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Thoracoabdominal-Aortoiliac MDCT Angiography Using Reduced Dose of Contrast Material

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

Top Abstract Introduction Materials and Methods Results Discussion References

 
OBJECTIVE. The objective of our study was to compare the image quality of MDCT angiography studies obtained by injection of low doses of contrast medium with saline flush versus conventional doses of contrast medium.

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

Introduction

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Computed tomography angiography has been widely accepted and is preferred to conventional angiography for evaluating the anatomy of major vessels such as the aorta and pulmonary arteries [1-7]. It provides many advantages for imaging the vascular system, including depiction of mural calcium and thrombus and visualization of stent-grafts [1-7].

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.

Materials and Methods

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Study Patients
From September 2002 to May 2004, 71 patients (54 men and 17 women; age range, 36-86 years; mean age ± SD, 73 ± 11 years) underwent MDCT examination for preoperative assessment of a thoracic aortic aneurysm or an abdominal aortic aneurysm or because postoperative complications such as pneumonia and renal infarction were suspected. Other indications for postoperative MDCT examinations were follow-up of comorbid diseases such as arterial occlusive diseases and aortic aneurysms located in other regions that were not the operative bed and tumors incidentally found during previous CT examinations. Patients who had renal failure (serum creatine level, > 2 mg/dL), congestive heart failure, respiratory failure, hepatic failure, poor general condition, or contraindication for iodinated contrast medium were excluded from the study. Patients who had an endoleak (n = 2) or aortic dissection (n = 4) revealed on postoperative MDCT examinations were also excluded from this study. Of 71 patients, 24 underwent endovascular stent-graft procedures and 29 had open surgical procedures.

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.

Results

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MDCT angiography was performed without complications in all patients, and no studies had to be repeated because of technical problems. All patients were able to complete the examination within a single breath-hold.

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.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Graphs show mean arterial enhancement and SD along z-axis for both patient groups. Mean arterial enhancement and SD along z-axis are shown for 100-mL group. Note small deviations in arterial enhancement values and ideal plateau-shaped aortoiliac enhancement profile along z-axis.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Graphs show mean arterial enhancement and SD along z-axis for both patient groups. Mean arterial enhancement and SD along z-axis are shown for 50-mL group. Note shallow saddle-shaped aortoiliac enhancement profile with slightly larger SD compared with that in 100-mL group.

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).

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —Graphs show effect of patient weight on enhancement. Correlations between arterial enhancement at level of 44 cm along z-axis and patient body weight in 100-mL group (A) and 50-mL group (B) are shown.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —Graphs show effect of patient weight on enhancement. Correlations between arterial enhancement at level of 44 cm along z-axis and patient body weight in 100-mL group (A) and 50-mL group (B) are shown.

View larger version (84K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —Coronal maximum-intensity-projection images of two patients. Image of 77-year-old man in 100-mL group after endovascular repair of thoracic aortic aneurysm and with residual abdominal aortic aneurysm shows uniform enhancement throughout thoracoabdominal-aortoiliac system: top of aortic arch, 356 H; origin of celiac axis, 354 H; and origin of common iliac artery, 343 H. Perivenous artifacts generated by pooling of contrast agent in superior vena cava (SVC) and high attenuation in pulmonary trunk cause blurred MDCT angiogram when injecting 100 mL of contrast medium only; SVC (arrow) was 1,777 H and main pulmonary artery (arrowhead) was 425 H.

View larger version (82K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —Coronal maximum-intensity-projection images of two patients. Image of 79-year-old man in 50-mL group after endovascular stent-graft procedure of abdominal aortic aneurysm shows uniform enhancement throughout thoracoabdominal-aortoiliac system: top of aortic arch, 349 H; origin of celiac axis, 330 H; and origin of common iliac artery, 325 H. Administration of 50 mL of contrast medium followed by 20-mL saline flush diminished perivenous artifacts generated by pooling of contrast agent in SVC and reduced high attenuation in pulmonary trunk; SVC (arrow) was 118 H and main pulmonary artery (arrowhead) was 258 H.

View larger version (85K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A —79-year-old woman in 50-mL group with preoperative thoracic aortic aneurysm and postoperative abdominal aortic aneurysm. Coronal maximum-intensity-projection (A) and multiplanar reconstruction (B) images clearly show relationship between aortic branches and thoracic aortic aneurysm.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B —79-year-old woman in 50-mL group with preoperative thoracic aortic aneurysm and postoperative abdominal aortic aneurysm. Coronal maximum-intensity-projection (A) and multiplanar reconstruction (B) images clearly show relationship between aortic branches and thoracic aortic aneurysm.

View larger version (64K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —81-year-old woman in 50-mL group after endovascular stent-graft procedure of thoracic aortic aneurysm and intravascular stent procedure in right external iliac artery. Coronal maximum-intensity-projection image reveals residual abdominal aortic aneurysm, partial right renal infarction (open arrow), occlusion of left superficial femoral artery (solid arrow), and severe stenosis of left internal iliac artery (arrowhead).

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —81-year-old woman in 50-mL group after endovascular stent-graft procedure of thoracic aortic aneurysm and intravascular stent procedure in right external iliac artery. Axial CT image shows partial right renal infarction, which is also visible on A.

View larger version (82K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A —78-year-old man with emphysema in 50-mL group who presented for follow-up after endovascular stent-graft procedure for abdominal aortic aneurysm. Coronal maximum-intensity-projection image shows occlusion of right internal iliac artery distal to intravascular stent (arrow) and slightly enhanced lesion in right lower region (arrowhead).

View larger version (97K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B —78-year-old man with emphysema in 50-mL group who presented for follow-up after endovascular stent-graft procedure for abdominal aortic aneurysm. Axial CT images show consolidation in right lung.

View larger version (118K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6C —78-year-old man with emphysema in 50-mL group who presented for follow-up after endovascular stent-graft procedure for abdominal aortic aneurysm. Axial CT images show consolidation in right lung.

Top Abstract Introduction Materials and Methods Results Discussion References

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.

References

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kubo, S. Articles by Togashi, K. Search for Related Content PubMed PubMed Citation Articles by Kubo, S. Articles by Togashi, K. Social Bookmarking                      What's this? Hotlight (NEW!) What's Hotlight?