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MDCT Angiography of the Pulmonary Arteries: Influence of Body Weight, Body Mass Index, and Scan Length on Arterial Enhancement at Different Iodine Flow Rates

¹ Department of Radiology, Medical University Graz, 8036 Graz, Austria.
² Institute for Medical Informatics, Statistics and Documentation, Medical University Graz, 8036 Graz, Austria.
³ Diagnostikum Graz Sued West, Weblinger Guertel 25, 8054 Graz, Austria.

Received April 27, 2005; accepted after revision July 25, 2005.

 
Address correspondence to M. Tillich (manfred.tillich{at}diagnostikum-graz.at).

Abstract

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OBJECTIVE. The purpose of this study was to assess whether body weight, body mass index, and scan length influence arterial enhancement during CT angiography (CTA) of the pulmonary arteries at different iodine flow rates.

MATERIALS AND METHODS. CTA examinations of the pulmonary arteries performed for routine clinical care of 120 patients between March and December 2003 were retrospectively evaluated. Patients had received either 120 mL of contrast medium with an iodine concentration of 300 mg I/mL (group A) or 90 mL of contrast medium with an iodine concentration of 400 mg I/mL (group B). The iodine dose was 36 g, and the injection rate was 4 mL/s in all examinations. The iodine flow rate was 1.2 g I/s in group A and 1.6 g I/s in group B. Arterial attenuation along the z-axis was measured per patient, and the influence of body weight, body mass index, and scan length on enhancement of the pulmonary arteries in the two groups was assessed.

RESULTS. In group A and in group B, body weight and body mass index correlated significantly with mean enhancement along the z-axis (r = -0.35 and -0.26 for group A and -0.48 and -0.40 for group B). Scan length showed no correlation with pulmonary attenuation. Mean pulmonary artery enhancement was significantly higher in group B with a difference of 51 H compared with group A.

CONCLUSION. Pulmonary artery attenuation in CTA of the pulmonary arteries shows a small but significant correlation with body weight and body mass index independently of the iodine flow rate used. A higher iodine flow rate improves pulmonary artery enhancement.

Keywords: arteries • body mass index • body weight • CT angiography • IV contrast agents • thorax

Introduction

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Computed tomography angiography (CTA) has become the first-line imaging technique in daily clinical routine for patients with suspected pulmonary embolism. The most important advantages of CT over other imaging techniques are that both mediastinal and parenchymal structures can be evaluated and that the thrombus can be directly visualized. The conspicuity of the thrombus depends on the density difference between the thrombus and the contrast-enhanced artery. The degree of arterial enhancement depends on injection-related factors and on patient-related factors. Both have been evaluated for CTA of the aorta, and models have been introduced to overcome variability between individuals [1-6]. Few data, however, are available on CTA of the pulmonary arteries. To the best of our knowledge, no data are available on the influence of body weight, body mass index, and scan length on enhancement of the pulmonary arteries. The aim of our study was to assess whether correlation exists between each of these parameters and pulmonary artery enhancement and whether iodine flow rate influences the correlation. A further aim was to retrospectively estimate contrast dose per unit of body weight, a value necessary for reaching an attenuation threshold of 300 H.

Materials and Methods

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Patient Population
In our department, contrast media with different iodine concentrations were used in clinical routine. The agent used was alternated daily by the radiologic technician. A retrospective database search yielded 224 patients who underwent CTA of the pulmonary arteries on the same CT scanner from March to December 2003. Among these, all patients who underwent CTA of the pulmonary arteries with contrast medium with an iodine concentration of 300 or 400 mg I/mL were included for further analysis (199 patients). The CTA images of these patients were consecutively loaded on a workstation and reviewed by two experienced radiologists in consensus with respect to evidence of central pulmonary embolism and cardiac insufficiency. Because coronal and lateral scout views were available for each patient, the same criteria for evaluation of cardiac dilatation and pulmonary congestion were used as in interpretation of radiographs of the chest. Evidence of cardiac insufficiency was defined as enlargement of the heart in the coronal or lateral scout view or both combined with dilatation of the pulmonary vessels.

The first 60 patients who underwent CTA of the pulmonary arteries with contrast medium with an iodine concentration of 300 mg I/mL (iopromide 300, Ultravist 300, Schering) and who had no radiologic evidence of central pulmonary embolism or cardiac insufficiency (group A) and the first 60 patients who underwent CTA of the pulmonary arteries with contrast medium with an iodine concentration of 400 mg I/mL (iomeprol 400, Imeron 400, Bracco) and who had no radiologic evidence of central pulmonary embolism or cardiac insufficiency (group B) were included in the final study group (120 patients). Twenty-four patients were excluded because of evidence of cardiac insufficiency. Demographics of the final study group are shown in Table 1. All patients were referred for CTA because of suspicion of embolism. According to the guidelines for retrospective studies of our institutional review board, formal approval and informed consent were not obtained.

Scan Protocol
Patients in group A received a volume of 120 mL of contrast medium, and patients in group B received 90 mL. The total iodine dose was 36 g, and the flow rate was 4 mL/s in both groups. The groups differed only in iodine flow rate, which was 1.2 mg I/s in group A and 1.6 mg I/s in group B. In all examinations, scan delay was estimated with a semiautomatic bolus tracking system (SmartPrep, GE Healthcare). The region of interest (ROI) for bolus tracking was set in the right atrium. Mean scan delay was 14.5 ± 2.8 seconds for group A and 15.1 ± 2.3 seconds for group B. A cubital or antebrachial vein was punctured with an 18- or 20-gauge needle for venous access. Before contrast medium was administered, saline injections were manually administered with the patient's arm in scanning position to ensure successful cannulation of the vein. After the contrast medium bolus was injected, a saline flush of 40 mL was administered with a double-syringe power injector (Missouri CT-Injector XD 2001, Ulrich). Scanning was performed with a 4-MDCT scanner (LightSpeed QXi, GE Healthcare). The scans were obtained with a detector configuration of 1.25 mm, a pitch of 1.5 (table feed, 7.5 mm per rotation; rotation time, 0.8 seconds), a reconstruction increment of 0.8 mm, and a section thickness of 1.25 mm. All scans were performed in a caudal to cranial direction from the dome of the diaphragm to the lung apex. Mean scanning time was 19.3 ± 2.6 seconds for group A and 20.7 ± 3.0 seconds for group B. Mean scan length was 17.1 ± 2.4 cm for group A and 18.4 ± 2.6 cm for group B. An X-ray tube voltage of 120 kV and a current of 230-250 mA were used in all examinations.

Data Acquisition
All CT scans were loaded on a workstation (MagicView, Siemens Medical Solutions). Quantitative analysis was performed by ROI measurements along the z-axis. Attenuation measurements were performed along the following arteries: subsegmental and segmental arteries of the lower lobes, lower lobe arteries, middle lobe artery, main pulmonary arteries, upper lobe arteries, and segmental and subsegmental arteries of the upper lobes. When more measurements were obtained per table position (e.g., subsegmental and segmental level in the upper and lower lobes) the measurements were averaged. The distance between measurements was 1.2 cm. An attempt was made to maintain an ROI including nearly the entire vessel diameter and to localize the ROI in areas without artifacts. Mean attenuation was recorded for each measurement. All measurements were performed by one observer.

Data Analysis and Statistical Analysis
Mean pulmonary arterial attenuation was calculated per patient; the attenuation values obtained were averaged along the z-axis. Mean pulmonary arterial attenuation was averaged for both groups. Data on body weight, body mass index, scan length, and mean pulmonary arterial enhancement are provided as mean ± SD. The significance of differences in body weight, body mass index, scan length, and pulmonary arterial attenuation between both groups was tested with Student's t test for unpaired samples. Mean pulmonary arterial attenuation was correlated with body weight, body mass index, and scan length. Pearson's product moment correlation coefficient (r) was used. For all statistical tests, two-sided p < 0.05 was considered statistically significant. The administered iodine dose per kilogram of body weight was calculated for both groups for extrapolation of the dose necessary to reach an attenuation threshold of 300 H. The software package SPSS version 11.0.5 was used for all calculations.

Results

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There was no statistically significant difference between the groups in body weight, body length, body mass index, or scan length (p > 0.05, Table 1). Mean pulmonary arterial attenuation was 274 ± 58 H in group A and 325 ± 87 H in group B. The difference of 51 H was statistically significant (p < 0.001). The SD for attenuation was higher in group B than in group A. Within group B, the SD was higher in the upper third of the scan length than in the lower third (Table 2). In group A and in group B, body weight correlated significantly with mean pulmonary artery enhancement (Figs. 1A and 1B). For group A, Pearson's correlation coefficient was -0.35 (p < 0.01); for group B, Pearson's correlation coefficient was -0.48 (p < 0.001). Body mass index also showed significant correlation with mean attenuation in the two groups. Pearson's correlation coefficient was -0.26 (p < 0.05) in group A and -0.40 (p < 0.01) in group B (Figs. 2A and 2B). Scan length showed no significant correlation with arterial enhancement in group A or in group B. Pearson's correlation coefficient was 0.24 (p = 0.07) in group A and -0.06 (p = 0.65) in group B. As shown in Figures 1B and 2B, one patient (body weight, 122 kg; body mass index, 49) in group B had a high deviation in body weight and body mass index related to the mean value for his group. Excluding this patient, Pearson's correlation coefficient was -0.44 (p < 0.01) for body weight and -0.33 (p < 0.05) for body mass index.

View larger version (6K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Correlation between body weight and pulmonary arterial attenuation. Group A; r = -0.35, p < 0.01.

View larger version (6K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Correlation between body weight and pulmonary arterial attenuation. Group B; r = -0.48, p < 0.001.

View larger version (6K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —Correlation between body mass index and pulmonary arterial attenuation. Group A; r = -0.26, p < 0.05.

View larger version (6K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —Correlation between body mass index and pulmonary arterial attenuation. Group B; r = -0.40, p < 0.01.

Mean administered iodine dose per kilogram of body weight was 0.50 ± 0.11 g I/kg in group A and in group B. In group B, the trend line of the iodine dose-attenuation curve showed a steeper course than in group A. The threshold of 300 H was reached at an iodine dose of 0.44 g I/kg body weight in group B, whereas in group A an iodine dose of 0.68 g I/kg body weight was necessary (Figs. 3A and 3B).

View larger version (6K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —Correlation between iodine dose per body weight and pulmonary arterial attenuation. Group A.

View larger version (7K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —Correlation between iodine dose per body weight and pulmonary arterial attenuation. Group B.

Discussion

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In CTA, the degree of arterial enhancement depends on injection-related factors such as iodine flow rate and on patient-related factors such as cardiac output and body weight [7]. The magnitude of arterial enhancement is proportional to the iodine flow rate, which can be altered by changing the injection rate or the concentration of contrast medium. The higher the iodine flow rate, the faster and higher peak arterial enhancement occurs [7]. Decreased cardiac output delays and increases peak arterial enhancement magnitude as a result of decreased mixing and dilution of the contrast medium. Several strategies have been introduced to overcome the variability of arterial enhancement due to differences in cardiac output among patients. Use of a test bolus and use of bolus-tracking systems have been assessed for adaptation of scan delay to cardiac output [4, 8, 9]. In our study, a semiautomatic bolus-tracking system was used. Another important patient-related factor that influences arterial enhancement is body weight. High body weight has been found to decrease the magnitude of arterial enhancement [1, 10], but these factors have been assessed in CTA of the abdominal aorta. To our knowledge, no data are available on CTA of the pulmonary arteries.

Our study showed a small but significant negative correlation between body weight and pulmonary arterial enhancement regardless of the iodine flow rate used. In addition, body mass index showed a small negative correlation with mean pulmonary artery enhancement for both iodine flow rates. The correlation between body mass index and pulmonary artery enhancement was smaller than the correlation between body weight and pulmonary artery enhancement. These results are in agreement with the findings of other studies [1, 10]. Bae et al. [1] reported that after IV administration, contrast medium distributes rapidly from the central blood compartment to the well-perfused extracellular compartment and slowly to the poorly perfused extracellular compartment. Awai et al. [10] found that in CTA of the aorta the poorly perfused extracellular compartment can usually be ignored because of early data acquisition. One can assume that this finding is especially valid in CTA of the pulmonary arteries. Consequently, the central blood compartment, that is, the pulmonary arteries themselves including the capillary volume, may be the only relevant compartment influencing distribution of contrast medium. This compartment seems to correlate better with body weight than with body mass index.

In group B, the SD was higher in the upper third of the scan length than in the lower third. We assume variabilities in the bolus trigger system caused this phenomenon. A delayed start of data acquisition showed more effect on arterial enhancement in the upper third of the scan length in group B than in group A because of the lower contrast volume and the lower injection time.

The results of this study confirmed the influence of iodine flow rate on vascular attenuation. Using an iodine flow rate of 1.6 g I/s led to significantly higher attenuation of the pulmonary arteries than using an iodine flow rate of 1.2 g I/s (325 vs 274 H). Mean pulmonary artery enhancement of 300 H, which is suggested for adequate visualization of the pulmonary arteries [11], was achieved in terms of an iodine dose of 0.68 g I/kg with an iodine flow rate of 1.2 g I/s. With an iodine flow rate of 1.6 g I/s, the required iodine dose was 0.44 g I/kg. Consequently, an iodine dose of 0.24 g I/kg can be saved when the iodine flow rate is increased 0.4 g I/s.

Two factors have to be considered when contrast media are tailored to patient weight. As shown in Table 3, to reach 300 H when an iodine flow rate of 1.2 g I/s is used (group A), a high volume of contrast medium would be necessary in heavy patients. Use of this volume would lead to an injection time longer than the scanning time, which would not make sense. On the other hand, with an iodine flow rate of 1.6 g I/s (group B), the injection time would be very short in small patients. It is generally agreed that in pulmonary CTA the injection time should not be significantly lower than the scanning time to ensure constant arterial attenuation. Consequently, scanner technology may be the limiting factor in reduction of iodine dose in small patients when high iodine flow rates are used because the injection time would be too short for adequate enhancement over the time needed for 4-MDCT, as was used in our study, to acquire a complete CTA study. In our study population, mean scan delay was 15 seconds, and mean scanning time was 20 seconds. Consequently, scanning was finished 35 seconds after initiation of the injection of contrast medium. When a high iodine flow rate is used (e.g., group B of our study population) and the contrast medium is tailored to patient weight, injection duration ranges from 14 seconds for a 50-kg patient to 28 seconds for a 100-kg patient (Table 3). For small patients, the injection time would be too short for scanning time because of the small volume of contrast medium needed. However, this premise is valid only when a scan protocol similar to the present one is used. These limitations may be overcome when CTA of the pulmonary arteries is performed with 16-MDCT or 64-MDCT, because these scanners are capable of imaging the entire pulmonary arterial bed in less than 10 seconds. Therefore, significant reduction of contrast medium dose may be reached even in small patients when high-concentration contrast medium is used in combination with 16-MDCT and higher scanners.

Limitations to our study have to be acknowledged. First, the study was a retrospective data analysis of routinely performed CT angiographic examinations of the pulmonary arteries, and data on the iodine dose necessary to achieve an attenuation threshold of 300 H were retrospectively extrapolated. Prospective studies are necessary to confirm the data. Second, no sufficient clinical data were available about the cardiac status of the patients. However, patients with radiologic evidence of cardiac insufficiency on CT were excluded. Scan delay was estimated with a semiautomatic tracking system to minimize differences in arterial enhancement due to differences in cardiac status. In addition, there were no significant differences in patient age and body weight or body mass index, and thus the study population was homogeneous. Third, the viscosity of high-concentration contrast media is greater than that of conventional contrast media (aside from the iodine load) (12.6 mPa · second at 37°C for iomeprol 400 vs 4.6 mPa · second at 37°C for iopromide 300). Therefore differences in pulmonary artery enhancement between groups A and B may have been due to differences in viscosity rather than differences in iodine flow rate. However, it can be assumed that viscosity is not as important in the contrast dynamics of larger vessels as it is for the lower-order pulmonary arteries measured in this study.

In conclusion, pulmonary artery attenuation in CTA of the pulmonary arteries shows a small but significant correlation with body weight and body mass index, there being a higher correlation with body weight, independent of the iodine flow rate used. A higher iodine flow rate improves pulmonary artery enhancement. Therefore adaption of contrast medium volume to body weight may help to optimize contrast enhancement. A significant reduction in contrast medium dose may be achieved with use of high-concentration contrast media in combination with CT scanners that allow rapid data acquisition. Prospective studies are needed to confirm the results obtained in this retrospective analysis.

References

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