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1 Service de Radiologie, l'Hôpital Cantonal Universitaire de
Genève, rue Micheli-du-Crest, CH-1211 Genève 14,
Switzerland.
2 Service de Radiologie, Centre Hospitalier Lyon Sud, EA 643, Chemin du Grand
Revoyet, 69495 Pierre Benite Cedex, France.
3 Unité de Pharmacologie Clinique, Université Claude Bernard Lyon
1, 8 Ave., Rockefeller, 69373 Lyon, Cedex 08, France.
Received April 5, 2001;
accepted after revision December 28, 2001.
Address correspondence to P. Loubeyre.
Abstract
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SUBJECTS AND METHODS. Two hundred consecutive patients referred for contrast-enhanced thoracic CT for malignancies or infections prospectively entered the study. They were randomly assigned to one of two simple bolus injection protocols (100 patients in each protocol): 60 mL of a nonionic contrast agent (250 mg I/mL) injected at a 3 mL/sec flow rate, or 80 mL of the same contrast agent injected at a 4 mL/sec flow rate. No saline flush or bolus triggering system was used. Hilar and mediastinal vessel enhancement was qualitatively (using a 4-point scale) and quantitatively (arterial attenuation values) assessed. Perivenous artifacts were also assessed.
RESULTS. No extensive perivenous artifacts were noted. No significant difference was noted regarding pulmonary venous enhancement. Excellent opacification of the pulmonary veins was observed in 66% of patients injected at 3 mL/sec and in 56% of patients injected at 4 mL/sec (p > 0.192). A highly significant difference was noted for pulmonary artery enhancement. Excellent opacification of the pulmonary arteries was noted in 83% of patients injected with a 3mL/sec flow rate and in 61% of patients injected with a 4mL/sec flow rate (p = 0.001).
CONCLUSION. A high level of opacification of hilar pulmonary vessels, with no major perivenous artifacts, can be obtained with a small amount of nonionic contrast medium using a simple bolus injection.
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The high cost and potential harmfulness of contrast media force radiologists to search for ways to further decrease contrast volume.
The main purpose of this study was to determine whether a high level of opacification of hilar pulmonary vessels can be obtained with a small amount of contrast material using a simple bolus injection. Two bolus injection protocols using small amounts of contrast medium were evaluated. Pulmonary vessel opacification and perivenous artifacts were assessed. Thoracic aorta opacification was also assessed. Qualitative and quantitative vascular enhancement assessments were performed.
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Indications for helical CT were malignancies (n = 124), uncharacterized pulmonary or mediastinal masses and nodules (n = 40), and pulmonary infections (n = 36). No informed consent was obtained because the contrast agent used in this study is routinely used in radiologic practice and because of the small amount of contrast agent injected. Nor was institutional review board approval required.
Injection of Contrast Material
The type of contrast agent, duration of injection, and delay time between
injection and initiation of CT scanning were the same for both protocols.
Injection flow rates, and thus the amount of contrast medium, varied.
For protocol A, 80 mL of iomeprol 250 (Iomeron; Bracco Diagnostics, Paris, France) was administered. The iodine concentration was 250 mg I/mL, osmolality was 435 mosm/kg of water, and viscosity measured at 37°C was 2.9 mPa/sec. The injection flow rate was 4 mL/sec.
For protocol B, 60 mL of iomeprol 250 was administered using a 3 mL/sec injection flow rate.
A left antecubital venous access was achieved for all patients using a 20-gauge venous catheter with the patient's arms positioned beside the head. The contrast medium was injected using a power injector (Multilevel CT Injector; Medrad, Pittsburgh, PA), and injection duration was 20 sec.
Helical CT
Ten seconds after initiation of the contrast medium injection, helical CT
was performed using a Somatom Plus 4 CT scanner (Siemens Medical Systems,
Erlangen, Germany) with 8-mm collimation, a pitch of 1.2, a 1.0-sec gantry
rotation period, 206 mA, and 140 kVp.
The entire thorax was imaged in a craniocaudal direction, in a single 25- to 30-sec breath-hold, from the lung apices to the lateral costophrenic sulci. Images were reconstructed using a 180° linear interpolation algorithm at 8-mm intervals on a 512 x 512 matrix.
All images were printed on hard-copy films at a window level appropriate for vascular visualization (window center, 50 H; window width, 350 H).
Qualitative Assessment
Two experienced thoracic radiologists independently evaluated the helical
CT examinations, which were randomized and presented to reviewers in a
different order. The radiologists were unaware of the patients' dose group.
Reviewers graded perivenous artifacts and arterial enhancement on a 4-point
scale (Table 1). The baseline
for scoring the images had been established by the two reviewers during a
previous study [5]. Perivenous
artifacts were scored 1 for extensive, 2 for moderate, 3 for minimal, or 4 for
absent. Vascular opacification was considered 1, poor
(Fig. 1); 2, fair
(Fig. 2); 3, good
(Fig. 3); or 4, excellent
(Fig. 4).
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The anatomic region chosen to assess perivenous artifacts (i.e., blooms and streaks) was the region adjacent to the superior vena cava. Anatomic regions chosen for qualitative assessment of vascular enhancement were the right pulmonary artery (main and interlobar arteries), the left pulmonary artery (main and interlobar arteries), the right superior pulmonary vein, the left superior pulmonary vein, the ascending aorta, and the descending aorta. In case of discordance between reviewers' scores, the lower score was considered the final score.
Quantitative Assessment
Images were reviewed on a viewing console. For each CT examination, single
circular vascular regions of interest (diameter, 1.4 cm) were positioned in
the ascending aorta, the descending aorta, and the pulmonary trunk at the
origin of the right pulmonary artery. The three measurements were obtained on
the same scan. For each CT examination and each measurement location, an
arterial attenuation value and its standard deviation were recorded. For each
measurement location, the mean and standard deviation of arterial attenuation
values were calculated for each injection protocol.
Arterial attenuation values were not measured in superior pulmonary veins because of volume averaging caused by the small diameter of these vessels.
Statistical Analysis
For both injection protocols, the mean arterial attenuation values in the
pulmonary trunk, in the ascending aorta, and in the descending aorta were
compared using the Student's t test.
For each vessel, because scores and injection amount were ordinal data, scores were compared using the Wilcoxon's rank sum test. To compare the proportion of cases of excellent opacification (score = 4), scores were grouped into two categories (scores < 4 and scores = 4) and compared using Pearson's chi-square test with 1 degree of freedom.
The relationship between scores and attenuation values was evaluated with a nonparametric correlation coefficient; Spearman's rho and Kendall's tau gave the same results.
An arterial attenuation threshold above which vascular opacification was scored as excellent (score = 4) was calculated for the pulmonary trunk, the ascending aorta, and the descending aorta. For each vessel, we calculated, for each observed attenuation value, the proportion of cases with a score of 4 among those having an attenuation greater than that value. Then we adjusted these proportions versus the corresponding attenuation values using a quadratic model, which gave the best results. The arterial attenuation threshold is the adjusted attenuation value corresponding to 95% of cases above that value with a score of 4. We calculated the 95% confidence limits for individual predicted values. For all statistical tests, p values were calculated.
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No extensive perivenous artifacts were noted with either injection protocol. Artifacts were less present with a 4 mL/sec flow rate than with 3 mL/sec, but the difference was not statistically significant (Table 2).
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No significant difference was noted for pulmonary venous opacification or ascending aorta opacification. Excellent opacification of the pulmonary veins was noted in 66% of patients injected with a 3 mL/sec flow rate and in 56% of patients injected with a 4 mL/sec flow rate. Excellent opacification of the ascending aorta was noted in 51% of patients injected with a 3 mL/sec flow rate and in 59% of patients injected with a 4 mL/sec flow rate (Table 2).
A significant difference was noted for the descending aorta opacification. Excellent opacification was noted in 40% of patients injected at 3 mL/sec and in 56% of patients injected at 4 mL/sec (Table 2).
A highly significant difference was noted for pulmonary arterial opacification. Excellent opacification was noted in 83% of patients injected at 3 mL/sec and in 61% of patients injected at 4 mL/sec (Table 2).
Arterial attenuation values were significantly higher in the ascending and descending aortas with a 4 mL/sec flow rate, and arterial attenuation values were significantly higher in the pulmonary trunk with a 3 mL/sec flow rate (Table 3).
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The relationship between scores of arterial opacification and arterial attenuation values measured in the vessels was highly significant (Table 4). The arterial attenuation threshold above which excellent opacification (score = 4) was seen is given in Table 5 for each vessel. Ninety-five percent of cases of excellent opacification had an observed arterial attenuation greater than this threshold (Table 5).
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During contrast-enhanced helical CT examinations for general thoracic evaluations, good opacification of thoracic vascular structures can be obtained with a small volume of contrast medium [4, 5]. Because pulmonary hila are difficult to analyze when discriminating between vessels and abnormal processes, a high level of opacification of hilar pulmonary vessels is mandatory. The main goal of this study was to determine whether a high level of opacification of hilar pulmonary vessels can be obtained using a small amount of contrast medium during contrast-enhanced helical CT for general thoracic examinations. Aorta opacification was also assessed.
Two injection protocols were assessed. With a 60-mL contrast medium bolus injected at 3 mL/sec, excellent pulmonary artery opacification was noted in 83% of patients and excellent pulmonary venous opacification was noted in 66% in patients. Insufficient pulmonary artery opacification was noted in only 4% of patients and insufficient pulmonary venous opacification was noted in only 8% of patients. Excellent opacification of the ascending aorta was noted in 70% of patients. These results were obtained with an iodine concentration of 250 mg I/mL.
Perivenous artifacts were scored as moderate or minimal and were never considered a problem, which can be explained by the low viscosity of the contrast medium and the low pooling of the contrast agent in the left brachiocephalic vein and the superior vena cava.
Our results confirm that reduction of contrast agent volume for general thoracic CT does not affect the diagnostic quality of vessel opacification [4, 5, 7].
Moreover, our results indicate that 60 mL injected at a rate of 3mL/sec allows a statistically significant greater opacification of pulmonary vessels than 80 mL injected at 4 mL/sec. These results appear paradoxic. For both protocols. the injection duration and the delay between the beginning of the contrast injection and the start of CT were the same, but the amount of iodine entering the blood pool per second was greater with 80 mL (4 mL/sec). Some explanations can be proposed. The first explanation is that the time between the beginning of the contrast injection and the peak of pulmonary artery enhancement is shorter with the 4 mL/sec injection rate. Thus, in some patients, CT scans at the level of the pulmonary arteries are probably obtained after the pulmonary artery enhancement peak. With the 4 mL/sec flow rate, a delay of 10 sec between the beginning of the bolus injection and the start of scanning could be too long to optimize the time window for data acquisition of enhanced pulmonary vessels. This hypothesis is supported by the fact that arterial attenuation measured in the aorta was greater with the 4 mL/sec flow rate. A second hypothesis is that, in some cases, a high flow rate (4 mL/sec) creates hemodynamic and rheologic effects that could modify the transit time of the contrast medium or modify the cardiac output [8].
This study was performed using a nonionic contrast medium to reduce the prevalence of nausea [9, 10]. To simplify the injection procedure, we did not attempt to push the contrast material with a saline solution [4, 11]. This technique has been proven to provide satisfactory thoracic vascular enhancement [4, 11].
In our study, the qualitative assessment of vascular enhancement proved to be accurate. The calculated threshold arterial attenuation above which vascular enhancement was noted to be excellent (score = 4) proved to be the same for the pulmonary trunk, the ascending aorta, and the descending aorta.
Our study had some limitations. Our results cannot be extrapolated to specific thoracic CT indications such as the detection of pulmonary embolism. For that specific indication, the mandatory amount of contrast material is greater. The results of our study are valid for the scan collimation, the pitch, and the contrast agent used in this study. The influence of factors such as weight and heart rate was not assessed. We did not measure agreement between observers. In a previous study based on the same criteria of qualitative assessment, agreement between observers was good when considering the vascular enhancement [5]. To avoid overestimating the final scores, only the lower score of the two reviewers was taken into account.
We did not use a bolus triggering system to optimize the timing of image acquisition [7] because this system is not yet available on all scanners. Our goal was to assess a simple injection procedure. With a short delay time of 10 sec between the beginning of the bolus injection and the start of scanning, we obtained high opacification of pulmonary vessels in most cases. Our injection protocol could probably be optimized for each patient using a bolus triggering system, and the amount of contrast medium could be further reduced with multidetector scanners.
In conclusion, during contrast-enhanced helical CT for general thoracic indications, a high level of opacification of hilar pulmonary vessels can be obtained with a small volume of nonionic contrast medium and a simple injection procedure, with no major perivenous artifacts. Optimization of the delay between the beginning of injection and the start of scanning can be achieved using a bolus triggering system.
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