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Original Research |
1 Department of Nuclear Medicine and Diagnostic Imaging, Kyoto University
Graduate School of Medicine, Kyoto, Japan.
2 Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline
Ave., Boston, MA 02215.
3 Department of Cardiovascular Medicine, Kyoto University Graduate School of
Medicine, Kyoto, Japan.
4 Present address: Division of Cardiology, Kitano Hospital, Tadukekofukai
Medical Research Institute, Osaka, Japan.
5 Department of Cardiovascular Surgery, Kyoto University Graduate School of
Medicine, Kyoto, Japan.
Received February 23, 2005;
accepted after revision July 7, 2005.
Address correspondence to S. Kubo.
Abstract
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MATERIALS AND METHODS. Seventy-one patients with pre- or postoperative aortic aneurysms underwent MDCT angiography throughout the thoracoabdominal-aortoiliac system using an 8-MDCT scanner. In 37 patients, 100 mL of contrast medium was injected at a flow rate of 3.0 mL/s (hereafter referred to as the 100-mL group). In 34 patients, 50 mL of contrast medium followed by a 20-mL saline flush was injected at a flow rate of 2.5 mL/s (the 50-mL group). For each group, quantitative analysis involved calculating the mean aortoiliac enhancement, plateau deviation, and contrast enhancement in the pulmonary trunk and superior vena cava (SVC). Qualitative analysis involved assessing the 3D postprocessing images.
RESULTS. Significant differences between the groups in mean
aortoiliac enhancement (100-mL group vs 50-mL group, 337 ± 6 H vs 319
± 5 H, p < 0.0001) and mean plateau deviation (51 ±
4 H vs 58 ± 4 H, p < 0.0001) were found. However, adequate
arterial enhancement (
200 H) was observed in 31 of 34 patients in the
50-mL group and uniform aortoiliac enhancement (< 50 H) was seen in 26
patients. Visual analysis showed no difference in contrast material magnitude
and homogeneity between the groups. Furthermore, in the 50-mL group, the
thoracic aorta was more clearly visualized because of a reduction in the
opacity of the main pulmonary artery and SVC.
CONCLUSION. In our experience, administration of 50 mL of contrast medium followed by a 20-mL saline flush produces thoracoabdominal-aortoiliac MDCT angiographic examinations of effective quality in most cases.
Keywords: cardiovascular imaging • contrast media • saline flush • MDCT angiography
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MDCT has provided improved spatial resolution with the use of near isotropic voxels, making it possible to assess smaller arterial branches [8, 9] and to perform 3D volumetric analysis at MDCT angiography [10]. However, the use of large amounts of contrast medium (> 100 mL) may increase the incidence of contrast-induced side effects.
Recently, several studies have shown that a saline flush helped in reducing the amount of contrast medium needed for contrast-enhanced single-detector CT [9, 11] or MDCT [12] examinations. With the high scanning speed provided by MDCT scanners, it has become possible to perform a systematic thoracoabdominal-aortoiliac CT angiography study [10], so further reduction in the amount of contrast medium can be expected. However, few studies have examined the reduction in the amount of contrast medium needed at MDCT angiography [10].
The purpose of this study was to determine whether a reduction to 50 mL of contrast medium combined with a saline flush can produce arterial enhancement comparable to the results obtained with 100 mL of contrast medium alone throughout the thoracoabdominal-aortoiliac system using 8-MDCT.
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This study was performed within the routine clinical standards of our hospitals. The study protocol was approved by a local ethics committee. Informed consent was obtained from each patient.
CT Protocol
CT examinations were performed with an 8-MDCT unit (Aquilion, Toshiba
Medical Systems) using a 2.0-mm collimation, slice pitch of 0.875, 0.5-second
gantry rotation period, table speed of 14 mm per rotation (28.0 mm/s), and 120
kV. The tube current varied between 100 and 220 mAs along the z-axis
using the z-axis automatic tube current modulation technique (Real
Exposure Control, Toshiba Medical Systems).
Patients were assigned randomly to one of two protocols receiving 100 or 50 mL of nonionic iodinated contrast medium. A 20-gauge IV catheter was placed into an antecubital vein in each patient. In 37 patients, 100 mL of nonionic iodinated contrast medium with an iodine concentration of 300 mg I/mL (iohexol [Omnipaque, Daiichi]) was administered at a flow rate of 3.0 mL/s (hereafter referred to as the 100-mL group). In 34 patients, 50 mL of nonionic iodinated contrast medium with an iodine concentration of 350 mg I/mL (iomeprol [Iomeron, Eisai]) was administered at a flow rate of 2.5 mL/s with a 20-mL saline flush (hereafter referred to as the 50-mL group). In each group, contrast medium or saline solution was injected using a double injector (Dual-Head Power Injector, Nemoto Kyorindo).
For optimal arterial contrast enhancement, the delay time between the start of contrast medium administration and the start of scanning was obtained for each patient individually using a bolus-tracking technique (SureStart, Toshiba). For this purpose, a single unenhanced low-dose scan was obtained. On the basis of this transverse image, a region of interest (ROI) with an area of 10-15 mm2 was set in the lumen of the distal aortic arch. This ROI served as a reference for the following dynamic measurements of contrast enhancement. Ten seconds after the start of contrast medium administration, repetitive low-dose monitoring scans (120 kV, 20 mAs, 0.5-sec scanning time, 1.0-second interscan delay) were obtained. After the preset contrast enhancement level of 200 H had been reached, MDCT scanning was initiated automatically. MDCT scanning was performed from the thoracic inlet to the common femoral artery in the inguinal ligaments. All scans were obtained with the patient breath-holding in inspiration.
Image Analysis
Transverse sections were reconstructed at a workstation (ZioM900, Zio
Software) by three radiologists. The section thickness was 2.0 mm, and the
interval was 1.0 mm (1.0-mm overlap). To compare the magnitude and uniformity
of arterial enhancement, a 44-cm measurement along the z-axis was
performed using circular ROIs in the center of the aortoiliac artery from the
aortic arch to the iliac arteries at distances of 2 cm for both groups.
Arterial enhancement values in the left and right iliac arteries were
averaged. All densitometric measurements were obtained by the same experienced
radiologist. The variability in enhancement values along the z-axis
was measured as the SD of the enhancement values (plateau deviation). The size
of the ROIs varied according to the target artery. An attempt was made to
magnify the images on the workstation so as not to place the ROI near the
vessel's edge. In addition, for each patient, a radiologist measured the
contrast enhancement values after placement of the ROI in the superior vena
cava (SVC) at the level of the aortic arch and main pulmonary artery.
For qualitative assessment of 3D postprocessing MDCT angiography, postprocessed, volume-rendered, and maximum-intensity-projection data sets were evaluated by two radiologists in consensus.
Statistical Analysis
Statistics were calculated using a commercially available PC software
program (StatView-J 5.0, SAS Institute). Patient characteristics and
quantitative results between the two protocols were compared using an unpaired
Student's t test. Differences in arterial or venous measurements in
each group were compared by a paired Student's t test. Regression
analysis was applied to the correlation between arterial enhancement and
patient body weight. Comparisons between uniformity of arterial enhancement in
each patient along the z-axis and the adequate arterial enhancement
between the groups were performed using Fisher's exact test. Comparison of the
sex distribution for the two groups was performed using the chi-square test.
Values are expressed as means ± SD. Probability values of less than
0.05 were considered statistically significant.
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Patient characteristics in the 100-mL group and 50-mL group are shown in Table 1. There were no statistically significant differences in patient age, patient body weight, distribution of sex, and maximal internal diameter in the thoracic aorta or abdominal aorta between the groups. There was no statistically significant difference in mean scan coverage between the 100-mL group and 50-mL group (624 ± 86 vs 608 ± 91 mm, respectively; p = 0.68) and scan duration (22 ± 3 vs 22 ± 3 seconds, respectively; p = 0.68) between the groups.
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200 H) was comparably observed in 37 (100%) of 37 patients in the 100-mL
group and in 31 (91%) of 34 patients in the 50-mL group (p = 0.10).
Uniform aortoiliac enhancement along the z-axis (< 50 H) was
comparably observed in 33 (89%) of 37 patients in the 100-mL group and in 26
(76%) of 34 patients in the 50-mL group (p = 0.21)
(Table 2).
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In the 100-mL group, there was no significant difference in contrast enhancement between the descending aorta and the main pulmonary artery (329 ± 49 vs 335 ± 95 H, respectively; p = 0.07). In the 50-mL group, however, contrast enhancement was significantly lower in the main pulmonary artery than the descending aorta (211 ± 67 vs 327 ± 57 H, respectively; p < 0.0001). In the 100-mL group, the mean contrast enhancement was significantly higher in the SVC than the ascending aorta (927 ± 577 vs 331 ± 48 H, respectively; p < 0.0001). In the 50-mL group, however, it was significantly lower in the SVC than the ascending aorta (207 ± 71 vs 327 ± 55 H, respectively; p < 0.0001).
In both groups, there were significant moderate inverse correlations between patient body weight and arterial enhancement at the level of 44 cm along the z-axis (Figs. 2A and 2B).
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The ideal enhancement profile postulated for CT angiography is a long, constant plateau [2]. This is because homogeneous arterial enhancement influences 3D postprocessing images. Schoellnast et al. [12] reported that a minimum difference of approximately 30-50 H is necessary to be perceptible to an average observer. In the 50-mL protocol, uniform aortoiliac enhancement along the z-axis (< 50 H) was comparably observed compared with 100-mL protocol. To obtain adequate homogeneous arterial enhancement on MDCT angiography, the injection duration of the contrast medium is recommended to be longer than the scanning duration [2]. To prolong the injection duration, contrast medium followed by 20 mL of saline was injected at a slower flow rate (2.5 mL/s) in the 50-mL protocol. Sadick et al. [13] showed that the 20-mL saline flush can prolong the period of maximal aortic attenuation by 4 seconds. As a result, the total injection duration in the 50-mL protocol was prolonged to 24 seconds.
In the current study, the delay time between bolus detection of 200 H in the descending aorta and the start of scanning is 5 seconds. Accordingly, the duration of contrast medium injection (19 seconds) is slightly shorter than the total scanning duration (mean, 22 seconds). The larger SD of contrast enhancement in the 50-mL protocol, especially at the last portion along the z-axis, might be caused by the difference in the duration, despite the fact that it did not affect the significant reduction in contrast enhancement. However, 16-MDCT with scanning parameters of 16 x 1.0 mm detector collimation, 0.9375 slice pitch, and 30.0 mm/s table feed will enable a shorter scanning duration and better spatial resolution than ours (8 x 2.0 mm detector collimation, 0.875 slice pitch, and 28.0 mm/s table feed). Thus, the current results suggest that 50 mL of contrast medium can produce sufficient contrast enhancement on thoracoabdominal-aortoiliac MDCT angiography and spatial resolution can be further improved by using faster MDCT scanners.
The magnitude of arterial enhancement, which depends on the concentration and amount of contrast medium, also influences the 3D postprocessing images. A previous study suggested that flow rates of 3-5 mL/s are necessary in MDCT angiography for an iodine concentration of contrast material of 300 mg I/mL [14]. Kim et al. [15] found that higher flow rates of contrast medium can produce higher peak enhancement, although it becomes more difficult to obtain the long, constant plateau of arterial enhancement. As a result, a higher flow rate of injection of contrast medium requires larger amounts of contrast material to obtain a constant plateau of arterial enhancement. In the 50-mL protocol, contrast medium with a slightly higher concentration (350 mg I/mL) was injected at a slower flow rate (2.5 mL/s). As a result, a nearly equal iodine delivery rate could be achieved between the 50-mL protocol and 100-mL protocol (0.875 vs 0.9 mg/s, respectively). When a lower threshold of 200 H was considered optimal for the contrast enhancement of the aortoiliac system [12, 16], the 50-mL protocol could produce an adequate magnitude of contrast enhancement on MDCT angiography.
Small amounts of contrast medium have some advantages over large ones. First, in combination with a saline flush, a small amount of contrast material can help to reduce perivenous streak artifacts by removing dense contrast medium from the brachiocephalic veins and SVC [9, 11]. A complete assessment of the thoracic aorta, including the brachiocephalic, carotid, and subclavian arterial branches, is necessary for preoperative thoracic aortic aneurysm endograft procedures and for the endograft to be successfully implanted [17]. In addition, the current results showed that a small amount of contrast medium followed by a saline flush can reduce the high attenuation in the pulmonary artery, resulting in better visualization of the thoracic aorta on 3D postprocessing images.
Second, a small amount of contrast medium is more favorable in reducing relative risk events such as renal toxicity reaction and allergic reactions [18, 19]. Contrast-induced nephropathy, which depends on the amount of contrast medium [20], occurs in 2-10% of patients exposed to intravascular radiographic contrast agents and contributes to significant morbidity and mortality [21]. Fifty milliliters of contrast medium with an iodine concentration of 350 mg I/mL has 17.5 g of iodine dose, which is approximately 60% of that in 100 mL of contrast medium with an iodine concentration of 300 mg I/mL. Third, it is favorable for cost savings.
The current study has some limitations. First, whether 50 mL of contrast
material produces adequate arterial enhancement in heavier patients has not
been determined. The mean patient body weight in the present study was 60 kg,
which is an average weight in Japan
[22]. Thus, the current
results do not necessarily apply to larger patients such as those in Western
countries. There is a correlation between the amount of contrast medium and
patient body weight
[22-24],
which is consistent with our results. Awai et al.
[23,
24] recommend that the dose of
contrast material should be determined according to the patient's weight to
obtain optimal contrast enhancement on MDCT angiography. However, 50 mL of
contrast medium with a 20-mL saline flush could produce adequate arterial
enhancement ( 200 H). This might be because central blood volume is only
approximately 1.2% of patient weight
[25] and because contrast
medium in the arterial phase accumulates only in the blood volume compartment
and the amount in the visceral organs can be ignored
[26]. Second, the current
study included patients not only with aortic aneurysms but also with
postoperative aortic aneurysms. Thus, whether the same results can be
extrapolated to every patient with aortic aneurysm remains to be determined.
Third, a slightly higher concentration (350 mg I/mL) of contrast medium was
used to conform the iodine delivery rate in the 50-mL protocol to that in the
100-mL protocol. The difference in viscosity in contrast medium might affect
the homogeneity of contrast enhancement on MDCT angiography.
In conclusion, in our experience, 50 mL of contrast medium followed by a 20-mL saline flush produces thoracoabdominal-aortoiliac MDCT angiographic examinations of effective quality in most cases.
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