readers were blinded as to whether the patient underwent imaging with the standard or the interactive protocol. Scans for which there was disagreement about the quality of enhancement were not reevaluated for consensus. Statistical analysis of these data was performed by using the Wilcoxon rank sum test. Software (SAS, version 8.2; SAS Institute, Cary, NC) also was used for statistical analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The mean volume of contrast medium used in the interactive protocol group (107 mL ± 20) was significantly smaller than that used in the standard protocol group (148 mL ± 3) (P < .001). Other parameters, such as mean weight, mean scanning delay, and mean acquisition time, were not significantly different between the interactive and standard protocol groups (Table 1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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One issue in the design of protocols specific to multi–detector row CT is the volume of contrast medium required to achieve adequate contrast enhancement. Macari et al (11) proposed the use of a fixed volume of contrast medium (150 mL) at a fixed rate of 4 mL/sec with multi–detector row CT. If the entire volume of contrast medium were injected at once, the administration of the bolus would take at least 37.8 seconds. With the fixed scanning delay of 25 seconds used by Macari et al, any acquisition time of less than 12.8 seconds would result in the continued injection of contrast medium after the completion of scanning. Other investigators (4,5) used contrast medium volumes based on patient weight, at a dose of 2 mL/kg, with typical injection volumes of 120–170 mL. Rubin et al (12) demonstrated that substantial reductions in contrast medium dose were possible with multi–detector row CT compared with single–detector row CT. In their study of the thoracoabdominal aorta, the dose was determined by calculating a bolus duration that was equivalent to the scanning duration at an injection rate of 4 mL/sec. For example, for a scanning duration of 31 seconds, the total contrast medium dose administered was 124 mL. Rubin et al determined the total contrast medium dose by multiplying the scanning duration (31 seconds) by the injection rate (4 mL/sec). The scanning duration was calculated by dividing the table coverage (580 mm) by the table speed (18.75 mm/sec). Rubin et al demonstrated that the dose of contrast material with four-section multi–detector row CT was 2.3 times less than that with single–detector row CT. However, in two instances in their study, the calculated dose of contrast material was very low (57 and 61 mL) and resulted in suboptimal vessel enhancement.
A considerable reduction in contrast medium dose was demonstrated with our interactive method compared with the fixed dose used in the standard protocol. The mean volume of contrast medium saved with the interactive protocol was 41 mL, and the dose was decreased by 29%, nearly one-third. Furthermore, by interrupting the bolus at the beginning of scanning, we avoided the injection of contrast medium after scanning was completed. Despite the lower contrast medium dose, aortoiliac enhancement with the interactive protocol achieved an overall rating of good or excellent from the abdominal radiologists participating in this study. Furthermore, there was no significant difference in initial, peak, or final attenuation measurements between the interactive and standard protocol groups. Although the aortoiliac attenuation over time was slightly higher with the interactive protocol than with the standard protocol, there was no significant difference in attenuation between the two. Furthermore, the attenuation difference between the two was lessened after data were adjusted for patient weight. This result was likely caused by a strong correlation between patient weight and attenuation, and, hence, the use of weight in an analysis of covariance yielded a more sensitive comparison between the two patient groups. Thus, we demonstrated that aortoiliac attenuation over time with the interactive protocol was equivalent to that with the standard protocol. This technique uses the potential for contrast medium dose reduction provided by multi–detector row CT technology without compromising image quality.
Previous studies have reported threshold attenuation values for obtaining adequate three-dimensional and multiplanar reformations of helical CT angiographic data. Sheiman et al (4) chose a value of 160 HU as the minimum vessel attenuation needed for CT angiographic reconstruction. More recently, Macari et al (11) used a mean attenuation value of 200 HU as their low threshold to define adequate vessel contrast enhancement. Our data demonstrate that the interactive protocol was successful at sustaining mean attenuation values greater than 200 HU in 18 (90%) of the 20 interactive protocol patients. In only two interactive protocol patients were mean attenuation values less than 200 HU (171 and 162 HU), and both values exceeded the 160-HU threshold proposed by Sheiman et al (4). Both of these patients weighed more than 100 kg. These findings correspond to a previously reported inverse correlation between aortic enhancement and patient weight (11,13). Platt et al (13) found that most patients with individual aortic enhancement measurements of less than 150 HU weighed 90 kg or more. It is noteworthy, however, that mean aortoiliac artery attenuation values of more than 200 HU were achieved in two other interactive protocol patients in our study who weighed more than 100 kg. Therefore, poor enhancement is not attributable to weight alone. Other factors, such as cardiac output, hydration status, and muscle mass, also contribute to the degree of aortic enhancement.
Some limitations to our study should be considered. First, we did not include for comparison a group of patients with a fixed scanning delay or a delay based on a test bolus. The scanning delays for both interactive and standard protocol groups were based on the automated triggering method. We chose this study design because, in our clinical practice, we routinely use the automated triggering method to determine scanning delay. Therefore, patients who were randomized to receive the standard protocol were imaged according to our routine abdominal aortic aneurysm protocol, which simplified the procedure for the CT technologist who performed the examinations. Furthermore, by designing our project in this way, only one variable was changed in the interactive protocol group, which was the contrast medium dose. Although this design facilitated our comparison between the interactive and standard protocol groups, it may also limit comparisons of our data with other results reported in the literature.
Second, not all institutions have the hardware and software capabilities for the method that we have proposed. As a result, the data we present may have limited utility at some departments.
Third, patient weight records were based on information provided by the patient. The patients were not weighed prior to scanning, as this was not thought to be a practical consideration in a busy imaging center. It is possible, however, that patients’ reported weights were inaccurate (with a likely bias toward underestimation), a possibility that limits our interpretation of our data regarding the relationship between patient weight and aortic attenuation.
A fourth potential limitation of this study is related to beam-hardening artifacts associated with endovascular stents. It is possible that some measurements of regions of interest in the aorta and iliac vessels were affected by such artifacts, in spite of our best efforts to avoid this problem.
A fifth possible limitation to this study is the inclusion both of patients who had untreated abdominal aortic aneurysms and of patients who had undergone endovascular or open surgical repair. It is conceivable that hemodynamics might differ between treated and untreated aortic aneurysms, and that different flow dynamics could lead to decreased aortic enhancement in patients with untreated aortic aneurysms. A useful project for the future would be to evaluate the effectiveness of the interactive protocol in patients with untreated abdominal aortic aneurysms.
The benefit of this protocol is primarily the reduction in contrast medium dose. Because the volume of contrast material needed is not determined before the start of scanning, we typically load the injector with the full 150-mL dose, and we discard any unused contrast medium after scanning. As a result, no cost savings to the department ensued from the decrease in contrast medium dose. An alternative approach currently used in our department is to load the injector with 100 mL of contrast medium for patients who weigh less than 100 kg.
In conclusion, we evaluated a protocol for multi–detector row CT angiography wherein both the scanning delay and the contrast medium dose are determined with an interactive method that is simple, efficient, and reproducible. Our method maximizes the potential of multi–detector row CT to enable a significant reduction in contrast medium dose without compromising image quality.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, all authors; study concepts, L.M.H., R.C.N.; study design, all authors; literature research, L.M.H.; clinical studies, L.M.H.; data acquisition, J.T., E.I.G., R.C.N., L.M.H.; data analysis/interpretation, all authors; statistical analysis, D.M.D., L.M.H., R.C.N.; manuscript preparation, definition of intellectual content, revision/review, and final version approval, all authors; manuscript editing, L.M.H., R.C.N.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  P. T. Johnson and E. K. Fishman IV Contrast Selection for MDCT: Current Thoughts and Practice Am. J. Roentgenol., February 1, 2006; 186(2): 406 - 415. [Abstract] [Full Text] [PDF]
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| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
) was not significantly different from that for the standard protocol group (
) (P > .50).
