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1 All authors: Department of Diagnostic Radiology, University of Technology, Pauwelsstra. 30, D-52057 Aachen, Germany.
Received July 14, 1999;
accepted after revision September 3, 1999.
Address correspondence to P. Haage.
Abstract
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MATERIALS AND METHODS. Fifty patients underwent helical CT of the thorax using 60 ml of contrast material (370 mg I/ml) followed by flushing with 30 ml of physiologic saline solution. These 50 patients had been examined before using our previous protocol, 75 ml of the same contrast material without a subsequent saline solution. Mean attenuation values for both protocols were measured in the superior vena cava, the pulmonary trunk, and the ascending aorta. Image artifacts and mediastinal and hilar depiction were graded and compared.
RESULTS. Mean attenuation values in the superior vena cava were considerably higher in the regimen without saline solution flush (459 H versus 352 H) and in the pulmonary trunk and the ascending aorta were almost identical for both protocols. Injection of saline solution diminished surrounding artifacts (p = 0.001). Grading results for the evaluation of mediastinal and hilar structures were not significantly different in the two protocols (p = 0.564).
CONCLUSION. Injection of contrast material followed by a saline solution bolus using a double power injector when performing thoracic helical CT allows a 20% reduction of contrast material volume to 60 ml with a similar degree of enhancement. In addition, perivenous artifacts in the superior vena cava are significantly reduced.
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Reassessment of scanning protocols using an understanding of the kinetics of contrast enhancement is essential to fully appreciate the advantages of helical scanning. Bolus injection timing, IV contrast volume, and contrast flow rate have to be considered.
Using helical CT, the entire thorax can be scanned in a single breath-hold, allowing a more consistent level of vascular enhancement throughout the acquisition [3]. By eliminating the respiratory inconsistencies associated with conventional CT, helical CT enables more lung lesions to be reliably detected [4, 5]. IV contrast material can be used to differentiate between lymph nodes or soft-tissue masses and normal vasculature and is necessary to assess vascular disease and anomalies [6]. Contrast-enhanced CT reveals more enlarged mediastinal lymph nodes in patients with lung cancer than unenhanced CT does [7]. Nonionic contrast agents are preferably used because they are better tolerated by patients [8, 9]; also, extravasation of nonionic contrast agents tends to cause less serious complications [10]. Compared with dynamic incremental CT, smaller amounts of contrast agent are required [6]. Nevertheless, the volume of contrast material used for thoracic helical CT varies between 75 and 150 ml, depending on the institution and the clinical indication for CT scanning [6, 11, 12].
Although routinely used nonionic contrast agents have optimized scanning protocols and contrast enhancement, their high cost forces the radiologist to search for ways to further decrease contrast volume. Hopper et al. [11] described using the power injection of a saline solution to push the contrast material (75 ml) in a single syringe for performing thoracic helical CT.
In our study, a double power injector was used to facilitate the examination procedure and to avoid mixing contrast material with the saline solution (Fig. 1).
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The purpose of this trial was to determine the effectiveness of this technique in thoracic CT when reducing contrast volume to 60 ml and to compare the results with those of our former protocol, which used 75 ml of contrast material and no saline solution bolus.
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All CT scans were obtained in the same manner using a 1-sec gantry rotation period, 120 kVp, and 200 mA. Sixty milliliters of the nonionic contrast agent iopromide (Ultravist 370; Schering, Berlin, Germany) containing 370 mg I/ml was administered with a power injector (CT 9000 Digital injection system; Liebel-Flarsheim, Cincinnati, OH), which is a motor-driven syringe mechanism with microprocessor control of the flow rate, volume, and timing of contrast medium injection. Vascular access was achieved through a 22- or 20-gauge IV catheter (Abbocath; Abbott, Sligo, Ireland) placed in an antecubital vein. Flow rate was set at 3 ml/sec. Immediately after completion of the nonionic contrast agent injection, 30 ml of sterile isotonic 0.9% saline solution was injected at a rate of 3 ml/sec with a second CT 9000 power injector. This injection technique was specially designed for standard helical CT imaging of the thorax. Each of the two 60-inch (152.4-cm) long tubings (Low Pressure Connector Tubing; Medrad, Indianola, PA) was connected in line with a non-return valve and then with a Y-adapter leading to the IV catheter. Helical CT scanning was performed using 5-mm collimation and a 7.5-mm table increment per gantry rotation (pitch, 1.5). Scanning was typically started 20 sec after the initiation of contrast material injection to achieve adequate enhancement in systemic arteries and veins. In patients with a history of congestive heart failure, orthopnea, or lower extremity edema, the delay between injection and scanning was 25 sec. Patients were imaged in the supine position with the arms above the head in a cephalocaudad direction starting at the lung apex and extending to the end of the diaphragm. The patient was asked to hyperventilate by performing at least two deep breaths before the helical scan, then told to take in another deep breath. Scanning was started at maximum inspiration, with the total scanning duration ranging between 24 and 35 sec. Axial images were reconstructed at 5-mm increments after 180° linear interpolation on a 512 x 512 matrix.
Fifty of the 380 examined patients had undergone helical CT of the thorax in our institution previously using the same Philips CT scanner, but using a different protocol with 75 ml of 37% iopromide (Ultravist 370) and no subsequent saline solution bolus, which was our established protocol of choice for several years. Results of these 50 patients' studies were analyzed in this study. The patients included 33 men and 17 women who were 23-77 years old (mean age, 59 years). Indications for helical CT were assessment of lung cancer (n = 20), lymphoma (n = 7), breast cancer (n = 7), renal cell carcinoma (n = 6), teratoma (n = 3), uncharacterized pulmonary mass (n = 3), penile carcinoma (n = 1), esophageal cancer (n = 1), pulmonary tuberculosis (n = 1), and arteriovenous malformation (n = 1).
In 31 of these 50 patients, slice thickness, table speed, and slice increment reconstruction were the same as described previously: 5 mm, 7.5 mm/sec, and 5 mm, respectively. In 19 patients the section acquisition thickness was 5 mm at a table speed of 5 mm (pitch, 1.0), and overlapping slices were reconstructed every 4 mm. Other parameters, such as patient preparation, delay time, flow rate, and reconstruction algorithms, were identical in both the scanning protocol with 75 ml and the protocol with 60 ml of contrast medium (Table 1).
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Comparative quantitative and qualitative assessment was performed for the 50 patients who had undergone a CT examination with and without saline solution flush. For each of the 50 patients, a single radiologist measured and recorded the mean arterial and venous enhancement values after the placement of consistent regions of interest in the superior vena cava at the level where the right pulmonary artery passes posteriorly the pulmonary trunk and the middle ascending aorta.
Calculation of the mean and standard deviation of arterial and venous attenuation values in the superior vena cava, the pulmonary trunk, and the middle ascending aorta was performed for both protocols. Wilcoxon's signed rank test for pairs was used to assess the statistical significance of the difference in mean values at each of the three locations. A two-tailed probability of less than 0.05 was defined as statistically significant.
Grading of image quality regarding surrounding artifacts and evaluation of mediastinal and hilar structures after contrast enhancement was performed by a consensus panel of three radiologists who were unaware of the type of protocol used and the date of the examination. Helical CT examinations were reviewed in random order using two three-point grading schemes, one for quality of mediastinal and hilar depiction and one for quality of image artifacts. The grading criteria for mediastinal and hilar depiction and perivenous image artifacts are shown in Table 2. A window width of 345-360 H and a center of 80 H were chosen for all images. Qualitative and quantitative scores were categorized according to the injection regimen, and the quantitative values were further subcategorized according to the measured arterial and venous regions of interest.
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The evaluation of each qualitative grading scheme and statistical comparison of both protocols (no change, better, or worse) were done using the symmetry test of Bowker, which is a McNemar test not restricted to fourfold or 2 x 2 table data analysis. Probability (p) values were calculated for each comparison using a significance level of 0.05. In addition, the data regarding vascular enhancement and perivenous artifacts in the 19 patients who were examined with a slower table speed (5 mm/sec) in the protocol with 75 ml of contrast medium, were compared with the results of the remaining 31 patients (table speed, 7.5 mm/sec).
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Three results may be drawn from analysis of these data. First, 60 ml of contrast agent followed by a saline solution bolus (protocol B) compared with 75 ml of contrast medium alone (protocol A) resulted in a lower mean net attenuation in the superior vena cava in protocol B than in protocol A, the mean difference being 106.9 H. Second, calculated mean attenuation values in the pulmonary trunk and middle ascending aorta were consistently comparable for both protocol A and protocol B, with 202.5 H versus 190.5 H in the pulmonary trunk and 240.8 H versus 238.3 H in the ascending thoracic aorta, respectively. And third, although the absolute enhancement values in the superior vena cava in protocol B were considerably lower than those in protocol A, these differences were not statistically significant (p = 0.239). The difference in mean attenuation values for the measured regions of interest in the pulmonary trunk (p = 0.420) and the ascending aorta (p = 0.811) to determine the level of confidence were also not of statistical significance.
The qualitative grading results of the consensus panel for mediastinal and hilar depiction and for image artifacts are displayed in Tables 4 and 5, respectively.
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Mediastinal and hilar image quality were graded on a three-point scale of 1 = excellent, 2 = good, and 3 = poor. The scored assessment for vascular opacification was 1.05 overall, with a score of 1.04 for protocol type A versus 1.06 for protocol type B. Two examinations rated excellent in protocol A were rated good when performed with saline solution flush (protocol B). On the other hand, one examination that was graded good in protocol A was defined as excellent in protocol B. Forty-six examinations were scored as excellent both before and after the change of protocols (Fig. 2A,2B). One examination was graded good in both protocols. Depiction and opacification were poor in none of the helical CT scans. The minor differences in grading results between both injection regimens were not statistically significant (p = 0.564, Table 4).
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The calculated mean values on perivenous artifacts analyzed on the same three-point scale (Table 5) were 2.08 for protocol A and 1.56 for protocol B, resulting in an overall score of 1.82. Statistical testing of these differences showed a highly significant probability (p = 0.001). Identical grading results with 75 and 60 ml of contrast agent were observed in 22 examinations: nine examinations were rated excellent, nine examinations were rated good, and four examinations were rated poor. Eighteen examinations from group A (without saline) scored as good (n = 16) or poor (n = 2) were rated excellent when protocol B (with saline) was used. Conversely, one group B examination rated as good and none of the group B examinations scored as poor were rated excellent when using protocol A. Eight CT examinations that scored as poor in protocol A were rated good when protocol B was used, whereas a score change from good to poor in the reverse direction (from protocol B to protocol A) occurred once. Thirty-six protocol A and 45 protocol B examinations showed no to moderate image artifacts. Extensive artifacts with associated vessel blurring were seen in 19 patients, of which 14 were observed with protocol A and five with protocol B. No statistically significant differences were noted when comparing the data of the two subpopulations of protocol A with different table speeds (5 mm versus 7.5 mm).
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In our case, however, no statistically significant grading differences were seen regarding the mediastinal and hilar depictability in the two examination regimens. Two possible explanations may account for the discordance between these qualitative results. Image artifacts were mostly restricted to the opacified vessel. Because the evaluation of vascular lesions, including pulmonary emboli or aortic aneurysms and dissections, for which a different protocol is used, was not the primary indication for our examinations, these intravascular artifacts did not considerably disturb anatomic detail. Also, perivascular structures were not necessarily obscured to a degree the observer would judge would make establishing a diagnosis difficult (Fig. 3A,3B).
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The enhancement of anatomic structures involves many complex parameters such as age, sex, weight, and cardiac status of the patient, and may vary dramatically despite similar or the same scanning and injection techniques. In contrast to previously published studies, we compared two different CT protocols in the same patient population because many of our patients have disorders that need to be followed up regularly. Forty-eight (96%) of 50 patients were oncologic patients in whom response to treatment was to be assessed. Consequently, the data acquired in our study population are of good statistical reliability and validity because they reduce the measurement error that may be caused by a wide degree of individual variation. Naturally, a certain degree of intraindividual variation, such as different volume status of the systemic vessels or cardiac problems due to recurrent chemotherapy at the time of the examination, is a factor that cannot be influenced by the investigator.
In some institutions, femoral injection of contrast material has been tried to diminish perivenous artifacts. This method, however, lacks practical acceptability compared with simple injection via an antecubital vein. Other hospitals favor scanning in a caudocranial direction to reduce diaphragmatic movements at the end of a scan and to eliminate artifacts caused by the inflow of high concentrations of contrast medium in the brachiocephalic veins and the superior vena cava. In our experience, problems with breath-holding do not occur when scanning in a craniocaudal direction if the patient has been instructed in the proper technique. The superimposition of mediastinal and hilar structures from the rapid enhancement of the brachiocephalic veins and the superior vena cava is virtually eliminated by the saline flush.
The use of two interconnected power injectors facilitated the procedural preparation because it was not necessary to load only one syringe with contrast material and saline solution for each examination as was done by Hopper et al. [11]. In a busy CT facility, this less time-consuming method is favorable. In addition, unwanted mixing of the two components is avoided. Injection parameters can be modified for the contrast material and the saline bolus individually, if desired.
Aside from the benefits of cost savings per patient and examination, the substitution of the last 15 ml of iodine with a saline push assists image quality in several positive ways: clearing the IV catheter of contrast material, reduction of contrast medium dilution through mixing with blood on its course from the peripheral vein to the central vessels, and avoidance of contrast material pooling in the arm veins. The substitution of a saline injection thus reduces the amount of contrast material and lessens streak artifacts while maintaining satisfactory central arterial enhancement. Reducing the contrast medium volume further will lead to even fewer perivenous artifacts and lower venous attenuation values; however, arterial enhancement will concomitantly decrease to a point that is simply not acceptable. Rubin et al. [12] described the effects of different contrast medium concentrations and reported that progressive reductions of the iodine concentration to as low as 75 mg/ml achieve the greatest degree of perivenous artifact reduction. On the other hand, reductions in such dimensions are not clinically feasible because of poor arterial opacification measurements. These researchers concluded that a dilution of 15.0 g of iodine to 150 mg I/ml and injecting a total volume of 100 ml resulted in superior arterial enhancement with diminished perivascular artifacts. Costello et al. [6] found that 60 ml of contrast material for thoracic helical CT provided better image quality and vessel opacification than 120 ml of contrast medium in dynamic incremental CT. Hopper et al. [11] showed the effectiveness of helical CT with 75 ml of contrast material pushed with saline compared with 125 ml of contrast medium alone. These researchers found that enhancement was statistically equal in both protocols and significantly reduced beam-hardening artifacts. Fifty milliliters of nonionic contrast material pushed with 50 ml of saline solution and 75 ml of contrast material alone provided inadequate arterial opacification.
In our study, the further reduction of contrast material by 20% (to 60 ml) decreased the mean net attenuation values only in the superior vena cava; measurements were almost identical in the main pulmonary artery and the middle ascending aorta. Image details were sufficiently clear in all examinations, supporting our theory that high-quality CT images can be obtained with as little as 60 ml of contrast medium if a saline chaser bolus is added.
Before this investigative study, 75 ml of contrast medium was routinely applied as our standard protocol for thoracic helical CT. After 8 months of successfully using the modified injection regimen on a daily basis, analysis of the acquired data resulted in a definitive change of protocols.
In conclusion, we advocate the use of a saline push in helical CT of the thorax. The extra effort required for syringe and tubing preparation is negligible, especially considering an expected 20% saving in the cost of contrast material.
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